"Methylmercury" is a shorthand for the hypothetical "methylmercury cation", sometimes writtenmethylmercury(1+) cation ormethylmercury(II) cation. Thisfunctional group is composed of amethyl groupbonded to an atom ofmercury. Itschemical formula isCH3Hg+ (sometimes written asMeHg+). The Methylmercury compound has an overall charge of +1, with Hg in the +2oxidation state. Methylmercury exists as a substituent in many complexes of the type[MeHgL]+ (L = Lewis base) and MeHgX (X = anion).[2]
Structure of the complex of methylmercury and cysteine.[4] Color code: dark blue = Hg, yellow = S.
Methylmercury is formed from inorganic mercury by the action of microbes that live in aquatic systems includinglakes,rivers,wetlands,sediments,soils and the openocean.[5] This methylmercury production has been primarily attributed toanaerobic bacteria in the sediment.[6] Capable bacteria that can methylate mercury are mostly thesulfate-reducing bacteria (SRB),[7][8] iron-reducing bacteria (FeRB)[9] andmethanogens.[10][11] Significant concentrations of methylmercury in ocean water columns[12] are strongly associated with nutrients and organic matterremineralization, which indicate that remineralization may contribute to methylmercury production.[13] Direct measurements of methylmercury production using stablemercury isotopes have also been observed in marine waters,[14][15] but the microbes involved are still unknown. Increased methylmercury concentrations in water and fish have been detected after flooding of soils associated withreservoir creation (e.g. forhydroelectric power generation) and inthermokarst wetlands that form afterpermafrost thaw.[14][16][17] The increased methylmercury concentration is due to its ability to bio-accumulate and biο-magnify in aquatic food webs.[18]
There are various sources of inorganic mercury that may indirectly contribute to the production of methylmercury from microbes in the environment. Natural sources of mercury released to the atmosphere includevolcanoes,forest fires, volatilization from the ocean[19] andweathering ofmercury-bearingrocks.[20]Anthropogenic sources of mercury include the burning of wastes containing inorganic mercury and from the burning offossil fuels, particularlycoal. Althoughinorganic mercury is only a trace constituent of such fuels, their large scale combustion in utility and commercial/industrial boilers in theUnited States alone results in release of some 80.2tons (73metric tons) of elemental mercury to theatmosphere each year, out of total anthropogenic mercury emissions in the United States of 158 tons (144 metric tons)/year.[21]
In the past, methylmercury was produced directly and indirectly as part of several industrial processes such as the manufacture ofacetaldehyde. However, currently there are few directanthropogenic sources of methylmercurypollution in the United States.[21]
Whole-lake ecosystem experiments atIISD-ELA inOntario, Canada, showed that mercury falling directly on a lake had the fastest impacts on aquatic ecosystems as opposed to mercury falling on the surrounding land.[22] This inorganic mercury is converted to methylmercury by bacteria. Differentstable isotopes of mercury were added to lakes,wetlands, anduplands, simulating rain, and then mercury concentrations in fish were analyzed to find their source.[23] The mercury applied to lakes was found in young-of-the-yearyellow perch within two months, whereas the mercury applied to wetlands and uplands had a slower but longer influx.[22][23]
Acute methylmercury poisoning can occur either directly from the release of methylmercury into the environment or indirectly from the release of inorganic mercury that is subsequently methylated in the environment. For example, methylmercury poisoning occurred atGrassy Narrows in Ontario, Canada (seeOntario Minamata disease), as a result of mercury released from the mercury-cellChloralkali process, which uses liquid mercury as an electrode in a process that entails electrolytic decomposition of brine, followed bymercury methylation in the aquatic environment. An acute methylmercury poisoning tragedy occurred also inMinamata, Japan, following release of methylmercury intoMinamata Bay and its tributaries (seeMinamata disease). In the Ontario case, inorganic mercury discharged into the environment was methylated in the environment; whereas, in Minamata, Japan, there was direct industrial discharge of methylmercury.
Because methylmercury is formed in aquatic systems, and because it is not readily eliminated from organisms, it isbiomagnified in aquaticfood chains frombacteria, toplankton, throughmacroinvertebrates, toherbivorousfish and topiscivorous (fish-eating) fish.[24][25] At each step in the food chain, theconcentration of methylmercury in the organism increases. The concentration of methylmercury in the top-level aquaticpredators can reach a level a million times higher than the level in the water.[24][25] This is because methylmercury has a half-life of about 72 days in aquatic organisms resulting in itsbioaccumulation within these food chains. Organisms, including humans,[26] fish-eating birds, and fish-eating mammals such asotters andcetaceans (i.e.whales anddolphins) that consume fish from the top of the aquatic food chain receive the methylmercury that has accumulated through this process, plus the toxins in their habitat.[24][25] Fish and other aquaticspecies are the main source of human methylmercury exposure.[24]
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.[24] In general, fish-eating fish such asshark,swordfish,marlin, larger species oftuna,walleye,largemouth bass, andnorthern pike, have higher levels of methylmercury than herbivorous fish or smaller fish such astilapia andherring.[27][28] 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 moreacidic also tend to have higher levels of methylmercury.[24]
Ingested methylmercury is readily and completely absorbed by thegastrointestinal tract. It is mostly found complexed with free cysteine and with proteins andpeptides containing that amino acid. The methylmercuric-cysteinyl complex is recognized by amino acids transporting proteins in the body asmethionine, anotheressential amino acid.[29] Because of this mimicry, it is transported freely throughout the body including across theblood–brain barrier and across theplacenta, where it is absorbed by the developingfetus. Also for this reason as well as its strong binding to proteins, methylmercury is not readily eliminated. Methylmercury has ahalf-life in humanblood of about 50 days.[30]
Several studies indicate that methylmercury is linked to subtle developmental deficits in children exposedin utero such as loss of IQ points, and decreased performance in tests of language skills, memory function and attention deficits.[31] Methylmercury exposure in adults has also been linked to increased risk ofcardiovascular disease includingheart attack.[32][33][34] Some evidence also suggests that methylmercury can causeautoimmune effects in sensitive individuals.[35] There is some evidence suggesting a possible connection between post-natal mercury exposure and autism; however, it is not clear whether methylmercury intake in particular is linked in a similar way.[36][37][38] Although there is no doubt that methylmercury is toxic in several respects, including through exposure of the developing fetus, there is still some controversy as to the levels of methylmercury in the diet that can result in adverse effects. Recent evidence suggests that the developmental andcardiovascular toxicity of methylmercury may be mitigated by co-exposures toomega-3 fatty acids and perhapsselenium, both found in fish and elsewhere.[33][39][40][41][42]
At present, exposures of this magnitude are rarely seen and are confined to isolated incidents. Accordingly, concern over methylmercury pollution is currently focused on more subtle effects that may be linked to levels of exposure presently seen in populations with high to moderate levels of dietary fish consumption. These effects are not necessarily identifiable on an individual level or may not be uniquely recognizable as due to methylmercury. However, such effects may be detected by comparing populations with different levels of exposure. There are isolated reports of various clinical health effects in individuals who consume large amounts of fish;[44] however, the specific health effects and exposure patterns have not been verified with larger, controlled studies.
Many governmental agencies, the most notable ones being theUnited States Environmental Protection Agency (EPA), the United StatesFood and Drug Administration (FDA),Health Canada, and theEuropean Union Health and Consumer Protection Directorate-General, as well as theWorld Health Organization (WHO) and the United NationsFood and Agriculture Organization (FAO), have issued guidance for fish consumers that is designed to limit methylmercury exposure from fish consumption. At present, most of this guidance is based on protection of the developing fetus; future guidance, however, may also address cardiovascular risk. In general, fish consumption advice attempts to convey the message that fish is a good source of nutrition and has significant health benefits, but that consumers, in particular pregnant women, women of child-bearing age, nursing mothers, and young children, should avoid fish with high levels of methylmercury, limit their intake of fish with moderate levels of methylmercury, and consume fish with low levels of methylmercury no more than twice a week.[45][46]
Four vials of larvae ofJordanella after one month in normal water for the first batch, and in water containing 0.6PPB and 1.26PPB and 2.5PPB (parts per billion) of methylmercury for the three bottles at right.
MeHg is a highly toxic compound to wild fish, mammals and birds, and hazardous contaminant that affects the environmental habitat where it is encountered, making it hard to achieve good environmental status as defined by the Water Framework Directive (WFD).
In recent years, there has been increasing recognition that methylmercury affects fish and wildlife health, both in acutely polluted ecosystems and ecosystems with modest methylmercury levels. Two reviews[24][47] document numerous studies of diminished reproductive success of fish, fish-eating birds, and mammals due to methylmercury contamination in aquatic ecosystems.
Reported methylmercury levels in fish, along with fish consumption advisories, have the potential to disrupt people's eating habits, fishing traditions, and the livelihoods of the people involved in the capture, distribution, and preparation of fish as a foodstuff for humans.[48] Furthermore, proposed limits on mercury emissions have the potential to add costly pollution controls on coal-fired utility boilers. Nevertheless, substantial benefits can be achieved globally by introducing mercury emission reduction measures because they reduce human and wildlife exposure to methylmercury.[49]
About 30% of the distributed mercury depositional input is from current anthropogenic sources, and 70% is from natural sources. The natural sources category includes re-emission of mercury previously deposited from anthropogenic sources.[50] According to one study, based on modeled concentrations, pre-Anthropocene tissue-bound levels in freshwater fish may not have differed markedly from current levels in some individual watersheds.[51] However, based on a comprehensive set of global measurements, the ocean contains about 60,000 to 80,000 tons of mercury from pollution, and mercury levels in the upper ocean have tripled since the beginning of the industrial revolution. Higher mercury levels in shallower ocean waters could increase the amount of the toxicant accumulating in food fish, exposing people to a greater risk of mercury poisoning.[52]
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