Green and Purple Sulfur Bacteria
Sulfur bacteria are Gram negative, anaerobic, filamentous photosynthetic bacteria. During photosynthesis, carbon dioxide reduction to carbohydrates doesn't utilize water as electron donor but hydrogen sulfide (H2S); hence they are called as sulfur (S) bacteria. Depending upon their carotenoid content, S bacteria are identified as green and purple bacteria. Purple bacteria appear purple or reddish brown, green bacteria appear as yellow green, green orange or brown. The color of major photosynthetic pigment, bacteriochlorophyll is not taken into account as human eye cannot detect its absorption spectrum in infrared region. Both purple and green bacteria are important biological constituents of atmospheric sulfur cycle. It is mandatory to know reactions of sulfur cycle, to understand the role of these bacteria. Sulfur is present in the nature in amino acids cysteine and methionine, free and combined, organic and inorganic states; its transformations between oxidized and reduced states are accomplished by S bacteria. They oxidize H2S to sulfate which is the most suitable form of essential sulfur utilized by plants. Anaerobic oxidation of sulfur with nitrate reduction is also found in some S bacteria. Sulfate is reduced and assimilated by plants and microbes in the soil. Assimilated sulfate is incorporated into proteins under aerobic conditions. Dissimilatory sulfate reduction occurs with the production of H2S. Hydrogen is used in the reduction of sulfate to H2S via sulphite; that means sulfate is utilized as terminal electron acceptor. Purple and green S bacteria coexist in illuminated sulfide rich anaerobic aquatic environments
Green S bacteria: Green bacteria are not cyanobacteria or blue-green algae as one may assume; they different group of phototrophic bacteria from family Chlorobiaceae. Photosynthetic pigments are bacteriochlorophylls c, d or e; carotenoids chlorobactene, hydrochlorobactene, isorenieratene and β-isorenieratene. They use H2S, other reduced inorganic sulfur compounds and H2 as electron donors. One interesting thing is that they cannot use sulfate as sulfur source, they require sulfide to meet biosynthetic needs when growing with H2 as electron donor and also vitamin B12. Elemental sulfur produced from H2S oxidation is deposited extracellularly as sulfur globules prior to its oxidation to sulfate. They cannot grow photoheterotrophically using organic compounds as their principle or sole carbon source in absence of inorganic reductant. They can photoassimilate organic compounds like acetate or pyruvate but only if H2S and CO2 are present at a time. They are unique and only bacteria (except some Archaebacteria) that use reductive tricarboxylic acid cycle (TCA) for CO2 fixation; other bacteria use Calvin-Benson cycle. Important genera of green S bacteria are: Chlorobium, Chloropseudomonas (Prosthecochloris), Pelodictyon, Clathrochloris (Acalochloris) and Chlorobacterium (Chloroherpeton). Green but nonsulfur (nonS) bacteria of family Chloroflexaceae are also present in anaerobic habitats. They are different from green S bacteria in structure, nutrition, metabolism and ecology but like green S bacteria; contain bacteriochlorophyll c or d and as major and minor pigments respectively. They are photoheterotrophs, facultative photoautotrophs or chemoheterotrophs; usually inhabit hot springs containing low organic matter. They derive organic nutrients from cyanobacteria and found always in association with cyanobacteria. Principle genera include Chloroflexus, Oscillochloris, Chloronema and Heliothrix. Heliothrix is found only in hot springs of Yellowstone National Park and Western Oregon.
Purple S bacteria: Purple S bacteria are from family Chromatiaceae, strictly anaerobic, obligate photoautotrophic inhabit sulfide rich waters. Sulfide is generated by sulfate reducing bacteria such as Desulfovibrio, Desulfococcous, Desulfomonas, Desulfosarcina, Desulfolobus, Desulfobacter and Desulfococcus spp. Photosynthetic pigments are bacteriochlorophyll a or b, carotenoids of group 1, 3 and 4. H2S, sulfur, thiosulfate, molecular hydrogen and organic compounds acts like electron donors. All purple S bacteria fix nitrogen photosynthetically. They can grow on pyruvate heterotrophically in dark. H2S is oxidized via elemental sulfur to sulfate by Calvin-Benson cycle. It always leads to transient accumulation of elemental sulfur as sulfur globules, gas vacuoles inside the cytoplasm. They also synthesize poly-β-hydroxybutyrate (PHB) granules. These bacteria also have special cell membrane intrusions or infolding. This results in enlarged membrane area to accommodate more centers of respiratory and photosynthetic activity. Principle genera are: Thiospirillum, Chromatium, Ectothiorhodospira, Thiocystis, Thiocapsa, Lamprocystis, Thiodictyon, Thiopedia and Amoebobacter spp. Purple nonS bacteria are also present and they sensitive to H2S. They represent principle genera Rhodospirillum, Rhodopseudomonas, Rhodomicrobium, Rhodopila, Rhodocyclus and Rhodobacter from family Rhodospirillaceae. They oxidize sulfide anaerobically at very low concentration in presence of light. They occur typically in freshwater habitats where sulfide is absent or present in extremely low concentration and organic matter is abundant. They can photoassimilate wide range of organic compounds like fatty acids, primary/ secondary alcohols, organic acids, carbohydrates, proteins and aromatic compounds. Some species also carry out simultaneous processes of denitrification and nitrogen fixation to produce reduced and combined form of nitrogen to support cell growth.
Ecological importance of sulfur oxidation and reduction: Sulfur is abundant in nature but its oxidation products like sulfate is the utilizable form for plants. Sulfate anions solubilize inorganic salts containing nutrients such as phosphorous for plant use. Sulfate also prevents excessive alkalinity due to ammonia formation by microorganisms. Anaerobic oxidation of sulfur along with nitrate reduction is sometimes detrimental process which results in loss of soil fertility. H2S produced by sulfur reduction is highly toxic to aquatic flora and fauna, often associated with fish mortality. In waterlogged anaerobic soils such as paddy fields, H2S may damage vegetation. H2S produced by reduction is important source of reducing power to support the growth of H2S aerobic chemoautotrophs or anaerobic photoauto and photoheterotrophs. Sulfur reducing bacteria and phototrophic bacteria are predominant in Sulfureta habitats such as sulfate rich stagnant lakes. These bacteria promote formation of elemental sulfur from sulfates; geological sulfur deposits are formed from activities of these bacteria. S bacteria also contribute to corrosion of metal pipes.
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