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Methylmercury (sometimes methyl mercury) is an organometallic cation with the formula [CH3Hg]+. It is a bioaccumulative environmental toxicant.
“Methylmercury” is a shorthand for “monomethylmercury”, and is more correctly “monomethylmercuric cation”. It is composed of a methyl group (CH3-) bonded to a mercury atom; its chemical formula is CH3Hg+ (sometimes written as MeHg+). As a positively charged ion it readily combines with anions such as chloride (Cl-), hydroxide (OH-) and nitrate (NO3-). It also has very high affinity for sulfur-containing anions, particularly the sulfhydryl (-SH) groups on the amino acid cysteine and hence in proteins containing cysteine, forming a covalent bond. More than one cysteine moiety may coordinate with methylmercury[1] and methylmercury may migrate to other metal-binding sites in proteins.[2]
In the past times, methylmercury was produced directly and indirectly as part of several industrial processes such as the manufacture of acetaldehyde. Currently there are few anthropogenic sources of methylmercury pollution other than as an indirect consequence of the burning of wastes containing inorganic mercury and from the burning of fossil fuels, particularly coal. Although inorganic mercury is only a trace constituent of such fuels, their large scale combustion in the United States alone results in release of some 48 tons of elemental mercury to the atmosphere each year. About three times as much additional inorganic mercury is contributed by natural sources such as volcanoes, forest fires and weathering of mercury-bearing rocks.[3] Methylmercury is formed from inorganic mercury by the action of anaerobic organisms that live in aquatic systems including lakes, rivers, wetlands, sediments, soils and the open ocean. It is this phenomenon that resulted in organomercury-poisoning tragedies such as at Minamata, Japan and at Grassy Narrows in Ontario, Canada (see Minamata disease and Ontario Minamata disease. In these cases, the mercury source was the manufacture of chlorine and related chemicals via the mercury-cell Chloralkali process, which uses liquid mercury as an electrode in a process that entails electrolytic decomposition of brine.
Because methylmercury is formed in aquatic systems and because it is not readily eliminated from organisms it is biomagnified in aquatic food chains from bacteria, to plankton, through macroinvertebrates, to herbivorous fish and to piscivorous (fish-eating) fish. At each step in the food chain, the concentration of methylmercury in the organism increases. The concentration of methylmercury in the top level aquatic predators can reach a level a million times higher than the level in the water. This is because methylmercury has a half-life of about 72 days in aquatic organisms, attributing to its bioaccumulation within these food chains. Organisms, including humans,[4] fish-eating birds, and fish-eating mammals such as otters and whales that consume fish from the top of the aquatic food chain receive the methylmercury that has accumulated through this process. Fish and other aquatic species are the only significant source of human methylmercury exposure.
The concentration of mercury in any given fish depends on the species of fish, the age and size of the fish and the type of water body in which it is found. In general, fish-eating fish such as shark, swordfish, marlin, larger species of tuna, walleye, largemouth bass, and chain pickerel have higher levels of methylmercury than herbivorous fish or smaller fish such as tilapia, and herring. Within a given species of fish, older and larger fish have higher levels of methylmercury than smaller fish. Fish that develop in water bodies that are more acidic also tend to have higher levels of methylmercury. Methylmercury has a half-life in human blood of about 50 days.
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