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Applied and Environmental Microbiology, January 2006, p. 457-464, Vol. 72, No. 1
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.1.457-464.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Mercury Methylation from Unexpected Sources: Molybdate-Inhibited Freshwater Sediments and an Iron-Reducing Bacterium

Emily J. Fleming,¹ E. Erin Mack,^1, Peter G. Green,² and Douglas C. Nelson^1*

Section of Microbiology,¹ Department of Civil and Environmental Engineering, University of California, Davis, California²

Received 20 July 2005/ Accepted 17 October 2005

Methylmercury has been thought to be produced predominantly by sulfate-reducing bacteria in anoxic sediments. Here we show that in circumneutral pH sediments (Clear Lake, CA) application of a specific inhibitor of sulfate-reducing bacteria at appropriate concentrations typically inhibited less than one-half of all anaerobic methylation of added divalent mercury. This suggests that one or more additional groups of microbes are active methylators in these sediments impacted by a nearby abandoned mercury mine. From Clear Lake sediments, we isolated the iron-reducing bacterium Geobacter sp. strain CLFeRB, which can methylate mercury at a rate comparable to Desulfobulbus propionicus strain 1pr3, a sulfate-reducing bacterium known to be an active methylator. This is the first time that an iron-reducing bacterium has been shown to methylate mercury at environmentally significant rates. We suggest that mercury methylation by iron-reducing bacteria represents a previously unidentified and potentially significant source of this environmental toxin in iron-rich freshwater sediments.

Present address: DuPont, Corporate Remediation Group, P.O. Box 6101, Glasgow 300, Newark, DE 19714-6101.

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