Mercury is a toxin that spreads around the globe mainly through the burning of coal, other industrial uses, and natural processes such as volcanic eruptions, and various forms of mercury are widely found in sediments and water. Methylmercury bioaccumulates in aquatic food chains, especially in large fish.

The ORNL study revealed new sources for formation of methylmercury. Click to enlarge.

Earlier this year, a multidisciplinary team of researchers at Oak Ridge National Laboratory discovered two key genes that are essential for microbes to convert oxidized mercury to methylmercury, a neurotoxin that can penetrate skin and at high doses affect brain and muscle tissue, causing paralysis and brain damage. (Earlier post.)

Most mercury researchers have believed that microbes could not convert elemental mercury—which is volatile and relatively inert—into methylmercury. Instead of becoming more toxic, they reasoned that elemental mercury would bubble out of water and dissipate. That offered a solution for oxidized mercury, which dissolves in water. By converting oxidized mercury into elemental mercury, they hoped to eliminate the threat of methylmercury contamination in water systems.

Methylmercury is a neurotoxin that poses significant health risks to humans. Some anaerobic sulphate- and iron-reducing bacteria can methylate oxidized forms of mercury, generating methylmercury. One strain of sulphate-reducing bacteria (Desulfovibrio desulphuricans ND132) can also methylate elemental mercury. The prevalence of this trait among different bacterial strains and species remains unclear, however.

Here, we compare the ability of two strains of the sulphate-reducing bacterium Desulfovibrio and one strain of the iron-reducing bacterium Geobacter to oxidize and methylate elemental mercury in a series of laboratory incubations. Experiments were carried out under dark, anaerobic conditions, in the presence of environmentally relevant concentrations of elemental mercury.

...In line with recent findings, we show that D. desulphuricans ND132 can both oxidize and methylate elemental mercury. We find that the rate of methylation of elemental mercury is about one-third the rate of methylation of oxidized mercury. We also show that Desulfovibrio alaskensis G20 can oxidize, but not methylate, elemental mercury. Geobacter sulphurreducens PCA is able to oxidize and methylate elemental mercury in the presence of cysteine. We suggest that the activity of methylating and non-methylating bacteria may together enhance the formation of methylmercury in anaerobic environments.

—Hu et al.

ORNL’s study and a parallel study reported by Rutgers University, however, suggest that elemental mercury is also susceptible to bacterial manipulation, a finding that makes environmental cleanup more challenging.

Our research allows us to understand generally where and how bacteria might produce methylmercury so that we can target those areas in the future. We are trying to understand the process of microbial mercury methylation. Once we understand the process, we can begin to form solutions to combat mercury pollution.

—ORNL’s Liyuan Liang, a co-author and director of the DOE-funded mercury research program

This research was funded by the DOE Office of Science.

Resources

• Haiyan Hu, Hui Lin, Wang Zheng, Stephen J. Tomanicek, Alexander Johs, Xinbin Feng, Dwayne A. Elias, Liyuan Liang & Baohua Gu (2013) Oxidation and methylation of dissolved elemental mercury by anaerobic bacteria. Nature Geoscience doi: 10.1038/ngeo1894