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Physiology and Diversity of Prokaryotes WS 2009/2010 (www.icbm.de/pmbio/) PHOTOTROPHS Martin Könneke Lithotrophic Processes Elektronendonor Oxidized product Process/ organism H+ (H2O) H2 NH4 + NO3 - Knallgas reaction/ Ralstonia Nitrification (2 types) NH4+ NO2- Ammonia oxidizer (Nitroso-) NO2- NO3- Nitrite oxidizer (Nitro-) CH4 CO2 Methane oxidizer (Methylo-) 2- H2S, S SO4 Sulfur oxidizer/Thiobacillus, Beggiatoa Fe2+ Fe3+ Iron oxidation/Thiobacillus H2O O2 Photosynthesis 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 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 ( 2 variations) CO2 fixation: Calvin Cyclus 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 Cyclus (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> + H2 O + 3 ADP + 3 Pi - IV 0 CO2 Fixierung: Calvin Cyclus RubisCO CO2 Fixierung: Calvin Cyclus 15 C 3C Phosphoribulokinase 18 C 3C 15 C RubisCO CO2 fixation: Calvin Cyclus 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 . Reductive citric acid pathway Reductive citric acid pathway 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 Reductive acetyl-CoA pathway 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 3-Hydroxypropionate cycle 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 The 3OH-propionate/ 4 OH-butyrate cycle in Crenarchaeota Acetyl-CoA Acetoacetyl-CoA !-ketothiolase (Nmar_0841 or Nmar_1631) Acetoacetyl-CoA 3-Hydroxybutyryl-CoA dehydrogenase (Nmar_1028) Acetyl-CoA 3-Hydroxybutyryl-CoA HCO3- Crotonyl-CoA hydratase Acetyl-CoA carboxylase (Nmar_1308) (Nmar_0272, 0273, 0274) Crotonyl-CoA 4-Hydroxybutyryl-CoA dehydratase Malonyl-CoA (Nmar_0207) Malonyl-CoA reductase Malonate semialdehyde reductase 4-Hydroxybutyryl-CoA 4-Hydroxybutyryl-CoA synthetase (unknown) (Nmar_0206) 3-Hydroxypropionate 3-Hydroxypropionyl-CoA synthetase 3-Hydroxypropionyl-CoA dehydratase Acryloyl-CoA reductase (unknown) Propionyl-CoA Propionyl-CoA carboxylase HCO3- (Nmar_0272, 0273, 0274) Methylmalonyl-CoA 4-Hydroxybutyrate Succinate semialdehyde reductase (Nmar_1110 or Nmar_0161) Succinate-semialdehyde Succinyl-CoA reductase (Nmar_1608) Succinyl-CoA Methylmalonyl-CoA epimerase Methylmalonyl-CoA mutase (Nmar_0953, 0954, 0958) 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 Oxygenic photosynthesis 2e2H+ Water serves as electron donor Anoxygenic photosynthesis Hydrogen sulfide (or sulfur) serves as electron donor Structure of a chloroplast Arrangement of light-harvesting Chlorophylls versus reaction center 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 Absorption spectrum of cyanobacterium 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 Phototrophic purple sulfur bacteria Purple sulfur bacteria e.g. Chromatium okenii Gamma proteobacteria Habitat: stratified lakes Electron donor: reduced sulfur compounds H2S, S0, S2O32Sulfur can be stores in globules inside the cell Mixotrophic: CO2 fixation (Calvin Cyclus), organic acids Phototrophic purple sulfur bacteria 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, S2O32Mixotrophic: CO2 fixation (Calvin Cyclus), organic compounds Phototrophic purple nonsulfur bacteria 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 Vesicular photosymthetic membranes Rhodobacter capsulatus Anoxygenic photosynthesis in purple bacteria Only 1 light reaction! Arrangement of protein complex 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, S2O32produced sulfur resides outside the cells Mixotrophic: CO2 fixation (reverse citric acid cycle) organic compounds (photoheterotrophy) Chlorosomes in green sulfur bacteria Chlorophyl-rich bodies, connected to cytoplasma membrane Model of chlorosome structure (green sulfur and green nonsulfur bacteria) 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 cycle Heterotrophic with organic acids In the dark, chemoorganotrophic by aerobic respiration 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 Bundles of cells of Heliophilum fasciatum Spore formation Heliobacterium gestii Physiological properties of phototrophic Bacteria Cyanobacteria Purplebacteria Green Sulfur bacteria Green nonSulfur bacteria Heliobacter PS-type PS I and II PS II PS I PS II PS I Pigments Chl a (b) BChl a, b BChl a, c, (d, e) BChl a, c BCHl g Autotrophy + (+) + +/- -(?) Physiology PhotoautoLithoauto- PhotoautoLithoautoOrganohetero- PhotoautoLithoauto- PhotoautoLithoautoOrganohetero- PhotoautoOrganohetero- CO2 fixation Calvin-cycle Calvin-cycle Reductive TCA 3OH-Propionate None ? Electron donor H2O H2S/ organic H2S H2/ organic Organic Adapted from Fuchs and Schlegel ‘Allgemeine Mikrobiologie’ Comparison of electron flow