Studies on microbial synthesis and decomposition of organomercury compounds.

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The studies conducted at Kumamoto University concluded that the Minamata disease is caused by methyl mercury. In the course of a nationwide survey made thereafter on environmental pollution by mercury, methyl mercury was detected in fish and human hairs which were supposedly free from artificial contamination by mercury. In particular, 60 to 70% of mercury found in tuna caught in oceans has been reported to be in the form of methyl mercury. An abnormally high mercury content in fish has also been reported in Sweden. Westoo carried out analyses of mercury in fish and other foodstuffs and claimed that 80 to 100% of mercury is present as methyl mercury.To elucidate the origin of such methyl mercury in nature and the effects of methyl mercury on mankind from the standpoint of public health, microbiological and ecological studies were undertaken on microbial conversion of inorganic mercury into organic mercury and of a process of the biological chain. A mercury-resistant bacterium belonging to Pseudomonas was separated from sewage water and biological synthesis and decomposition of organic mercury by this bacterium were investigated. Following this, microorganisms growing in a zone of mercury deposits, soils, rivers and seas and certain eumycetes were found to possess the ability to synthesize organic mercury. Hence, the possibility of conversion of inorganic mercury into organic mercury in nature was recognized. Furthermore, the extent of synthesis of organic mercury and the extent of accumulation of methyl mercury in fish by way of biological concentration were studied with the aid of ecological means. The microbially synthesized methyl mercury is concentrated by the food chain and an unusually high mercury content is sometimes found in fish. However, accumulation of methyl mercury in fish seldom proceeds to such a high level as to cause mercury poisoning, unless continual flow of methyl mercury in abnormally large quantities is present, as has been known in the case of the Minamata disease. Therefore, detection of an abnormally large amount of mercury in fish or shellfish suggests artificial contamination by mercury in one way or another, and a countermeasure must be worked out.