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CHAPTER 15
Functional
Diversity of
Microorganisms

© 2018 Pearson Education, Inc.

 I. Functional Diversity as a Concept

• 15.1 Making Sense of Microbial Diversity

© 2018 Pearson Education, Inc.

 15.1 Making Sense of Microbial Diversity

• Phylogenetic diversity: Evolutionary relationships

between organisms
• diversity of phyla, genera, species

• genetic and genomic diversity of evolutionary lineages

• can be defined on the basis of either genes or

organisms

• most commonly defined by ribosomal RNA phylogeny

© 2018 Pearson Education, Inc.

 15.1 Making Sense of Microbial Diversity

• Functional diversity: Form and function as related

to microbial physiology and ecology
• Organisms with common traits/genes often share
physiological characteristics and similar ecological roles.

• relationships in form and function between organisms

(Figure 15.1)

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© 2018 Pearson Education, Inc. Figure 15.1
 15.1 Making Sense of Microbial Diversity

• Reasons functional traits are seen in different

species
• Gene loss: A trait is present in a common ancestor
is lost.
• Convergent evolution: A trait has evolved independently
in two or more lineages and is not encoded by
homologous genes.
• Horizontal gene transfer: Genes that code for a trait are
homologous and have been exchanged between
distantly related lineages.

© 2018 Pearson Education, Inc.

 15.1 Making Sense of Microbial Diversity

• Functional diversity can be further defined in

terms of
• physiological diversity: functions and activities in terms of
metabolism and biochemistry
• ecological diversity: relationships between organisms
and their environment
• morphological diversity: relationships associated with
outward appearance; shape and structures often have
ecological significance

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 II. Diversity of Phototrophic Bacteria

• 15.2 Overview of Phototrophic Bacteria

• 15.3 Cyanobacteria
• 15.4 Purple Sulfur Bacteria
• 15.5 Purple Nonsulfur Bacteria and Aerobic
Anoxygenic Phototrophs
• 15.6 Green Sulfur Bacteria
• 15.7 Green Nonsulfur Bacteria
• 15.8 Other Phototrophic Bacteria

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 15.2 Overview of Phototrophic Bacteria

• Ability to conserve energy from light evolved when

Earth was anoxic.
• Originated within Bacteria
• First phototrophs were anoxygenic phototrophs that
do not generate O2 as a product.
• instead of water, used H2, Fe+2, or H2S as electron donor
• Most phototrophs are also autotrophs.

• Extensive diversity among anoxygenic phototrophs

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 15.2 Overview of Phototrophic Bacteria

• Several common features

• use chlorophyll-like and accessory pigments to harvest
energy from light and transfer to membrane-bound
reaction center to drive electron transfer that
produces ATP
• two types of reaction centers: type I (FeS) and type II
(quinone- or Q-type)
• both found in Cyanobacteria
• only one or other found in anoxygenic phototrophs
• pigments often found in intracellular membrane systems
that allow phototrophic bacteria to better use light of low
intensities
• Many but not all fix carbon.
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 15.3 Cyanobacteria
• Key genera: Prochlorococcus, Crocosphaera, Synechococcus,
Trichodesmium, Oscillatoria, Anabaena
• Large, morphologically, ecologically heterogeneous group of
oxygenic, phototrophic Bacteria
• First oxygen-evolving phototrophs
• Phylogeny and Classification of Cyanobacteria
• oxygenic phototrophs
• impressive morphological diversity (Figure 15.2)
• unicellular or filamentous
• range from 0.5 to 100 μm in diameter
• Five morphological groups
• chroococcales: unicellular, divide by binary fission
• Includes prochlorophytes (unique, unicellular)
• pleurocapsales: unicellular, dividing by multiple fission (colonial)
• oscillatoriales: ilamentous nonheterocystous
• nostocales: filamentous, divide on single axis, can differentiate
• stigonematales: filamentous, divide in multiple planes, forming branching
filaments
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 15.3 Cyanobacteria

• Phylogeny and classification of cyanobacteria

• Some morphological classifications correspond to
phylogenetic groups; others do not. (Figure 15.3)
• Pleurocapsales, Nostocales and Stigonematales form
coherent groups.

• Chroococcales and Oscillatoriales are dispersed.

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© 2018 Pearson Education, Inc. Figure 15.3
 15.3 Cyanobacteria

• Physiology and photosynthetic membranes

• oxygenic phototrophs with both FeS and Q-type
photosystems
• All fix CO2 by the Calvin cycle.
• Many fix N2.
• Most synthesize own vitamins.
• harvest energy from light and fix CO2 during day
• generate energy by fermentation or aerobic respiration
of carbon storage products (e.g., glycogen) during night
• Some can assimilate simple organic compounds in light
(photoheterotrophy).
• Some can switch to anoxygenic photosynthesis using
H2S as electron donor.

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 15.3 Cyanobacteria

• Physiology and photosynthetic membranes

• specialized membrane systems called thylakoids that
increase ability to harvest light energy (Figure 15.4)

• Cell walls contain peptidoglycan.

• Photosynthesis occurs in thylakoid membrane.

• produce pigments (chlorophyll a and phycobilins:

accessory pigments) (Figure 15.5)

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© 2018 Pearson Education, Inc. Figure 15.4
© 2018 Pearson Education, Inc. Figure 15.5
 15.3 Cyanobacteria

• Motility and cellular structures

• Many cyanobacteria display gliding motility.
• Most show phototaxis (toward light); chemotaxis may occur.
• gas vesicles important in positioning cells in water column
where light intensity is optimal by regulating buoyancy
• Many produce sheaths (mucilaginous envelopes) that
bind groups of cells or filaments together.
• Some form hormogonia (short, motile filaments that break
off to facilitate dispersal under stress; Figure 15.6).
• Some form akinetes (resting structures with thickened
outer walls that protect the organism from darkness,
desiccation, or cold).
• Many form cyanophycin (nitrogen storage product).

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© 2018 Pearson Education, Inc. Figure 15.6
 15.3 Cyanobacteria

• Heterocysts and nitrogen fixation

• Nitrogenase is sensitive to oxygen, so fixation cannot
occur with oxygenic photosynthesis.
• Many fix nitrogen only at night.
• Some transiently suppress photosynthetic activity within
filaments.
• Many filamentous Cyanobacteria form heterocysts on the
ends of filaments or along the filament. (Figure 15.8)
• surrounded by thickened cell wall that slows diffusion of O2
and provides an anoxic environment
• lack photosystem II, cannot fix CO2
• exchange materials with adjacent cells
• Fixed carbon is imported and oxidized to yield electrons for
nitrogen fixation.
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© 2018 Pearson Education, Inc. Figure 15.8
 15.3 Cyanobacteria

• Ecology of cyanobacteria
• important for productivity of oceans
• Synechococcus and Prochlorococcus are most abundant
ocean phototrophs, contributing 80 percent of marine
photosynthesis and 35 percent of all Earth’s photosynthesis.
• Cyanobacterial nitrogen fixation is dominant input of new
nitrogen in oceans.
• Widely distributed in terrestrial and freshwater
environments
• more tolerant of extremes than eukaryotic algae (e.g., hot
springs, saline lakes, desert soils)
• can be phototrophic component of lichens (symbiosis
between phototroph and fungus)
• important metabolic products (e.g., potent neurotoxins
and toxic blooms, geosmin in water)
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 15.4 Purple Sulfur Bacteria

• Key genera: Chromatium, Ectothiorhodospira

• Anoxygenic phototrophs that use hydrogen sulfide
(H2S) as an electron donor for photosynthesis
• Found in illuminated anoxic zones where H2S
present (lakes, marine sediments, sulfur springs;
Figure 15.9)
• Also found in microbial mats and salt marsh
sediments
• Purple color comes from carotenoids (accessory
pigments for light harvesting).
• Use Q-type photosystem, contain
bacteriochlorophyll, and fix carbon with Calvin cycle
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© 2018 Pearson Education, Inc. Figure 15.9
 15.4 Purple Sulfur Bacteria
• During autotrophic growth, H2S oxidized to elemental sulfur
(S0) deposited as granules. (Figure 15.10)
• Sulfur can be oxidized to sulfate (SO42−) as an electron donor
for photosynthesis.
• Many can use other reduced sulfur compounds, such as
thiosulfate (S2O32−).
• Form two families: Chromatiaceae and
Ectothiorhodospiraceae
• Chromatiaceae store S0 granules inside (in periplasm) and
have vesicular intracellular photosynthetic membrane systems.
(Figure 15.11b)
• stratified lakes and salt marsh anoxic sediment
• Ectothiothodospiraceae deposit S0 outside cells and have
lamellar intracellular photosynthetic membrane systems.
• often halophilic or alkaliphilic
• saline lakes, soda lakes, salterns
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© 2018 Pearson Education, Inc. Figure 15.10
© 2018 Pearson Education, Inc. Figure 15.11
 15.5 Purple Nonsulfur Bacteria and Aerobic
Anoxygenic Phototrophs
• Purple nonsulfur Bacteria
• key genera: Rhodospirillum, Rhodoferax, Rhodobacter
• most metabolically versatile microbes
• synthesize an array of carotenoids giving them colors (purple,
red, orange) (Figure 15.12)
• typically photoheterotrophs
• use a wide range of carbon sources and electron donors for
photosynthesis (e.g., organic acids, amino acids, alcohols,
sugars, aromatic compounds)
• use Q-type photosystem and bacteriochlorophyll
• morphologically and phylogenetically diverse
Alphaproteobacteria or Betaproteobateria (Figure 15.13)

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© 2018 Pearson Education, Inc. Figure 15.12
© 2018 Pearson Education, Inc. Figure 15.13
 15.5 Purple Nonsulfur Bacteria and Aerobic
Anoxygenic Phototrophs
• Purple nonsulfur Bacteria
• conserve energy through a variety of processes
• Some grow photoautotrophically with H2, H2S, or Fe+2 as
electron donor for photosynthesis and Calvin cycle for CO2
fixation.
• Most can grow aerobically in the dark with aerobic
respiration of organic or some inorganic compounds; O2
represses photosynthesis.
• Some can also grow using fermentation or anaerobic
respiration.
• Enrichment and isolation is easy using mineral salts with
an organic acid as carbon source and anaerobic incubation
in light.

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 15.5 Purple Nonsulfur Bacteria and Aerobic
Anoxygenic Phototrophs
• Aerobic anoxygenic phototrophs
• key genera: Roseobacter, Erythrobacter
• obligate aerobic heterotrophs that use light as a
supplemental energy source
• diverse Alphaproteobacteria and Betaproteobacteria
• strict heterotrophs that employ anoxygenic
photosynthesis under oxic conditions
• contain bacteriochlorophyll and Q-type photosystem
• unable to fix CO2, need organic carbon
• carotenoids provide color (yellow, orange, pink)

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 15.5 Purple Nonsulfur Bacteria and Aerobic
Anoxygenic Phototrophs
• Aerobic anoxygenic phototrophs
• only photosynthesize when grown on day/night cycle

• bacteriochlorophyll synthesized in dark and used for

photophosphrylation when light returns

• perform five percent of photosynthesis in coastal

marine waters

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 15.6 Green Sulfur Bacteria

• Key genera: Chlorobium, Chlorobaculum,

“Chlorochromatium”
• Phylogenetically coherent anoxygenic phototrophs
in the phylum Chlorobi
• Little metabolic versatility; typically nonmotile,
strictly anaerobic anoxygenic phototrophs
• Morphologically restricted, short to long rods
(Figure 15.14)
• Oxidize H2S to SO42− for autotrophy; S0 deposited
only outside cell
• Autotrophy uses a reversal of citric acid cycle.
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 15.6 Green Sulfur Bacteria

• Pigments and ecology

• contain bacteriochlorophyll in chlorosomes (Figure 15.15)
• oblong bodies bounded by a thin membrane and attached
to cytoplasmic membrane in cell periphery

• funnel energy into photosystem, leading to ATP synthesis

• use FeS-type photosystem

• green- and brown-colored species (Figure 15.16)

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© 2018 Pearson Education, Inc. Figure 15.15
© 2018 Pearson Education, Inc. Figure 15.16
 15.6 Green Sulfur Bacteria

• Pigments and ecology

• live in anoxic, sulfidic, illuminated aquatic environments
• Chlorosomes allow these bacteria to grow at much lower
light intensities.
• have higher tolerance of H2S than other oxygenic
phototrophs
• found at greatest depths of all phototrophs (e.g.,
hydrothermal vent)
• Chlorobium tepidum is model organism.

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 15.6 Green Sulfur Bacteria

• Green sulfur bacteria consortia

• consortium: intimate two-membered association
• involves the green sulfur bacterium and a
chemoorganotrophic bacterium
• Phototrophic member is called epibiont.
• Epibiont is physically attached to nonphototrophic cell.
(Figure 15.17)
• examples: Chlorochromatium aggregatum (mixed
culture) and Pelochromatium roseum

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© 2018 Pearson Education, Inc. Figure 15.17
 15.7 Green Nonsulfur Bacteria

• Key genera: Chloroflexus, Heliothrix, Roseiflexus

• Filamentous anoxygenic phototrophs
• Phylum Chloroflexi
• Class: Chloroflexi are green nonsulfur bacteria.
• other metabolically diverse organisms
• aerobic and anaerobic chemoorganotrophs
• Dehalococcoidetes: dehalogenating bacteria that use
halogenate organic compounds as electron acceptors in
anaerobic respiration
• most species yet to be cultivated and characterized

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 15.7 Green Nonsulfur Bacteria

• All capable of gliding motility

• Chloroflexus forms thick microbial mats in neutral to
alkaline hot springs with thermophilic cyanobacteria.
(Figure 15.18)
• Grows best photoheterotrophically
• Can grow photoautotrophically using H2 or H2S as
electron donors
• 3-hydroxypropionate bi-cycle supports autotrophy
• Most grow well in dark by aerobic respiration.

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© 2018 Pearson Education, Inc. Figure 15.18
 15.7 Green Nonsulfur Bacteria

• Photosynthetic features are a hybrid of green sulfur

and purple phototrophic bacteria.
• reaction centers and chlorosomes containing
bacteriochlorophyll

• contain Q-type photosynthetic reaction center

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 15.7 Green Nonsulfur Bacteria

• Other Chloroflexi
• Heliothrix and Roseiflexus
• filamentous and thermophilic but lack chlorosomes

• Oscillochloris and Chloronema

• large cells (two to five μm wide and several hundred μm
long)

• inhabit freshwater lakes with H2S

• chemotrophic genus of Chloroflexi

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 15.7 Green Nonsulfur Bacteria

• Other Chloroflexi
• Thermomicrobium
• chemotrophic genus of Chloroflexi

• strictly aerobic gram-negative rod

• grows optimally in complex media at 75°C

• Unusual membrane lipids form on 1,2-dialcohols instead

of glycerol and do not have ester or ether linkages.
(Figure 15.19)

• cell wall primarily composed of protein; only small

amounts of peptidoglycan

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© 2018 Pearson Education, Inc. Figure 15.19
 15.8 Other Phototrophic Bacteria
• Key genera: Heliobacterium, Chloracidobacterium
• Heliobacteria
• phototrophic gram-positive bacteria within phylum
Firmicutes
• anoxygenic phototrophs with FeS-type photosystem
• produce unique pigment bacteriochlorophyll g
• grow photoheterotrophically using pyruvate, lactate,
acetate, butyrate
• all rod-shaped or filamentous cells (Figure 15.20)
• Heliophilum forms bundles of cells that move as a unit.
• strict anaerobes that can ferment pyruvate
• produce endospores
• reside in soils or in highly alkaline environments (e.g.,
soda lakes and alkaline soils)
• have nitrogen-fixing capabilities
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© 2018 Pearson Education, Inc. Figure 15.20
 15.8 Other Phototrophic Bacteria

• Phototrophic Acidobacteria
• Chloracidobacterium thermophilum (Figure 14.21)
• found in thermal springs of Yellowstone National Park
• thermophilic oxygen-tolerant anoxygenic phototroph
• similar to green sulfur bacteria, producing chlorosomes and
using FeS-type photosystem
• can grow aerobically
• photoheterotroph that uses short-chain fatty acids as
carbon sources
• not autotrophic

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© 2018 Pearson Education, Inc. Figure 15.21
 15.8 Other Phototrophic Bacteria
• Phototrophic Gemmatimonadetes
• Gemmatimmonas phototrophica
• found in freshwater desert lake
• aerobic facultative photoheterotroph
• most energy from aerobic respiration of organic
compounds
• in light, uses photophosphorylation to supplement aerobic
respiration
• cannot grow as an obligate phototroph, cannot fix CO2, and
cannot grow anaerobically
• has photosynthetic gene cluster like aerobic anoxygenic
phototrophs
• like purple bacteria, produces bacteriochlorophyll and
Q-type reaction center
• likely gained photophosphorylation through horizontal gene
transfer
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 III. Microbial Diversity in the Sulfur Cycle

• 15.9 Dissimilative Sulfate-Reducers

• 15.10 Dissimilative Sulfur-Reducers

• 15.11 Dissimilative Sulfur-Oxidizers

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 15.9 Dissimilative Sulfate-Reducers

• Key genera: Desulfovibrio, Desulfobacter

• Couple oxidation of H2 or organics to reduction of
SO42– (anaerobic respiration)
• Over 30 genera of sulfate-reducers across five
phyla of Bacteria and Archaea (Figure 15.22)
• Most are Deltaproteobacteria.
• Some are Firmicutes, Thermodesulfobacteria, Nitrospira.
• Archaeoglobus in Archaea (Euryarchaeota)

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© 2018 Pearson Education, Inc. Figure 15.22
 15.9 Dissimilative Sulfate-Reducers

• Physiology of sulfate-reducing bacteria

• diverse
• generally obligate anaerobes (Figure 15.23g)
• use H2 or organic compounds (lactate and pyruvate,
some short-chain alcohols, few hydrocarbons) as
electron donors
• Some (Desulfosarcina and Desulfonema) grow
chemolithotrophically and autotrophically with H2 as
electron donor, SO42– as electron acceptor, and CO2 as
carbon source.

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© 2018 Pearson Education, Inc. Figure 15.23
 15.9 Dissimilative Sulfate-Reducers

• Physiology of sulfate-reducing bacteria

• Complete oxidizers can oxidize acetate and other fatty acids
completely to CO2.
• include Desulfobacter, Desulfococcus, Desulfosarcina, and
Desulfonema
• incomplete oxidizers unable to oxidize acetate to CO2
• Desulfovibrio, Desulfomonas, Desulfotomaculum, and
Desulfobulbus
• alternative metabolic pathways
• Some can reduce nitrate and sulfonates (e.g., isethionate).
• Some organics (e.g., pyruvate) can be fermented (by
phosphoroclastic reaction).
• A few are O2-tolerant and one can grow with O2 as electron
acceptor microaerophilically.
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 15.9 Dissimilative Sulfate-Reducers

• Ecology of sulfate-reducing bacteria

• widespread in aquatic and terrestrial environments that
contain SO42– and become anoxic from microbial
decomposition

• Desulfotomaculum in certain foods leads to “sulfide

stinker” spoilage.

• some thermophiles found in geothermally heated

environments

• can occasionally be isolated from mammalian gut

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 15.10 Dissimilative Sulfur-Reducers

• Key genera: Desulfuromonas, Wolinella, Sulfolobus

• Use respiratory reduction of S0 to H2S
• Over 25 genera of sulfur-reducers across five phyla
of Bacteria and Archaea
• Most are Deltaproteobacteria.
• Some are Epsilonproteobacteria, Gammaproteobacteria,
Firmicutes.
• several Archaea in Crenarchaeota

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 15.10 Dissimilative Sulfur-Reducers

• Physiology and ecology of sulfur-reducing bacteria

• Most are obligate anaerobes; some are facultative
aerobes.
• often able to reduce nitrate, ferrous iron, or thiosulfate
• characterized by whether they completely oxidize acetate
and fatty acids to CO2 (Desulfuromonas) or not
(Sulfospirillum and Wolinella)
• Some cannot oxidize to acetate.
• reside in many of the same habitats as dissimilative
sulfate-reducers and often form associations with
bacteria that oxidize H2S to S0 (anoxic sulfur cycle)

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 15.11 Dissimilative Sulfur-Oxidizers

• Key genera: Thiobacillus, Achromatium, Beggiatoa

• Chemolithotrophs that oxidoze H2S, S0, thiosulfate,
thiocyanate (–SCN)
• Common in marine sediments, sulfur springs,
hydrothermal systems where H2S is produced by
other bacteria or geothermal reactions
(Figure 15.24).
• Sulfur-oxidizers are found in three phyla of Bacteria
and one phylum of Archaea.
• Most are Betaproteobacteria, Gammaproteobacteria, and
Epsilonproteobacteria.
• several Crenarchaeota
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© 2018 Pearson Education, Inc. Figure 15.24
 15.11 Dissimilative Sulfur-Oxidizers

• Physiological diversity of sulfur-oxidizing bacteria

• 1–750 μm diameter
• Most are obligate aerobes; some can reduce NO3–.
• Many oxidize H2S to S0 and deposit as granules for later
use as an electron donor.
• Some are obligate chemolithotrophs, performing Calvin
cycle for autotrophy and containing carboxysomes.
• Others are facultative chemolithotrophs (e.g., Beggiatoa).

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 15.11 Dissimilative Sulfur-Oxidizers

• Thiobacillus and Achromatium

• Thiobacillus
• gram-negative, rod-shaped Betaproteobacteria
• generates sulfuric acid, so thiobacilli often acidophilic
• Acidobacillus ferrooxidans can also oxidize Fe2+ from iron
pyrite (FeS2).
• Achromatium
• spherical cells common in freshwater sediments at
neutral pH
• related to purple sulfur bacteria (e.g., Chromatium)
• store S0 internally and oxidize to SO42–
• also store granules of calcite (CaCO3)

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 15.11 Dissimilative Sulfur-Oxidizers

• Ecological diversity and strategies of sulfide-

oxidizing bacteria
• demonstrate ecological diversity among microbes
sharing the same basic metabolism: H2S oxidation
spontaneous and rapid
• Thiothrix (Figure 15.26)
• filamentous chemolithotroph that forms filaments grouped
together at their ends by a holdfast to form cell
arrangements called rosettes
• uses holdfast to position itself in high-flow environments
providing H2S (e.g., sulfur springs, creeks draining salt
marshes)
• obligate aerobic mixotroph
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© 2018 Pearson Education, Inc. Figure 15.26
 15.11 Dissimilative Sulfur-Oxidizers

• Ecological diversity and strategies of sulfide-

oxidizing bacteria
• Beggiatoa (Figure 15.27a)
• filamentous, gliding bacteria with many short cells attached
end-to-end
• found in microbial mats, sediments, sulfur springs, hot
springs
• Gliding motility positions Beggiatoa where H2S and
O2 meet.
• example: can move vertically several cm/day up to obtain
O2 at night and down during day

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© 2018 Pearson Education, Inc. Figure 15.27
 15.11 Dissimilative Sulfur-Oxidizers

• Ecological diversity and strategies of sulfide-

oxidizing bacteria
• Thiomargarita (Figure 15.28)
• large, nonmotile bacterium that can be as much as
0.75 mm wide
• separates timing of H2S oxidation from reduction of O2
• contains giant vacuole filled with nitrate
• lives in sulfide-rich marine sediments
• cells oxidize H2S to S0 anaerobically by reducing NO3– in
vacuole to ammonium (NH4+).
• stores S0 in intracellular granules and aerobically oxidizes it
when H2S is lacking
• uses energy to refill vacuole with NO3- from water column
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© 2018 Pearson Education, Inc. Figure 15.28
 15.11 Dissimilative Sulfur-Oxidizers

• Ecological diversity and strategies of sulfide-

oxidizing bacteria
• Thioploca
• large, filamentous bacteria that function similar to
Thiomargarita (intracellular S0 granules and large NO3–
vacuoles)
• gliding motility and occur in large sheaths of parallel
filaments
• filaments glide up (to aerobically respire S0) and down (to
anaerobically respire H2S).

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 15.11 Dissimilative Sulfur-Oxidizers

• Ecological diversity and strategies of sulfide-

oxidizing bacteria
• Thiovulum
• found in freshwater and marine habitats (Figure 15.29)
• fairly large (10–20 μm) cocci
• high-speed swimmers
• controls flow of nutrients to cells by rotation around flagellar
axis within a slimy veil

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© 2018 Pearson Education, Inc. Figure 15.29
 15.11 Dissimilative Sulfur-Oxidizers

• Ecological diversity and strategies of sulfide-

oxidizing bacteria
• symbiotic associations with eukaryotes
• Host regulates H2S and O2; sulfide-oxidizer fixes CO2 and
provides carbon and energy to host.
• Example: Riftia (tube worm) contains sulfide-oxidizing
endosymbionts and lives in hydrothermal vents,
Calyptogena magnifica (giant clam) gills, surface of yeti
crab, Solemyidae bivalves.

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 IV. Microbial Diversity in the Nitrogen Cycle

• 15.12 Diversity of Nitrogen-Fixers

• 15.13 Diversity of Nitrifiers and Denitrifiers

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 15.12 Diversity of Nitrogen-Fixers

• Key genera: Mesorhizobium, Desulfovibrio, Azotobacter

• Diazotrophs are microorganisms that fix N2 gas into
ammonia.
• Nitrogen fixation requires ATP and nitrogenase.
• Different solutions for protecting nitrogenase from O2
• Nitrogen fixation is widespread among Bacteria and a
few Archaea.
• Distribution of nitrogenase has been influenced strongly
by horizontal gene exchange. (Figure 15.30)

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© 2018 Pearson Education, Inc. Figure 15.30
 15.12 Diversity of Nitrogen-Fixers

• Symbiotic diazotrophs
• several relationships with plants, animals, and fungi
• need source of carbon and energy and regulation of
oxygen concentrations
• microbial symbiont provides fixed nitrogen
• symbiosis between rhizobia and leguminous plants
• Alphaproteobacteria, Betaproteobacteria, Actinobacteria
• other symbioses
• shipworms and Teridinibacter
• termite guts and Treponema
• fungi, algae, and plants with Cyanobacteria

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 15.12 Diversity of Nitrogen-Fixers

• Free-living diazotrophs
• need to protect nitrogenase
• obligate anaerobes in marine and freshwater sediments
and microbial mats
• Some fix N2 only when oxygen absent or low
concentration.
• example: facultative aerobes only when anaerobic
(Klebsiella)
• example: microaerophiles (typically less than two percent
oxygen)
• some aerobes
• Cyanobacteria and chemoorganotrophic bacteria
• Azotobacter produce extensive capsules or slime layers to
help protect nitrogenase. (Figure 15.31)
• believed to protect nitrogenase from O2
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© 2018 Pearson Education, Inc. Figure 15.31
 15.12 Diversity of Nitrogen-Fixers

• Azotobacter and alternative nitrogenases

• Azotobacter chroococcum was first nitrogen-fixer shown
to grow on N2 without molybdenum.
• forms alternative nitrogenases containing either vanadium
(V) or Fe in place of Mo

• backup nitrogenases widely distributed (especially

Cyanobacteria and Archaea)

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 15.13 Diversity of Nitrifiers and Denitrifiers

• Denitrifiers grow by anaerobic respiration of

inorganic nitrogen (NO3–, NO2–) to gaseous NO,
N2O, and N2.
• typically facultative aerobes and chemoorganotrophs
• Nitrifiers grow chemolithotrophically at the expense
of reduced inorganic nitrogen compounds (NH3,
NO2–). (Figure 15.33)
• typically obligate aerobes that can also grow
autotrophically, fixing CO2 by Calvin cycle
• Few also grow mixotrophically by assimilating organic
carbon + CO2.

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© 2018 Pearson Education, Inc. Figure 15.33
 15.13 Diversity of Nitrifiers and Denitrifiers

• Physiology of nitrifying Bacteria and Archaea

• nitrification often results from sequential activities of
• Ammonia oxidizers
• oxidize NH3 to NO2–; typically have genus names beginning
with Nitroso-
• Nitrite oxidizers
• oxidize NO2– to NO3–; typically have genus names beginning
with Nitro-
• Some Nitrospira carry out both, oxidizing ammonia to
nitrate.
• Many have internal membrane stacks containing
nitrification enzymes (ammonia monooxygenase and
nitrite oxidoreductase).
• grow on mineral salts containing NH3 or NO2– and
bicarbonate (HCO3–)
© 2018 Pearson Education, Inc.
 15.13 Diversity of Nitrifiers and Denitrifiers

• Nitrifying Bacteria and Archaea: Ammonia oxidizers

• key genera: Nitrosomonas, Nitrosospira, Nitrosopumilus
• found in Betaproteobacteria and Gammaproteobacteria,
Nitrospirae, and archaeal Thaumarchaeota
• widespread in soil and water
• Bacteria found near high concentrations of NH3 (e.g.,
sites with extensive protein decomposition, sewage
treatment facilities, lakes and streams that receive
wastewater)
• Archaea found near low concentrations of NH3 (e.g.,
oceans, soils)

© 2018 Pearson Education, Inc.

 15.13 Diversity of Nitrifiers and Denitrifiers

• Nitrifying bacteria: Nitrite oxidizers

• key genera: Nitrospira, Nitrobacter
• found in Alphaproteobacteria, Betaproteobacteria,
Gammaproteobacteria, Deltaproteobacteria, and
Nitrospirae
• Nitrospira oxidizes NO2– to NO3– and grows
autotrophically. (Figure 15.34)
• lacks extensive internal membranes
• may have acquired NO2– oxidation from nitrifying
Proteobacteria (e.g., Nitrobacter) by horizontal gene flow
(or vice versa)
• much more abundant than Nitrobacter

© 2018 Pearson Education, Inc.

© 2018 Pearson Education, Inc. Figure 15.34
 15.13 Diversity of Nitrifiers and Denitrifiers

• Denitrifying Bacteria and Archaea

• key genera: Paracoccus, Pseudomonas
• growth by anaerobic respiration of nitrate or nitrite to NO,
N2O, and N2
• nearly all chemoorganotrophs that use organic carbon for
carbon and electron donor
• exceptions: denitrifying sulfur-oxidizers (Section 15.11)
• typically facultative aerobes
• important in agricultural soils, causing loss of nitrogen
fertilizers and production N2O (greenhouse gas)

© 2018 Pearson Education, Inc.

 15.13 Diversity of Nitrifiers and Denitrifiers

• Denitrifying Bacteria and Archaea

• phylogenetically and metabolically diverse
• found in two archaeal phyla and six bacterial phyla

• Genes for denitrification are widespread via horizontal

gene exchange.

• Many nitrate reducers only have part of the pathway and

cannot completely reduce NO3– to N2.

© 2018 Pearson Education, Inc.

 V. Other Distinctive Functional Groupings of
Microorganisms
• 15.14 Dissimilative Iron-Reducers

• 15.15 Dissimilative Iron-Oxidizers

• 15.16 Methanotrophs and Methylotrophs

• 15.17 Microbial Predators

• 15.18 Microbial Bioluminescence

© 2018 Pearson Education, Inc.

 15.14 Dissimilative Iron-Reducers

• Key genera: Geobacter, Shewanella

• Couple reduction of oxidized metals or metalloids to
cellular growth
• Have to overcome obstacle of using an insoluble
solid material as a respiratory electron acceptor
• Carry out metal respiration by coupling oxidation of
H2 or organics to reduction of ferric iron (Fe3+)
(Figure 15.35a) or manganese (Mn6+)
• Phylogenetically diverse
• found in Proteobacteria, Acidobacteria, Deferribacteres,
Deinococcus-Thermus, Thermotogae, Firmicutes, and
archaeal Crenarchaeota
© 2018 Pearson Education, Inc.
© 2018 Pearson Education, Inc. Figure 15.35
 15.14 Dissimilative Iron-Reducers

• Physiology
• typically very versatile at anaerobic respiration
• possess outer membrane cytochromes that facilitate
electron transfer with insoluble minerals
• Most species use iron oxide or manganese oxides as
electron acceptors.
• Various species use nitrate, fumarate, oxidized inorganic
sulfur, cobalt, chromium, uranium, tellurium, selenium,
arsenic, humic compounds.
• mostly obligate anaerobes; some facultative aerobes
• Electron donors are organic compounds (e.g., fatty acids,
alcohols, sugars, some aromatic compounds) or H2 (not
autotrophically).
© 2018 Pearson Education, Inc.
 15.14 Dissimilative Iron-Reducers

• Physiology
• Geobacter, Desulfuromonas, Desulfuromusa all oxidize
acetate and other small organics to CO2.
• specialize in anaerobic respiration
• Geobacter produce pili containing cytochromes that
facilitate electron transfer to surface of iron oxide minerals.
• Pelobacter are primarily fermentative.
• Pelobacter carbinolicus only uses lactate as electron donor
and ferric iron or S0 as electron acceptor.
• unable to oxidize substrates completely to CO2

© 2018 Pearson Education, Inc.

 15.14 Dissimilative Iron-Reducers

• Physiology
• Shewanella, Ferrimonas, Aeromonas are facultative
aerobes.
• Shewanella use a variety of electron donors and acceptors
but cannot completely oxidize carbon substrates to CO2
and cannot oxidize acetate for anaerobic respiration.
• Ecology
• common in anoxic freshwater and marine sediments
• also common in shallow aquifers and deep subsurface
• several thermophilic and hyperthermophilic iron-reducing
species (e.g., Thermus, Thermotoga) in hot springs and
other geothermally heated systems

© 2018 Pearson Education, Inc.

 15.15 Dissimilative Iron-Oxidizers

• Key genera: Acidithiobacillus, Gallionella

• Coupling of Fe2+ to cell growth found in five
bacterial phyla and two archaeal phyla
• Aerobic iron-oxidizers diversity and distribution
influenced by pH and O2.
• Fe2+ oxidizes to insoluble precipitates at alkaline pH (>7).
• stable under anoxic conditions or aerobically at acidic
pH (<4)
• Divided into four functional groups
• acidophilic aerobic iron oxidizers
• neutrophilic aerobic iron oxidizers
• anaerobic chemotrophic iron oxidizers
• anaerobic phototrophic iron oxidizers
© 2018 Pearson Education, Inc.
 15.15 Dissimilative Iron-Oxidizers

• Acidophilic aerobic iron-oxidizing Bacteria

• favored in iron-rich acidic environments with soluble Fe2+
• found in acid mine drainage and acidic springs in
volcanic areas
• Many oxidize elemental sulfur.
• can be autotrophic or heterotrophic
• genera: Acidithiobacillus, Leptospirillum, Ferroplasma

© 2018 Pearson Education, Inc.

 15.15 Dissimilative Iron-Oxidizers

• Neutrophilic aerobic iron-oxidizing Bacteria

• specialized niche because of insolubility of Fe2+,
spontaneous oxidation in air, formation of iron oxide crust
• found where iron-rich anoxic waters are exposed to air,
near wetlands or soils with anoxic springs; also
rhizosphere of wetland plants and some submarine
hydrothermal systems
• genera in Betaproteobacteria (freshwater) and
Zetaproteobacteria (marine)
• narrow metabolism (microaerophilic obligate
chemolithotrophy, some mixotrophy)

© 2018 Pearson Education, Inc.

 15.15 Dissimilative Iron-Oxidizers

• Neutrophilic aerobic iron-oxidizing Bacteria

• Leptothrix and Sphaerotilus are exceptions.
• common in freshwater environments
• catalyze oxidation of iron and manganese but do not
conserve energy from these reactions; conserve energy
from oxidation of organics
• Gallionella (freshwater) and Mariprofundus (marine)
• form twisted, stalklike structures containing Fe(OH)3
(Figure 15.36) that prevents cells from entombment in
iron oxide
• Gallionella common in waters draining bogs, iron springs,
and other environments rich in Fe2+
• Mariprofundus first isolated from a submarine volcano
near Hawaii
• autotrophic chemolithotrophs with Calvin cycle enzymes
© 2018 Pearson Education, Inc.
© 2018 Pearson Education, Inc. Figure 15.36
 15.15 Dissimilative Iron-Oxidizers

• Anaerobic iron-oxidizing Bacteria

• mediated by both chemotrophic and phototrophic bacteria
• common in anoxic sediments and wetlands
• anoxic conditions promote solubility of Fe2+ across pH, so
growth is not limited to neutral
• metabolically diverse and can grow using a variety of
different electron donors and acceptors

© 2018 Pearson Education, Inc.

 15.15 Dissimilative Iron-Oxidizers

• Anaerobic iron-oxidizing Bacteria

• phototrophy in some purple nonsulfur bacteria (e.g.,
Rhodopseudomonas palustris), some purple sulfur
bacteria, some green sulfur bacteria (e.g., Chlorobium
ferrooxidans)
• Fe2+ and other compounds donate electrons in
photosynthesis
• chemotrophs couple oxidation of Fe2+ to NO3– reduction
(denitrification)
• Most use various organic electron donors.
• Many grow aerobically.
• examples: Acidovorax, Aquabacterium, Marinobacter are
mixotrophs or autotrophs

© 2018 Pearson Education, Inc.

 15.16 Methanotrophs and Methylotrophs

• Methylotrophs grow using organic compounds

lacking C–C bonds as electron donors and carbon
sources.
• Methanotrophs are a subset of methylotrophs that
use methane for growth.
• Aerobic methylotrophs are common in soil and
aquatic environments with O2.
• Anaerobic methylotrophs are common in anoxic
environments, particularly marine sediments.
• Many are methanogenic Archaea.

© 2018 Pearson Education, Inc.

 15.16 Methanotrophs and Methylotrophs
• Aerobic facultative methylotrophs
• key genera: Hyphomicrobium, Methylobacterium
• among Alphaproteobacteria, Betaproteobacteria,
Gammaproteobacteria, Actinobacteria, and Firmicutes
• Facultative methylotrophs are metabolically diverse.
• cannot use methane
• Most can grow aerobically using methylated substrates and
other organics.
• Most can grow aerobically with methanols.
• Some also metabolize methylated amines and sulfur
compounds and halomethanes.
• Most are obligate aerobes; some denitrifiers.
• example: Hyphomicrobium
• aerobically use methanol, methylamine, dimethyl sulfide
• anaerobically use methanol coupled to denitrification
• grows aerobically on a range of C2 and C4 compounds
© 2018 Pearson Education, Inc.
 15.16 Methanotrophs and Methylotrophs

• Aerobic methanotrophs
• key genera: Methylomonas, Methylosinus
• use methane as an electron donor and typically also
carbon source (Table 15.1)
• mostly Proteobacteria
• Type I assimilate one-carbon compounds via ribulose
monophosphate cycle and are Gammaproteobacteria.
• Type II assimilate one-carbon compounds via serine
pathway and are Alphaproteobacteria.
• most specialized for aerobic growth on methane; some
can grown on methanol
• typically obligate methylotrophs
• Methylacidiphilum uses Calvin cycle.

© 2018 Pearson Education, Inc.

 15.16 Methanotrophs and Methylotrophs

• Physiology
• Methane monooxygenase catalyzes incorporation of an oxygen
atom from O2 into CH4 to form methanol
• located in extensive internal membrane systems
• In type I methanotrophs, membranes arranged as bundles of disc-
shaped vesicles throughout cell. (Figure 15.37b)
• Type II have paired membranes running along cell periphery.
• Verrucomicrobial methanotrophs have membrane vesicles.
• Methylotrophs unable to use CH4, lack membrane arrays.
• Methanotrophs possess large amounts of sterols (rigid planar
molecules found in eukaryotes but not most bacteria).
• Sterols may be essential for membrane system.
• also found in mycoplasmas (bacteria that lack cell walls)
• Methylotrophs contain various carotenoids and cytochromes,
making them pink.

© 2018 Pearson Education, Inc.

© 2018 Pearson Education, Inc. Figure 15.37
 15.16 Methanotrophs and Methylotrophs
• Ecology
• aerobic methylotrophs found in the open ocean, in soils,
around plant roots and leaf surfaces, and at the oxic interface
of many anoxic environments
• Aerobic methanotrophs consume atmospheric methane in soils
and are common at the oxic interface of anoxic lakes,
sediments, and wetland environments, playing an important
role in global carbon cycle.
• Methanotrophs have symbioses with eukaryotes (e.g., marine
mussel gill tissue provides effective gas exchange with
seawater; Figure 15.38).
• Methylomirabilis oxyfera is an obligate anaerobic methanotroph
that uses methane monoxygenase to oxidize methane to CO2.
• reduces nitrite to nitric oxide, which is dismutated to N2 and O2
• O2 consumed by methane monooxygenase
• assimilates C1 units as CO2, probably by Calvin cycle
© 2018 Pearson Education, Inc.
© 2018 Pearson Education, Inc. Figure 15.38
 15.17 Microbial Predators

• Key genera: Bdellovibrio, Myxococcus

• Some bacteria consume others.
• Among Proteobacteria, Bacteroidetes, Cyanobacteria
• Various methods
• Some epibiotic predators (e.g., Vampirococcus, Micavibrio,
Vampirovibrio) attach to prey surface and acquire nutrients
from cytoplasm or periplasm.
• Some cytoplasmic predators (e.g., Daptobacter) invade host
cells and replicate in cytoplasm, consuming prey from within.
• Bdellovibrioi are periplasmic predators that invade and
replicate within periplasm.
• Lysobacter and Myxococcus are social predators that swarm
and collectively feed upon lysed prey.
© 2018 Pearson Education, Inc.
 15.17 Microbial Predators

• Bdellovibrio
• small, highly motile, curved
• after attachment, penetrates cell wall and replicates in
periplasm
• forms a spherical bdelloplast
• two stages of penetration (Figures 15.39 and 15.40)
• attacks gram-negatives, not gram-positives
• obligate aerobes that oxidize amino acids and acetate
• assimilates nucleotides, fatty acids, peptides, even some
proteins without hydrolysis
• Predation is not obligatory.
• widespread in aquatic habitats, soils, sewage
• forms plaques like bacterial viruses
© 2018 Pearson Education, Inc.
© 2018 Pearson Education, Inc. Figure 15.39
© 2018 Pearson Education, Inc. Figure 15.40
 15.17 Microbial Predators

• Myxobacteria
• most complex behavior among known bacteria

• Life cycle results in formation of multicellular structures (fruiting

bodies).
• often strikingly colored and morphologically elaborate (Figure 15.41)

• can often be seen with hand lens on decaying wood or plant material

© 2018 Pearson Education, Inc.

© 2018 Pearson Education, Inc. Figure 15.41
 15.17 Microbial Predators

• Myxobacteria
• life cycle (Figure 15.42)
• Vegetative cells are simple nonflagellated gram-negative
rods (Figure 15.43) that glide and obtain nutrients by
lysing other bacteria.
• Vegetative cells excrete slime trails. (Figure 15.44)
• form a swarm that self-organizes, allowing them to
behave as a single coordinated entity in response to
environment
• when nutrients exhausted, vegetative cells aggregate in
mounds/heaps (Figure 15.45) likely mediated by
chemotaxis or quorum-sensing
• differentiate into fruiting bodies (Figure 15.46) containing
myxospores (specialized resistant cells)
© 2018 Pearson Education, Inc.
© 2018 Pearson Education, Inc. Figure 15.42
© 2018 Pearson Education, Inc. Figure 15.43
© 2018 Pearson Education, Inc. Figure 15.44
© 2018 Pearson Education, Inc. Figure 15.45
© 2018 Pearson Education, Inc. Figure 15.46
 15.18 Microbial Bioluminescence

• Key genera: Vibrio, Aliivibrio, and Photobacterium

• Bioluminescence: Emission of light (Figure 15.47)

• A few Shewanella (marine) and Photorhabdus

(terrestrial)

• Mostly marine; some colonize light organs of some

fish and squid, producing light for signaling,
avoiding predators, attracting prey.

© 2018 Pearson Education, Inc.

© 2018 Pearson Education, Inc. Figure 15.47
 15.18 Microbial Bioluminescence

• Mechanism and ecology of bioluminescence

• only when O2 present
• requires luxCDABE genes and is catalyzed by luciferase,
which uses O2, a long-chain aliphatic aldehyde (RCHO; e.g.,
tetradecanal) and reduced flavin mononucleotide (FMNH2)
• In many bacteria, only occurs at high population density,
exhibiting autoinduction/quorum sensing with control by LuxR
regulatory protein and acyl homoserine lactone (AHL)
• High local population density results in AHL accumulation.
• At high concentration, AHL binds LuxR, activating transcription
of luxCDABE and resulting in light.
• ensures luminescence occurs when population densities high
enough for light to be visible to animals to attract them to be
eaten (providing a nutrient-richer environment) or for
symbiosis
© 2018 Pearson Education, Inc.
 VI. Morphologically Diverse Bacteria

• 15.19 Spirochetes

• 15.20 Budding and Prosthecate/Stalked

Microorganisms

• 15.21 Sheathed Microorganisms

• 15.22 Magnetic Microbes

© 2018 Pearson Education, Inc.

 15.19 Spirochetes

• Key genera: Spirochaeta, Treponema, Cristispira,

Leptospira, Borrelia
• Morphologically unique found only in Spirochaetes
• Gram-negative, motile, tightly coiled, typically slender
Bacteria (Figure 15.48)
• Widespread in aquatic sediments and in animals
• Some cause diseases (e.g., syphilis).
• Classified into eight genera based on habitat, pathogenicity,
phylogeny, morphology, and physiology (Table 15.2)
• Have endoflagella (Figure 15.49) in the periplasm
surrounded by outer sheath that results in a corkscrew-like
motion that allows burrowing through viscous materials or
tissues
© 2018 Pearson Education, Inc.
© 2018 Pearson Education, Inc. Figure 15.48
© 2018 Pearson Education, Inc. Table 15.2
© 2018 Pearson Education, Inc. Figure 15.49
 15.19 Spirochetes

• Spirochetes often confused with spirilla

• helically curved, rod-shaped

• usually motile using polar flagella (Figure 15.50)

• widespread shape among Bacteria and Archaea

• lack outer sheath, endolagella, and corkscrew-like

motility

• typically rigid cells

© 2018 Pearson Education, Inc.

 15.19 Spirochetes

• Spirochaeta and Cristispira

• Spirochaeta
• free-living, anaerobic and facultative aerobes
• common in aquatic environments (freshwater and sediments)
and oceans
• Spirochaeta plicatilis found in sulfidic freshwater and marine
habits and has ~20 endoflagella at each pole
• Spirochaeta stenostrepta is an obligate anaerobe found in H2S-
rich mud and ferments sugars to ethanol, acetate, lactate, CO2,
and H2.
• Cristispira (Figure 15.51)
• found in nature in the crystalline style of some freshwater and
marine molluscs (e.g., clams and oysters)
• Crystalline style is flexible, semisolid rod that mixes and
grinds food.
• not yet cultured
© 2018 Pearson Education, Inc.
© 2018 Pearson Education, Inc. Figure 15.51
 15.19 Spirochetes

• Treponema and Borrelia

• Treponema: anaerobic or microaerophilic host-
associated spirochetes that are commensals or
pathogens of humans and animals
• T. palldum causes syphilis; cell is flat, wavy, and thin.
• T. denticola in oral cavity and associated with gum disease;
ferments amino acids, forming acetate, CO2, NH3, and H2S
• also common in rumen, such as T. saccarophilum
(Figure 15.52a) in cows
• T. primitia is an acetogen found in hindgut of some
termites; T. azotonutricium also found and fixes nitrogen.

© 2018 Pearson Education, Inc.

© 2018 Pearson Education, Inc. Figure 15.52
 15.19 Spirochetes

• Treponema and Borrelia

• Majority of Borrelia are animal or human pathogens.

• Borrelia burgdorferi is the causative agent of Lyme

disease.

• B. burgdorferi has a linear chromosome.

• Others cause disease in cattle, sheep, horses, birds.

• mostly transmitted via tick bite

© 2018 Pearson Education, Inc.

 15.19 Spirochetes

• Leptospira and Leptonema

• strictly aerobic spirochetes that oxidize long chain fatty acids
(e.g., oleic acid) for electrons and carbon
• thin, finely coiled, usually bent at ends into semicircular hooks
• some free-living, many parasitic, such as Leptospira
interrogans (parasitic for humans and animals) and Leptospira
biflexa (free-living)
• Rodents are the natural host of Leptospira, though dogs and
pigs are also carriers.
• Leptospirosis: Organism localizes in kidneys, causing renal
failure or death.
• enters body through mucous membranes or breaks in skin during
contact with infected animal
• distemper-leptospira-hepatitis vaccine for dogs
© 2018 Pearson Education, Inc.
 15.20 Budding and Prosthecate/Stalked
Microorganisms

• Key genera: Hyphomycrobium, Caulobacter

• cell division different from other bacteria, including
budding and appendage formation

• distinct life cycles

© 2018 Pearson Education, Inc.

 15.20 Budding and Prosthecate/Stalked
Microorganisms
• Budding division
• divide as a result of unequal cell growth
• Cell division forms totally new daughter cell with mother
retaining original identity.
• polar growth (formation of new cell wall material from a
single point)
• Internal structures are not partitioned during division and
must be formed de novo, thus, can form more
complicated structures.

© 2018 Pearson Education, Inc.

 15.20 Budding and Prosthecate/Stalked
Microorganisms
• Budding bacteria: Hyphomicrobium
• two well-studied genera:
• Hyphomicrobium (chemoorganotrophic; Figure 15.53)
• Rhodomicrobium (phototrophic)
• release buds from ends of hyphae (direct cellular extensions)
• life cycle
• Mother cell forms hypha.
• Bud forms, enlarges, forms flagellum, breaks loose, swims away.
• Daughter cell loses flagellum, matures, forms hypha and buds.
• Sometimes a bud forms directly from mother cell or a single cell
forms hyphae at both ends. (Figure 15.54)
• Hyphomicrobium is methylotrophic, widespread; only known
denitrifier that uses methanol as an electron donor.

© 2018 Pearson Education, Inc.

© 2018 Pearson Education, Inc. Figure 15.53
© 2018 Pearson Education, Inc. Figure 15.54
 15.20 Budding and Prosthecate/Stalked
Microorganisms
• Prosthecate and stalked Bacteria
• Cytoplasmic extrusions called prosthecae include stalks
(Figure 15.55), hyphae, and appendages. (Table 15.3)
(Figure 15.56)
• allow attachment to particulate matter, plant material, and
other microbes in aquatic environments
• Prosthecae increase surface-to-volume ratio of the cells
for nutrient uptake and waste expulsion.
• Prosthecae may keep cells from sinking into
anoxic zones.

© 2018 Pearson Education, Inc.

© 2018 Pearson Education, Inc. Figure 15.55
© 2018 Pearson Education, Inc. Table 15.3
© 2018 Pearson Education, Inc. Figure 15.56
 15.20 Budding and Prosthecate/Stalked
Microorganisms
• Caulobacter
• chemoorganotroph
• produces a cytoplasm-filled stalk
• Gallionella is a stalked chemolithotrophic iron-oxidizer
whose stalk is made of Fe(OH)3.
• often seen on surfaces in aquatic environments with
stalks of several cells attached to form rosettes
• holdfast structure on end of stalk used for anchoring
• Cell division cycle (Figure 15.57) is unique: unequal
binary fission.
© 2018 Pearson Education, Inc.
© 2018 Pearson Education, Inc. Figure 15.57
 15.21 Sheathed Microorganisms

• Key genera: Sphaerotilus, Leptothrix

• Sheaths made of polysaccharide or protein that
encase one or many cells
• bind cells into long multicellular filaments
• Unique life cycle in which cells grow vegetatively
within a long tube or sheath
• Under unfavorable conditions, flagellated swarmer
cells form, break out, and are dispersed.
• Common in freshwater habitats rich in organic
matter (e.g., wastewater, polluted streams, sludge
digesters in sewage treatment plants)
© 2018 Pearson Education, Inc.
 15.21 Sheathed Microorganisms

• Leptothrix
• can precipitate iron oxides on sheaths from oxidation of
metals (Figure 15.58)

• Chemoorganotrophic; does not gain energy from iron or

manganese oxidation.

© 2018 Pearson Education, Inc.

© 2018 Pearson Education, Inc. Figure 15.58
 15.21 Sheathed Microorganisms

• Sphaerotilus
• a chain of rod-shaped gram-negative cells in a sheath
(Figure 15.59a)
• Motile swarmer cells are liberated, migrate, attach to a
surface, and start new filaments.
• nutritionally versatile; use simple organics for carbon and
energy
• obligate aerobes
• cause “sewage fungus,” a filamentous slime found on
rocks in streams receiving sewage pollution
• in activated sludge, responsible for “bulking” that
suspends sludge instead of settling
© 2018 Pearson Education, Inc.
© 2018 Pearson Education, Inc. Figure 15.59
 15.22 Magnetic Microbes
• Key genera: Magnetospirillum
• magnetotaxis: directed movement in a magnetic field
• magnetosomes consist of chains of magnetic particles made
of Fe3O4 (aerobic) or Fe3S4 (anaerobic).
• Localized within cell membrane invaginations
• align north-south
• may maintain organisms in zones of low O2
• microaerophilic or anaerobic
• found near oxic-anoxic interface in sediments or stratified lakes
• members of Alphaproteobacteria, Gammaproteobacteria,
Deltaproteobacteria, and Nitrospira
• Best characterized is Magnetospirillum magnetotacticum
(Figure 15.60), a chemoorganotrophic microaerophile that can
grow anaerobically.
• Desulfovibrio magneticus is a sulfate-reducing obligate
anaerobe.
© 2018 Pearson Education, Inc.
© 2018 Pearson Education, Inc. Figure 15.60