9

AQUATIC FUNGI

9.1 INTRODUCTION
A number of different groups of fungi are found in water, including many
Mastigomycotina (zoosporic fungi), some Zygomycotina, Ascomycotina,
Deuteromycotina, yeasts and a few Basidiomycotina. Some may inhabit
water for the whole of their lives, others may be amphibious, with one
stage of their life cycle spent in, adapted to and dispersed under water, and
another stage dispersed in air. Yet others may have a transient aquatic
existence, possibly brought on a substratum by wind or swept by floods
into water. The spores of many terrestrial fungi are carried into water by
rain, and so may be isolated by conventional mycological techniques. It is
therefore necessary to define carefully the term aquatic fungus. Park
(1972b) has introduced a number of useful terms in distinguishing between
the activities of heterotrophic microorganisms in fresh water.
Microorganisms recorded from an aquatic site by observation or by
isolation may or may not have originated there. Those that have, and
that maintain themselves in that habitat and spend all their life-history
there, may be regarded as indigenous organisms or indwellers. Other
organisms present may normally have an extra-aquatic habitat. Those in
which the main habitat is clearly extra-aquatic might be regarded as
immigrants in relation to water, whilst those that alternate periodically
between aquatic and extra-aquatic habitats might be regarded as
migrants. Versatiles might be an appropriate term for organisms in
which it could be shown that the movement between aquatic and
extra-aquatic habitats is haphazard rather than regular.
Park has summarized these terms in the form of Table 9.1
Dick (1976) has produced a similar classification of the role of fungi in
freshwater habitats.
The degree of adaptation of a microorganism to an aquatic environment
may vary. Indwelling organisms are fully adapted to life in water in the

N. J. Dix et al., Fungal Ecology

© Neville J Dix and John Webster 1995
226 AQUATIC FUNGI

Table 9.1 Classification of heterotrophic microorganisms resident in water

Category Presence in water

Indwellers Permanent
{
Resident . {Migrants Periodic}
Immigrants Not permanent
Versatiles Irregular

Source: Park (1972b).

sense that they are able to maintain their biomass at a more or less constant
level from year to year as substrates and nutrients become available, and
most are capable of sporulating in the water. Transients might arrive at the
aquatic habitat and immediately start to decline in activity as a result of
changes in conditions, e.g. availability of oxygen, loss of nutrients by
leaching, or in the face of competition by organisms better adapted to life
in water. Such organisms may arrive already active within their substra-
tum, but unable to sporulate, and so be unable to colonize new substrata
within the aquatic habitat. There is obviously a wide range of possible
behaviours, and this has been presented by Park in Table 9.2 for
decomposer organisms.
As well as decomposer microorganisms, there are parasites attacking
hosts such as plankton, algae, macrophytes and various animals.

9.1.1 Aquatic habitats

There is a great diversity of aquatic habitats found on earth, ranging from
temporary pools, ponds, lakes, bogs, fens, streams, rivers, estuaries,
inland salt lakes, melting ice and thermal springs to seas and oceans. At
their margins, these water masses are in contact with rock, soil and mud,

Table 9.2 Possible combinations of categories of presence and activity in decom-

posers in water

Possible activity categories

Presence category
Constant Periodic Sporadic No activity

Indwellers + + +
Residents { Migrants + +
Versa tiles + +
Transients +

Source: Park (1972b)

 FRESHWATER FUNGI 227

and may be fringed with vegetation. There is also an interface with air. The
soil, mud and vegetation contain fungi which may release spores into the
water. The water masses may also contain numerous organisms which, in
the living or dead state, may provide a source of nutrients for parasitic or
saprophytic fungi. It is clear that the ecology of aquatic fungi is an
enormous subject, and selection of topics must be made. Valuable review
articles on the ecology of different groups of aquatic fungi can be found in
Jones (1976a), Sparrow (1968), Dick (1976), Kohlmeyer and Kohlmeyer
(1979), Kohlmeyer (1981a), Webster and Descals (1981), Moss (1986a),
Biirlocher (1992a) and Shearer (1993).

9.1.2 Techniques in aquatic fungus ecology

As discussed in Chapter 1, there are many different aspects of fungal

ecology, and the technique chosen will depend not only on the aspect to be
studied, but also on the group of fungi involved. Different techniques
applied to the same body of water may yield entirely different results. Park
(1972a) used dilution plate, direct observation, baiting and particle plate
techniques on the same bodies of water and obtained strikingly different
lists of species by the four methods. The baiting technique yielded a
preponderance of zoosporic fungi (Oomycetes), and direct observation
yielded Oomycetes and aquatic hyphomycetes. Particle plating produced
colonies of only one Oomycete (Pythium), large numbers of fungi normally
associated with moribund, terrestrial plant material (e.g. Alternaria,
Cladosporium, Aureobasidium and Epicoccum) and no aquatic hyphomy-
cetes. The dilution plates bore many fungi previously recorded from soil,
no Oomycetes and only one aquatic hyphomycete (Heliscus). This com-
parative study emphasizes how important it is to use a range of techniques,
and to select the most appropriate technique for the investigation. For a
discussion on the limitations of techniques for studying the ecology of
zoosporic aquatic fungi, see Dick (1976).
In the account which follows, aspects of the ecology of selected groups of
aquatic fungi will be considered.

9.2 FRESHWATER FUNGI

9.2.1 Chytridiomycetes

a) Chytridiales
Chytridiales live in the sea and in fresh water, in mud and in soil. Some are
biotrophic parasites of higher plants, but these are not considered here.
For the most part, their thalli are eucarpic and monocentric, but forms with
holocarpic thalli or eucarpic polycentric thalli are also known. They
228 AQUATIC FUNGI

reproduce by posteriorly uniflagellate zoospores. Taxonomic accounts of

the group have been given by Sparrow (1960, 1973) and Barr (1989), and
figures by Karling (1977). Because of their simple morphology, identifica-
tion is difficult, and this, in turn, creates difficulties in ecological studies
(Miller, 1976; Masters, 1976). Chytrids are of ecological importance as
parasites of planktonic algae such as diatoms, desmids and filamentous
green algae. They are also ecologically significant as decomposers of the
remains of algae, other plants and animals. They can be studied by direct
microscopic observations on these substrata, or by baiting water samples
with various materials such as pollen grains, Cellophane (cellulose), snake
skin, insect wings (chitin) or hair (keratin) (Sparrow, 1960).
Parasitic chytrids
Classical studies on the effects of chytrids parasitizing the phytoplankton
diatom Asterionella formosa in the English Lake District have been
reported by Canter and Lund (1948, 1951, 1953). A. formosa forms
colonies composed of radially arranged frustules looking like the spokes of
a cartwheel (Figure 9.1). The diatom is parasitized by several chytrids.
These include Rhizophydium planktonicum, Zygorrhizidium affluens and
Z. planktonicum, which are somewhat similar in morphology. Before the
distinction between the first two taxa had been pointed out by Canter
(1969), they had both been regarded as belonging toR. planktonicum. To
avoid confusion, the mixed populations are referred to as R. planktonicum
agg., whilst the 'true' R. planktonicum is referred to as R. planktonicum
Canter emend. (or R. planktonicum emend.). The parasite forms an
epibiotic sporangium by enlargement of an encysted zoospore on the
outside of the host cell, and a thread-like internal rhizoid which may
extend throughout the whole length of the diatom frustule. Some 2-3 days
after infection, the sporangia mature and release posteriorly uniflagellate
zoospores. Infection of the host is enhanced by high light intensity,
possibly because it stimulates zoospore motility and also because it causes
the host cells to become more susceptible (Canter and Jaworski, 1980,
1981).
One result of infection of an Asterionella colony by R. planktonicum agg.
is to reduce the number of cells per colony (Figure 9.2). This effect is
presumably due to the failure of infected diatom cells to divide in the
normal way. Epidemics of Rhizophydium (i.e. over 25% of Asterionella
infected) may be associated with drastic reductions in the concentration of
cells in the top 5 m of lake water. In Esthwaite Water during an epidemic
from 25 October to 6 November 1947, the proportion of infected cells rose
to 38%, whilst the number of Asterionella cells fell from 121 to 37 em - 3
and the mean number of cells per colony fell from over 8 to below 4. In
September 1948, more than 90% of Asterionella cells in Windermere South
Basin were infected (Figure 9.2).
 FRESHWATER FUNGI 229

Figure 9.1 Rhizophydium planktonicum agg. (a) A healthy colony of Asterionella

with well-developed chromatophores and an encysted zoospore of Rhizophydium
(x). (b) Heavily parasitized Asteria nella colony, with more or less disorganized cell
contents and Rhizophydium sporangia (live or dehisced). Scale bar = 20 J.Lm.
(Reproduced from Canter and Lund, 1948 with permission.)

The reasons for the incidence of epidemics are not clear. They may occur
at any time when the numbers of Asterionella cells exceed 10 cm- 3 .
Epidemics appear not to be related to the limitations of host nutrients such
as available phosphate, nitrate or silica, which are often rising in concen-
tration during epidemics. Clear-cut epidemics occur only in the more
eutrophic waters. It is likely that the pathogen may survive in low numbers
in Asterionella populations at all times in the zoosporangial state.
Canter and Jaworski (1979) showed that certain Asterionella clones were
highly compatible with the parasite, whilst others appeared to be incom-
patible, sometimes showing a hypersensitive response in which, soon after
penetration, the host cell died rapidly and the encysted zoospore did not
enlarge into a sporangium.

Saprophytic chytrids
Pine pollen is a natural substratum for saprophytic chytrids, and samples of
pollen scooped from the surface of a lake or pond are invariably colonized
by monocentric, epibiotic chytrids, including species of Rhizophydium
(Figure 9.3). By serial dilutions of lake water with sterile water to which
sterilized pine pollen is added, it is possible to make quantitative estimates
of the concentration of zoospores of chytrids capable of colonizing pine
pollen. This technique, which resembles the most probable number (MPN)
technique used by bacteriologists to estimate bacterial numbers, has been
230 AQUATIC FUNGI

'7
E
u
t.i

Figure 9.2 Parasitism of Asterionella formosa in Windermere South Basin in

autumn 1946 . ._.Number of live cells per colony (C. Co); 0-0 = number of live
cells cm- 3 (C.cm- 3 , logarithmic scale); 0- -0 = theoretical numbers of cells which
might have been produced in the absence of parasitism. 6.-6. = Percentage of cells
infected by Rhizophydium planktonicum agg. (%P). (Redrawn from Canter and
Lund, 1951 with permission.)

used by Ulken and Sparrow (1968) in Douglas Lake, Michigan, USA.

Douglas Lake is a eutrophic lake surrounded by a conifer-aspen forest. In
spring, it receives a large input of Pinus pollen, which may remain floating.
Estimates of chytrid propagules in the epilimnion of lake water rose from
less than 100 to over 900 1- 1 between 8 and 23 June 1967 (Figure 9.4). This
results from the availability of large numbers of pollen grains at that time
of the year. The pollen bore numerous sporangia of Rhizophydium. A
second peak of propagules of about 600 1- 1 was detected in early July,
corresponding to the release of pollen from a different species of Pinus.
The high numbers declined to about 30 l- 1 in the epilimnion by mid-
August, possibly due to leaching of nutrients from the pollen, sinking of
the pollen, and predation by fungi and protozoa. The numbers of
propagules estimated in the epilimnion were always greater than those in
 FRESHWATER FUNGI 231

Figure 9.3 Rhizophydium sp. Epibiotic sporangium attached by means of rhizoids

to the central fertile cell of a pine pollen grain.

the hypolimnion. It is likely that pollen grains sinking through the

hypolimnion onto the mud surface develop resting sporangia which give
rise to zoospores which provide the inoculum for new infections the
following spring at the overturn of stratification of water layers in the lake
associated with the melting of ice cover and wind action.

9.2.2 Oomycetes
Oomycetes are not related to 'true' fungi such as Zygomycotina, Ascomy-
cotina and Basidiomycotina. They reproduce asexually by biflagellate
heterokont zoospores and are sometimes classified in a separate kingdom
(Heterokonta) along with other heterokont organisms (Dick, 1989). We
shall consider aspects of the ecology of members of three orders -
Leptomitales, Saprolegniales and Peronosporales.

a) Leptomitales
The Leptomitales are freshwater fungi with a characteristic constricted
tubular mycelium plugged at the constrictions by spherical plugs of cellulin.
Asexual reproduction is by biflagellate zoospores, and sexual reproduc-
tion, where present, is oogamous (Dick, 1973b, 1989; Sparrow, 1960).
Most members of the group grow on vegetable debris such as twigs or
 232 AQUATIC FUNGI

900

800

700

600
l...
(/)
..21 500 Epilimnion
::J
Cl
co
Q.
400
...
0
0..
300

200

100 ._Hypolimnion
,•... ,
0
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-----·--------.
~ ,'' ' '•... ........
I 8 1215 1922 262913 6 1013 1720 2427 31 3 710 1417 21 Date
~June July !~August-----+
Figure 9.4 Fluctuations of number of propagules of zoosporic fungi in the
epilimnion and hypolimnion of Douglas Lake, Michigan, USA during spring and
summer 1967. (Reproduced from Ulken and Sparrow, 1968 with permisison.)

fruits. Their ecological requirements are diverse. For example, Apodach-

lya and Sapromyces thrive in well-oxygenated water, whilst Aqualinderella
is obligately fermentative and develops best in COrrich waters on
submerged fruits in tropical and subtropical streams. It does not require
oxygen for growth, and the presence of free oxygen may indeed inhibit
growth. Growth is stimulated by high levels (5-20%) of C0 2 • In pure
culture the fungus undertakes a homolactic fermentation. It has been
considered as 'an oxygen indifferent, facultative anaerobe resembling the
lactic acid bacteria' (Emerson and Weston, 1967; Emerson and Held,
1969).
Leptomitus lacteus forms part of a complex of organisms, making up the
so-called 'sewage fungus' associated with rivers polluted with organic
matter, especially at acid pH values. Rope-like strands of slimy growth
may extend down-river for several kilometres from the source of pollution
such as outfalls from distilleries and sewage works. The growths include
not only L. lacteus, but other fungi such as Fusarium aqueductuum and
Geotrichum candidum, bacteria, ciliates and algae (E.J.C. Curtis, 1969).
Although abundant in polluted sites associated with human activities, the
fungus has also been recorded in cleaner rivers and around lake margins
 FRESHWATER FUNGI 233

(Park, 1972a) and has been reported as an external parasite on freshwater

fish (Willoughby, 1978). Propagules have been estimated to occur in low
concentrations in Windermere, about 0.7 spores l- 1 at the lake margin, to
less than one in 20 I from the lake centre, implying that the fungus is active
in decomposition in the littoral zone (Willoughby and Roberts, 1991).
Studies of the nutrition of L. lacteus in pure culture reveal a number of
distinctive features. It is unable to utilize sugars as sources of carbon, but
can grow on organic and fatty acids. Inorganic nitrogen sources are unable
to support growth, and amino-acids are required. By contrast with
Aqualinderella, it is highly aerobic and, whilst it can survive periods of
oxygen deprivation, oxygen is required for active growth (Gleason, 1968).
So far, no sexual stage has been discovered.

b) Saprolegniales
The Saprolegniales are the best-known group of water-moulds and are
abundant and worldwide in their distribution. An outline of their tax-
onomy has been given by Dick (1973a, 1989). Their substrata are plant and
animal remains in freshwater, mud and soil, and some are parasites of
plants, fish, crustacea and other animals. Common genera include Sapro-
legnia, Achlya, Dictyuchus and Aphanomyces.
Varied techniques for studying the ecology of Saprolegniales have been
developed, and the effectiveness of some of them has been compared by
Dick (1966, 1976).

Baiting
The classical method for detecting the presence of Saprolegniales is by the
use of baits (e.g. boiled hemp seeds, dead insects) floated in water.
Colonization is dependent on the ability of zoospores to move chemotacti-
cally towards the bait. Baits placed in contact with soil or mud may be
colonized by mycelium or from zoospores. Dick (1966, 1971) developed
the use of hemp seed bait in a quantitative assay which enabled estimates
to be made of the minimum propagule number in a water or mud {slurry)
sample. The number of species obtained is relatively insensitive to the
amount of slurry used. The origin of the inoculum which colonizes the
hemp seed bait is uncertain, and could be derived from a zoospore, a
vegetative hypha, a germinating gemma or a germinating zoospore.
Diluted slurry can also be incorporated into cornmeal agar plates. The
agar is then cut into small blocks, each of which is baited with a hemp seed
in water. Using this method, Dick (1971) has estimated minimum prop-
agule numbers between 300 and 1000 g- 1 dry soil from a slope adjoining
Blelham Tarn in the English Lake District. These values are low in relation
to those of other groups of soil fungi.
234 AQUATIC FUNGI

Agar cultures
Willoughby (1962) added samples of lake water to molten dilute oat agar.
When cut into sectors, and incubated in sterile water, those sectors of agar
which had contained a viable propagule of a member of the Saprolegni-
aceae developed a coarse mycelium and, in some cases, sporangia.
Estimates of propagule number per litre of lake water could be made.
Values as high as 5200 1- 1 (i.e. about 5 propagules cm- 3) have been
reported for Windermere.

Cold-setting gels
Cold-setting gels, e.g. calcium carboxymethylcellulose (Polycell gel, a
wallpaper paste) or the more transparent hydroxyethylcellulose (Natrosol
250) have been used to overcome problems caused by the high temperature
of molten agar (c. 45°C) (Willoughby, Pickering and Johnson 1984; Celio
and Padgett, 1989). The low concentrations of nutrients minimize the
growth of contaminants. Antibiotics such as pimaricin, which selects for
Oomycetes and suppresses growth of other fungi, improve the recovery of
Oomycete propagules.

Centrifugation
Continuous flow centrifugation of lake water was used by Fuller and
Poyton (1964) to concentrate propagules of aquatic fungi prior to plating.
This technique has been developed by Hallett and Dick (1981) in a study of
Oomycete (mostly Saprolegniales and Peronosporales) propagule number
in a freshwater lake.

Direct observation of natural substrata

In some circumstances, it may be possible to assess the frequency of
colonization of natural substrata such as the proportion of fish eggs
colonized by Saprolegniaceae. Dick (1970) followed the colonization of
insect exuviae by Saprolegniaceae in Marion Lake, near Vancouver,
Canada.
Using these techniques, information has been obtained on various aspects
of the ecology of Saprolegniaceae (see Sparrow, 1968; Dick, 1976).

Distribution
A comparison of the frequency of isolation of Saprolegniaceae from littoral
(i.e. lake margin) and benthic (i.e. bottom) muds was made by Dick (1971)
using the hemp seed baiting technique and 10 replicate mud samples of
about 10 cm3 , along transects extending from the shore to the open water
at Lake Marion near Vancouver (Figure 9.5). The maximum number of
 FRESHWATER FUNGI 235

species (about 15) was found at the lake margin. This is also the region of
maximum abundance as indicated by the fall in the number of negative
samples near the lake margin. In the benthic muds, a high proportion (up
to 70%) of the samples were negative, indicating a low level of abundance.
Although the bottom muds contain viable propagules of Saprolegniaceae,
there is little evidence of biological activity. Dick has concluded that most
Saprolegniaceae are primarily fungi of the emergent littoral and benthic/
littoral interface, and very few are clearly benthic.
Collins and Willoughby (1962) and Willoughby and Collins (1966) have
made similar conclusions from a study of the frequency of isolation of
Saprolegniaceae in water and mud samples in and around Blelham Tarn in
the English Lake District. The high incidence of colony counts following
periods of heavy rainfall suggests that spores may be carried into the lake
from the surrounding drainage basin. Counts from mud samples near the
centre of the lake, taken from the hypolimnion, where the dissolved
oxygen content may be zero, were lower than in the water column above,
or at the lake margin. Possibly spores do not survive for very long in the
anaerobic conditions of the mud beneath the hypolimnion. Comparison of

70
25
60
/
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50 ~
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~-------,-~----------------------- } ~:t~r

------------- _____ 1 m---- level

Profile (variable scale)

Figure 9.5 Transect from the margin of Lake Marion, Vancouver, Canada,
showing the number of negative samples and the mean percentage of the total
species of Saprolegniaceae recorded. 0--0 =Negative samples,: 6.-6. =mean%
of total species recorded. (Redrawn from Dick, 1971 with permission.)
236 AQUATIC FUNGI

the colony counts taken from inflow streams with lake counts showed the
striking effect of a small sewage outfall on the incidence of Saprolegni-
aceae. Above the sewage outfall, the mean spore concentration was 20 1- 1
whilst below the outfall the concentration was 312 1- 1• A major component
(about one-third of all isolates) from the sewage effluent was Saprolegnia
ferax/mixta, whilst Achlya was not represented.
Systematic studies carried out in a series of defined habitat areas permit
comparisons of the degree of correlation between the species composition
characteristic of the sites chosen. Dick (1971) used an index of similarity to
compare the Oomycete populations from different sites. Sites showing a
high degree of similarity were Sphagnum-dominated or acid humus areas
in which Saprolegnia litoralis is abundant, associated with Scoliolegnia
asterophora or Saprolegnia turfosa.
The occurrence of Saprolegniaceae may be affected by variations in the
pH of the water. However, the pH of a body of water is not constant, and
can vary over wide limits due to the photosynthetic activity of algae and
macrophytes in the water. The depletion of dissolved carbon dioxide
causes a rise in pH, especially in water with low bicarbonate concentration.
For this reason, data on frequency of occurrence in relation to pH may be
misleading if based only on a single pH reading. Also Saprolegniaceae
show seasonal variations in abundance, so that isolated records indicating
absence may reflect season rather than adverse effects of pH.
A classical study of the distribution of Saprolegniaceae in relation to pH
is that of Lund (1934) who recorded the occurrence of aquatic fungi from a
range of aquatic habitats in Denmark. However, his samples and pH
records were not repeatedly collected at regular intervals. He classified the
aquatic habitats into five groups in relation to pH, and listed the fungi
characteristic of each type of habitat:
• Highly acid (pH 3.5--4.5), mostly represented by bogs: Saprolegnia
delica, S. diclina, S. litoralis.
• Slightly acid (pH 5.5-6.8); no Oomycete flora was found to be peculiar
to these waters; the range of species resembles that of highly acid
waters.
• Neutrally acid (pH 5.3-7.5): S. diclina, Dictyuchus sterile.
• Neutrally alkaline (pH 6.5-7.7), e.g. ponds or pools with stagnant water
and abundant decaying plant material: S. ferax, S. monoica, Achlya
radios a.
• Constantly alkaline (pH 7.0-8.4), e.g. certain lakes: S. ferax, S.
hypogyna, Aphanomyces laevis.
Certain species, e.g. Achlya racemosa, occurred over the whole pH range.
Roberts (1963) made regular baitings with hemp seed from water
samples with a pH range from 3.6 to 8.0 from 21 sites in Britain over a
19-month period. There was seasonal variation in occurrence, some species
 FRESHWATER FUNGI 237

being found in summer, others in winter. She classified the fungi found into
three groups in relation to pH variation:
• Acid group: Species found in soft waters below pH 5.2, e.g. Achlya
americana, Saprolegnia litoralis.
• Neutral group: Species found in waters with a wide pH range between
5.2 and 7.4, e.g. Achlya racemosa, Saprolegnia monoica.
• Alkaline group: Species found in waters of pH 7.8 or above, e.g. Achlya
polyandra, Saprolegnia ferax.
In general, Roberts's work confirms the findings of Lund.
It is important to realize that pH, i.e. hydrogen concentration, has
complex effects not only on the activity of zoospores (Smith, Armstrong
and Rimmer, 1984), mycelial growth, enzyme activity and reproduction,
but also affects the availability of salts such as those of calcium, potassium,
magnesium, iron and phosphorus, and the form in which nitrogen is
present. pH also affects the growth of higher plants, and this in turn affects
the substrates available. At low pH, competition with bacteria is also lower
than at high pH.
Saprolegniaceae are rare in the sea and are among the most sensitive
fungi to decreasing solute potential (i.e. to increased concentration of
dissolved salts; Duniway, 1979). Estuaries also represent a hostile environ-
ment (Hohnk, 1935, 1939, 1952, 1953). Natural sea water has a salinity of
about 35%o, but in estuaries the salinity level can fluctuate from zero at low
tide to that of normal sea water at high tide. When suitable baits were
added to water samples collected at various points along a salinity gradient
provided by an estuary, Saprolegniaceae were found on baits in water
samples with up to 2.8%o salinity values (Te Strake, 1959). Later work, in
which baits were left in situ at different points along an estuary for periods
up to 3 days, gave positive colonization in water where the salinity values
varied up to 12%o (Padgett, 1978a). It is doubtful that the inoculum which
colonized baits at the higher salinity levels was released close to them; it
had possibly been carried from freshwater sources higher up the river
system feeding the estuary. Physiological studies show that zoospore
germination, mycelial growth rate, respiration rate and asexual reproduc-
tion become progressively reduced as salinity increases (e.g. Te Strake,
1959; Harrison and Jones, 1971, 1974; Padgett, 1978b; Padgett et al., 1988;
Smith, Ince and Armstrong, 1990), and that at values around 10%o (i.e. less
than one-third full-strength sea water), plasmolysis of hyphae may occur.
When saprolegniaceous fungi established on floating substrata are swept
into the sea, they are not immediately killed. Padgett (1984) showed that
Saprolegniaceae in hemp seed and twig baits could survive salinity changes
of 0-35%o for 48 h and, when returned to fresh water, were capable of
colonizing fresh baits. The formation of gemmae may aid survival.
An interesting aspect of salinity tolerance is the question of whether
238 AQUATIC FUNGI

Saprolegnia parasitica, the cause of ulcerative necrosis of salmonid fish,

can survive the transition from fresh water to the sea and back as young
salmon leave the spawning ground, migrate to the sea and then return to
fresh water to reproduce.

Periodicity
Changes in the frequency of isolation using a standard technique or
estimates of propagule availability show considerable variations with time,
both in the relatively short term (e.g. diurnal periodicity) or in the longer
term (e.g. cyclical variation throughout the year). Hallett and Dick (1981),
using a centrifugation-plating technique in a small ornamental lake at
Reading, England, found two discrete diurnal peaks of Oomycete prop-
agule availability (Saprolegniales and Peronosporales) from 12.00 to 16.00
and 20.00 to 24.00 h. So far, no satisfactory hypothesis to explain these
variations has been advanced, and more work on diurnal fluctuation is
needed.
Suzuki (1961a,b, in Sparrow, 1968) has claimed that there is a diurnal
migration of zoospores of aquatic fungi, closely correlated with the diurnal
vertical distribution of dissolved oxygen in spring and autumn. They report
that zoospores assemble in the oxygen-rich water layers, a conclusion
supported by the finding by Smith, Armstrong and Rimmer (1984) that
zoospore production and encystment in Saprolegnia diclina are markedly
affected by oxygen concentration, being much reduced at low concentra-
tions.
Willoughby (1962) and Clausz (1974) have indicated that there are two
peak periods of annual zoospore availability in north temperate latitudes:
late spring and autumn, with maximum availability in autumn. However,
Hallett and Dick (1981) demonstrated the existence of three, rather than
two, periods of peak availability of Oomycete propagules in a Reading
lake. Attempts to correlate periods of maximum availability with environ-
mental variables have not met with success.
Wood (1988), working in the River North Tyne, England, using the
Polycell gel assay technique, found that the total Saprolegniaceae counts
rose sharply in February, and peaked in the summer months June and
August. After August, colony counts began to decline towards lower
winter levels between December and February.
Using techniques in which standardized baits were added to water or
muds at different times of the year, seasonal patterns in frequency of
isolation have been reported (e.g. Coker, 1923; Perrott, 1960; Hughes,
1962; Roberts, 1963; Alabi, 1971a,b; Okane, 1978, 1981; Gupta and
Mehrotra, 1989). Temperature has an important effect, some species being
isolated with equal frequency throughout the year, whilst others are more
frequent during periods of low or high temperature. Since colonization of
 FRESHWATER FUNGI 239

the baits is by zoospores, variations in frequency of isolation may be

related to zoospore production and motility. At times when species are not
reported, they must be surviving in a different form, e.g. as oospores, and
the factors relating to oospore maturation, survival and germination may
also be correlated with seasonal periodicity. It has been claimed (e.g. by
Ziegler, 1958; Hughes, 1962; Klich and Tiffany, 1985) that seasonal
abundance and also latitudinal distribution are correlated with oospore
morphology. In some species, there is a large spherical lipid globule placed
centrally within the oospore (centric types), whilst in others the globule is
subcentric or eccentric. Species with centric and subcentric oospores are
isolated more frequently in cooler months, whilst species with eccentric
oospores are more frequently isolated in warmer conditions. This is also
correlated with the fact that species with eccentric oospores are reported to
be more common in warmer countries. The underlying reasons are
unknown.
Whilst variations in seasonal periodicity may well be directly controlled
by temperature differences, it should be borne in mind that temperature
and other correlated features such as daylength also affect surrounding or
submerged vegetation in ponds and rivers, and in turn there are seasonal
variations in animal populations, all of which may provide substrata for
growth of Saprolegniaceae. Seasonal changes in rainfall, water flow, etc.,
may also exert effects.

c) Peronosporales
The Peronosporales may exist as saprophytes or as parasites of plants in
soil or water (Waterhouse, 1973; Dick, 1989). One family, the Pythi-
aceae, is well represented in freshwater habitats by two genera, Pythium
and Phytophthora. Park (1975) used a selective medium for the isolation
of some unusual cellulolytic species of Pythium from river water in
Northern Ireland. The selective medium makes use of the nutritional
preference of these species of Pythium for cellulose and the insensitivity
of Pythium to the polyene antibiotics pimaricin and vancomycin.
Pythium species isolated using this technique were the zoosporic P.
fluminum var. fluminum and P. fluminum var. flavum, and a second
species in which zoospores were not observed, P. uladhum (Park, 1977).
Pythium fluminum var. fluminum may be adapted to growth in rivers
where the concentration of dissolved nutrients is often low. Both
species, including the two varieties, possess characteristic pigments.
The possession of these pigments, and the selective medium for
isolation, have enabled Park and McKee (1978) to follow the 'numbers'
of these fungi in river water using filter papers, protected in mesh bags,
anchored in the river. At intervals, the filter papers were removed and
sampled by plating 5 mm discs on selective medium. After 3 days, all
240 AQUATIC FUNGI

discs bore cellulolytic Pythium, indicating early and progressive arrival

of inoculum. Rates of arrival of propagules can also be calculated and it
has been estimated that the arrival rate was about 0.5 propagule
cm- 2 h- 1 per surface exposed. It is also possible to make an estimate of
the concentration of Pythium propagules in river water by filtering a
sample and placing the filter paper on selective medium without
cellulose. The pigmented colonies which develop can then be counted
and identified. Using this technique, propagule numbers varying from
less than 1 to over 4000 1- 1 have been estimated.
Park (1980b) has extended these studies of the numbers of cellulolytic
Pythium species over a 2-year period in two rivers in Northern Ireland.
The numbers were plotted as log 10 against time. Decreases in numbers
were found associated with decreased levels of river water, and a
general increase was found immediately after a period of heavy rain. No
significant correlation was found for the two varieties of P. fluminum in
either river against pH. They showed different correlations with river
temperature. For P. fluminum var. fluminum, there was no significant
relationship with temperature, but for var. f/avum, high numbers were
significantly correlated with low temperature, and low numbers with
high temperature. Very significant relationships between numbers of
var. fluminum and previous rainfall and river flow rate were demon-
strated. At the optimum river flow rates, the peak numbers were about
10 times higher than at the lowest flow rates. It is only possible to
speculate on the underlying causes of the correlation between numbers
and rainfall or flow rate. Possibly substrata containing the fungus could
be washed into the river from surrounding habitats, or from soil.
Possibly increased flow and turbulence stimulates sporulation, or stirs
up sediment so that propagules become suspended.
The colonization of filter paper in a stream has also been studied by
Willoughby and Redhead (1973) who observed that, although the filter
paper lost more than half its initial dry weight in 50 days, its nitrogen
content increased on a percentage basis from about 0.2% to 1.47%, and in
absolute amounts from 9 x 10- 6 g em - 2 of filter paper at 8 days to 190 x
10- 6 g em - 2 at 50 days. The capacity of microorganisms to concentrate
nitrogen in a substratum decomposing in a stream is an important
phenomenon which will be discussed more fully in relation to leaf decay in
streams by aquatic hyphomycetes (section 9.2.3(b)).
Studies by Kirby (1984) have shown that Pythium species are impor-
tant early colonizers of the leaves of Ranuncu/us penicillatus, both of
attached leaves and killed leaf segments placed in mesh bags in streams.
The early samples from killed leaves were dominated by Pythium spp.,
but from segments left longer in the river, other fungi such as aquatic
hyphomycetes and species of Fusarium were more abundant (Figure
9.6).
 FRESHWATER FUNGI 241
100
"' Pythium spp.

• Tetracladium
marchalianum

~
>-
(J

~ 50
::l
C"
....
Q)

u..

.. Other fungi

.. / .. o Fusarium
tabacinum

•/D~D
0~--~------~----------~------------~----------~
0 5 10 15
Time (days)
Figure 9.6 Percentage frequency of isolation of Pythium spp. and other fungi from
sieved particles of dead Ranunculus penicillatus leaves at different periods after
submergence in the River Exe, Devon, England. (Reproduced from Kirby, 1984.)

9.2.3 lngoldian aquatic hyphomycetes

This group of freshwater fungi was named in honour of Professor C.T.
Ingold, a pioneer on their taxonomy. About 300 species are known, with a
worldwide distribution. Their characteristic habitat is in rapidly flowing,
i.e. turbulent, well-aerated, non-polluted streams, although they have also
been reported from lakes and terrestrial habitats. Typical substrata are
leaves and branches of deciduous trees which have fallen into the water,
but they also grow on submerged macrophytes, and as endophytes in the
healthy roots of riparian trees such as Salix and Alnus which extend into
water.
The conidia of Ingoldian fungi are unusually large, many spanning
50-100 JJ.m or more. They also have very distinctive shapes, being
Figure 9.7 Spores of lngoldian aquatic hyphomycetes from a foam sample col-
lected from the River Teign, Devon, England. Most of the spores m this sample are
branched (tetraradiate) but two sigmmd spores are also present (arrowed). Scale
bar = 100 ~J.m.

branched, often with four arms (tetraradiate) or worm-shaped , in the

form of a steep helix (sigmoid) (Figures 9.7 and 9.8). These unusual
spores, produced in great abundance, are adapted to underwater
trapping, and germination is stimulated by contact with a surface
(Webster, 1959, 1987; Webster and Davey, 1984; Read, Moss and
Jones, 1991, 1992).
In their conidial state, these fungi are classified in the Hyphomycetes
in the Deuteromycotina. However, an increasing number of conidial
forms ( anamorphs) are being connected with teleomorphs which belong
to unrelated genera of Ascomycotina and Basidiomycotina (Webster,
1992). Ingoldian hyphomycetes are thus not a group of closely related
organisms, but represent fungi which have become adapted morphologi-
cally and physiologically to aquatic habitats. Their conidia are believed
to have resulted from convergent evolution (Webster, 1987). The
teleomorphs usually develop on twigs and branches previously sub-
merged in streams which have become exposed on the banks or
projecting from the water as the stream level falls. The ascospores and
basidiospores show no unusual morphological adaptations. They may be
discharged into air, and thus possibly play an important role in
long-distance transport.
Because of their characteristic conidia, the identification of Ingoldian
hyphomycetes can often be done from these spores alone , which is
certainly not the case for most other groups of fungi. Moreover, their
conidia are readily trapped in foam in streams, enabling surveys of
 "T'I
;;o
m
(J)
I

~
m
;;o
"T'I
c
z
C)

~
w

Figure 9.8 Conidiophores and conidia of some common aquatic hyphomycetes. (a) Articulospora tetrac/adia. (b) Tetrachaetum
e/egans. (c) Varicosporium e/odeae. (d) Tricladium splendens. (e) Tricladium chaetoc/adium . (f) Lunulospora curvula.
244 AQUATIC FUNGI

Figure 9.8 contd. (g) TetracladlUm marchalianum. (h) Heliscus lugdunensis. (i)
Anguillospora crassa. (j) Lemonmera aquattca conidiophore with two ph~alides.
(Scanning electron micrograph by P.J. Sanders.) (k) Clavariopsts aquatica. (1)
Tumularia aquatica. Scale bars = 10 J.Lffi (j, own scale), 40 fJ.ffi (a,c,d,f,k); 20 fJ.ffi
(g,h,l); 80 J.Lffi (b,e).

distribution. A feature of ecological interest is the important role which

this group of fungi plays in conditioning and processing leaf debris,
making it palatable and nutritious to aquatic invertebrates. There have
therefore been extensive ecological studies of Ingoldian aquatic
hyphomycetes (for reviews, see Ingold, 1975a, 1976; Webster and
Descals , 1981; Barlocher, 1992a).
A guide to the taxonomy of these fungi has been provided by Ingold
(1975b) and keys and descriptions by Descals, Marvanova and Webster
(1995).
 FRESHWATER FUNGI 245

a) Techniques for study

Direct observation on randomly selected litter
A handful of deciduous tree leaves from a rapidly flowing stream in
autumn will yield a rich harvest of aquatic hyphomycetes. If a single leaf is
incubated at a temperature of about 15°C in a shallow dish of water,
conidiophores develop overnight, often projecting from the leaf margin.
Hanging drop cultures of leaf fragments may be used to follow conidial
development, which is important in classification since tetraradiate and
sigmoid spores may develop in a variety of ways.

leaf packs and twig packs

Instead of using randomly selected leaf and twig material from streams,
there are advantages in using packs of leaves or twigs of known tree species
which can be attached to masonry bricks or placed in litter bags and
immersed in streams. Amongst the advantages of using leaf and twig packs
are that the place, time and duration of immersion can be controlled.
Samples can be placed at different points along a river system and can also
be moved about from one site to another within it. Leaf material is not
scoured when the stream is in spate. The colonization of different tree leaf
species under identical conditions can be compared. Leaf pack studies
usually involve cutting out discs from the submerged leaves. The discs can
then be incubated in water or dilute salt solution to permit sporulation and
identification of the fungi which have colonized the leaf. Shearer and
Webster (1985c) have compared the examination of randomly collected
litter and leaf pack litter in the evaluation of community structure of
aquatic hyphomycetes.
Twig packs are also used. Comparisons at the same site between leaf and
twig colonization, between twigs of different tree species, and between
corticated and decorticated twigs are possible. Direct observation can be
done by scraping the twig surface. Incubating the twigs in aerated tubes
also encourages the sporulation (and subsequent identification) of the
fungi capable of fruiting, and such studies can be used to give quantitative
estimates of conidial production (Shearer and Webster, 1991).

leaf mapping
If a previously submerged leaf is cut up into small squares, 6 x 6 mm, which
are then incubated separately in a dilute salt solution to permit sporulation,
the distribution of aquatic hyphomycete colonies within the leaf can be
mapped (Figure 9.9) (Shearer and Lane, 1983). Chamier, Dixon and
Archer (1983) have used a similar technique in which 2 x 2 mm squares
excised from repeatedly washed strips (transects) of Alnus leaves were
plated on to dilute agar media. After sporulation the distribution of fungi
246 AQUATIC FUNGI

in the transects could be reconstructed.

Particle plating
Identifications can be made from leaf fragments plated on to dilute
non-selective media. The proportion of fragments which give rise to
identifiable mycelia can be used as a measure of leaf colonization. The
method used by Chamier, Dixon and Archer (1983) described above is
effectively a particle plate technique although the particle size is rather
large. Barlocher and Kendrick (1974) used squares of about 1 mm side to
follow colonization of ash, maple and oak leaves. Kirby, Webster and
Baker (1990) used much smaller particles in the size range 212-700 IJ-m
obtained by passing homogenized material of the aquatic macrophyte
Ranunculus penicillatus through sieves. The frequency of isolation of fungi
increased with increasing particle size.

Cultures
Ingoldian hyphomycetes grow well in agar culture on a range of ordinary
laboratory media. They appear to have no unusual nutritional require-
ments, many being able to utilize simple carbohydrates, and some can
degrade cellulose and other polysaccharides. They can also utilize amino
acids either as a carbon or a nitrogen source, and can immobilize dissolved
nitrogen salts from stream water (Thornton, 1963, 1965; Suberkropp and
Klug, 1981). In pure culture, sporulation normally only occurs when
culture pieces are immersed in water, and spore production is greatly
increased if culture pieces in water are agitated by compressed air or
shaking (Webster and Tawfik, 1972; Webster, 1975). Increasing the flow
rate of water over cultures also stimulates sporulation in many cases
(Sanders and Webster, 1980). These conditions simulate the turbulence of
their normal habitats.
For many aquatic hyphomycetes from temperate streams, growth in
culture is often better at low to moderate temperatures (around 15°C) than
at higher temperatures (see further). Prolonged incubation of culture
pieces in sterile water under diffuse light at low temperatures may result in
the development of teleomorphs (Webster, 1992).

Spores in foam
Foam which accumulates near barriers in turbulent streams is a rich source
of detached conidia (Figure 9. 7) and from such material it is possible to
make isolations (Descals, Webster and Dyko, 1977). Foam samples can be
preserved for subsequent examination by the addition of fixative or by
smearing on microscope slides and staining, and can be used to make
comparisons between the aquatic hyphomycete spora of different river
 Anquillosporo lonqiss1mo Tetroclod!Um morcholionum Ftlosporello onne!ldiCO

Clovonops1s oquoi iCO Oimorphosporo foli!co/o A !olosporo conslnclo

..,.,
::00
m
<Jl
:::r

~
m
::00
Lemonmer o oquot,co
..,.,
Bacillospor o oquofiCO Vorqomyces oquoticus c
z
C)
Figure 9.9 Leaf maps showing the distribution of aquatic hyphomycetes in a leaf of silver maple (Acer saccharum). • = developing
conidia; ~ = loose conidia. The left hand map of each pair represents the upper surface and the right hand map represents the lower ~
.....,
surface. (Reprinted by permission, from Shearer and Lane , 1983, Myco/ogia, 75, 498-508, The New York Botanical Garden.)
248 AQUATIC FUNGI

systems or to estimate species frequency of aquatic hyphomycetes through-

out the year (Nilsson, 1964; Willoughby and Archer, 1973; Chauvet, 1991,
1992). However, foam samples may be unrepresentative compared with
those obtained by Millipore filtration (see below). Branched spores are
overrepresented as compared with spores of sigmoid and other shapes
(Iqbal and Webster, 1973a).

Millipore filtration
Determinations of conidial concentrations can be made by filtering river
water through a membrane filter (e.g. a Millipore filter) 5-8 j.Lm pore size.
A known volume of water is filtered in the field using a suction pump. The
filter is stained with lactic acid cotton blue or lactic acid fuchsin which kills
and stains the spores and renders the filter semitransparent (Iqbal and
Webster, 1973b).

b) Ecological studies
When an autumn-shed leaf falls into a stream, it undergoes a series of
changes resulting from physical, chemical, mechanical and biological
action. Decomposition in the stream has been subdivided into three
phases: leaching, microbial colonization and invertebrate feeding (Cum-
mins, 1974). Leaching of soluble materials (carbohydrates, amino acids
and phenolic compounds) may be quite rapid (Suberkropp, Godshalk and
Klug, 1976), and 24-28 h immersion may result in the loss of up to 25% of
the original weight (Webster and Benfield, 1986). If the leaf has been shed
in the normal way by abscission after maturity, it will contain mycelia of
several common terrestrial and phylloplane fungi such as species of
Aureobasidium, Cladosporium, Epicoccum, Alternaria, etc. (Chapter 4).
In the water, the leaf is rapidly colonized by the conidia of aquatic
hyphomycetes which, in the autumn, may reach very high concentrations
of over 1000 spores l- 1 (see below). The original 'terrestrial' leaf inhabit-
ants persist for a time, but are gradually replaced by aquatic fungi
(Biirlocher and Kendrick, 1974; Chamier, Dixon and Archer, 1983).
Within a few weeks, a mosaic of colonies of aquatic hyphomycetes is
established (Figure 9.9). This early association is made up of a relatively
small number of species (about 5 or 6), possibly resulting from competitive
interactions (see later). It has been suggested that at an early stage of
colonization, a leaf receives an 'imprint' from the stream spora which
determines the dominant members of the fungal community throughout its
decay (Biirlocher and Schweizer, 1983). Fungi account for 63-95% of the
microbial biomass in decomposing submerged leaves of Platanus, Quercus
and Ulmus (Findlay and Arsuffi, 1989). Bacteria play only a minor role in
the early stages of decomposition. They increase in numbers with increas-
 FRESHWATER FUNGI 249

ing time of submersion, possibly as a result of increased exposure of

surface area of litter.
Aquatic hyphomycetes show little resource specificity, and many species
can be collected on a wide range of different plant materials. However,
certain species do show resource preference. Preference can be shown in a
variety of ways, e.g. by more frequent occurrence on particular leaf
species, on standard areas of different leaf baits in packs in the same
stream, or by different levels of spore output. Inability to colonize a given
kind of substratum may result from the presence of barriers to infection,
such as a thick cuticle and epidermis, or to other anatomical features such
as well-developed sclerenchyma or other lignified tissues, or to chemical
factors such as the nitrogen content of the leaf or the presence of inhibitors
such as phenolics. Tetracladium marchalianum (Figure 9.8) showed a
higher colonization frequency and importance index on discs cut from leaf
packs of hickory ( Carya glabra) than on discs of oak (Quercus alba)
submerged in Augusta Creek, Michigan, USA. Triscelophorus mono-
sporus showed the opposite effect (Suberkropp, 1984). In comparisons
between beech (Fagus sylvatica) and alder (Alnus glutinosa), it has been
shown that Tetracladium marchalianum is abundant on alder leaves, but
absent from beech leaves which were dominated by Tricladium splendens
(Figure 9.8) (Bengtsson, 1983; Gonczol, 1989). The rate of mycelial
growth of T. marchalianum inoculated onto beech leaves was very low, and
no protease activity could be detected (Bengtsson, 1983). Possibly the
metabolism of T. marchalianum is affected by inhibitors such as phenolics
which complex with proteins, reducing the availability of leaf protein and
inhibiting enzyme activity. Thomas, Chilvers and Norris (1992a) compared
the frequency of occurrence of aquatic hyphomycetes on discs cut from
randomly selected leaves of Eucalyptus viminalis and phyllodes of Acacia
melanoxylon in an Australian stream. There was clear evidence of prefer-
ence. Alatospora acuminata was recorded more frequently on Acacia,
whilst Tetrachaetum elegans (Figure 9.8) was more common on Eucalyptus.
Coniferous leaves were thought to be poor substrata for aquatic
hyphomycetes (Ingold, 1966), but later studies showed that they do
become colonized, especially after prolonged submersion in a stream. The
thick cuticle and epidermis are barriers to infection. If the cuticle is
removed by steam or chemical treatment, the leaves become much more
extensively colonized, as do leaves which are sliced longitudinally. Conif-
erous leaves also contain phenolic substances which inhibit the growth of
aquatic hyphomycetes. These are eventually leached as the inner leaf
tissues become exposed (Michaelides and Kendrick, 1978; Barlocher and
Oertli, 1978a,b; Barlocher, Kendrick and Michaelides, 1978).
Woody materials, varying in size from small twigs to large branches or
even whole tree trunks, fall into streams and become exposed to coloniza-
tion by the aquatic hyphomycete spores. The fungal community which
250 AQUATIC FUNGI

develops on woody substrata is similar, in part, to the leaf community, but

is nevertheless distinctive (Revay and Gonczol, 1990). Shearer (1992) has
reviewed the role of woody debris in the life cycles of aquatic hyphomyc-
etes. One of the most important features is its longer persistence in a
stream, sometimes extending over several years. In comparison, leaf
material may only persist for a period of weeks, and by the time a crop of
deciduous tree leaves is shed in the autumn, the previous year's crop has
long since disappeared. Thus wood may represent an important reservoir
of inoculum when other substrata are scarce. Living roots of riparian trees
may also be another source of continuously available inoculum (Fisher,
Petrini and Webster 1991; Sridhar and Barlocher, 1992a). Another impor-
tant property of woody materials is that they support the development of
the teleomorphs of aquatic hyphomycetes, on which long-distance trans-
port and sexual recombination may depend (Webster, 1992). Compared
with the species known to be present in streams (for example, as spores in
foam), the number of species which fruit on wood is smaller, i.e. wood
selects only a fraction of the fungi available (Willoughby and Archer, 1973;
Sanders and Anderson, 1979). Experimental studies based on submerged
twig packs give little evidence of host preference, e.g. in a comparison
between beech and alder (Revay and Gonczol, 1990) or oak and alder
(Shearer and Webster, 1991). Shortly after immersion, corticated hard-
wood twigs develop pustules of Heliscus lugdunensis, Cylindrocarpon and
Fusarium, but as the bark is shed, these early colonists d~sappear. It is
likely that substances present in bark (e.g. of oak and alder) ~nhibit growth
and development of many aquatic hyphomycetes, apart from the few
species which develop in profusion on the bark of fresh twigs. Fungi
fruiting late on wood include Anguillospora crassa (Figure 9 .8) which is not
common on leaves.

Table 9.3 Development of the fungal community on oak wood blocks of different
sizes

Block size Exposure period (weeks) Total no. of species

mm Relative area 2 4 6 8 13

5x5 1 oa 1 3 6 6 6
lOxlO 4 0 4 4 7 6 9
20x 20 16 2 3 5 10 12
40 X 40 64 4 9 8 11 16
80 X 80 256 6 15 15 13 19

aNumber of species.
Source: Sanders and Anderson (1979).
 FRESHWATER FUNGI 251

Table 9.4 Distribution of species on oak blocks of different sizes (numbers in the
table are number of blocks bearing each species, maximum 20)

Species Block size (mm) Total Mean

frequency concentration
of conidia

5 10 20 40 80 (%) o-~r

Flagellospora curvula 11 11 11 9 17 59 396

Tricladium gracile 6 6 7 4 9 32 51
Dendrospora erecta 2 4 5 6 11 28 32
Casaresia sphagnorum 4 4 4 2 7 21 26
Articulospora tetracladia 2 1 1 6 11 21 410
Tricladium splendens 2 1 3 4 8 18 21
T. giganteum 3 7 5 9 24 13
Varicosporium elodeae 1 4 11 16 26
Lemonniera terrestris 1 1 2 6 10 0
Taeniospora gracilis 1 1 9 11 627
Anguillospora crassa 2 1 5 8 1
Alatospora acuminata 2 1 4 7 77
Tetrachaetum elegans 1 2 4 7 26
Anguillospora longissima 4 12 16 32
Volucrispora graminea 3 2 5 6
Clavatospora longibrachiata 2 1 3 621
Lemonniera aquatica 4 4 0
· Flagellospora penicillioides 2 2 51
Tetracladium marchalianum 1 1 0
Mycocentrospora angulata 0 294

a5 X 1 1samples over 8 weeks.

Source: Sanders and Anderson (1979).

Sanders and Anderson (1979) immersed square blocks of oak wood of

varying size- 5, 10, 20, 40 or 80 mm sides- to study the effect of resource
size on colonization by aquatic hyphomycetes. The blocks were removed
from the stream at intervals from 2 to 13 weeks, and incubated in water for
3-7 days to allow aquatic hyphomycetes to sporulate. Filtration of the
stream water during the exposure period provided figures for the concen-
tration of spores in suspension. The numbers of species found on the
different sizes of block are shown in Table 9.3.
Three interesting points emerge from these results. First, there is a delay
in appearance of fungi on the smaller blocks. Secondly, there is an increase
in the number of species appearing with increasing exposure. Thirdly, the
252 AQUATIC FUNGI

number of species present is related directly to block size. This last

conclusion indicates that resource size has a strong influence on the
establishment of the species of aquatic fungi on it. Biirlocher and
Schweizer (1983) reached a similar conclusion in studying the colonization
of squares of different size cut from Quercus and Acer leaves. The species
identified on the blocks of different size are listed in Table 9.4. The
frequency of colonization of the blocks bears no relation to the measured
concentration of the spores of different species in the stream. This is well
shown by the fact that Lemonniera terrestris, which was found in a low
proportion of blocks of 10 mm side or larger, was not detected by
filtration, whilst Mycocentrospora angulata, which was detected at concen-
trations of 294 conidia 1-\ did not fruit on the block, i.e. it may be
taxon-specific for reasons already described. The most frequently recorded
fungus, Flagellospora curvula, which has sigmoid conidia, is probably less
efficiently trapped than the tetraradiate conidia of some other fungi. The
reason for the low number of species on the smaller blocks may be that
interspecific competition in the smaller volume of wood may occur early,
and may limit colony development and sporulation. This speculation,
however, needs experimental investigation (see later).
The studies reported above show that there is a succession of fungi
fruiting on submerged leaves. The detailed picture is dependent on the
kind of leaf involved, the time (season) of submersion in the stream, which
will in turn be related to the range of temperature to which the submerged
leaves are subjected, and the abundance and species composition of spores
available in the stream during the period of submergence. On leaves which
are rapidly decomposed and consumed by invertebrates, such as those of
alder, successional changes are more difficult to detect than on more
persistent leaves such as those of oak (Chamier and Dixon, 1982a). Fungi
which fruited early on leaf pack material of oak and hickory were
Flagellospora curvula, Lemonniera aquatica and Alatospora acuminata
(Suberkropp and Klug, 1976). Similar findings are reported by Gessner et
al. (1993) from alder leaf packs submerged in autumn in the River Touyre
in the French Pyrenees. Within 2 weeks of submergence,' the leaves
supported densely sporulating colonies of Flagellospora curvula, along
with four other species, Lemonniera aquatica, L. centrosphaera, L.
terrestris and Tetrachaetum elegans. These species persisted throughout the
decay of the leaves, but after 4 weeks the community included 13 abundant
species (with a frequency greater than 10% ). Species which appeared late
in the succession included Clavatospora longibrachiata, Heliscella stellata
and Goniopila monticola. The succession was thus characterized by three
stages: an assemblage of five pioneer species, a mature community of
about 13 species and an impoverished successional stage with a reduced
species diversity.
The arrival of additional species into the leaf and wood community, and
 FRESHWATER FUNGI 253

the displacement of one dominant group of fungi by others, coupled with

the reported patchy distribution of fungi colonizing leaves (Shearer and
Lane, 1983; Chamier, Dixon and Archer, 1983) indicate some form of
competition. It has also been claimed that fungi fruiting on submerged
wood 'occur as discrete, usually dense, monospecies patches' (C.A.
Shearer, unpublished). Khan (1987) examined the interactions of eight
aquatic hyphomycetes when paired together on agar media, but found no
evidence of interspecific competition inhibition at a distance or of hypha!
interference. There were, however, differences in growth rate as the
colonies approached each other, and it thus seems likely that more rapidly
growing species might deprive slower growing forms of nutrients. Competi-
tive interactions between 25 species of aquatic fungi normally occurring on
leaves or on wood (or sometimes both) have been studied on agar by
Shearer and Zare-Maivan (1988). The fungi included ascomycetes and
fungi imperfecti. A wide range of response was reported. Aquatic
hyphomycetes which ranked high in ability to inhibit the growth of other
aquatic fungi were Clavariopsis aquatica (23 species inhibited), Anguil-
lospora cf. gigantea and Tetracladium marchalianum (22). Lower ranking
species included Margaritispora aquatica and Triscelophorus monosporus
(20), Heliscus lugdunensis (16) and Clavatospora longibrachiata (9). These
results are difficult to extrapolate to the field situation, but the least
inhibitory fungi were those characteristic of leaves, whilst the more
inhibitory species were wood inhabitants. These findings support the idea
that persistent and late-colonizing fungi on long-lasting substrata are more
likely to produce antagonistic substances than those on less persistent
substrata.
Massarina aquatica, an aquatic loculoascomycete which fruits on wood
and submerged tree roots, is the teleomorph of Tumularia aquatica (see
Figure 9.8). This conidial state grows on submerged wood, but also on
submerged leaves. Fisher and Anson (1983) tested the ability of diffusates
from oak wood blocks colonized by M. aquatica to inhibit germination and
growth of other fungi, by placing colonized blocks on water agar to allow
leachates to diffuse into the agar. Germ tube production of Tricladium
giganteum was significantly reduced close to the edge of the block and the
growth of other fungi was also inhibited. It is thought that M. aquatica can
produce one or more antifungal antibiotics.
In temperate and cool-temperate latitudes, streams bordered by decidu-
ous trees show enormous variations in seasonal abundance of conidia of
aquatic hyphomycetes directly related to the addition of autumn-shed
leaves. In areas where litter input is spread over a longer period, the
seasonal variations in abundance are less pronounced, e.g. in parts of
Australia where litterfall in Eucalyptus forest may reach a maximum in the
late-summer to early-autumn months, December-March (Thomas, Chil-
vers and Norris, 1989; Thomas, 1992b ). In a stream in which needles of
254 AQUATIC FUNGI

spruce (Picea abies), which are shed evenly throughout the year, domi-
nated the terrestrial input, there was no clear peak in conidial concentra-
tion during the year (Barlocher and Rosset, 1981).
The fall of tree leaf litter into a stream is quickly followed by a rapid rise
in spore concentration in the water as shown by filtration. Chamier and
Dixon (1982a) have estimated that the numbers of spores which can be
produced per gram oven dry weight of leaf tissue are c. 140 000 for
Quercus and Alnus. Even higher values have been claimed by Barlocher
(1982) who estimated that leaves of Quercus and Larix could produce 5-6 x
106 conidia g- 1 within a 2-day period. Suberkropp (1991) has estimated
that Lunulospora curvula allocates 60-80% of its biomass to sporulation,
and Anguillospora filiformis 30-45%. Such prolific production is a ruderal
trait, probably correlated with growth on relatively ephemeral substratum
and the low chance that an individual spore has of being trapped on a
suitable underwater substratum in a rapidly moving water current. Possibly
rapid and prolific reproduction is an adaptation to minimize the effects of
predation by aquatic invertebrates which graze on leaves colonized by
aquatic hyphomycetes.
Spore concentrations between 103 and 104 l- 1 have been reported by
Iqbal and Webster (1973b) for a lowland stream in Devon, England,
during October-December (Figure 9.10), and these autumnal values have
been matched or exceeded by other workers elsewhere. In the following
summer, as the leaf litter is decomposed, consumed by invertebrates,
fragmented and swept downstream, the spore concentration may fall too
low to be detected by filtration. Nevertheless, sufficient inoculum is
available to infect leaves in the autumn, and it is possible that materials
such as bud-scales, catkins, etc., falling in spring and early summer,
contribute, as do colonies growing on woody material and riparian tree
roots.
The frequency pattern of the total spore production masks variations in
frequency of individual species. The seasonal abundance of the spores of
Clavariopsis aquatica mirrors the total abundance of spores. Here the
availability of leaf litter is probably the major factor controlling its
abundance. For some other species, other factors are also important.
Figure 9.10 shows that the peak concentrations of conidia of Tricladium
chaetocladium are in the winter months from December to March in the
River Creedy. A contrasting pattern of abundance is shown by Lunu-
lospora curvula, detected in the River Creedy only from August to
November. The suggested explanation that these different periods of spore
production are controlled by temperature was explored by Webster,
Moran and Davey (1976), who found differences in temperature optima
for growth and sporulation, L. curvula having higher optima.
Experiments designed to separate the effects of temperature and litter
availability have been reported by Suberkropp (1984), who immersed leaf
 FRESHWATER FUNGI 255

l...
til 3
....
Q)

0
c.
til
.....
0 2
ci
c:
Cl
0
...J

0~~~~~~~~1~1~11~~
Aug Sept Oct Nov Dec Jan Feb Mar Apr May June July Aug Sept
(a)
1970 1971

(b) Aug Sept Oct Nov Dec Jan Feb Mar Apr May June July Aug Sept
1~0 1~1

3
l...
til
~
0
c. 2
.....0til
ci
c:
Cl
0
...J

0
(c) Aug Sept Oct Nov Dec Jan Feb Mar Apr May June July Aug Sept
1970 1971
Figure 9.10 Changes in spore concentrations of aquatic hyphomycetes detected by
Millipore filtration of the River Creedy, Devon, England, 1970--71. (a) Total
spores; (b) spores of Tricladium chaetocladium; (c) spores of Lunulospora curvula.
(Based on data from Iqbal and Webster, 1973b.)

packs of oak (Quercus) and hickory ( Carya) for 4-week periods at different
points in the stream characterized by different temperature ranges. Inter-
changes of packs colonized for a short period at one point in the stream
before transfer to another point were also made. A summer assemblage of
species dominant on leaf packs during the warmer months could be
256 AQUATIC FUNGI

recognized: Lunulospora curvula, Flagellospora penicillioides, Triscelo-

phorus monosporus and Heliscus tentaculus. Flagellospora curvula and
Lemonniera aquatica were dominant on both leaf types in the colder
months. Alatospora acuminata and Tetracladium marchalianum increased
in importance as the water temperature increased. Transfer of packs
briefly submerged at a warmer site to allow them to be colonized by
members of the summer assemblage, L. curvula and F. penicillioides, then
transferred to a cooler site, showed that both species could survive and
sporulate in the cooler site.
Temperature affects different phases of the life cycle of a fungus:
germination and infection, mycelial growth and sporulation (Koske and
Duncan, 1974). Laboratory experiments show that species more abundant
in cooler months have different temperature characteristics from species
from the summer assemblage. For example, L. curvula, H. tentaculus and
F. penicillioides only grow at temperatures above 5°C, whilst F. curvula
and Lemonniera aquatica are able to grow at 1oc (Suberkropp, 1984;
Suberkropp and Klug, 1981).
The San Marcos River in Texas is unusual in having a relatively constant
temperature of 22oc ± 1oc. Here some, but not all, of the fungi present are
the same as those reported to belong to the summer assemblage of
Suberkropp, e.g. Lunulospora curvula and Triscelophorus monosporus
(Akridge and Koehn, 1987).
By contrast, a relatively constant winter temperature of 0.1 oc occurs in
the Njakajokk stream in the Swedish Arctic (68°N) from the end of
October to May. For some of this period, the stream is covered with ice.
Three species dominate the spore output during the year: Flagellospora
curvula, Lemonniera aquatica and Alatospora acuminata, all of which are
members of the cold water assemblage of Suberkropp (1984).
Temperature is also related to latitudinal and altitudinal distribution. A
group of tropical and subtropical species has been recognized (Nilsson,
1964; Webster and Descals, 1981; Sridhar, Chandrashekar and Kaveri-
appa, 1992). Nawawi (1985) has listed 25 species which are tropical or
subtropical in distribution, including Brachiosphaera tropicalis, Campy-
lospora chaetocladia, Heliscus tentaculus, Flagellospora penicillioides,
Lunulospora curvula, Triscelophorus acuminatus, T. monosporus and
Ingoldiella hamata. All these species occur in Malaysia but some also occur
in temperate climates. It is interesting that the following, all common in
temperate latitudes, have not been found in Malaysia: Tricladium splend-
ens, Anguillospora, Culicidospora, Dendrospora and Lemonniera.
Chauvet (1991) has surveyed the distribution of spores of aquatic
hyphomycetes at 27 stations in south-western France based on foam
samples collected at intervals throughout the year. The altitude of the
stations varied between 9 and 935 m, with a pH range of 5-8.5 and a water
temperature range of 2-20°C. Correspondence analysis was used to
 FRESHWATER FUNGI 257

examine the relationship between distribution patterns and abiotic vari-

ables such as altitude, pH, temperature and season. A group of five species
associated with lowland streams, high pH and high temperature was
identified: Campylospora chaetocladia, Campylospora sp., Heliscus ten-
taculus, Lunulospora curvula and Triscelophorus monosporus. Two spe-
cies, Clavatospora longibrachiata and Tetrachaetum elegans, appeared
characteristic of acid water (pH <6), low altitude and autumn months.
Tetracladium marchalianum and Tricladium angulatum were typical of
lowland streams, whilst Taeniospora gracilis and Tricladium chaetocladium
were found in mountain streams. Water pH appeared to be of secondary
importance as compared with altitude.
Generally, where the gathering grounds for river systems are in moun-
tainous or upland areas, as the streams descend into lowland and the
tributaries merge to form larger rivers, there are associated changes in the
instream and riparian vegetation and in chemical and physical characters of
the water (Hynes, 1970; Chamier, 1992). These include changes in pH,
conductivity, water hardness, dissolved oxygen, dissolved organic matter,
flow characteristics, and amount and quality of trapped litter and sedi-
ment. Water quality is markedly affected by flow through forests of
different type (coniferous litter results in low pH), and flow through
agricultural areas may result in increase in dissolved nutrients through
fertilizer run-off and leaching. Flow through populated areas may result in
industrial or sewage pollution. As rivers approach the sea, they come
under the influence of tides. There are also changes in the range and
abundance of different animals within the length of a river. Barlocher
(1992b) has described variations in physical, chemical and biological
parameters along river systems, all of which may affect community
structure and activity of aquatic fungi.
Shearer and Webster (1985a,b) studied variation in spore concentration
and in the colonization of alder leaf packs submerged at different points
along the River Teign in Devon, England. The upper reaches arise in
moorland where the water is acidic (pH 5.4-6.0). There are hardly any
riparian trees, although there are several instream macrophytes. The main
source of allochthonous litter is culms of a rush (Iuncus effusus). The lower
sites are characterized by higher pH (7.0-7.2), and are bordered by various
trees whose detached leaves and branches are trapped amongst boulders.
The upper (moorland) stretch of the river had much lower conidial
concentrations, probably related to the scarcity of allochthonous litter.
Species numbers increased in a downstream direction, and this might be
associated with the greater variety of available substrata, or to features of
water quality including higher pH. The community of aquatic hyphomyc-
etes at the upland site was distinctly different from the downstream
communities and less than 20% of all species were found on leaves at all
the sites. Fontanospora eccentrica and Varicosporium elodeae were both
258 AQUATIC FUNGI

common at certain times in the upland site, but were either absent or
infrequent lower down. In contrast, Tricladium splendens and Dimorphos-
pora foliicola showed the opposite pattern of distribution. The conidia of
F. eccentrica and V. elodeae were present only in low concentration at the
downstream sites. For example, the conidia of F. eccentrica made up
10--15% of the conidial pool at the upstream site and only 0--0.1% lower
down. This suggests local production at the upstream site, and rapid
extinction (i.e. loss due to trapping or dilution) as the spores are carried
downstream. The idea is supported by the work of Metwalli and Shearer
(1989), who studied species distribution and spore concentration in an
Illinois stream whose banks were bordered by alternately clear-cut or
wooded sections, using oak leaf packs. The concentration of conidia was
high in the wooded areas but fell in the clear-cut areas downstream. The
mean number of species was also lower in the clear-cut areas.
Thomas, Chilvers and Norris (1991a) have discussed factors related to
changing spore concentrations in a stream and have provided a mathemati-
cal model (Thomas, Chilvers and Norris, 1991b) to explain the dynamics of
spore populations in a body of moving water. The model enabled them to
calculate the half-life of populations in terms of the distance downstream
that the population would be halved in concentration. For three fungi
tested, the half-lives gave similar values: Alatospora acuminata 0.69 km,
Clavariopsis aquatica 0.78 km, and Tetrachaetum elegans 0.81 km. A good
fit between simulated data and real data was obtained.
The above reports suggest that in a given river system, species number
may be correlated with rising pH. When data from streams in Europe and
Canada are correlated, however, this simple relationship becomes less

-·-----
• 0
0

• •
<f)
CD
·c:;
CD
c.
(/)
0
0

0
0

pH

Figure 9.11 Plot of numbers of species of aquatic hyphomycetes recorded from

streams of different pH based on data from Biirlocher (1987) • and Wood-
Eggenschwiler and Biirlocher (1983) o. (Redrawn from Biirlocher, 1987 with
permission.)
 FRESHWATER FUNGI 259

clear (Biirlocher and Rosset, 1981; Wood-Eggenschwiler and Biirlocher,

1983; Biirlocher, 1987). Figure 9.11 is a plot based on records from Europe
and Canada, and shows a slow, possibly negligible, decrease in species in
circumneutral waters (pH 5.7-7.2) with a much more rapid decline in
alkaline waters (pH> 7.2) (Biirlocher, 1987).
The effects of pH on the abundance and activity of aquatic fungi are
complex since H + concentration is related to the abundance of plants (as
possible substrata) and animals (as possible consumers). There are also
features of water chemistry such as water hardness, alkalinity and the
solubility of Ca and AI which are pH-dependent (Chamier, 1992). At low
pH ( <5.5), ionized forms of Al 3 + predominate in solution, and some of
these are toxic. In acid streams, submerged litter significantly accumulates
AI more rapidly than in circumneutral streams and decomposition is slower
(Chamier, Sutcliffe and Wishman, 1989).
Aquatic hyphomycetes possess an array of enzymes, including pecti-
nases, capable of macerating leaf tissues (Chamier and Dixon, 1982a,b;
Chamier, 1985; Zemek et al., 1985; Abdullah and Taj-Aldeen, 1989). The
pectolytic enzymes are made up of several species of enzyme with different
pH optima. It is therefore possible that the degradative activity of aquatic
hyphomycetes is directly related to the pH of streamwater. An indirect
effect is through the increasing availability of Ca2 + above pH 6.5.
Increasing Ca2 + concentration increases pectolytic activity of Tetrachaetum
elegans on a sodium polypectate medium at pH 6.5 and (in ecologically
probable amounts) is also directly related to strength loss in alder leaf
strips inoculated with the fungus. Stimulation of pectic enzyme activity of
aquatic hyphomycetes by Ca 2 + may in part explain the more rapid
maceration of leaf tissues recorded in streams above pH 6.5 (Chamier and
Dixon, 1983).
Where rivers flow into the sea, the organisms which thrive there must be
able to tolerate variations in salinity. Observations of the distribution of
aquatic hyphomycetes on wood blocks show that they do occur, even if
they are not abundant in estuaries (Jones and Oliver, 1964). Byrne and
Jones (1975a,b) have studied spore germination, vegetative growth and
sporulation in two species, Heliscus lugdunensis and Tetracladium seti-
gerum, in sea water at various dilutions or on agar to which sea water was
added, at varying temperatures. Germination and growth occurred over a
range of concentrations of sea water but sporulation was severely inhibited
at quite low salinities, i.e. 30% sea water for H. lugdunensis and 10% for
T. setigerum. These results indicate that freshwater aquatic hyphomycetes
might be able to colonize substrata in brackish water, but would probably
be unable to reproduce there.
The abundance of aquatic hyphomycetes in rapidly flowing, well-aerated
streams suggests that they might be adapted physiologically to water with a
high concentration of dissolved 0 2 . In stagnant habitats such as ponds and
260 AQUATIC FUNGI

ditches, they are much less frequent, although Tricladium splendens is an

exception. The mud surface of stagnant ponds, especially when carpeted
by a layer of leaves, may rapidly become oxygen-deficient, and below the
mud surface anaerobic conditions may prevail, as evidenced by the smell of
hydrogen sulphide, which would rapidly combine with available oxygen.
Even in rapidly flowing streams, accumulations of leaf and branch litter
may become anaerobic if overlaid by sediments. Experimental studies
indicate that different species of aquatic hyphomycetes vary in their
capacity to survive anaerobic conditions. For example, the mycelium of
Articulospora tetracladia failed to survive for 3 months, whilst a small
proportion of colonies of Tricladium splendens and Anguillospora rosea
survived for 1 year (Field and Webster, 1983}. In similar studies in which
beech leaf discs inoculated with aquatic hyphomycetes were incubated in
buffer solutions at different concentrations of H 2S for 24 h, variation in
survival was found, A. tetracladia being more sensitive than T. splendens.
The aquatic hyphomycetes tested were more sensitive to H 2S than species
of aero-aquatic fungi which are common in stagnant habitats (Field and
Webster, 1985).
Bandoni (1981} has reviewed evidence that the conidia of some fungi
commonly considered to be aquatic hyphomycetes may be found in
terrestrial habitats. Bandoni has collected conidia from leaf litter remote
from water and in water flowing down the stems of trees. There are also
records of isolation from soil and from root surfaces, and as endophytes
from roots in soil. The presence of spores in the habitats mentioned, or the
isolation of a fungus from soil, does not prove that the fungus has active
mycelium. However, more convincing evidence is available that some are
active in terrestrial situations. Park (1982b) has shown that Varicosporium
elodeae, which can be regularly found in the litter layer above soil, can
colonize leaves or even filter paper buried in soil. Sanders and Webster
(1978) have also shown that V. elodeae and Articulospora tetracladia can
extend from colonized leaf discs sandwiched between sterile oak leaves
buried in woodland litter, to infect the oak leaves. When leaves of oak
which had been artificially or naturally colonized with certain aquatic
hyphomycetes were hung in nylon bags above ground level, several species
were capable of surviving for up to 13 weeks.
Since some of these so-called aquatic hyphomycetes are active and can
survive in terrestrial habitats, and because some of them form teleomorphs
which can release air-borne ascospores or basidiospores, it is probably best
to regard them as amphibious fungi.
As indicated earlier, a series of chemical changes follows the immersion
of tree leaves in streams, beginning with the rapid loss of soluble materials
by leaching. Microbial colonization is accompanied by an absolute increase
in protein content, first demonstrated by Kaushik and Hynes (1971) and
later confirmed by many studies (see Barlocher, 1985; Chauvet, 1987). The
 FRESHWATER FUNGI 261

increase in protein largely results from fungal growth. There are also
increases in other compounds, e.g. phosphate (Meyer, 1980). Physical
changes also occur, notably the softening of leaf tissues as the result of the
macerating activity of fungal enzymes (Suberkropp and Klug, 1980, 1981;
Suberkropp, Arsuffi and Anderson, 1983; Chamier, 1985). These changes
in the chemical and physical properties of submerged leaf litter have been
termed 'conditioning'. There is much evidence that conditioned leaves are
consumed in preference to non-conditioned leaves by leaf-shredding
invertebrates such as the larvae of aquatic insects and Gammarus spp.
Patches of leaf material colonized by fungi are preferred to uncolonized
areas (Arsuffi and Suberkropp, 1985), and it is known that leaves
colonized by certain fungal species are more attractive than others
(Suberkropp, 1992). Aquatic invertebrates thus exert some control over
the populations of aquatic hyphomycetes. In experiments in which leaf
packs enclosed in fine- or coarse-mesh litter bags were immersed in
streams, Barlocher (1980) showed that the leaves in coarse-mesh bags,
permitting the entry of invertebrates which fed on the leaves, had a lower
cumulative species count of aquatic hyphomycetes than the leaves in the
fine-mesh bags from which invertebrates were excluded. The rate of decay
of the leaves in the coarse-mesh bags was greater than for the leaves
enclosed in fine mesh. Thus the leaf-eating invertebrates are potent
competitors of aquatic hyphomycetes. As well as eating fungal mycelium,
they also remove substratum that might otherwise be used by fungi
characteristic of later stages of succession.
There are several reasons why aquatic invertebrates preferentially ingest
conditioned leaf material. Most do not possess gut enzymes capable of
degrading plant polymers such as cellulose and lignin. Fungal enzymes
transform plant polymers into digestible matter and their activity may
continue in the invertebrate gut. The enrichment of the protein and
probably the lipid content of leaf material associated with increase in
fungal biomass renders conditioned leaf material more nutritious, whilst
the macerating activity brought about by fungal enzymes makes it easier to
ingest. For these reasons, animals fed on conditioned leaf material grow
more rapidly than those fed on non-conditioned leaves, and expend less
energy in foraging for food. Thus, although in most streams the bulk of the
carbon and energy inputs is derived from allochthonous riparian tree leaf
litter, this material is largely unavailable to aquatic invertebrates unless
first colonized by microorganisms of which the aquatic hyphomycetes
appear to be the most important (Barlocher and Kendrick, 1976, 1981).

9.2.4 Aero-aquatic hyphomycetes

If leaves and twigs from the mud surface of a stagnant pond or slow-
running ditch are washed and incubated in a moist chamber for a few days
262 AQUATIC FUNGI

at room temperature, a remarkable array of large conidia develop. From

the mycelium within the substratum, conidiophores extend into the air,
giving rise to various kinds of buoyant, non-wettable propagules. In most
propagules, air is entrapped. In Helicoon and Helicodendron (Figure
9.12), a cylindrical or barrel-shaped propagule is formed by the tight
coiling of the developing conidium, whilst in Clathrosphaerina a hollow,
clathrate sphere develops from a system of repeatedly dichotomous
branches whose tips bend towards each other and touch (Figure 9.12). The
propagules of Aegerita candida resemble a raspberry fruit, and are made
up of inflated, clamped segments, between which air is trapped (Figure
9.12). Cancellidium applanatum has a flattened, gas-filled bag made up of
parallel hyphae, and Fusticeps bullatus has a club-shaped, septate spore
whose outer wall bears stud-like scales between which air is held.
Propagules of this kind are not developed in water, but only in air. In
nature, they develop at the margins of drying ponds and ditches. When a
dried-up pond is flooded again, the conidia float off and are dispersed.
Premdas and Kendrick (1991) showed that leaves of Acer saccharum gently
lowered onto the surface of a Canadian pond quickly acquired propagules
of aero-aquatic fungi which had been floating in the surface film. The time
between initial propagule attachment, colonization and subsequent sporu-
lation can be as little as 1 week. These fungi are not confined to stagnant
water habitats: they can be found on wood submerged in streams and
rivers, moist leaf litter, and soil. The teleomorphs, where known, are
inoperculate Discomycetes, Pyrenomycetes and Basidiomycotina with
relatively simple fructifications formed on wet wood. It is obvious that the
aero-aquatic way of life has evolved independently in unrelated fungal
groups. Accounts of their morphology and ecology have been given by
Webster and Descals (1981) and Fisher and Webster (1981). Goos (1987)
has given an account of helicosporous forms.
Techniques for studying the ecology of aero-aquatic fungi in the
laboratory and the field have been devised by Fisher (1977). The prop-
agules germinate readily on suitable media, and sporulation occurs on
dilute laboratory media such as 0.1% malt extract agar. Spores can be
wetted by detergent and can then be used as inoculum. A valuable
technique in studying survival or colonization is to inoculate large numbers
of small discs cut out from leaves of beech (Fagus sylvatica). Such discs can
be colonized and placed amongst leaf litter at the mud surface in ponds or
in the laboratory. When the discs are removed from the field site, they are
placed on moist filter paper, and the proportion of them which gives rise to
sporulating colonies can be assessed. The use of freshly abscised leaves
(brown) or leaves which had been submerged in water for some time
(black), from which to prepare discs, enabled Fisher (1979) to compare the
capacity of different aero-aquatic fungi to colonize fresh and partially
decayed leaf discs in a eutrophic and an oligotrophic site. In general, the
 FRESHWATER FUNGI 263

Figure 9.12 Propagules of some aero-aquatic fungi as seen on the surfaces of

leaves (a) Heltcodendron gzganteum (b) Clathrosphaerma zalewsku (c) Aegerzta
candzda Scale bar = 100 J.Lm (Scannmg electron micrographs by P J Fisher )
264 AQUATIC FUNGI

brown leaves were colonized more readily than the black ones, but in the
oligotrophic habitat which was fairly well aerated, two species, Helicoden-
dron giganteum and Clathrosphaerina zalewskii, were able to colonize
brown or black leaf material equally well. Premdas and Kendrick (1991)
found variations in the preference of different species of aero-aquatic fungi
for freshly abscised (3-4 weeks) and 1-year-old leaves. In most cases, the
fungi studied colonized freshly abscised leaves preferentially.
The mud surface of a pond containing large amounts of leaf litter can
frequently become anaerobic, especially during the summer months,
resulting from respiratory oxygen demand and from the lowered solubility
of oxygen at higher temperatures. The saturation concentration of oxygen
dissolved in water is about 9 mg 1- 1 (or ppm), but values in the range
0.1-0.6 mg 1- 1 are often detected immediately above the mud throughout
the year, falling to zero in summer. Only a few millimetres beneath the
surface, the mud may be anaerobic for much of the time, indicated by the
black colour, largely due to iron sulphide (Fe 2S), and by the smell of
hydrogen sulphide (H 2S). The growth and survival of aero-aquatic fungi
under conditions of low oxygen availability is therefore of interest.
Observations on leaves recovered from mud which had been anaerobic for
several months show that after a few days' incubation in moist air conidia
of aero-aquatic fungi develop. Similar results have been obtained after
submerging beech leaf discs in anaerobic mud for 1 month during the
summer, showing that colonization can occur under these conditions,
probably from mycelium (Fisher, 1977). Incubation of the recovered discs
in aerated water for a few days results in an increase in the number of
species sporulating after incubation in air. This suggests that a period of
vegetative growth under more aerobic conditions may permit the sporula-
tion of some fungi which would otherwise not have been detected.
The survival of aero-aquatic fungi on beech leaf discs maintained in the
laboratory under strictly anaerobic conditions has been studied by Field
and Webster (1983). Five species of Helicodendron showed almost 100%
survival for 6 months, and all species showed some survival for 12 months.
The capacity of aero-aquatic fungi to survive anaerobiosis was significantly
better than that of the Ingoldian hyphomycetes or saprolegniales tested.
There is no evidence that aero-aquatic fungi can actually grow, i.e.
increase the weight of mycelium, under anaerobic conditions. In experi-
ments in which mycelia of four aero-aquatic fungi were grown in liquid
beech leaf decoction equilibrated to different partial pressures of gases, the
best mycelial growth was found in liquids equilibrated with air (160 mm
0 2 ). Growth at lowered partial pressures of 0 2 was significantly reduced
(Fisher and Webster, 1979). Survival under anaerobic conditions probably
occurs by means of thick-walled hyphae. The occurrence of sclerotia or
chlamydospores in this group of fungi is rare.
Similar comparative studies have also been made of survival, under
 MARINE FUNGI 265

anaerobic conditions, at low concentrations of H 2S, a gas which is toxic to

many organisms in low concentrations. The aero-aquatic fungi proved to
be the most tolerant (Field and Webster, 1985).
Aero-aquatic fungi are found primarily in freshwater habitats, but in
estuaries they may be subjected to variations in salinity. Several species
can germinate and grow at high salinity levels, so that their relative scarcity
in the lower reaches of tidal rivers may possibly be due to their inability to
compete with other organisms in the colonization of suitable substrata,
rather than intolerance of salinity (Field, 1983).
Fisher (1978) has investigated the ability of mycelium and spores of
aero-aquatic fungi to survive desiccation. Homogenized, sterilized beech
leaf debris was inoculated with mycelium and, after 7 weeks, the colonized
debris was air-dried. The dried material was placed on garden soil. Of the
eight species tested, all could survive in this form for over a month, and
three species survived for 10 months, despite colonization of the leaf debris
by other organisms. In most species, similar material survived drying in a
desiccator for 1 month, and Helicodendron triglitziense survived for 9
months at a relative humidity of 4% and a moisture content of around 3%.
The conidia were much less tolerant of desiccation. Conidia of six of the
eight species retained the capacity to germinate after being in a desiccator
for 10 days, but none survived for 20 days.
The aero-aquatic fungi are ubiquitous in appropriate habitats, although
possibly more abundant in temperate and colder regions than in the
tropics. Long-distance transport between isolated water bodies is possibly
brought about by ascospores and basidiospores in those species which have
teleomorphs. The buoyant propagules may aid dispersal during periods of
flooding following a period of dry weather sufficient to allow mycelia from
previously submerged substrata to sporulate. They may also help in
dispersal and colonization after wetting and sinking. Since conidia and
teleomorphs do not develop underwater, it is possible that spread under-
water is by mycelial fragments or by leaf-to-leaf contact. Growth is most
rapid in well-aerated water, but prolonged survival is possible at low
oxygen levels or under completely anaerobic conditions. Some of these
fungi can also survive prolonged drought and, under these conditions,
dispersal as mycelium in wind-blown leaf litter might be possible, but it is
unlikely that the conidia are dispersed independently in air.

9.3 MARINE FUNGI

Compared to the many thousands of fungal species known from terrestrial
habitats, only about 500 have been described from oceans and estuaries
which make up most of the world's surface (Kohlmeyer and Kohlmeyer,
1979; Kohlmeyer and Volkmann-Kohlmeyer, 1991). Although it was at
first doubted that there were indigenous marine fungi, the publication by
266 AQUATIC FUNGI

Barghoorn and Linder (1944) entitled 'Marine fungi: their taxonomy and
biology' provided a foundation and a stimulus to the study of these
organisms. Kohlmeyer and Kohlmeyer (1979) have distinguished between
obligate marine fungi which grow and sporulate exclusively in a marine or
estuarine habitat and facultative marine fungi which are derived from
freshwater or terrestrial sources and which are able to grow and possibly
sporulate in the marine environment. There is also a fungal flora of inland
salt lakes which resembles that of the sea.

9.3.1 The marine environment

Within the sea there is a range of chemical, physical and biological factors
which control the distribution, activities and abundance of fungal inhabit-
ants. The chief feature distinguishing the sea from fresh water is, of course,
its salt content. The total dissolved solids per litre of a sample of sea water
may vary, depending on the source, especially in relation to evaporation
and nearby freshwater inflows. The values can vary from below 5%o in
estuaries, to 37%o or more. A typical value for the salinity of ocean water is
between 33 and 37%o, and the average salinity is usually stated to be 35%o.
The proportion of the different salts in sea water is reasonably constant,
but may, of course, be altered by pollution, and by proximity to land,
rivers and terrestrial run-off. The pH of sea water lies in the range 7.5-8.4,
but is usually between 8.1 and 8.3 at the surface. The photosynthetic
activity and respiration of the phytoplankton affect the concentration of
dissolved C02 so that, when photosynthesis is high, the C02 content
decreases and the pH may rise to 8.3-8.5.
The temperature range of sea water is related to depth, latitude,
insolation, season and time of day. It is also affected by such factors as
proximity to ice, cooling by wind, and the upwelling of currents. Close to
the poles, especially near to ice, the surface sea water temperature may be
below 0°C. In the tropics, the surface temperature may never fall below
20°C. With increasing depth, the sea water temperature can also fall to low
values.
All these factors, i.e. the salinity, pH and temperature of the water,
influence the activity, abundance and distribution of marine fungi.
There are several groups of fungi or fungus-like organisms well-
represented in the sea. The so-called lower fungi, reproducing by
zoospores, include organisms of diverse affinity. Some members of the
Chytridiales and the Lagenidiales are parasitic, e.g. on marine algae, whilst
some grow saprotrophically. The Thraustochytriales and Labyrinthulales
are both obligately marine. Although previously classified as fungi, they
are now placed in a separate phylum Labyrinthomycota (Porter, 1989).
The ecology of these organisms is not considered further (but for refer-
ences, see Moss, 1986b).
 MARINE FUNGI 267

The higher filamentous marine fungi include about 300 species, although
more remain to be described. Most are ascomycetes and deuteromycetes
and a few basidiomycetes. There are also marine yeasts. Some of the
higher filamentous fungi are parasitic on marine algae or marine
angiosperms, or grow symbiotically with brown algae, e.g. Mycosphaerella
ascophylli on Ascophyllum and Pelvetia (Garbary and Gautam, 1989;
Kingham and Evans, 1986). Marine lichens in which ascomycetes grow
symbiotically with green algae or cyanobacteria include genera such as
Arthropyrenia, Verrucaria and Lichina. Some marine fungi are associated
with corals (for references, see Kohlmeyer and Volkmann-Kohlmeyer,
1989). However, the majority are saprotrophs on algae, wood, mangrove
roots or marine angiosperms ('sea grasses').
Details of techniques for studying different groups of marine fungi have
been given by Johnson and Sparrow (1961), Jones (1971), Hughes (1975)
and Hyde, Farrant and Jones (1987).

9.3.2 Higher marine fungi

Several taxonomic groups of higher fungi have marine representatives, for
example:
Ascomycotina
Eurotiales: Amylocarpus, Eiona
Sphaeriales: Halosphaeriaceae:
Antennospora, Ceriosporopsis, Corollospora,
Halosphaeria, Lindra, Lulworthia
Loculoascomycetes: Aigialus, Halotthia, Leptosphaeria, Massarina,
Mycosphaerella, Pleospora
Basidiomycotina
Aphyllophorales: Digitatispora, Halocyphina
Gasteromycetes: Nia
Deuteromycotina Asteromyces, Dendryphiella, Orbimyces, Sig-
moidea, Varicosporina, Zalerion
A fuller list of higher marine fungi will be found in Kohlmeyer and
Volkmann-Kohlmeyer (1991).
Marine ascomycetes are often characterized by having asci whose walls
deliquesce and ascospores which bear appendages. A few typical marine
ascospores are illustrated in Figure 9.13. Ascospore appendages can
develop in a great variety of ways, and their structure and developments
have been much studied (Kirk, 1976; Jones and Moss, 1978; Jones,
Johnson and Moss, 1983; Johnson, 1980). The marine basidiomycetes
Digitatispora marina and Nia vibrissa, although not closely related, form
basidiospores with radiating arms, so that the spore is tetraradiate or
pentaradiate (Figure 9.14).
 N
&;
>
,Q
c
~
n
..,.,
c
z
C)

Figure 9.13 Marine ascomycetes. (a) Perithecium of Corollospora maritima attached to a sand grain. (b) Corol/ospora maritima
ascospore with polar and equatorial appendages. (c) Amylocarpus encephaloides ascospores. (d) Halosphaeria quadriremis ascospore.
(e) Ceriosporopsis halima ascospore. (f) Halosphaeria mediosetigera ascospore . (g) Arenariomyces trifurcata ascospore.
 MARINE FUNGI 269

Figure 9.13 (h) Ceriosporopsis calyptrata ascospores. (i) Lulworthia sp. sigmoid
ascospores. Scale bar= 200 11m (a); 20 11m (b-h); 50 11m (g,i).

The role of ascospore appendages or of branched basidiospores has been

studied experimentally. Removal of the arms or appendages by sonication
resulted in more rapid sedimentation than for undamaged spores (Rees,
1980), and Rees concluded that the extensions helped to keep the spores in
suspension. It is also likely that they aid trapping to substrata such as wood
(Hyde and Jones, 1989; Hyde, Moss and Jones, 1989).
Apart from Varicosporina, Orbimyces, Robillarda and Dinemasporium,
which have conidia bearing appendages, the conidia of most marine
deuteromycetes appear not to have distinctive morphological adaptations
to flotation or trapping. The distribution and ecology of conidial marine
fungi have been reviewed by Kohlmeyer (1981a) and Subramanian (1983).
There have been numerous studies on the physiology of higher marine
fungi in culture (for references, see Jones and Byrne, 1976; Jennings, 1983,
270 AQUATIC FUNGI

Figure 9.14 Marine basidiomycetes and deuteromycetes. (a) Digitatospora marina

basidium showing three attached tetraradiate basidiospores. (b) Nia vibrissa
pentaradiate basidiospore. (c) Asteromyces cruciatU5 conidia attached to conidi-
ophores. (d) Zalerion maritimum detached conidium. (e) Dendryphiella salina
conidiophores and conidia. Scale bar = 20 f1m (a,b,c); 30 f1m (d); 30 f1m (e).

1986). Unfortunately, most studies of marine fungi in culture have

provided little information of ecological significance (Jennings, 1986). This
 MARINE FUNGI 271

is partly because the commonly used batch culture technique does not
match natural conditions and may result in large pH changes. A second
reason is that in many experiments in culture, the fungi have been provided
with abnormally high concentrations of carbohydrates, enabling them to
synthesize organic solutes in the cytoplasm, thus to maintain a high solute
potential, and consequently a turgor potential sufficient to allow growth to
continue. Jennings (1986) has pointed out that for a fungus growing in the
sea, sea water has three unusual properties: its low water potential, its high
concentration of ions and its alkaline pH. The ability to grow in a medium
of low water potential is correlated with a high internal osmoticum,
generated by the synthesis of polyols such as glycerol, mannitol and
arabitol. Ions may also make a major contribution to the solute potential of
the protoplasm (Wethered, Metcalf and Jennings, 1985; Clipson and
Jennings, 1990).
Many physiological studies have centred on the effects of salinity. It is
surprising that in pure culture many marine fungi grow equally well on
media made up with sea water or with distilled water. It is important,
however, to distinguish between the effects of sea water on spore germina-
tion, vegetative growth and on sporulation, because, for successful life in
the sea, a fungus must be able to germinate, grow and fruit in competition
with other marine organisms.

a) Spore germination
The spores of most marine fungi have no constitutive dormancy, and are
capable of immediate germination. There is, however, evidence that sea
water contains a mycostatic factor that inhibits spore germination of some
species of marine lignicolous fungi but not others. Kirk (1980) studied
germination of conidia of Orbimyces spectabilis, Trichocladium achraspo-
rum, Dendryphiella salina and Zalerion maritimum, and ascospores of
Halosphaeria mediostigera in Millipore-filtered sea water with a salinity
range of 15.5 to 27.5%o. 0. spectabilis, T. achrasporum (the conidial state
of H. mediostigera) and Z. maritimum showed no mycostatic inhibition,
but the spore germination of D. salina and H. mediosetigera was inhibited
in filtered natural sea water. The mycostatic effect was nullified by the
addition of nutrients such as 0.1% glucose, 0.1% yeast extract or 0.1%
(NH4 hP0 4 . The requirement for nutrients to stimulate germination may
be an adaptation preventing premature germination of spores not in
contact with a suitable substratum. Kohlmeyer (1981b) has suggested that
the mycostatic factor is effective for spores of obligate intertidal species,
but does not operate on spores of subtidal or deep-sea species of marine
fungi.
In many marine fungi, spore germination is relatively insensitive to
variations in salinity. Byrne and Jones (1975a) studied spore germination
272 AQUATIC FUNGI

in Coro/lospora maritima (ascospores), Asteromyces cruciatus, Dendryph-

iella salina and Zalerion maritimum (conidia) in sea water concentrations
from 0 to 100%. The germination response in all was 'flat-topped', with
spore germination between 50 and 100% within the range of salinities
tested. This behaviour contrasts with that shown by ascospores of the
marine pyrenomycete Lindra thallassiae, which grows on leaves of turtle
grass (Thalassia testudinum), a submerged marine angiosperm. The spores
of this fungus fail to germinate on distilled water media (Meyers and
Simms, 1965).

b) Vegetative growth
As stated above, mycelial growth of many higher marine fungi can occur
over a wide salinity range (Jones and Jennings, 1964). Jones, Byrne and
Alderman (1971) have shown that mycelia of Cremasteria cymatilis,
Sporidesmium sa/inurn and Lulworthia floridana can grow over the range
of 10-100% sea water. The rate of growth of Cremasteria was little affected
by salinity variations in this range, whilst the other two species showed
improved growth at the higher salinities. The marine pyrenomycetes
Lindra thalassiae, Lulworthia floridana and Halosphaeria mediosetigera
are also capable of growth over the range 0-100% sea water (Meyers and
Simms, 1965), whilst the basidiomycete Halocyphina villosa can grow in
water with a salinity range of 1-200% sea water (Rohrmann and Molitoris,
1986).
Davidson (1974) has compared the growth rate and respiration of two
pyrenomycetes (Giiumannomyces graminis, a terrestrial cereal pathogen,
and Lulworthia medusa, a lignicolous marine fungus) at different salinities
from 0 to 28%o. Although both can grow within the salinity range tested,
the growth of G. graminis is more rapid on media lacking sea water, whilst
the opposite is true for L. medusa. The respiration rate of G. graminis is
reduced in sea water, whilst the respiration rate of L. medusa was
approximately the same in fresh and in sea water. It was postulated that in
fresh water L. medusa devotes a high proportion of its respiratory energy
to functions other than biomass increase, which may place the fungus at a
competitive disadvantage in a freshwater environment.
Interactions between salinity and temperature have been noted by
Ritchie (1957). He isolated a species of Phoma from submerged pine
panels near Panama and tested its radial growth rate on media containing
artificial sea salts up to a concentration of 9% w/v (i.e. up to nearly 2.5
times that of normal sea water) within the temperature range 7-37°C. The
results (Figures 9.15 and 9.16) indicate that the optimum salinity for
growth rises with increasing temperature. This so-called 'Phoma pattern' of
growth response to salinity and temperature has been found also in a
species of Pestalotia isolated from the same substratum, in the marine
 MARINE FUNGI 273

I
5
•

I
;$!.
~
•
·c:....
> 3

ca
"'
E
:J
E 2
·a0 .~

I I

I 10 20 30 40
Temperature (•C)

Figure 9.15 Growth response of Phoma sp. to increasing salt concentration over a
range of-temperatures in vitro. (Reproduced from Ritchie, 1957.)

basidiomycetes Digitatispora marina (Doguet, 1964), Halocyphina villosa

(Rohrmann and Molitoris, 1986), and the hyphomycete Zalerion mariti-
mum (Ritchie and Jacobsohn, 1963).

Table 9.5 Fruiting of marine ascomycetes in diluted sea water

Seawater%

0 20 40 60 80 100

Lulworthia floridana 0 3 3 3 3 3
Lulworthia sp. 3 3 3 3 3 3
Lindra thalassiae 1 3 3 3 3 3
Halosphaeria mediosetigera 3 3 3 3 3 3
Torpedospora sp. 3 3 3 3 3 3

0 = Perithecia absent; 1 = perithecia immature; 3 = perithecia with asci and

ascospores.
Source: Jones, Byrne and Alderman (1971).
274 AQUATIC FUNGI
70

50

.s::. 40
E
E
.s::.
~ 30
e
(!)

20

10

2 4 6 8 10
Salt concentration (%)
Figure 9.16 Relationship between optimum salinity for growth and temperature in
Phoma sp. (Reproduced from Ritchie, 1957.)

c) Reproduction
The effects of salinity variations on the reproduction of higher marine fungi
are variable. Some species are able to sporulate within the salinity range of
G-100% sea water, whilst others fail to develop fructifications, or have only
immature fructifications in the absence of sea water (Jones, Byrne and
Alderman, 1971) (Table 9.5). Doguet (1964) has shown that the marine
basidiomycete D. marina can sporulate in diluted sea water within the
range 5-25%o (i.e. up to about 70% sea water). Sporulation is most prolific
at 15-20°C, which may be related to the known distribution of the fungus in
temperate waters, and may also explain why in nature it appears to fruit
best only in the cooler months. H. villosa produces basidiocarps in culture
over the range 25-100% sea water at temperatures between 22 and 27°C.
This may reflect its natural habitat on the wood of mangroves, which are
tropical and may be coastal or estuarine in their distribution (Ginns and
Malloch, 1977; Hyde, 1986; Rohrmann and Molitoris, 1986).
Although vegetative growth of the marine deuteromycetes Orbimyces
spectabilis and Varicosporina ramulosa takes place within the range
0--100% sea water, the amount of growth rising with increasing salinity,
typical conidial development only occurs above a sea-water concentration
of 20%. Below this concentration, conidia either fail to develop or the cells
making up the body of the conidium may encyst. Chlamydospores may also
 MARINE FUNGI 275

occur in the mycelium (Meyers and Hoyo, 1966).

The ability of many higher marine fungi to germinate, grow and fruit
over a range of 0-100% sea water is puzzling. It is known that some species
can extend into estuaries, but what factors prevent their further extension
into fresh water? Possibly competition from terrestrial or aquatic fungi is
involved.
Jones (1974) has compared the behaviour of some different ecological
groups of fungi in relation to salinity tolerance (Figure 9.17). The diagram
shows that the spore germination, growth and reproduction of terrestrial
ascomycetes are deleteriously affected by increasing salinity, and that
aquatic hyphomycetes are even more drastically affected.
There is a wide range of substratum and mode of nutrition for higher
marine fungi. Some are parasites of seaweeds, angiosperms and marine
animals. Saltmarsh plants such as Spartina and Halimione bear an exten-
sive flora of fungi, many of which are adapted to sea water. However, most
ecological work has been done on the saprophytic fungi which colonize
wood in or around the sea. Some tropical coasts are fringed by woody
plants, e.g. Cocos, or by mangroves like Rhizophora and Avicennia, which
are adapted to live in mud bathed by the sea. The leaves, branches, roots,
seedlings and fruits of these plants which fall into the sea are colonized by
marine fungi or, in the case of prop-roots of mangroves, are colonized in
situ. Detritus from mangroves can provide the major carbon input into
some estuarine ecosystems and microbial colonization increases the pro-
tein content of the detritus which provides food for invertebrates which are

%Sea water

Mucoraceae

Terrestrial ascomycetes

Terrestrial fungi imperfectii

Aquatic hyphomycetes

Marine ascomycetes

Marine fungi imperfectii

Figure 9.17 Physiological and ecological responses to salinity of various ecological

groups of fungi. The scale represents percentage sea-water concentration. (After
Jones, 1974.)
276 AQUATIC FUNGI

in turn consumed by crustacea and fish. There have been many studies of
fungi colonizing mangrove detritus, including leaves and whole seedlings
(Kohlmeyer, 1969, 1986; Anastasiou and Churchland, 1969; Fell and
Master, 1975; Newell, 1976; Newell et al., 1987; Hyde and Jones, 1988;
Jones and Kuthubutheen, 1989; Newell and Fell, 1992). These studies will
not be considered here, but reference will be made below to fungi
colonizing woody mangrove debris.

9.3.3 lignicolous marine fungi

Apart from woody fragments produced by maritime plants, there are
considerable amounts of driftwood brought down to the sea by rivers, or
brought there as a result of human activity. Studies on the higher fungi that
colonize driftwood can be made by collecting samples which can be
examined directly or after incubation in moist chambers. The fungi
colonizing wood in situ in the sea can be studied by examining wooden piles
or breakwaters. Alternatively, wood panels can be immersed in the sea at
different depths and left in place for varying times before collection and
examination, usually after moist-chamber incubation.
By submerging a string of panels of different kinds of wood, an
indication of host preference by different marine fungi has been obtained.
Meyers and Reynolds (1960) found that panels of basswood (Tilia ameri-
cana) bore fruit-bodies of Lulworthia, Ceriosporopsis and Halosphaeria,
but panels of yellow pine (Pinus palustris) did not. Similarly, Byrne and
Jones (1974) showed that panels of beech (Fagus sylvatica) were more
frequently colonized than those of Scots pine (Pinus sylvestris). Certain
fungi show a preference for beech, e.g. Halosphaeria appendiculata, H.
hamata and Nautosphaeria cristaminuta, whilst others prefer Scots pine,
e.g. Ceriosporopsis circumvestita (Jones, 1976b).
Although there is an apparent succession of fungi fruiting on incubated
wood panels, the sequence of fungi may reflect the time taken for
fruit-bodies to develop. Amongst the first fungi to fruit on incubated wood
panels are the hyphomycetes Piricauda, Zalerion and Humicola. Some
panels may be dominated by Lulworthia sp. to the virtual exclusion of
other fungi; this is possibly an example of antagonism.

a) Fruiting of marine lignicolous fungi in nature

Schaumann (1968) has studied the fruiting of marine fungi on incubated
fragments of wood removed from posts driven into the sea-bed in the
estuary of the River Weser in northern Germany. The salinity range varied
from 26-31%o at the seaward end to around 0.7%o in the area dominated by
fresh water. Schaumann has distinguished between four salinity zones, and
has listed the characteristic fungi which he found in them.
 MARINE FUNGI 277

• Holeuryhaline species: These occur in fresh, brackish and sea water,

spanning the whole range from 0.5 to 30%o. They include the deutero-
mycetes Cirrenalia macrocephala and Piricauda pelagica.
• Euryhaline species: The distribution of this group resembles the former,
but they do not colonize wood in fresh water. They include the
deuteromycete Dictyosporium pelagicum, and the ascomycetes
Halosphaeria appendiculata, H. mediosetigera, Lignincola laevis, Ceri-
osporopsis calyptrata, Remispora hamata and R. maritima.
• Genuine brackish water species: These inhabit regions with fluctuating
salinity, including Remispora pilleata and the deuteromycete Humicola
alopallonella. ·
• Stenohaline species: These live in salt water with a relatively narrow and
decidedly high concentration. These typically marine forms include the
ascomycete Corollospora maritima and the deuteromycete Zalerion
maritima.

Vertical distribution of lignicolous fungi in the intertidal (eulittoral) zone

on wooden pilings in the sea in the Weser Estuary and around the island of
Helgoland in the North Sea has been compared (Schaumann, 1969, 1975).
Around Helgoland, the salinity range is around 30-34%o, which is higher
than in the estuary. Wood samples were removed from the pilings, and the
heights above chart datum of the sampling points were recorded. Selected
results are shown in Figure 9 .18.
Fungi colonizing the pilings below the level of low water spring tides
(LWS) would remain permanently submerged, but above this point they
would be subjected to periodic emersion, and the effects of sun, wind, rain,
etc. At the upper end, in the supralittoral zone, they would be uncovered
by the sea for most of the time, but subjected to splash and spray. It is
notable that some fungi, e.g. Corollospora maritima, Lignincola laevis and
Zalerion maritima, were recovered from virtually the whole of the tidal
range, whilst others, such as Remispora maritima, Monodictys pelagica and
Dictyosporium pelagicum, were only recovered from the upper part of the
eulittoral zone. The reasons for these differing patterns of distribution are
not understood.
Differences in vertical distribution within the intertidal zone of fungi
growing on overhanging branches and on prop-roots and subterranean
exposed roots of mangroves (Rhizophora spp.) have been noted by Hyde
(1988), working in Brunei. Of the 41 species collected, most (26 species)
occurred in the middle level. Some species, such as Halocyphina villosa,
Lulworthia grandispora and Lulworthia sp., formed fruit-bodies at all
levels. Others, such as Humicola allopallonella and Tricladium sp., were
restricted to lower levels, whilst Aigialus spp. fruited at the mid- and
upper-littoral levels where the surface of the wood dries out. Whilst the
distribution of fruit-bodies may be related to the degree of exposure to the
278 AQUATIC FUNGI

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::: ----~---t--- --------------- --- --- --- --- -------- ----HVVN

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Figure 9.18 Vertical distribution of fruit-bodies of lignicolous marine fungi on

posts submerged in the sea at Helgoland in the North Sea. HWS = High water at
spring tide; HWN =high water at neap tide; LWN =low water at neap tide; LWS
=low water at spring tide. (Redrawn from Schaumann, 1969.)

air, or to the extent of covering by mud, biotic factors may also be

involved. For example, wood in the middle levels is more prone to attack
by marine borers.
The distribution of wood-colonizing fungi in estuaries has been studied
by Gold (1959) and Shearer (1972). Gold, working in the Newport River,
North Carolina, USA, immersed wood panels at stations with a salinity
gradient of 0-35%o. Four lignicolous fungi, Ceriosporopsis halima, C.
cambrensis, Lulworthia floridana and Trichocladium opacum, were recov-
ered within virtually the entire salinity range. There were indications of an
interaction between higher temperatures, higher salinity and distribution,
i.e. with increasing temperature the fungi were able to fruit better in water
of higher salinity. This is reminiscent of the 'Phoma pattern' of mycelial
growth reported by Ritchie (1957) (section 9.3.2(b)). Shearer submerged
balsa wood (Ochroma) blocks in the Putuxent River, Maryland, USA, at
 MARINE FUNGI 279

stations covering a salinity gradient of 0.05-29.0%o. Not all the fungi which
fruited were marine; many were of 'terrestrial' and some of freshwater
origin. Increasing salinity was correlated with an increase in the proportion
of ascomycetes and a decrease in the proportion of deuteromycetes
recovered, so that, at the most saline station, about two-thirds of the fungi
found were ascomycetes. No evidence of a 'Phoma pattern' of
temperature-salinity relationships was found. Hughes (1975) has reviewed
other studies on the distribution of fungi in estuaries.
Marine lignicolous fungi can exist at considerable depth in the sea,
extending well beyond the sublittoral. Kohlmeyer and Kohlmeyer (1979)
have claimed that 'fungal mycelia have been found in every piece of wood
collected in the deep sea, unless a low oxygen content of the surrounding
water had impeded development'. Down to a depth of several hundred
metres, the fungi recorded include some familiar representatives from
intertidal wood such as Zalerion maritimum and Corollospora maritima.
At greater depths (over 1000 m), a few specialized fungi exist at high
pressures, low temperatures and in constant darkness. They include the
ascomycetes Bathyascus vermisporus and Oceanitis scuticella, and the
deuteromycetes Allescheriella bathygena and Periconia abyssa. Samples of
wood recovered from the deep sea rarely bear fungal fructifications, and
Kohlmeyer (1977) has speculated that sporulation may only occur on small
wood fragments broken away as a result of the activity of wood-boring
animals.
The so-called lignicolous marine fungi are not confined to woody
materials, and many have been reported from 'sea grasses' such as
Cymodocea, Posidonia, Thallasia and Zostera (Hughes, 1975). Species of
Corollospora, Lindra, Lulworthia, Halotthia and Pontoporeia, Varicospo-
rina and Dendryphiella have been found on debris derived from such plants
(Cuomo et al., 1985).
The decay of wood immersed in the sea is of great economic significance.
It is brought about by bacteria (including actinomycetes), fungi and
animals, especially wood-boring molluscs and crustacea. In most cases, the
decay of submerged wood by ascomycetes and deuteromycetes is of the
'soft-rot' type. Fungal hyphae grow in the lumina of the wood cells, and
from these penetrate to the lightly lignified S2 layer of the secondary cell
wall, often following the spiral orientation of the microfibrils making up
the wall. Leightley and Eaton (1977) have given a detailed description of
the decay of wood by the ascomycete Halosphaeria mediosetigera which
causes soft-rot decay. The hyphae in this fungus are ensheathed in a
mucilaginous layer which possibly contains wall-degrading enzymes. The
basidiomycete Nia vibrissa causes white-rot decay in which a well-marked
erosion zone surrounds the track of hyphae penetrating host cell walls
(Leightley and Eaton, 1979; Jones, 1982).
Much interest has been devoted to the suggestion that marine fungi and
280 AQUATIC FUNGI

bacteria may 'condition' wood and make it more susceptible to attack by

marine borers (Kohlmeyer and Kohlmeyer, 1979). The gribble, Limnoria
tripunctata (Crustacea, Isopoda) can live on fungus-free wood, but its
lifetime can be extended if it is grown on softwood colonized by marine
fungi. Apparently L. tripunctata does not reproduce unless marine fungi
are included in its diet. The reason for this is unknown, but there have
been speculations that the marine fungi might supply protein, vitamins or
oils which may be required for reproduction.
Wood which has been in the sea for some time may be cast ashore
bearing marine fungi visible at the time of collection or after incubation in
moist chambers. On sandy shores, the driftwood can become partially or
completely buried in sand, especially at the foot of sand dunes along the
upper shore. Marine fungi may fruit on wood which has lain in the moist
sand, and some can extend from the substratum to form ascocarps on the
sand grains. It is also known that the appendaged spores of marine fungi
may be trapped in foam and can then be carried on shore by wind, possibly
to be caught onto suitable substrata or into the sand (the so-called
arenicolous fungi) (Kohlmeyer, 1966; Wagner-Merner, 1972). Common
spores in sea foam include ascospores Corollospora maritima, Arenariomy-
ces trifurcata (Figure 9.13) and, in warmer seas where appropriate sub-
strata such as Zostera and seaweeds are available, the conidia of
Varicosporina ramulosa.
Ecological studies on the fungi characteristic of sand dunes have been
made by Koch (1974) and Rees, Johnson and Jones (1979). Koch has
distinguished between three zones on sandy shores in Denmark:
Zone 1: the wet, turbulent water's edge
Zone 2: the dry, windswept beach
Zone 3: the undisturbed area at the base of the dune and the gullies
leading up to the inner dunes.
The condition of the wood in each zone is influenced by its water content,
and probably by its salt content. In zone 1, the wood is often eroded,
presumably by abrasion, whilst in zone 2, the wood is usually much drier
and is capable of floating. In zone 3, the wood is often moist and its lower
side often bears hyphae which extend into the sand. Only a small
proportion of the wood collected in zone 1 bears fruit-bodies at the time of
collection, and most of the fungi which do fruit have perithecia immersed
in the wood. After incubation in moist chambers, perithecia of Corol-
lospora spp. develop at the surface, indicating the presence of mycelium in
the outer layers of the wood. The absence of fruit-bodies on fresh
collections is probably due to abrasion. Wood from zone 2 similarly bears
few fruit-bodies at the time of collection, and the effect of heat and drying
may damage the mycelium or prevent development of fruit-bodies. Wood
from zone 3 pressed into or partially covered by moist sand may develop
 MARINE FUNGI 281

perithecia of Corollospora maritima, Arenariomyces trifurcata and Car-

bosphaerella leptosphaerioides actually attached to the sand grains (Figure
9.13a). Perithecia of some of these fungi also develop on other hard
materials such as barnacle tests. The physiological reasons for this prefer-
ence for hard substrata for fruit-bodies are not known, but it is possible
that dispersal of sand grains bearing perithecia may occur.
The marine deuteromycete Varicosporina ramulosa forms sclerotia
attached to sand grains. Kohlmeyer and Charles (1981) have used the term
sclerocarp for such structures which they interpret as propagules, and have
presented anatomical evidence of homology between sclerocarps in this
fungus with perithecia of members of the Halosphaeriaceae.
Isolations of fungi from sand near driftwood from Danish sand dunes by
Rees, Johnson and Jones (1979) yielded different species depending on the
technique used. If sand was plated directly on to agar media made up with
sea water, the most commonly encountered fungi were terrestrial
hyphomycetes, and some marine hyphomycetes such as Asteromyces
cruciatus, Monodictys pelagica and Dendryphiella salina (Figure 9.14).
When sand was added to sterile balsa-wood strips moistened by sea water,
a number of marine ascomycetes (Corollospora spp. and Carbosphaerella)
were identified. Presumably the spores of these fungi were trapped within
the sand.

b) Geographical distribution of marine lignicolous fungi

In surveying the worldwide distribution of marine lignicolous fungi, there
are several difficulties. The distribution of suitable substrata is obviously
related to proximity to land with trees whose remains can reach the sea,
although wood can drift for great distances. Some marine fungi are
restricted to the prop-roots of mangroves, whilst others are host-specific
parasites of marine algae, so that the distribution of these fungi is
dependent on the distribution of the host plant. However, the majority of
wood-inhabiting marine fungi are less specific and are not host-dependent
in their distribution (Kohlmeyer, 1983). The activities of biologists with
interest in and competence to identify marine fungi are not evenly
distributed. For example, the polar regions are probably under repre-
sented because of difficulty of access and facilities for study. The world
distribution of marine fungi has been discussed by Hughes (1974, 1975,
1986), Kohlmeyer (1983) and Booth and Kenkel (1986). Hughes (1974) has
mapped the records of distribution of marine lignicolous fungi and has
related these to five biogeographical regions defined in terms of surface-
water isotherms. The surface-water temperature regimes used to delimit
these five regions are:

• Tropical: The regions between the isotheres for 20°C water temperature
282 AQUATIC FUNGI

at the coldest time of the year. This region corresponds with that of
reef-building corals.
• Subtropical: The region between the 17°C isocryme for August in the
southern hemisphere and February in the northern hemisphere.
• Temperate: A region bounded towards the equator by the 17°C
isocryme for the coldest calendar month and towards the pole by the
woe isothere for the warmest calendar month.
• Arctic and Antarctic: Regions separated from temperate regions by a
line following the woe isothere for August in the north and for February
in the south.

Each of these regions has its characteristic flora of marine fungi.

Ceriosporopsis halima has a wide distribution, but is not common in the
tropical zone. Halosphaeria hamata 'is perhaps best considered as a species
of temperate regions', but there are problems in identification of this and
similar fungi which make for uncertainty in mapping its distribution.
Antennospora quadricornuta 'is the most common marine ascomycete in
tropical waters and appears to be virtually restricted to this zone', an
opinion confirmed by Hyde (1986). Ascospore germination occurs readily
at 28°C within a few hours, but not at zooc (Kohlmeyer, 1968).
Corollospora maritima (Figure 9.13a,b) is a ubiquitous and common
fungus of sandy beaches with a worldwide distribution (Kohlmeyer, 1984).
However, five cultures from widely separated localities, although morpho-
logically similar, showed significantly different growth responses to tem-
perature variations (Bebout et al., 1987), suggesting that there are
physiological races adapted to different temperatures.
The relationship between temperature, seasonal occurrence and distri-
bution of three marine deuteromycetes has been investigated by Boyd and
Kohlmeyer (1982). Asteromyces cruciatus is an arenicolous fungus, and
Sigmoidea marina grows on seaweeds and marine angiosperms. Both are
distributed in the temperate zone. Varicosporina ramulosa also grows on
seaweeds and marine angiosperms, but has a subtropical distribution.
Conidia of all three fungi can be collected in sea foam, and regular
sampling of foam can indicate seasonal abundance. Some correlation was
found between the effects of temperature on growth and survival and
distribution. For V. ramulosa, the optimum temperature for linear growth
was 30-40°C, and for dry weight increase 20-30°C. This fungus can also
survive temperatures as low as woe but makes little or no growth at this
temperature. It possesses temperature-resistant propagules (sclerocarps)
which are attached to sand. They can endure prolonged exposure to a wide
temperature range (45 to -70°C), which would enable V. ramosa to
survive the extreme temperature ranges found on sandy beaches
(Kohlmeyer and Charles, 1981). The upper temperature limit for vegeta-
tive growth for A. cruciatus and S. marina was 35°C, but prolonged
 MARINE FUNGI 283

exposure of both fungi to this temperature resulted in an inability to grow

even at room temperature.
It is thus apparent that temperature is an important factor in relation to
the distribution of marine fungi. The studies of Booth and Kenkel (1986),
in which ordination analysis was used to group fungi with similar distribu-
tions together and relate them to environmental variables, showed that
temperature gradient is the dominant factor, whilst the combined effect of
temperature-salinity variation is of secondary importance.