Sulfur cycle

• Mineralization of hydrogen sulfide: (H₂S).
• sulfate (SO₄^2–).
• 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 (SO₄^2–) 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 (S^o), 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 acid rain.