Microbiological disproportionation of inorganic sulfur compounds

The disproportionation of inorganic sulfur intermediates at moderate temperatures (0–80 °C) is a microbiologically catalyzed chemolithotrophic process in which compounds like elemental sulfur, thiosulfate, and sulfite serve as both electron donor and acceptor, and generate hydrogen sulfide and sulfate. Thus the overall process is comparable to the fermentation of organic compounds such as glucose and is consequently often described as ‘inorganic fermentation’. The process is primarily carried out by microorganisms with phylogenetic affiliation to the so called sulfate-reducing bacteria within the delta subclass of Proteobacteria. The organisms grow with sulfate as their external electron acceptor and low-molecular weight organic compounds or hydrogen as energy sources. Studies of the biochemistry of a few isolates indicate that the disproportionating microbes reverse the sulfate reduction pathway during disproportionation. However, investigations with elemental sulfur disproportionating bacteria present evidence for an alternative pathway involving the enzyme sulfite-oxidoreductase, an enzyme that has hitherto only been reported participating in the oxidation of sulfite in aerobic or phototrophic sulfide oxidizers. Investigations bridging geology and microbiology have found strong evidence for disproportionating bacteria participating in and enhancing the rate at which pyrite forms and being partly responsible for the isotopic signatures of sulfidic minerals in recent and old sediments. New results indicate that elemental sulfur disproportionating microbes can be traced back in time as long as 3.5 billion years and elemental sulfur disproportionation would thus be one the oldest biological processes on Earth.