The sulfur cycle, like the biogeochemical cycles of other elements, includes a series of oxidation and reduction stages of sulfur that are principally biotic and performed by microorganisms. Under certain pHs and redox potentials, some reactions of the sulfur cycle take place in the absence of microorganisms.

The sulfur cycle integrates and interconnects the different components of an ecosystem (soil, rock, sediment, aquatic phase, and atmosphere). Sulfur is present as organic sulfur (in amino acids, coenzymes, etc.) and inorganic sulfur: sulfides (S²⁻), elemental sulfur (S⁰), sulfates (S⁶⁺), and a series of intermediaries of minor environmental relevance. Sulfate (SO₄²⁻) is the major bioavailable form in nature, and is particularly abundant in marine environments, because elemental sulfur and hydrogen sulfide (H₂S) are of biological or geothermic origin and metal sulfides (MeS) are insoluble.

The sulfur cycle is complex, involving aerobic and anaerobic, auto- and...