Sulfate-reducing bacteria are those bacteria and archaea that can obtain energy by oxidizing organic compounds or molecular hydrogen while reducing sulfate to hydrogen sulfide. In a sense, these organisms breathe sulfate rather than oxygen in a form of anaerobic respiration, many bacteria reduce small amounts of sulfates in order to synthesize sulfur-containing cell components, this is known as assimilatory sulfate reduction. By contrast, the sulfate-reducing bacteria considered here reduce sulfate in large amounts to obtain energy and expel the resulting sulfide as waste and they use sulfate as the terminal electron acceptor of their electron transport chain. Most of them are anaerobes, however there are examples of sulfate-reducing bacteria that are tolerant of oxygen, under oxygenated conditions these bacteria switch to aerobic respiration before reducing sulfate. Most sulfate-reducing bacteria can reduce other oxidized inorganic sulfur compounds, such as sulfite, thiosulfate. In addition, there are sulfate-reducing bacteria that can reduce fumarate, nitrate and nitrite, iron and some metals, dimethyl sulfoxide. Sulfate occurs widely in seawater, sediment, or water rich in decaying organic material, sulfate-reducing bacteria are common in anaerobic environments where they aid in the degradation of organic materials. The toxic hydrogen sulfide is a product of sulfate-reducing bacteria. Sulfate-reducing bacteria are responsible for the odors of salt marshes. Much of the hydrogen sulfide will react with metal ions in the water to produce metal sulfides and these metal sulfides, such as ferrous sulfide, are insoluble and often black or brown, leading to the dark color of sludge. Some sulfate-reducing bacteria can reduce hydrocarbons such as benzene, toluene, ethylbenzene, and xylene and their use has also been proposed for other kinds of contaminations. Sulfate-reducing bacteria are considered as a way to deal with acid mine waters that are produced by other bacteria. Hydrogen sulfide from sulfate-reducing bacteria also plays a role in the biogenic sulfide corrosion of concrete and it also occurs in sour crude oil. This process is considered a major sink for sulfate in marine sediments. Ultimately resulting in minimizing potential production loss, before sulfate can be used as an electron acceptor, it must be activated. This is done by the enzyme ATP-sulfurylase, which uses ATP, APS is subsequently reduced to sulfite and AMP. Sulfite is then reduced to sulfide, while AMP is turned into ADP using another molecule of ATP. The overall process, thus, involves an investment of two molecules of the energy carrier ATP, which must to be regained from the reduction, the sulfate-reducing bacteria have been treated as a phenotypic group, together with the other sulfur-reducing bacteria, for identification purposes
Sludge from a pond; the black color is due to metal sulfides that result from the action of sulfate-reducing microorganisms.
Overview of the three key enzymatic steps of the dissimilatory sulfate reduction pathway. Enzymes: sat and atps respectively stand for sulfate adenylyltransferase and ATP sulfurylase (EC 22.214.171.124); apr and aps are both used to adenosine-5'-phosphosulfate reductase (EC 126.96.36.199); and dsr is the dissimilatory (bi)sulfite reductase (EC 188.8.131.52);