Methylmercury (sometimes methyl mercury) is an organometallic cation with the formula [CH3Hg]+. Methylmercury is extremely toxic, and its derivatives are the major source of organic mercury for humans. It is a bioaccumulative environmental toxicant. Structure and chemistry
"Methylmercury" is a shorthand for the hypothetical "methylmercury cation", sometimes written "methylmercury(1+) cation" or "methylmercury(II) cation". This functional group is composed of a methyl group bonded to an atom of  mercury. Its chemical formula is CH3Hg+ (sometimes written as MeHg+). Methylmercury exists as a substituent in many complexes of the type [MeHgL]+ (L = Lewis base) and MeHgX (X = anion).As a positively charged ion it readily combines with anions such as chloride (Cl−), hydroxide (OH−) and nitrate (NO−3). It has particular affinity for sulfur-containing anions, particularly thiols (RS−). Thiols are generated when the amino acid cysteine and the peptide glutathione form strong complexes with methylmercury:
[MeHg]+ + RSH  →   MeHg−SR + H+

Sources
Environmental sources
Methylmercury is formed from inorganic mercury by the action of microbes that live in aquatic systems including lakes, rivers, wetlands, sediments, soils and the open ocean. This methylmercury production has been primarily attributed to anaerobic bacteria in the sediment. Significant concentrations of methylmercury in ocean water columns are strongly associated with nutrients and organic matter remineralization, which indicate that remineralization may contribute to methylmercury production. Direct measurements of methylmercury production using stable mercury isotopes have also been observed in marine waters, but the microbes involved are still unknown.
The mercury applied to lakes was found in young-of-the-year yellow perch within two months, whereas the mercury applied to wetlands and uplands had a slower but longer influx.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 at Grassy Narrows in Ontario, Canada (see Ontario Minamata disease) as a result of mercury released from the mercury-cell Chloralkali process, which uses liquid mercury as an electrode in a process that entails electrolytic decomposition of brine, followed by mercury methylation in the aquatic environment. An acute methylmercury poisoning tragedy occurred also in Minamata, Japan following release of methylmercury into Minamata Bay and its tributaries (see Minamata 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. Dietary sources
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 resulting in its bioaccumulation within these food chains.