Methyl mercury, biological effects

It has been known since 1970 that inorganic mercury contained in sediments could be converted into methyl mercury compounds by biological processes (Wood et al. 1968, Jensen and Jernelov 1969). The significance of this process is still unclear however, it seems from recent reports (Billen 1974, Lindberg et al. 1975) that the rates of methylation are very low and are likely to be balanced by demethylation reactions. Thus, the amount of methyl mercury compounds present in natural waters is likely to be very small and of minor importance in terms of toxic effects to aquatic organisms. This is borne out by the fact that methyl mercury has never been detected in natural waters. 

Mercury is essentially nontoxic in its elemental form (Hg ). In the absence of any chemical or biological system that chemically alters Hg , it can be consumed orally without any significant side effects. However, once Hg is chemically modified to the ionized, inorganic species, Hg T it becomes toxic. Further bioconversion to an alkyl Hg, such as methyl Hg (CHaHg ), yields a very toxic species of Hg that is highly selective for lipid-rich tissue, such as the neuron. The relative order of toxicity is as follows ... 

The biological methylation of mercury (e.g., from weathering, volcanism, fossil fuels, chloralkali electrolysis) is effected by microorganisms that utilize methylco-balamin (2c) see Section 5.1.2. 

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