The essential steps of the sulfur cycle are:
- Mineralization of organic sulfur to the inorganic form, hydrogen sulfide: (H2S).
- Oxidation of sulfide and elemental sulfur (S) and related compounds to sulfate (SO42–).
- Reduction of sulfate to sulfide.
- Microbial immobilization of the sulfur compounds and subsequent incorporation into the organic form of sulfur.
These are often termed as follows:
- Assimilative sulfate reduction (see also sulfur assimilation) in which sulfate (SO42–) is reduced to organic sulfhydryl groups (R–SH) by plants, fungi and various prokaryotes. The oxidation states of sulfur are +6 in sulfate and –2 in R–SH.
- Desulfuration in which organic molecules containing sulfur can be desulfurated, producing hydrogen sulfide gas (H2S), oxidation state = –2. Note the similarity to deamination.
- Oxidation of hydrogen sulfide produces elemental sulfur (So), oxidation state = 0. This reaction is done by the photosynthetic green and purple sulfur bacteria and some chemolithotrophs.
- Further oxidation of elemental sulfur by sulfur oxidizers produces sulfate.
- Dissimilative sulfur reduction in which elemental sulfur can be reduced to hydrogen sulfide.
- Dissimilative sulfate reduction in which sulfate reducers generate hydrogen sulfide from sulfate.
Human impact on the sulfur cycle is primarily in the production of sulfur dioxide (SO2) from industry (e.g. burning coal) and the internal combustion engine. Sulfur dioxide can precipitate onto surfaces where it can be oxidized to sulfate in the soil (it is also toxic to some plants), reduced to sulfide in the atmosphere, or oxidized to sulfate in the atmosphere as sulfuric acid, a principal component of acid rain.