PerspectiveEnvironmental Science

Cracking the Mercury Methylation Code

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Science  15 Mar 2013:
Vol. 339, Issue 6125, pp. 1280-1281
DOI: 10.1126/science.1235591

Mercury (Hg) is a global pollutant that is transported over long distances. Although it occurs naturally, its concentration in the biosphere has increased dramatically over the past 200 years as a result of industrial activities. Mercury enters the environment in its inorganic form, but its bioaccumulation in organisms, biomagnification in food webs, and toxicity to humans depend on microbial methylmercury (MeHg) synthesis (see the figure). The use of stable isotopes of mercury has improved scientists' ability to trace and measure mercury in the environment (1, 2), but methods to predict methylmercury synthesis in the environment remain scarce. On page 1332 of this issue, Parks et al. (3) identify two genes required for mercury methylation. This discovery will be helpful for developing tools to study the synthesis and accumulation of methylmercury and to improve the management of contaminated environments.

The mercury geochemical cycle.

Mercury is methylated in anoxic environments by microorganisms, one of which is illustrated in the inset. Parks et al. show that the methylation reaction requires two genes, hgcA (encoding a putative methyltransferase corrinoid protein) and hgcB (encoding a putative [4Fe-4S] ferredoxin). Different colors for the HgcA protein indicate different redox states of the corrinoid HgcA enzyme. The toxic methylmercury accumulates in aquatic species (bioaccumulation), and its concentrations increase with each trophic level (biomagnification), causing a threat to humans whose diets rely on fish. THF, tetrahydrofolate.

CREDIT: K. SUTLIFF/SCIENCE

The crippling and deadly effects of methylmercury have been recognized globally since the severe mercury poisoning event in Minamata, Japan in 1956, after the release of mercury from a nearby industry. Since then, studies have shown that mercury is methylated under anoxic conditions (4) by sulfate- and iron-reducing bacteria (5, 6). Biochemical studies suggested the possible involvement of corrinoid proteins in the methylation pathway (7, 8). However, no specific mercury methylation genes were identified, limiting understanding of the methylation pathway and hence the ability to track methylmercury production.

Parks et al. used comparative genomics and structural biology tools to identify candidate genes. They performed targeted gene deletion and complementation experiments to show that two genes, hgcA (which encodes a putative corrinoid protein) and hgcB (which encodes a 2[4Fe-4S] ferredoxin), are required for mercury methylation.

On the basis of these findings, the authors propose a mechanistic model where a methyl group is transferred from the methylated HgcA protein to inorganic Hg(II) and the HgcB protein is required for HgcA turnover (see the figure). This proposed pathway raises questions about the biophysical and biochemical mechanisms by which mercury is methylated. Most intriguing is the authors' prediction that the C terminus of HgcA may be membrane-embedded, possibly coupling methylation to the transport of Hg(II) and/or methylmercury across the cell wall.

The authors identified homologs of hgcA and hgcB in genomes of 52 bacteria and methanogenic archaea. Although most have not been tested for their ability to methylate mercury in pure culture, field studies have implicated methanogens in mercury methylation (9). The distribution of methylation ability is sporadic, with methylating and nonmethylating strains occurring in the same species (10). These observations raise the questions of how methylation evolved and what its purpose might be. Methylation may be a detoxification mechanism (11, 12), possibly an ancient pathway used to deal with toxic inorganic mercury. Further experiments and analyses of the genomes in which hgcA and hgcB are found will help establish the evolutionary path of mercury methylation (13).

The importance of mercury as a chemical of major concern to human health was underscored by the recent adoption of the Minamata Convention on Mercury, a legally binding treaty to be signed in fall 2013 by more than 140 countries. This treaty requires that government agencies be equipped to monitor processes affecting global mercury transport and cycling. The United Nations Environment Programme recently identified two pressing global issues with regard to mercury pollution: establishing the link among deposition, methylation, and uptake by living organisms, and characterizing methylation and demethylation and how these reactions are affected by climate change (14).

The study by Parks et al. is important and timely for its promise to inform the development of such monitoring and management strategies. Knowing the sequences of mercury methylation genes will be useful for the development of molecular biomarkers for the detection and quantification of mercury methylation and the elucidation of the environmental triggers of hgcA/hgcB expression. Given that quantitative and traditional polymerase chain reactions can now be performed in the field, these biomarkers would offer specific, fast analyses of whether or not methylation is likely to occur in a given environment, as well as enable evaluation of the efficiency of potential mitigation strategies. Further work may reveal additional determinants of mercury methylation under anoxic conditions and might explain puzzling observations of methylation under oxic conditions in surface marine waters (15).

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