It has been suspected by researchers that anoxic aquatic sediments represented the sites where resident bacteria were believed to transform the inorganic forms of mercury by methylation. However, it was not clear which group of microbes were responsible at that time. Methanogenic bacteria are considered to be important members of anaerobic food webs since they contain abundant B-12 coenzymes. It was thought that methylcobalamin-mediated reactions in methanogens were responsible for the biomethylation of Hg (II) in nature. This reaction was proven. The concept that methanogens were significant mercury methylators in the environment was demonstrated in vitro with cell extracts. It was noted however, that mercury methylation could not be found in whole cells in vivo of methanogens or in assays of with strongly methanogenic systems like sewage sludges.
The article described that researches with marine sediments showed a capacity for biogenesis of methylmercury (MeHg). Likewise, its activity was abolished by molybdate which is an inhibitor of sulfate-reducing bacteria (SRBs). It is now clear that those certain types of SRBs form MeHg though cobalamin-linked reactions. This is possible in both freshwater and marine sediments. With the use of high purity radioassay with 203Hg (II) in situ measurements of sediment Hg (II) methylation have been refined. This allows for methylation activity to be examined closely at near-ambient levels of Hg (II).
Studies showed enough information about the mechanisms for bacterial demethylation of MeHg. It represents a microbial toxin which is potent. It must be neutralized in order for bacteria to grow. In some bacteria the presence of MeHg activates the mer operon which is composed of mer A and mer B genes. The mer B gene is responsible in the encoding for the formation of an organomercurial lyase enzyme which cleaves MeHg into methane and Hg (II). On the other hand the mer A component codes the formation of mercuric reductase which by volatilization removes Hg by reducing Hg (II) to Hg (0). The mer operon is contained on a plasmid. It can also be inserted into various bacteria cells containing mer B but do not necessarily have mer A.
Research studies involving freshwater systems have recognized demethylation activity and the presence of mer operon genes in the resident bacterial populations. It is not clear though if mer operon-mediated reactions are the dominant mechanism of MeHg demethylation in anoxic sediments. Assessments of both methylation and demethylation capacity is important in determining whether a net methylation or demethylation occurs. High purity Me203Hg will facilitate such determinations.
A recent documented discovery revealed that MeHg mimics a 1-carbon organic substrate, such as methanol. This process is referred to as oxidative demethylation. It was discovered in sediment incubations with 14C-MeHg. 14C-carbon dioxide was found to be the major product, especially in systems which harbored active populations of sulfate reducers. 14C-methane was also an important end product 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. Results showed that both methanogens and sulfate-reducers are involved in oxidative demethylation in nature. Statistics indicated that the highest potential activity occur in systems having extensive mercury-contamination from mine tailing point sources. . The oxidative demethylation can also be detected in Everglades’s sediments at much slower rates. Other researches are on the way to demonstrate that oxidative demethylation occurs at near-ambient MeHg levels and that the product i
Original article -http://sofia.usgs.gov/projects/bact_demeth/bactdemab2.html
