Animation of the Sulfur Cycle

Copyright 1998, Thomas M. Terry, The University of Conn.

Sulfur cycle animated gif

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.