Mercury is one of the most hazardous contaminants that may be present in the aquatic environment, but its ecological and toxicological effects are strongly dependent on the chemical species present. Species distribution and transformation processes in natural aquatic systems are controlled by various physical, chemical, and biological factors. Depending on the prevailing environmental conditions, inorganic mercury species may be converted to many times more toxic methylated forms such as methylmercury, a potent neurotoxin that is readily accumulated by aquatic biota. Despite a considerable amount of literature on the subject, the behavior of mercury and many of the transformation and distribution mechanisms operating in the natural aquatic environment are still poorly understood. This review examines the current state of knowledge on the physicochemical behavior of mercury in the aquatic environment, and in particular the environmental factors influencing its transformation into highly toxic methylated forms.

Mercury in the Aquatic Environment: A Review of Factors Affecting Methylation

Substrate-electron acceptor combinations and specific metabolic inhibitors were applied to anoxic saltmarsh sediment spiked with mercuric ions (Hg2+) in an effort to identify, by a direct approach, the microorganisms responsible for the synthesis of hazardous monomethylmercury. 2-Bromoethane sulfonate (30 mM), a specific inhibitor of methanogens, increased monomethylmercury synthesis, whereas sodium molybdate (20 mM), a specific inhibitor of sulfate reducers, decreased Hg2+ methylation by more than 95%. Anaerobic enrichment and isolation procedures yielded a Desulfovibrio desulfuricans culture that vigorously methylated Hg2+ in culture solution and also in samples of presterilized sediment. The Hg2+ methylation activity of sulfate reducers is fully expressed only when sulfate is limiting and fermentable organic substrates are available. To date, sulfate reducers have not been suspected of Hg2+ methylation. Identification of these bacteria as the principal methylators of Hg2+ in anoxic sediments raises questions about the environmental relevance of previous pure culture-based methylation work.

Sulfate-Reducing Bacteria: Principal Methylators of Mercury in Anoxic Estuarine Sediment

Marine biogeochemical cycling of mercury.

Metallothioneins in metal regulation and toxicity in aquatic animals

James G. Wiener, David P. Krabbenhoft, Gary H. Heinz, and Anton M. ScheuhammerCONTENTS16.1 Introduction 409
16.2 Evolution of the Environmental Mercury Problem 410
16.3 Global-Scale Environmental Cycling and Fate 413
16.4 Mercury Speciation and Environmental Concentrations 415

Ecotoxicology of Mercury

Methylation of mercury by humic substances in an aquatic environment

Separation and purification of (Cd, Cu, Zn)-metallothionein in carp hepato-pancreas.

Mercury methylation by compounds in humic material

Protection by metallothionein against cadmium toxicity.

The sediments of Nagara river and its bystreams were extracted with ether. The mutagenicities of the extracts were determined by the Ames test with Salmonella typhimurium TA100 with S9 mix. The extracts were fractionated on an activated silica-gel column with four kinds of organic solvent, namely iso-octane, iso-octane:benzene (1:1), benzene:ethyl acetate (1:1) and benzene:methanol (1:1). The highest mutagenicity was observed in the iso-octane-benzene fraction of Arata river by using TA100 with S9 mix. Chemical substances in this fraction were identified by GC-MS and GC, and several kinds of polycyclic aromatic hydrocarbons (PAH) were found. Several peaks were unidentified, but these PAH, especially benzo [b] fluoranthene and benzo [a] pyrene, may contribute substantially to the mutagenicity of this fraction.

Tetsuya Ishikawa

Papers

Methylation of mercury by humic substances in an aquatic environment

Separation and purification of (Cd, Cu, Zn)-metallothionein in carp hepato-pancreas.

Mercury methylation by compounds in humic material

Protection by metallothionein against cadmium toxicity.

Mutagenicity of Nagara River sediment