Animation of the Sulfur Cycle

• Some major steps in the sulfur cycle include:
  1. Assimilative reduction of sulfate (SO₄⁼) into -SH groups in proteins.
  2. Release of -SH to form H₂S during excretion, decomposition, and desulfurylation.
  3. Oxidation of H₂S by chemolithotrophs to form sulfur (S^o) and sulfate (SO₄⁼)
  4. Dissimilative reduction of sulfate (SO₄⁼) by anaerobic respiration of sulfate-reducing bacteria.
  5. Anerobic oxidation of H₂S and S by anoxygenic phototrophic bacteria (purple and green bacteria)
• The sulfur cycle includes more steps than are shown here. Sulfur compounds undergo some interconversions due to chemical and geologic processes (not shown here). In addition, a number of organic sulfur compounds accumulate in significant amounts, especially in marine environments.
• For example, about 45 tons of dimethyl sulfide are produced annually by degradation of dimethylsulfonium propionate, a chemical produced by marine algae for osmoregulation. This is gradually broken down by a variety of biotic and abiotic mechanisms.