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Microorganisms that "breathe" sulfates

Sulfate reducing microorganisms (SRM) or sulfate reducing prokaryotes (SRP) are a group composed of sulfate reducing bacteria (SRB) and sulfate reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate (SO42 ) as terminal electron acceptor, reducing it to hydrogen sulfide (H2S). Therefore, these sulfidogenic microorganisms "breathe" sulfate rather than molecular oxygen (O2), which is the terminal electron acceptor reduced to water (H2O) in aerobic respiration. Most sulfate reducing microorganisms can also reduce some other oxidized inorganic sulfur compounds, such as sulfite (SO32 ), dithionite , thiosulfate , trithionate , tetrathionate , elemental sulfur (S8), and polysulfides (Sn2 ). Other than sulfate reduction, some sulfate reducing microorganisms are also capable of other reactions like disproportionation of sulfur compounds. Depending on the context, "sulfate reducing microorganisms" can be used in a broader sense (including all species that can reduce any of these sulfur compounds) or in a narrower sense (including only species that reduce sulfate, and excluding strict thiosulfate and sulfur reducers, for example). Sulfate reducing microorganisms can be traced back to 3.5 billion years ago and are considered to be among the oldest forms of microbes, having contributed to the sulfur cycle soon after life emerged on Earth. 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 microorganisms that are tolerant of oxygen, and some of them can even perform aerobic respiration. No growth is observed when oxygen is used as the electron acceptor. In addition, there are sulfate reducing microorganisms that can also reduce other electron acceptors, such as fumarate, nitrate (NO3 ), nitrite (NO2 ), ferric iron (Fe3+), and dimethyl sulfoxide (DMSO). In terms of electron donor, this group contains both organotrophs and lithotrophs. The organotrophs oxidize organic compounds, such as carbohydrates, organic acids (such as formate, lactate, acetate, propionate, and butyrate), alcohols (methanol and ethanol), aliphatic hydrocarbons (including methane), and aromatic hydrocarbons (benzene, toluene, ethylbenzene, and xylene). The lithotrophs oxidize molecular hydrogen (H2), for which they compete with methanogens and acetogens in anaerobic conditions.

Sulfate occurs widely in seawater, sediment, and water rich in decaying organic material. including the world's oldest isolated ground water. Sulfate reducing microorganisms are common in anaerobic environments where they aid in the degradation of organic materials. 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. 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.

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.

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. As of 2009, 60 genera containing 220 species of sulfate reducing bacteria are known. The second largest group of sulfate reducing bacteria is found among the Bacillota, including the genera Desulfotomaculum, Desulfosporomusa, and Desulfosporosinus. In the Nitrospirota phylum we find sulfate reducing Thermodesulfovibrio species. Two more groups that include thermophilic sulfate reducing bacteria are given their own phyla, the Thermodesulfobacteriota 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 7900 feet (2408 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. Kidd Mine is also the site of the oldest known water on Earth.