Sulfate reduction

The bioconversion of sulfate to hydrogen sulfide and organic sulfur compounds is a crucial part of the biogeochemical sulfur cycle.  Bacteria such as Desulfovibrio and Desul¬ fotignum perform this function along with a single genus in the Archaea (Archaeoglobus). These organisms use sulfate (SO₄^2-) as a source of energy and as a source of sulfur although little energy can be derived from the reduction assimilatory and dissimilatory reactions can be linked to the use of hydrogen, pyruvate or lactate as electron donor. Dissimilation of sulfate uses ATP and proceeds via an intermediate known as APS. A further reduction produces sulfite (SO₃^2-) and the release of adenosine monophosphate. A final reduction yields hydrogen sulfide, which is then excreted to be used by other micro-organisms or released into the atmosphere. Bacteria can assimilate the sulfur by a further ATP-consuming reaction to produce PAPS (phosphoadenosine-5′-phosphosulfate). Reduction to sulfite is this time accomplished by using NADPH. Assimilation into com- pounds such as the amino acid cysteine is again via hydrogen sulfide in Figure .

Sulfur itself can act as an electron acceptor and is reduced to hydrogen sulfide directly through many Bacteria including species of Desulfuromonas, Wolinella and Campylobacter when growing anaerobically. A broad variety of electron donors are used and the bioenergetics of the pathway is still not fully understood.

Figure .  Microbial sulfate reduction. H2ase, hydrogenase; Hmc, hexadecaheme cytochrome; Cyt c3, cytochrome c3; FeS, iron-sulfur protein.