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Applied and Environmental Microbiology, September 2003, p. 5414-5422, Vol. 69, No. 9
0099-2240/03/$08.00+0     DOI: 10.1128/AEM.69.9.5414-5422.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Mercury Methylation Independent of the Acetyl-Coenzyme A Pathway in Sulfate-Reducing Bacteria

Eileen B. Ekstrom,^1* François M. M. Morel,² and Janina M. Benoit³

Department of Civil and Environmental Engineering,¹ Department of Geosciences, Princeton University, Princeton, New Jersey 08544,² Department of Chemistry, Wheaton College, Norton, Massachusetts 02766³

Received 1 April 2003/ Accepted 8 July 2003

Sulfate-reducing bacteria (SRB) in anoxic waters and sediments are the major producers of methylmercury in aquatic systems. Although a considerable amount of work has addressed the environmental factors that control methylmercury formation and the conditions that control bioavailability of inorganic mercury to SRB, little work has been undertaken analyzing the biochemical mechanism of methylmercury production. The acetyl-coenzyme A (CoA) pathway has been implicated as being key to mercury methylation in one SRB strain, Desulfovibrio desulfuricans LS, but this result has not been extended to other SRB species. To probe whether the acetyl-CoA pathway is the controlling biochemical process for methylmercury production in SRB, five incomplete-oxidizing SRB strains and two Desulfobacter strains that do not use the acetyl-CoA pathway for major carbon metabolism were assayed for methylmercury formation and acetyl-CoA pathway enzyme activities. Three of the SRB strains were also incubated with chloroform to inhibit the acetyl-CoA pathway. So far, all species that have been found to have acetyl-CoA activity are complete oxidizers that require the acetyl-CoA pathway for basic metabolism, as well as methylate mercury. Chloroform inhibits Hg methylation in these species either by blocking the methylating enzyme or by indirect effects on metabolism and growth. However, we have identified four incomplete-oxidizing strains that clearly do not utilize the acetyl-CoA pathway either for metabolism or mercury methylation (as confirmed by the absence of chloroform inhibition). Hg methylation is thus independent of the acetyl-CoA pathway and may not require vitamin B₁₂ in some and perhaps many incomplete-oxidizing SRB strains.

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* Corresponding author. Mailing address: Department of Geosciences, Guyot Hall, Princeton University, Princeton, NJ 08544. Phone: (609) 258-1052. Fax: (609) 258-5242. E-mail: eekstrom{at}princeton.edu.

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Applied and Environmental Microbiology, September 2003, p. 5414-5422, Vol. 69, No. 9
0099-2240/03/$08.00+0     DOI: 10.1128/AEM.69.9.5414-5422.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

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