in decaying organic material. Sulfate is also found in more extreme environments such as hydrothermal vents, acid mine drainage sites, oil fields, and the deep subsurface, 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. 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. 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. Sulfate-reducing microorganisms are considered a possible way to deal with acid mine waters that are produced by other microorganisms. 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. Some sulfate-reducing microorganisms play a role in the anaerobic oxidation of methane: 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