The chemical cycle and bioaccumulation of mercury

Because it is very toxic and accumulates in organisms, particularly in fish , mercury is an important pollutant and one of the most studied. Nonetheles s we still have an incomplete understanding of the factors that control the bioconcentration of mercury. Elemental mercury is efficiently transported as a gas around the globe, and even remote areas show evidence of mercury p ollution originating from industrial sources such as power plants. Besides elemental mercury, the major forms of mercury in water are ionic mercury (w hich is bound to chloride, sulfide, or organic acids) and organic mercury, particularly methylmercury. Methylmercury rather than inorganic mercury is bioconcentrated because it is better retained by organisms at various level s in the food chain. The key factor determining the concentration of mercur y in the biota is the methylmercury concentration in water, which is contro lled by the relative efficiency of the methylation and demethylation proces ses. Anoxic waters and sediments are an important source of methylmercury, apparently as the result of the methylating activity of sulfate-reducing ba cteria. In surface waters, methylmercury may originate from anoxic layers o r be formed through poorly known biological or chemical processes. Demethyl ation is effected both photochemically and biologically.