Mercury methylation in an aquatic environment.

THE HIGH TOXICITY OF MERCURY COM POUNDS has long been known,1-2 but their hazardousness as pollutants of aquatic environments became a matter of concern only after the deaths from poisoning at Minamata Bay, Japan.3'4 The cause was traced to the presence of effluents contain ing methylmercuric chloride, and Kurland 3 suggested the possibility of microbial trans formation of mercury to methylmercury. The experimental support for this hypoth esis was furnished by Jensen and Jernelov,5 who showed that inorganic mercury was converted to monomethyland dimethyl mercury in aquarium sediments. The microbial methylation of mercury is the most hazardous pathway through which mercury contributes towards en vironmental pollution because methyl mercury concentrates progressively in aquatic organisms from minute concentra tions in water.6 Once in living organisms, it circulates in the blood unchanged, with a very slow rate of elimination.7 More over, methylmercury is able to penetrate the blood-brain barrier resulting in severe damage to nerve cells even at very low concentrations.8 Earlier work suggests that approximately 95 percent of mercury found in fish is methylmercury,9 although recent work indicates that in some cases, the levels may be considerably lower. Thus, fish, as an important dietary source, form the major route of methylmercury transfer to higher trophic levels. Figure 1 schematically illustrates the biotransforma tion of mercury in sediments as described by Jernelov.10 The environmental levels of mercury and methylmercury in a variety of fish, and also the accumulation of mercury by fish in aquaria, have been the subject of con siderable study. Yet, there is no published data on the relative methylating capacities of different mercury-contaminated sedi ments, although work has been in progress in Sweden for some time.10