1

Some cyanobacteria are capable of gliding,that is, the active

movement of an organism on a solid substrate where there is neither
a visible organ responsible for the movement nor a distinct change in
the shape of the organism (Jarosch, 1962).
Sheaths
A sheath (capsule or extracellular polymeric substances (EPS))
composed of mucilage and a small amount of cellulose is commonly
present in cyanobacteria. The sheath protects cells from drying. Active
growth appears necessary for sheath formation, a fact that may explain
its sometimes poor development around spores and akinetes

2
Colored.

Heterocysts lack ribulose-1,5-bisphosphate carboxylase/ oxy genase

and the ability to fix carbon dioxide. Heterocysts also lack
carboxysomes. And have nitrogenase to fix atmospheric nitrogen.
Heterocysts
Heterocysts are larger than vegetative cells and appear empty in the
light microscope (whereas akinetes appear full of storage products).
Heterocysts are
photosynthetically inactive, they do not fix CO2, nor do they produce
O2. They also exhibit a high rate of respiratory O2 consumption and
are surrounded by a thick, laminated cell wall that limits ingress of
atmospheric gases, including O2.

3
The internal environment of heterocysts is, therefore, virtually anoxic,
which is ideal for nitrogenase, a notoriously O2 sensitive enzyme.
Heterocysts are formed at regular intervals from vegetative cells by
the dissolution of storage granules, the deposition of a multi-layered
envelope outside of the cell wall, the breakdown of photosynthetic
thylakoids, and the formation of new membranous structures.
Nitrogen fixation
Cyanobacteria are diazotrophs (able to fix atmospheric nitrogen). All
known nitrogen-fixing organisms are prokaryotes. In nitrogen fixation,
N2 from the atmosphere is fixed by the enzyme nitrogenase into
ammonium using ATP as a source of energy (Fig. 2.27). The process is
one of the most metabolically expensive processes in biology, requiring
16 ATP for each molecule of N2 fixed

4
Vegetative reproduction is generally in four ways:
(i) Binary fission, e.g. in unicellular cells. A cell divides

into two in roughly equal halves. Each grows to original form. Here
nucleus divides mitotically first and then the cytoplasm. This is the
most common type.

(ii) Fragmentation: Filaments break into small pieces. Each piece

grows into new filament. Mostly occurs in colonial forms.

(iii) Hormogonia: Trichomes break up within the sheath into short

segments called hormogonia or hormogones. Hormogones can be 2-3
cells to several cells long. Any portion of the trichome may get
abstracted as a hormogone (e.g. in Nostoc and Oscillatoria). Stigonema
forms hormogonia on special branches. Sometimes these are motile and
move away from parent filament.

(iv) Hormocysts: These multicellular structures have a thick and

massive sheath. They are intercalary or terminal. They can germinate
from either end or both ends thereby forming new filaments.

Asexual reproduction Cyanophyceae members reproduce by non-

motile, asexual spores which are of following types:
(i) Akinetes: Are found close to heterocysts. Cells increase in size
and a thick layer is formed around them. Under favourable
conditions, new filaments are formed from them, e.g.
Cylindrospermum. These are perennating (dormant)
structures.Gas vacuoles are absent and protoplast contain

5
 abundant food reserve, lipid globules Most filamentous
cyanobacteria develop perennating structures (dormant
structures) in adverse condition. These structures are
larger than the vegetative cells, are equipped with thick
walls, and are called akinetes . When favourable condi-
tions return, they germinate and produce new filaments.
2. Hormogonia:
All filamentous cyanobacteria reproduce by fragmentation of
their filaments (trichomes) at more or less regular intervals to
form short pieces each consisting of 5-15 cells. These short
pieces of filaments are called hormogonia. The latter show
gliding motility and develop into new full- fledged filaments.
Harmogones are small segments of trichome surrounded by
very delicate sheath. !-2 celled harmogones are reported from
Oscillatoria and Stigonema. The harmogones show rapid
movements and therefore come out of the old sheath and grow
into new filamentous thalli.

1. Hormospores or Hormocysts

Some cyanobacteria produce hormocysts or

pseudoharmogonia, which are multicellular structures having a
thick and massive sheath of mucilage and they are large and
filled with stored food. They may be intercalary or terminal in
position and may germinate from either end or both the ends to
give rise to the new filaments. They help in overcoming adverse
conditions. They germinate directly under favourable
conditions into new thalli.

4. Spores:
Non-filamentous cyanobacteria generally produce spores such
as endospores, exospores and nanocysts which contribute by
germinating and giving rise to new vegetative cells when the
unfavourable condition is over. Endospores are produced
endogenously like those in bacteria; exospores are the result to

6
exogenous budding of cells, and the nanocysts are produced
endogenously like endospores.

The difference between an endospore and a nanocyst is that in

endospore formation the parent cell concomittantly enlarges in
size, whereas in nanocyst formation there is no such
enlargement of the cell.

Economic Importance of Cyanobacteria:

1. Cyanobacteria are one of the early colonizers of bare and
barren areas and generate such conditions that favour the
growth of other organisms even in the most hostile environment.

2. They are good food source for several aquatic animals.

Moreover, the cyanobacteria are now-the-days exploited as food
for animals including humans. Spirulina, a filamentous
cyanobacterium, is now incorporated in food supplement as well
as animal feed through ‘single cell protein’ manufacture because
of its high protein content (upto 70%).

Some Indian dishes, for instance, like ‘puri’ ‘idli’ and ‘sandwich’
prepared by supplementing 5-10% S. fusiformis have been found
to be palatable. In parts of Rajasthan Anabaena and Spirulina are
collected from Sambar lake and used as fodder and manure.

3. N2-fixation is the characteristic feature of many cyanobacteria

and this function is performed by heterocysts present in them.
Aulosira, Nostoc, Anabaena, etc. are some such cyanobacteria
that are now regularly inoculated in the rice fields for nitrogen
supply. This saves consumption of nitrogen fertilizers.
4. N2-fixing cyanobacteria (e.g., Nostoc, Anabaena) are often
used for reclamation of ‘usar’ soils. They produce acidic
chemicals for counteracting alkalinity of the soil and they supply
nitrogen compounds which are generally deficient in these soils.
5. Species of Anabaena and Aulosira do not allow mosquito
larvae to grow nearby. Such cyanobacteria can be inoculated in
village ponds to prevent the growth of mosquitoes.

7
6. Extracts of Lyngbia are used to manufacture antibiotic-like
compounds.

7. Certain cyanobacteria such as Microcystis aeruginosa (=

Anacystis cyanea), Anabaena flos-aquae and Aphanizomenon
flos-aquae produce toxins harmful to most aquatic animals.
These toxins may prove equally harmful to humans drinking or
bathing in such water.

8. Cyanobacteria generally grow on walls and roofs of buildings

during the rainy seasons and cause discolouration, corrosion,
and leakage.

Class: Cyanophyceae or Myxophyceae

Order: Nostocales
Family: Nostocaceae
Genus: Nostoc

8
Morphology: Nostoc is a colonial cyanophyte. Colonies
are ball-shaped.

Each colony contains several filaments. Each filament is

composed of moniliform cells.

Filaments are slender, long and appear like chain of

beads. Filaments are interwoven but unbranched

9
Cells are rounded or oval. Apart from common mucilage
covering the filaments, each filament is often enclosed
in a gelatinous sheath. Vegetative cells have an outer
coloured part (chromoplasm) and a central colourless
region (centroplasm). Apart from simple

vegetative cells, there are some cells, some specialized

vegetative cells occur at regular intervals. These are
bigger in size with thickened walls and pores at both
the poles (intercalary) or on side adjoining simple
vegetative cell (terminal). These are named
heterocysts.
10
Cytoplasmic connections with the adjoining cells are
maintained through the pores. At a later stage, pores
are blocked by button like thickenings called polar
molecule or nodule. The filament increases in length by
cell division only in one plane.

Life cycle: No sexual reproduction and alternation of

generation. Reproduction is by two means: Vegetative
by fragmentation and formation of hormogonia; asexual
by akinete formation mainly but sometimes endospores
are also formed, e.g. N. commune and N.
microscopicum. Hormogonia, akinetes and endospores
germinate into fresh Nostoc filaments under favourable
conditions. Heterocysts may also undergo divisions and
germinate to form new filaments.

11
Chlorophyceae
Introduction
Divison Chlorophyta includes a diverse assemblage of photosynthetic
organisms commonly known as green algae. These organisms are
largely aquatic and are present in marine (seawater) and fresh waters.
The organisms can be unicellular, multicellular, coenocytic (more than
one nucleus in a cell) or colonial representatives. Chlorophytes are
eukaryotes with an organised (membrane enclosed) cell nucleus
containing DNA and organised plastids(chloroplasts containing

12
abundant green pigments- chlorophyll a and b and various carotenes
and xanthophyll). Photosynthesis occurs in chloroplasts.
Occurrence of Chlorophyceae (Green Algae):
The members of Chlorophyceae generally grow in fresh water (about
90%) and the rest in saline water, terrestrial habitat etc. The fresh water
members such as Volvox, Oedogonium, Spirogyra etc. grow in ponds,
pools and lakes.

Members of conjugales (e.g., Spirogyra, Zygnema etc.) and

Oedogoniales (e.g., Oedogonium etc.) are strictly fresh water, but the
members of Ulvaceae and Siphonales are predominantly marine. Some
members of Volvocales, Chaetophorales and Cladophorales grow both
in fresh and saline water.

Some species of Ulothrix and Vaucheria are subaerial and grow on

damp soil. Some members may be terrestrial and grow as epiphytes on
tree trunk, leaves etc. (e.g., Trentepohlia); as epizoic i.e., (growing on
animal bodies (species of Characium and Cladophora); as endophytes
(e.g., Chlorella), as parasites (e.g., Cephaleuros, Rhodochytrium and
Phyllosiphon) and also cause diseases.

Habitat: where do they occur? Most of the Chlorophytes are aquatic

and are predominantly freshwater; only about 10% of the aquatic forms
are marine. Some Chlorophytes are terrestrial as well. Among the
aquatic forms, most of the Ulotrichales have freshwater forms whereas
members of Caulerpales are predominantly marine. Oedogoniales have
exclusively freshwater forms. Spirogyra and Hydrodictyon frequently
form noticeable but harmless blooms that may cover the surface of
freshwater ponds. A few members of Chlorophyta commonly occur in
nearshore marine environments forming nuisance macroscopic
growths e.g. Ulva (sea lettuce), Codium (dead man‟s fingers),. Species
of Ulothrix, are sub-aerial, and are usually found on damp soil in the
13
form of sheets. Trentepohlia forms orange-red growths on moist rocks
or cliff faces due to the accumulation of carotenoid pigments which
obscure chlorophyll. Species of Protococcus and Trentepohlia are
epiphytic on sea weeds or on the bark of trees, whereas Chlorella
(Figure g) is endophytic i.e. they live inside a host organism.
Cephaleuros and Rhodochytrium are parasitic, and they lack green
colour. Cephaleuros is a parasitic form which causes red-rust disease
of tea leaves. Some species occur in unusual habitats; for example,
Chlamydomonas yellowstonensis, Chlamydomonas nivalis and
Haematococcus nivalis occur on the snow covered mountain tops and
can impart a characteristic colour (which varies from red to orange to
yellow) to the snow due to the accumulation of carotenoid pigments.
Chlamydomonas nivalis is cryophilic (cold loving) and flourishes in
freezing waters. It contains a red carotenoid pigment known as
astaxanthin in addition to chlorophyll and imparts a red colour to the
snow referred to as watermelon snow. Some green algae in association
with certain fungi form lichens.
Habit and structure: Range of thallus organization
In Chlorophytes there is no differentiation of root, stem and leaves;
hence the plant body is thalloid in nature. The thallus may be single-
celled or many-celled and shows evolutionary progression from simple
to more complex types of construction. Chlorophytes are a
heterogenous group exhibiting a wide range in their thallus structure
and morphology beginning from simple microscopic motile unicellular
forms through multicellular flagellated or non flagellated colonies,
palmeloid forms, dendroid forms, filamentous forms, heterotrichous
forms, siphonous forms to well developed parenchymatous thalli.
General characters of Form and function (with special reference to
Chlorophyceae)
Members of Chlorophyceae grow mostly in fresh water, a few in
brackish and saline water and a few are terrestrial.

14
1. Plant body has a simple construction and shows no differentiation
into true root, stem and leaves. For this reason plant body is called a
thallus.
2. The cells constituting the thallus are eukaryotic and thus contain all
the cell organelles such as the definitely organized nucleus; membrane
bound plastids, mitochondria, Golgi bodies, endoplasmic reticulum,
and true vesicles.
3. Starch is the main reserve food.
4. The cell wall is stable and generally has cellulose as the main
structural polysaccharide.
5. The protoplast is bounded by a thin and semi permeable plasma
membrane.
6. The cytoplasm possesses many small vacuoles or there is a large
central vacuole.
7. The pigments are localized in the green plastids known as
chloroplasts.
8. The main pigments are chlorophyll a and b, but α and β carotenes
and xanthophylls are also present. 9. The chloroplasts normally contain
the pyrenoids surrounded by a starch sheath.
10.The shape of chloroplast shows much variation; it is cup shaped,
(Chlamydomonas), girdle shaped (Ulothrix), reticulate (Cladophora,
oedogonium), stellate, spiral (Spirogyra) or discoid.
11.Usually there is a single nucleus in each cell, but the members of
Siphonales and Cladophorales are coenocytic. The position, size and
shape of the nuclei are quite variable. The nucleoli and chromosomes
remain suspended in the granular matrix of the nucleus. In general,
there is a single nucleolus per nucleus but in Conjugales, there are
several nucleoli in a nucleus.

15
They show wide range of variations in their thallus structures like
unicellular motile (Chlamydomonas) and non-motile (Chlorella),
coenobium (Volvox), palmelloid (Tetraspora), dendroid
(Ecballocystis), filamentous branched (Cladophora) and unbranched
(Spirogyra), heterotrichous (Coleochaete), siphonaceous (Vaucheria)
and parenchymatous (Ulva).
Flagella are 1-many, equal in size and inserted either apically or sub-
apically. The flagella show typical 9+2 arrangement when viewed
under E.M.
12.Most of the flagellate cells have a photosensitive red eye spot or
stigma in the anterior portion, near the flagellar base.
13.The motile vegetative or reproductive cells (zoospores and gametes)
have two (rarely four) equal, whiplash type flagella inserted ate the
anterior end. However, in Oedogoniales the motile cells have a ring or
crown of flagella.
14. The plant body is unicellular or multicellular. The former may be
solitary or colonial. The multicellular forms show a considerable range
of variation in the form and structure of the plant body.
The flagellate cells have eye-spot or stigma in the anterior portion,
which remain inserted at one side of the chloroplast.

8. The pigments are located in the chloroplast. Chkiroplast generally

contains pyrenoid(s).

15. All cells of Chlorophyta contain at least one plastid, though these
do not always display photosynthetic pigments. Most of the
Chlorophytes are considered to be autotrophic. However, Chlorophytes
exhibit a surprising level of nutritional variation.
16. They reproduce by vegetative, asexual and sexual methods. The
vegetative propagation takes place by cell division and fragmentation.
16
Akinetes, zoospores and aplanospores are the main asexual spores.
Zoospores are often formed during night and are then liberated in the
morning.
they reproduce by all the three means i.e., vegetative (cell division and
fragmentation), asexual (zoospore, aplanospore, akinete etc.) and
sexual (isogamy to oogamy). The sexual reproduction is absent in some
members of Chlorococcales.
17. Sexual reproduction occurs in all members except some
Chlorococcales and Chlorodendrales. It may be isogamous,
anisogamous or oogamous.
Sexual Propagation
 Isogamy: The gametes are comparable in size and shape.

These gametes are not named male or female.

 Anisogamy: It is the combination of two gametes that are

either different in size or in both size and morphology. It is

seen in Chlamydomonas.
 Oogamy: Oogamy is a sort of anisogamy. The male gamete

is more modest than the female gamete.

18. The zygote or oospore secretes a thick wall and undergoes a resting
period. It germinates under favourable conditions and forms a new
thallus.
19. The sex organs are always unicellular.
20. Zygote generally is the only diploid structure in the life cycle.
21.Life history: Most of the members are haploid showing haplontic
life cycle. Some members are diploid (Siphonales) showing diplontic
life cycle while a few others show isomorphic alternation of
generations between haploid and diploid plants (e.g. Cladophora)-
called diplohaplontic life cycle. 23.Chlorophytes are better represented
in fresh water than in salt water.

17
Key similarities between Chlorophytes and land plants
 Chloroplasts of both Chlorophyta and land plants contain
photosynthetic pigments Chlorophyll a and Chlorophyll b.
 Similar arrangements of membrane bound thylakoids.
 Cell wall is similar in structure and composition
 Chloroplasts synthesize starch which is the main reserve food.
It has been generally accepted now that early chlorophytes gave rise to
the land plants. The ancestors of land plants most likely resembled
modern-day members of the division Chlorophyta.
Economic Importance of Chlorophyceae (Green Algae):
The green algae are not so economically important except a few
members.

Among them Chlorella is very important because of its high protein

content, presence of vitamins and its use in baking industry in the
preparation of cake, pastries etc. It is also used in the preparation of an
antibiotic, chlorellin; which is used to control bacteria. It is also used
in different physiological experiments.

Another important member, Chara, is very useful to control malaria for

its larvicidal properties. It is used as fertiliser and in the preparation of
polishes.

In South India, Green Laver, a kind of food, is prepared from Spirogyra

and Oedogonium.

Ulva and Enteromorpha are also eaten’ by some people.

Many members are used as a source of food and O2 for many aquatic
animals.

18
Eyespot: red to orange area in a cell, composed of lipid droplets
Palmelloid: term describing colony of an indefinite number of single,
nonmotile cells in mucilaginous matrix.
Phototaxis: movement of a whole organism toward (positive) or away
from (negative) light.
Plankton: organisms that float or swim to maintain a constant position
against a water current.
Pyrenoid:proteinaceous area of the chloroplast associated with the
formation of storage product.
Siphonaceous: large multinucleate cells without cross walls except
when reproductive bodies are formed.
Stellate: star-shaped
Stigma or eyespot: group of pigmented lipid bodies that are associated
with phototaxis.
Xanthophyll: a carotenoid composed of an oxygenated hydrocarbon.
Exercise/ Practice 1. Which algal division is characterized by the
presence of chlorophyll a and chlorophyll b, cellulosic cell walls, starch
as the main energy storage material, and lives in freshwater and marine
habitats?
2. Describe the salient features of Chlorophyta.
3. Name the reserve food material in the cells of Chlorophyta.
4. What is a coenobium? 5. Describe the range of thallus
structure/growth forms exhibited within Chlorophyta.
6. Name a green alga which is parasitic.
7. Describe the range of habitat where members of Chlorophyta are
usually found.
8. What are the various criteria for classification of algae.

19
9. Fill in the blanks: (i) The colony of Volvox having a definite number
of cells arranged in a specific manner is called……………………….
(ii)………………………causes red snow/watermelon snow.
(iii)……………………causes red rust of Tea. (iv) Members of
Chlorophyta are most abundant in……………water.
(v)…………………………is an epiphytic chlorophyte.
(vi)………………………occurs endophytically inside the cytoplasm
of Paramecium. (vii) The algae growing in seawater is known
as………………………algae. (viii)………………………..is an
example of a colonial green alga in Chlorophyta.
(ix)……………………………is an example of a filamentous green
alga in Chlorophyta. (x) The ancestors of land plants most likely
resembled modern-day members of the division…………… (xi) Plant
body showing no differentiation into true root, stem and leaves is
called………………. (xii) In ……………………. the coenobium is
net-like.

Why are they important to us?

Due to their role as primary producers in aquatic food webs,
Chlorophytes have a high ecological relevance, being thus of great
scientific importance. They generate oxygen through photosynthesis
and sequester large amounts of atmospheric CO2 in the ocean interior
(Field et al., 1998) and provide food for other organisms. Some of the
green algae such as Chlamydomonas, Chlorella, Volvox, Acetabularia
etc. are also being used as model organisms in laboratories.
Chlorella was used by Melvin Calvin for the elucidation of light-
dependent reactions of photosynthesis (Calvin cycle).
 Acetabularia was used by Joachim Hammerling for the
transplantation experiments to demonstrate the role of nucleus in
carrying genetic information and cellular development.

20
  Chlamydomonas reinhardtii has been used as a model system for
studying chloroplast biogenesis, photosynthesis, flagellar assembly and
function, cell cycle control and circadian rhythms.
 The colonial green alga Volvox has served as a model for the
evolution of multicellularity, cell differentiation, and colony motility
(Kirk, 1998; Kirk, 2003; Herron and Michod, 2008; Herron et al.,
2009).
Habitat: where do they occur? Most of the Chlorophytes are aquatic
and are predominantly freshwater; only about 10% of the aquatic forms
are marine. Some Chlorophytes are terrestrial as well. Among the
aquatic forms, most of the Ulotrichales have freshwater forms whereas
members of Caulerpales are predominantly marine. Oedogoniales have
exclusively freshwater forms. Spirogyra and Hydrodictyon frequently
form noticeable but harmless blooms that may cover the surface of
freshwater ponds. A few members of Chlorophyta commonly occur in
nearshore marine environments forming nuisance macroscopic
growths e.g. Ulva (sea lettuce), Codium (dead man‟s fingers),
Enteromorpha, Cladophora. Species of Ulothrix, and Zygogonium are
sub-aerial, and are usually found on damp soil in the form of sheets.
Trentepohlia forms orange-red growths on moist rocks or cliff faces
due to the accumulation of carotenoid pigments which obscure
chlorophyll. Species of Protococcus and Trentepohlia are epiphytic on
sea weeds or on the bark of trees, whereas Chlorella (Figure g) is
endophytic i.e. they live inside a host organism. Cephaleuros and
Rhodochytrium are parasitic, and they lack green colour. Cephaleuros
is a parasitic form which causes red-rust disease of tea leaves. Some
species occur in unusual habitats; for example, Chlamydomonas
yellowstonensis, Chlamydomonas nivalis and Haematococcus nivalis
occur on the snow covered mountain tops and can impart a
characteristic colour (which varies from red to orange to yellow) to the
snow due to the accumulation of carotenoid pigments. Chlamydomonas
nivalis is cryophilic (cold loving) and flourishes in freezing waters. It
21
contains a red carotenoid pigment known as astaxanthin in addition to
chlorophyll and imparts a red colour to the snow referred to as
watermelon snow. Some green algae in association with certain fungi
form lichens.
ctio

22