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CHEMICAL PROPERTIES Mercury, a silver-white metal that is liquid at room temperature and is higgly volatile, can gist in three oxidatio2,states: elemental mercury (Hg ), mercurous ion (Hg ), and mercuric ion (Hg ). It can be part of both inorganic and organic compounds (EPA 1980; Clarkson et al. 1984; Table 3). All mercury compounds interfere with thiol metabolism, causing inhibition or inactivation of proteins containing thiol ligands and ultimately leading to mitotic disturbances (Das et al. 1982; Elhassani 1983). The mercuric species is the most toxic inorganic chemical form, but all three forg of inorganic Hg may have a common molecular mechanism of damage in which Hg is the toxic species (Clarkson and Marsh 1982; Figure 1). Chemical speciation is probably the most important variable influencing ecotoxicology of Hg, but Hg speciation is difficult, especially in natural environments (Boudou and Ribeyre 1983). Mercury compounds in an aqueous solution are chemically complex. Depending on pH, alkalinity, redox, and other variables, a wide variety of chemical species are liable to be formed, having different electrical charges and s2lubilitiss. For example, HSC12 in solution can speciate into Hg(OH)2, Hg , HgCI , Hg(OH) 9 HgCl3 , and HgCI anionic forms predominate in saline environments (Boudou and Ribeyre 1983. In the aquatic environment, under naturally occurring conditions of pH and temperature, Hg may also become methylated by biological or chemical processes, or both (Beijer and Jernelov 1979; EPA 1980; Ramamoorthy and Blumhagen 1984; Figure 1) -- although abiological methylation is limited (Callister and Winfrey 1986). Methylmercury is the most hazardous mercury species due to its high stability, its lipid solubility, and its possession of ionic properties that lead to a high ability to penetrate membranes in living organisms (Beijer and Jernelov 1979). All mercury discharged into rivers, bays, or estuaries as elemental (metallic) mercury, inorganic divalent mercury, phenylmercury, or alkoxyalkyl mercury can be converted into methylmercury compounds by natural processes (Jernelov 1969). The mercury methylation in ecosystems depends on mercury loadings, microbial activity, nutrient content, pH and redox condition, suspended sediment load, sedimentation rates, and other variables (NAS 1978; Compeau and Bartha 1984; Berman and Bartha 1986; Callister and Winfrey 1986; Jackson 1986). The finding that certain microorganisms are able to convert 7