Hydrogen sulfide: a toxic gas produced by dissimilatory sulfate and sulfur reduction and consumed by microbial oxidation.

Sulfur is an essential element for the synthesis of cysteine, methionine, and other organo-sulfur compounds needed by living organisms. Additionally, some prokaryotes are capable of exploiting oxidation or reduction of inorganic sulfur compounds to energize cellular growth. Several anaerobic genera of Bacteria and Archaea produce hydrogen sulfide (H2S), as a result of using sulfate (SO(4)(2 -) ), elemental sulfur (S(0)), thiosulfate (S₂O(3)(2 -)), and tetrathionate (S(4)O(6)(2 -)) as terminal electron acceptors. Some phototrophic and aerobic sulfur bacteria are capable of using electrons from oxidation of sulfide to support chemolithotrophic growth. For the most part, biosulfur reduction or oxidation requires unique enzymatic activities with metal cofactors participating in electron transfer. This review provides an examination of cytochromes, iron-sulfur proteins, and sirohemes participating in electron movement in diverse groups of sulfate-reducing, sulfur-reducing, and sulfide-oxidizing Bacteria and Archaea.