title: Mercury methylation from unexpected sources: molybdate-inhibited freshwater sediments and an iron-reducing bacterium.
abstract: 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.
authors.affiliation: Section of Microbiology, 357 Briggs Hall, University of California, Davis, CA 95616, USA.
authors.forename: Emily J
authors.initials: EJ
authors.lastname: Fleming
authors.forename: E Erin
authors.initials: EE
authors.lastname: Mack
authors.forename: Peter G
authors.initials: PG
authors.lastname: Green
authors.forename: Douglas C
authors.initials: DC
authors.lastname: Nelson
mesh_terms: D004275:DNA, Ribosomal; D005618:Fresh Water / Q000737:chemistry / Q000382:microbiology*; D045302:Geobacter / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D019015:Geologic Sediments / Q000737:chemistry / Q000382:microbiology*; D007501:Iron / Q000378:metabolism*; D008628:Mercury / Q000378:metabolism*; D008745:Methylation; D008767:Methylmercury Compounds / Q000378:metabolism*; D008969:Molecular Sequence Data; D008982:Molybdenum / Q000494:pharmacology; D010084:Oxidation-Reduction; D012336:RNA, Ribosomal, 16S / Q000235:genetics; D017422:Sequence Analysis, DNA; D013431:Sulfates / Q000378:metabolism
chemical_list: D004275:DNA, Ribosomal; D008767:Methylmercury Compounds; D012336:RNA, Ribosomal, 16S; D013431:Sulfates; C044659:molybdate; D008982:Molybdenum; D007501:Iron; D008628:Mercury