BACTERIAL SULFUR REDUCTION IN HOT VENTS

Elemental sulfur can be reduced through different microbial processes, including catabolically significant sulfur respiration and reduction of sulfur in the course of fermentation. Both of these processes are found in thermophilic microorganisms inhabiting continental and submarine hot vents, where elemental sulfur is one of the most common sulfur species. Among extreme thermophiles, respresented mainly by Archaea, sulfur-respiring bacteria include hydrogen-utilizing lithoautotrophs and heterotrophs, oxidizing complex organic substrates. Some marine heterotrophic sulfur-reducing Archaea were found to ferment peptides and polysaccharides, using elemental sulfur as an electron sink and thus avoiding the formation of molecular hydrogen which is highly inhibiting. Moderately thermophilic communities contain eubacterial sulfur reducers capable of lithotrophic and heterotrophic growth. Total mineralization of organic matter is carried out by a complex microbial system consisting of fermentative heterotrophs, which use elemental sulfur as an electron sink, and sulfur-respiring bacteria of the genus Desulfurella, which oxidize other fermentation products, yielding only CO2 and H2S. The most remarkable thermophilic microbial community is the thermophilic cyanobacterial mat found in the Uzon caldera, Kamchatka, which contains elemental sulfur among the layers. Organic matter produced by the thermophilic Oscillatoria is completely and rapidly mineralized by means of sulfur reduction.