diagram on the right is a simplified version of the pathways, transformations,
and chemical species in a marine mat sulfur cycle. Thickness of
the arrow depicts relative abundance.
1) Sulfate that diffuses into a mat is reduced
to hydrogen sulfide (H2S) by sulfate-reducing bacteria (SRB).
2) Some sulfate is assimilated by organisms to form
cell components such as amino acids and cofactors.
3) Upon mineralization, organic sulfur is converted
4) H2S is transformed to elemental sulfur and
5) then to SO42- by sulfide oxidizing bacteria such
as Beggiatoa sp.
6,7) Anoxygenic phototrophic bacteria also convert
H2S to SO42- via elemental sulfur.
8) Elemental sulfur may be transfomred back to H2S
by sulfur reducing bacteria such as Desulfuromonas spp.
9) Some H2S complexes with iron to form black FeS