Sulfate-reducing bacteria

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. 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 other oxidized inorganic sulfur compounds, such as sulfite (SO32–), dithionite (S2O42–), thiosulfate (S2O32–), trithionate (S3O62–), tetrathionate (S4O62−), elemental sulfur (S8), and polysulfides (Sn2−). 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. Many organisms 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 microorganisms considered here reduce sulfate in large amounts to obtain energy and expel the resulting sulfide as waste; this is known as dissimilatory sulfate reduction. 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 , and dimethyl sulfoxide. In terms of electron donor, this group contains both organotrophs and lithotrophs. The organotrophs oxidize organic compounds, such as carbohydrates, organic acids (e.g., 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. Some sulfate-reducing microorganisms can directly utilize metallic iron as electron donor, oxidizing it to ferrous iron . Sulfate occurs widely in seawater, sediment, or water rich in decaying organic material. Sulfate-reducing microorganisms are common in anaerobic environments where they aid in the degradation of organic materials. 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. 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. 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. 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. 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.