Sulfur Cycle

The sulfur cycle is the biogeochemical cycle by which sulfur moves between rocks, waterways, and living organisms, a movement significant both geologically and biologically because sulfur is an essential component of proteins and cofactors and also serves microorganisms as an oxidant or reductant in respiration. Its major reservoirs include oceanic sulfate, the largest sink and an electron acceptor for microbes in oxygen-poor waters; sedimentary rocks such as pyrite-rich shales, evaporites, and carbonate-associated sulfate; and short-lived atmospheric forms such as dimethylsulfide and sulfur dioxide. The cycle moves sulfur through mineralization, which converts organic sulfur into inorganic forms like hydrogen sulfide; oxidation, which converts sulfides and elemental sulfur to sulfate; reduction, which converts sulfate back to sulfide by both biological and thermochemical routes; and incorporation, which builds sulfur into the organic compounds of living things.