The 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 to H2S.
  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 precipitates.