Since the late 1960s it has been suspected that anoxic aquatic sediments represented the sites where inorganic forms of mercury underwent methylation by resident bacteria, but it was not clear at the time which group of microbes were responsible. Because methanogenic bacteria are important members of anaerobic food webs and that they contain abundant B-12 coenzymes, it was reasoned that methylcobalamin-mediated reactions in methanogens were responsible for the biomethylation of Hg (II) in nature. Indeed, because this reaction was demonstrated in vitro with cell extracts the concept that methanogens were important mercury methylators in the environment was the dominant paradigm in the literature for over a decade. However, mercury methylation could not be found in whole cells (in vivo) of methanogens or in assays of with strongly methanogenic systems, such as sewage sludges. Subsequent investigations with marine sediments revealed a capacity for biogenesis of methylmercury (MeHg) and that activity was abolished by molybdate, an inhibitor of sulfate-reducing bacteria (SRBs). It has now been well-established that certain types of SRBs form MeHg via cobalamin-linked reactions, and that this occurs in both freshwater and marine sediments. In situ measurements of sediment Hg (II) methylation have been refined by employing high-purity radioassays with 203Hg (II), thereby allowing for methylation activity to be assessed at near-ambient levels of Hg (II). In contrast to the methylation reaction, much more is known about the mechanisms for bacterial demethylation of MeHg. MeHg represents a potent microbial toxin which must be neutralized in order for bacteria to grow. In some bacteria the presence of MeHg activates the mer operon which is composed, in part, of mer A and mer B genes. The mer B gene encodes for the formation of an organomercurial lyase enzyme which cleaves MeHg into methane and Hg (II). The mer A component codes of the formation of mercuric reductase which by volatilization removes Hg by reducing Hg (II) to Hg (0). Although the mer operon is contained on a plasmid and can be inserted into various bacteria, cells which have mer B do not necessarily have mer A. Studies with freshwater systems have detected demethylation activity as well as the presence of mer operon genes in the resident bacterial populations. However, it is not clear if mer operon-mediated reactions are the dominant mechanism of MeHg demethylation in anoxic sediments. Assessments of both methylation and demethylation capacity of such systems is necessary in order to determine whether a net methylation (or demethylation) occurs. The employment of high purity Me203Hg will facilitate such determinations.

A second mechanism for microbial demethylation was recently discovered whereby MeHg mimics a 1-carbon organic substrate, such as methanol. This process is termed oxidative demethylation, and was discovered in sediment incubations with 14C-MeHg. Instead of 14C-methane, 14C-carbon dioxide was found to be the major product, especially in systems which harbored active populations of sulfate reducers. However, 14C-methane was also an important endproduct (in addition to 14C-carbon dioxide) in sediments which had high methanogenic activity. The addition of molybdate to inhibit sulfate-reducers or of bromoethanesulfonic acid to inhibit methanogens shifted the ratio of CO2/CH4 formed from MeHg. These results indicated that both methanogens and sulfate-reducers are involved in oxidative demethylation in nature. Highest potential activity was found to occur in systems having extensive mercury-contamination from mine tailing point sources. This was shown to occur in the Carson River, Nevada. The oxidative demethylation can also be detected in Everglades sediments, but at much slower rates. Future challenges for this research will be to demonstrate that: 1) oxidative demethylation occurs at near-ambient MeHg levels; 2) the product is Hg (II) or Hg (0); and 3) high rates of oxidative demethylation can be achieved in novel bacterial isolates which will clarify the underlying mechanism(s).

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