Diversity of Sulfur-Oxidizing and Sulfur-Reducing Microbes in Diverse Ecosystems

Sulfur (S) is one of the most important elements, of which the organosulfur compounds and/or metal sulfides are considered essential for life. Microbial sulfur oxidation and reduction are the most active and ancient metabolic processes in S cycle that operate in diverse ecosystems. This process is carried out by sulfur-oxidizing (SOB) and sulfur-reducing bacteria (SRB) in all ecosystems and considered as key phenomenon in sulfur biogeochemical cycling. Usually, on the basis of nutrition, SOB and SRB are categorized as lithoautotrophs. SOB oxidize the reduced sulfur compounds such as hydrogen sulfide (H2S), elemental sulfur (S0), sulfite (SO3 −2), thiosulfate (S2O3 2−), and various polythionates (SnO6 2− or -SnO6-) into sulfate (SO4 −2). On the contrary, SO4 −2 can serve as an electron acceptor of SRB under anaerobic condition, and they reduce the SO4 −2 and other oxidized sulfur compounds (S2O3 2−, SO3 −2, S0) into H2S. In natural system, SRB reduce the SO4 −2 in two different reduction processes, viz, dissimilatory and assimilatory reactions. In dissimilatory reaction, SRB utilize three kinds of enzymes ( ATP sulfurylase, APS reductase, and sulfite reductase ) to reduce the S substrate, whereas the sulfate is assimilated or incorporated into organic compounds under assimilatory process through S substrate reduction. In recent years, molecular methods have emerged as essential tools for a better understanding of the microbial role in S transformation under various habitats. Keeping the importance of microbial-mediated S oxidation and reduction in biogeochemical cycle of S, the present chapter describes the role of key functional microbial genes in S transformation such as genes involved in S oxidation (sox, aps, asf, and sor) and reduction (dsr) and also discusses in detail about the abundance, diversity, and impact of these in diverse ecosystems.