Although the biosynthesis of methylmercury in sediments is well established, this is not necessarily the exclusive natural source of methylmercury entering the marine food chain, particularly commercial fish and shellfish species for human consumption. An examination of mercury levels in freshwater fish, collected from a lake with a history of industrial mercury contamination, suggested that levels in fish are controlled in part by mercury in suspension and it followed that methylation should occur in the water column. Although methylmercury is present in seawater in coastal areas receiving discharges of waste containing either inorganic mercury or methylmercury there is no evidence that methylmercury is actually formed in the water column. The authors present data which demonstrate that inorganic mercury can be methylated in the water column and compare this production with that known to occur in marine sediments.
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