Anoxygenic photosynthesis uses energy from sunlight to couple the reduction of C in CO2 to the anaerobic oxidation of S in S0 or H2S. If one is accustomed to thinking of S oxidation in a strictly aerobic sense, then anaerobic S oxidation appears contradictory. Anoxygenic photosynthesis would have been compatible with the anoxic (O2-free) conditions of the earth's primordial atmosphere. It could have been mediated by anaerobic organisms like present-day photosynthetic sulfur bacteria and is believed to have preceded oxygenic photosynthesis (Staley and Orians, 1992). Dominance of anoxygenic photosynthesis would have favored anaerobic respiration or fermentative pathways for obtaining energy from the products of photosynthesis. Consequently one might expect, and certainly one finds, a wide representation of anaerobic microorganisms in soil environments. Many elements can cycle under entirely anaerobic conditions due to the syntrophic relationships among photolithotrophs and anaerobic chemoorganotrophs. With the input of electromagnetic radiation to power photosynthesis by photolithotrophs that reduce C and oxidize elements such as S, combined with decomposition by anaerobic chemoorganotrophs to reoxidize C and re-reduce S, one can summarize the anoxy-genic cycle as:
Energy trapped as organic compounds during anoxygenic photosynthesis could be released by oxidation of the reduced C (e- donor) coupled to reduction of the oxidized minerals (e- acceptor) formed during anoxygenic photosynthesis.
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