Featured Research Highlight

Simultaneous Mercury Methylation and Demethylation by Geobacter bemidjiensis Bem Discovered

The Science

For the first time, we found that the strain Geobacter bemidjiensis Bem can methylate mercury and degrade methylmercury toxin concurrently under anoxic conditions. A reductive demethylation pathway is utilized by Bem to degrade methylmercury, possibly due to its genes encoding homologs of a organomercurial lyase (MerB) and a mercuric reductase (MerA).

The Impact

Geobacter bemidjiensis bacteria widely occur in sediments and water, including permafrost soils, and may thus play an important role in controlling net methylmercury production and bioaccumulation in biota in natural aquatic environments.

Summary

Microbial methylation and demethylation are two competing processes controlling the net production and bioaccumulation of neurotoxic methylmercury in natural aquatic ecosystems. Although mercury methylation by anaerobic microorganisms and demethylation by aerobic Hg-resistant bacteria have both been extensively studied, little attention has been given to MeHg degradation by anaerobic bacteria, particularly the iron-reducing bacterium Geobacter bemidjiensis Bem.

We report, for the first time, that the strain G. bemidjiensis Bem can mediate a suite of Hg transformations, including Hg(II) reduction, Hg(0) oxidation, and MeHg production and degradation under anoxic conditions. Results suggest that G. bemidjiensis utilizes a reductive demethylation pathway to degrade MeHg, with elemental Hg(0) as the major reaction product, possibly due to the presence of genes encoding homologs of a organomercurial lyase (MerB) and a mercuric reductase (MerA). In addition, the cells can strongly sorb Hg(II) and MeHg, and reduce or oxidize Hg, resulting in both time- and concentration-dependent Hg species transformations. Moderate concentrations of Hg-binding ligands, such as cysteine, enhance Hg(II) methylation but inhibit MeHg degradation. These findings indicate a cycle of Hg methylation and demethylation among anaerobic bacteria, thereby influencing net MeHg production in anoxic water and sediments.

Publication

Lu, Xia, Yurong Liu, Alexander Johs, Linduo Zhao, Tieshan Wang, Ziming Yang, Hui Lin, Dwayne A. Elias, Eric M. Pierce, Liyuan Liang, Tamar Barkay, and Baohua Gu. 2016. “Anaerobic Mercury Methylation and Demethylation by Geobacter bemidjiensis Bem.” Environmental Science & Technology. DOI: 10.1021/acs.est.6b00401

 

The ORNL Mercury SFA is sponsored by the Subsurface Biogeochemical Research (SBR) program within the U.S. Department of Energy's Office of Biological and Environmental Research.