1. Ecological Importance and Markers

Sulfate occurs widely in seawater, sediment, and water rich in decaying organic material.^[1] Sulfate is also found in more extreme environments such as hydrothermal vents, acid-mine drainage sites, oil fields, and the deep subsurface,^[2] including the world's oldest isolated ground water.^[3][4] Sulfate-reducing microorganisms are common in anaerobic environments where they aid in the degradation of organic materials.^[5] In these anaerobic environments, fermenting bacteria extract energy from large organic molecules; the resulting smaller compounds such as organic acids and alcohols are further oxidized by acetogens and methanogens and the competing sulfate-reducing microorganisms.^[1]

Sludge from a pond; the black color is due to metal sulfides that result from the action of sulfate-reducing microorganisms. https://handwiki.org/wiki/index.php?curid=1633825

The toxic hydrogen sulfide is a waste product of sulfate-reducing microorganisms; its rotten egg odor is often a marker for the presence of sulfate-reducing microorganisms in nature.^[5] Sulfate-reducing microorganisms are responsible for the sulfurous odors of salt marshes and mud flats. Much of the hydrogen sulfide will react with metal ions in the water to produce metal sulfides. These metal sulfides, such as ferrous sulfide (FeS), are insoluble and often black or brown, leading to the dark color of sludge.^[6]

During the Permian–Triassic extinction event (250 million years ago) a severe anoxic event seems to have occurred where these forms of bacteria became the dominant force in oceanic ecosystems, producing copious amounts of hydrogen sulfide.^[7]

Sulfate-reducing bacteria also generate neurotoxic methylmercury as a byproduct of their metabolism, through methylation of inorganic mercury present in their surroundings. They are known to be the dominant source of this bioaccumulative form of mercury in aquatic systems.^[8]

2. Uses

Some sulfate-reducing microorganisms can reduce hydrocarbons, and they have been used to clean up contaminated soils. Their use has also been proposed for other kinds of contaminations.^[9]

Sulfate-reducing microorganisms are considered as a possible way to deal with acid mine waters that are produced by other microorganisms.^[10]

3. Problems Caused by Sulfate-Reducing Microorganisms

In engineering, sulfate-reducing microorganisms can create problems when metal structures are exposed to sulfate-containing water: Interaction of water and metal creates a layer of molecular hydrogen on the metal surface; sulfate-reducing microorganisms then oxidize the hydrogen while creating hydrogen sulfide, which contributes to corrosion.

Hydrogen sulfide from sulfate-reducing microorganisms also plays a role in the biogenic sulfide corrosion of concrete. It also occurs in sour crude oil.^[9]

Some sulfate-reducing microorganisms play a role in the anaerobic oxidation of methane:^[9]

CH₄ + SO₄²⁻ → HCO₃⁻ + HS⁻ + H₂O

An important fraction of the methane formed by methanogens below the seabed is oxidized by sulfate-reducing microorganisms in the transition zone separating the methanogenesis from the sulfate reduction activity in the sediments. This process is also considered a major sink for sulfate in marine sediments.

In hydraulic fracturing, fluids are used to frack shale formations to recover methane (shale gas) and hydrocarbons. Biocide compounds are often added to water to inhibit the microbial activity of sulfate-reducing microorganisms, in order to but not limited to, avoid anaerobic methane oxidation and the generation of hydrogen sulfide, ultimately resulting in minimizing potential production loss.

4. Biochemistry

Before sulfate can be used as an electron acceptor, it must be activated. This is done by the enzyme ATP-sulfurylase, which uses ATP and sulfate to create adenosine 5'-phosphosulfate (APS). APS is subsequently reduced to sulfite and AMP. Sulfite is then further 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.^[11]

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 2.7.7.4); apr and aps are both used to adenosine-5'-phosphosulfate reductase (EC 1.8.4.8); and dsr is the dissimilatory (bi)sulfite reductase (EC 1.8.99.5); https://handwiki.org/wiki/index.php?curid=2048749

The enzyme dissimilatory (bi)sulfite reductase, dsrAB (EC 1.8.99.5), that catalyzes the last step of dissimilatory sulfate reduction, is the functional gene most used as a molecular marker to detect the presence of sulfate-reducing microorganisms.^[12]

5. Phylogeny

The sulfate-reducing microorganisms have been treated as a phenotypic group, together with the other sulfur-reducing bacteria, for identification purposes. They are found in several different phylogenetic lines.^[13] As of 2009, 60 genera containing 220 species of sulfate-reducing bacteria are known.^[9]

Among the Deltaproteobacteria the orders of sulfate-reducing bacteria include Desulfobacterales, Desulfovibrionales, and Syntrophobacterales. This accounts for the largest group of sulfate-reducing bacteria, about 23 genera.^[11]

The second largest group of sulfate-reducing bacteria is found among the Firmicutes, including the genera Desulfotomaculum, Desulfosporomusa, and Desulfosporosinus.

In the Nitrospirae division we find sulfate-reducing Thermodesulfovibrio species.

Two more groups that include thermophile sulfate-reducing bacteria are given their own phyla, the Thermodesulfobacteria and Thermodesulfobium.

There are also three known genera of sulfate-reducing archaea: Archaeoglobus, Thermocladium and Caldivirga. They are found in hydrothermal vents, oil deposits, and hot springs.

In July 2019, a scientific study of Kidd Mine in Canada discovered sulfate-reducing microorganisms living 7,900 feet (2,400 m) below the surface. The sulfate reducers discovered in Kidd Mine are lithotrophs, obtaining their energy by oxidizing minerals such as pyrite rather than organic compounds.^[14][15][16] Kidd Mine is also the site of the oldest known water on Earth.^[17]