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PHOTOTROPHS

Martin Könneke

Physiology and Diversity of Prokaryotes WS 2010/2011 (www.icbm.de/pmbio/)

Lithotrophic Processes

Elektronendonor Oxidized product Process/ organism

H2 H+ (H2O) Knallgas reaction/ Ralstonia

NH4+ NO3

- Nitrification (2 types)

NH4+ NO2

- Ammonia oxidizer (Nitroso-)

NO2- NO3

- Nitrite oxidizer (Nitro-)

CH4 CO2 Methane oxidizer (Methylo-)

H2S, S SO42- Sulfur oxidizer/Thiobacillus,

Beggiatoa

Fe2+ Fe3+ Iron oxidation/Thiobacillus

H2O O2 Photosynthesis

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Lithotrophic processes are essential for the

reoxidation of reduced electron acceptors!

All chemolithotrophes are prokaryotes!

Almost all known lithotrophes are autotroph!

Energieform Elektronendonor Kohlenstoffquelle

Chemo- Organo- heterotroph

Photo- Litho- autotroph

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CO2 fixation pathways

At present, 6 different pathways are known,

just a single one within the Eukarya

Differ with regard to energy requirement, end

products and oxygene sensitivity

Calvin Cycle

Reductive (reverse) citric acid cycle

Reductive acetyl-CoA pathway

3-Hydroxypropionate cycle ( 3 variations)

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CO2 fixation: Calvin Cycle

Most widespead carbon fixation pathway

(RubisCO most abundant enzyme on Earth)

Occurs in chloroplast, cyanobacteria, and most

chemolithoautotrophic bacteria

Some bacteria contain speciallized compartements,

carboxysome, with high RubisCO concentration

Reduces CO2 even at high oxygen concentrations

Can also funtion as oxygenase

CO2 fixation via the Calvin Cycle

(Calvin-Bassham-Benson-cylcle)

Key enzyme: RubisCO

Ribulosebisphosphat-Carboxylase/Oxygenase

Used by all plants, cyanobacteria, and most of the aerobic

chemolithoautotrophic bacteria

Reduction of CO2 to the oxidation state of sugar:

CO2 + 3 ATP + 4[H] ! <CH2O> + H2O + 3 ADP + 3 Pi

- IV 0

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CO2 Fixierung:

Calvin Cycle

RubisCO

CO2 Fixierung: Calvin Cycle

RubisCO

Phosphoribulokinase

15 C

15 C

18 C

3 C

3 C

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CO2 fixation: Calvin Cycle

Key enzyme: RubisCO

Ribulosebisphosphat-Carboxylase/Oxygenase

Requirements for the synthesis of

3-phosphate glycerine aldehyde

3 CO2 + 9 ATP + 6 NADPH

carbon energy reducing power

Energy expensive pathway!

Reductive (reverse) citric acid cycle

Represents the reversion of the citric acid cycle

Replacement of 3 enzymes:

1) ATP-citrate lyase instead of the citrate synthase

2) !-ketoglutarate-synthase instead of "-

ketoglutaratedehydrogenase 3) Fumarate synthase instead of succinate dehydrogenase

Final product of the cycle is acetyl-CoA, that is further

carboxylized to pyruvate. A third step is the ATP dependent

conversion to triose-phosphate .

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Reductive citric acid pathway

Reductive citric acid pathway

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Reductive (reverse) citric acid cycle

Present in:

Phototrophic green sulfur bacteria (e.g. Chlorobium limicola)

Sulfate reducers (Desulfobacter hydrogenophilus)

Knallgas bacteria (Hydrogenobacter thermophilus)

Thermophilic, sulfur-reducing archaea (Thermoproteus

neutrophilus)

The acetyl-CoA pathway

-! In contrast to other carbon fixation pathways, not a cycle

-! two linear reaction series resulting in A) a methyl- and B) a carbonyl group

-! key enzyme: CO-DH (Carbon monooxide dehydrogenase)

CO2 + H2 ! CO + H2O

-! The CO2 reduction must be considered as bifunctional

pathway: A) energy metabolism B) C-fixation for biosynthesis

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Reductive acetyl-CoA pathway

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3-Hydroxypropionate cycle

Reduction of bicarbonate to gyoxylate

Bicarbonate fixing enzymes are: acetyl-CoA

carboxylase and propionyl-CoA carboxylase

3-hydroxypropionyl-CoA as characteristic

intermediate

Recycling of the primary carbonate acceptor

acetyl-CoA

3-Hydroxypropionate cycle

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3-Hydroxypropionate cycle

3-Hydroxypropionate bicycle in Chloroflexus spec.

Zarzycki et al 2009

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3-Hydroxypropionate cycle

At present only found in the green nonsulfur

phototrophic members of the genus Chloroflexus

and in thermophilic Crenarchaeota

(Metallosphaera)

Suggested to be oldest pathway of autotrophy in

anoxygenic phototrophes

HCO3-

Acetyl-CoA

Succinyl-CoA Methylmalonyl-CoA

3-Hydroxybutyryl-CoA

Crotonyl-CoA

Acetyl-CoA

Malonyl-CoA

3-Hydroxypropionate

Propionyl-CoA

Acetoacetyl-CoA

4-Hydroxybutyrate

Succinate-semialdehyde

4-Hydroxybutyryl-CoA

Acetyl-CoA carboxylase (Nmar_0272, 0273, 0274)

Malonyl-CoA reductase Malonate semialdehyde reductase (unknown)

3-Hydroxypropionyl-CoA synthetase 3-Hydroxypropionyl-CoA dehydratase

Acryloyl-CoA reductase (unknown)

HCO3-

Propionyl-CoA carboxylase (Nmar_0272, 0273, 0274)

Methylmalonyl-CoA epimerase Methylmalonyl-CoA mutase (Nmar_0953, 0954, 0958)

Succinyl-CoA reductase (Nmar_1608)

4-Hydroxybutyryl-CoA synthetase (Nmar_0206)

Succinate semialdehyde reductase (Nmar_1110 or Nmar_0161)

Crotonyl-CoA hydratase (Nmar_1308)

Acetoacetyl-CoA !-ketothiolase (Nmar_0841 or Nmar_1631)

3-Hydroxybutyryl-CoA dehydrogenase (Nmar_1028)

4-Hydroxybutyryl-CoA dehydratase (Nmar_0207)

The

hydroxypropionate/

hydroxybutyrate cycle in Crenarchaeota

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Berg et al. 2010

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Photosynthetic organisms

Distinction between light and dark reaction

Light reaction conserve energy of light into

chemical energy (ATP)

Dark reaction involves the consumption of ATP for

fixation of CO2

Depending on electron donor:

Oxygen-producing: oxygenic

Non oxygen-producing: anoxigenic

2e-

2H+

Water serves as electron donor

Oxygenic photosynthesis

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Anoxygenic photosynthesis

Hydrogen sulfide (or sulfur) serves as electron donor

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Structure of a chloroplast

Arrangement of light-harvesting

Chlorophylls

versus reaction center

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Phylogenetic affiliation of phototrophic bacteria

Oxigenic photosynthetic bacteria

Cyanobacteria

- Only bacteria which gain energy by oxigenic

photosynthesis (formation of O2)

- Large and heterogeneous group of bacteria

-! Major primary producer in many habitats (aquatic and

terrestrial habitats, symbiotic with Eukaryotes)

-! Ancestor of chloroplasts (Endosymbiosis theory)

-! Many can fix N2 (Heterocyst or temporal seperation)

-! Occur as unicellular and filamentous forms

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Absorption spectrum of cyanobacterium

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Phototrophic purple bacteria

- gain energy by anoxic photosynthesis (no formation of O2)

- contain bacteriochlorophyll and a variety of carotonoids

-! electron carriers are arranged in specific

intracytoplasmatic photosynthetic membranes (increase

of pigment density)

-! electron carriers are in the order of more electronegative

to higher electropositive reduction potential

Intracytoplasmic membranes

in anoxygenic phototrophs

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Phototrophic purple sulfur bacteria

Purple sulfur bacteria

e.g. Chromatium okenii

Gamma proteobacteria

Habitat: stratified lakes

Electron donor: reduced sulfur compounds

H2S, S0, S2O32-

Sulfur can be stores in globules inside the cell

Mixotrophic:

CO2 fixation (Calvin Cyclus), organic acids

Purple sulfur bacteria

Ectothiorhodospira sp., Halorhodospira sp.

Gamma proteobacteria

Habitats: sola lakes, marine environments

halophilic = salt-loving

Produce sulfur outside the cell

Electron donor: reduced sulfur compounds

H2S, S0, S2O32-

Mixotrophic:

CO2 fixation (Calvin Cyclus), organic compounds

Phototrophic purple sulfur bacteria

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e.g. Rhodospirillum rubrum

Alpha or beta proteobacteria

Electron donor: hydrogen, sulfur, organic

substrates (no storage of sulfur)

Some can grow in the dark by fermentation,

anaerobic respiration, or aerobic respiration

Can also fix N2

Mixotrophic:

CO2 fixation (Calvin Cyclus), organic compounds

Phototrophic purple nonsulfur bacteria

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Rhodobacter

capsulatus

Vesicular photosymthetic membranes

Anoxygenic photosynthesis

in purple bacteria

Only 1 light reaction!

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Arrangement of protein complexes in

phototrophic purple bacteria

Green sulfur bacteria

z.B. Chlorobium limicola

Phylum green sulfur bacteria

All isolates are obligate anaerobic and phototrophic

contain chlorosoms (location of photosynthesis)

electron donors: reduced sulfur compounds

H2S, S0, S2O32-

produced sulfur resides outside the cells

Mixotrophic:

CO2 fixation (reverse citric acid cycle)

organic compounds (photoheterotrophy)

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Chlorosomes in green sulfur bacteria Chlorophyl-rich bodies, connected to cytoplasma membrane

Model of chlorosome structure (green

sulfur and green nonsulfur bacteria)

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Green Sulfur bacteria Consortia

"Chlorochromatium aggregatum"

Symbiosis between

Phototrophic green sulfur bacterium (epibiont)

and a chemotrophic beta proteobacterium

(by J. Overmann, mikrobiologischer-garten.de)

Green nonsulfur bacteria (“Chloroflexi“)

e.g. Chloroflexus aurantiacus

All isolated members are thermophilic

Formation of thick microbial mats in hot habitats.

Electron donor: H2 and organic compounds

CO2 fixation via 3-hydroxypropionate bicycle

Heterotrophic with organic acids

In the dark, chemoorganotrophic by aerobic respiration

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Green nonsulfur bacteria

(“Chloroflexi“)

Chloroflexus aurantiacus

Heliobacteria

z.B. Heliobacillus chlorum

contain bacteriochlorophyl g!

Strict anaerobic, N2-fixation!

Anoxygenic phototrophic Gram-positive bacteria!

Spore-forming

Electron donor: H2 and pyruvate (fermentation)

Mixotrophic:

CO2 fixation (reverse citric acid cycle)

organic compounds

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Bundles of cells of Heliophilum fasciatum

Spore formation

Heliobacterium gestii

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Cyanobacteria Purplebacteria Green

Sulfur bacteria

Green non-

Sulfur bacteria

Heliobacter

PS-type

Pigments

PS I and II

Chl a (b)

PS II

BChl a, b

PS I

BChl a, c, (d, e)

PS II

BChl a, c

PS I

BCHl g

Autotrophy + (+) + +/- -!(?)

Physiology Photoauto-

Lithoauto-

Photoauto-

Lithoauto-

Organohetero-

Photoauto-

Lithoauto-

Photoauto-

Lithoauto-

Organohetero-

Photoauto-

Organohetero-

CO2 fixation Calvin-cycle Calvin-cycle Reductive TCA 3OH-Propionate None ?

Electron donor H2O H2S/ organic H2S H2/ organic Organic

Physiological properties of phototrophic Bacteria

Adapted from Fuchs and Schlegel

‘Allgemeine Mikrobiologie’

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Comparison of electron flow