Mercury undergoes global cycling via reactions catalyzed largely by prokaryotes. Mercury (II) is toxic to most living things (ToxFAQs: Mercury) because of its avid coordination by thiol groups within biological systems. Some prokaryotes protect themselves against mercury (II) by reducing it to mercury (0) via the enzyme mercuric reductase (reviewed by Miller, 1999). Mercury (0) is volatile and relatively non-toxic. Volatile mercury is distributed globally and can be reoxidized abiotically by ozone or enzymatically by peroxidases (e. g. catalase) (Smith et al, 1998). Mercury (II) is known to undergo biological methylation to methylmercury and dimethylmercury (Siciliano and Lean, 2002), both of which are extremely toxic to humans. The enzyme organomercurial lyase catalyzes the conversion of methylmercury species to methane and mercury (II) (Begley et al, 1986).

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Essa AM, Macaskie LE, Brown NL. Mechanisms of mercury bioremediation. Biochem Soc Trans. 2002;30:672-74.