Elemental mercury may be oxidized to Hg2þ, which can then become methylated into
methylmercury compounds by chemical or microbiological reactions in the aquatic environment.
The intestinal bacterial flora of various animal species, including fish, is also, though
to a much lower degree, able to convert ionic mercury into methyl mercuric compounds
(Nielsen, 1992).Methylmercury is accumulated by fish andmarine mammals and it attains its
highest concentrations in large predatory species at the top of the aquatic food chain. By this
means, it enters the human diet. Certain microorganisms can demethylate methylmercury, for
example, in thegut,while others can reduceHg2þ toHg0.Thus,microorganisms are believed to
play an important role in the fate of mercury in the environment and in affecting human
exposure.