Copyright 1999, ASM Digital Image Collection. Terry
Some major steps in the sulfur cycle include:
Assimilative reduction of sulfate (SO4=)
into -SH groups in proteins.
Release of -SH to form H2S during
excretion, decomposition, and desulfurylation.
Oxidation of H2S by chemolithotrophs
to form sulfur (So) and sulfate (SO4=)
Dissimilative reduction of sulfate (SO4=)
by anaerobic respiration of sulfate-reducing bacteria.
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.