Figure 2: Sulphur transformations. Sulphate-reducing bacteria have a key role in the sulphur cycle. They use sulphate (SO42-) as a terminal electron acceptor in the degradation of organic matter, which results in the production of hydrogen sulphide (H2S). Subsequently, the sulphide can be oxidized aerobically by chemolithotrophic sulphur-oxidizing bacteria (for example, Thiobacillus or Beggiatoa spp.) or anaerobically by phototrophic sulphur bacteria (for example, Chlorobium spp.) to elemental sulphur (S°) and SO42-. Other transformations, which are carried out by specialized groups of microorganisms, result in sulphur reduction (for example, Desulfuromonas spp.) and sulphur disproportionation (Desulfovibrio sulfodismutans). Organic sulphur compounds, such as dimethylsulphoxide (DMSO) can be transformed into dimethylsulphide (DMS) and vice versa by several groups of microorganisms. SH, sulfhydryl. Figure modified, with permission, from Ref. 148(2006) Pearson Education.