Contribution of Coexisting
Sulfate and Iron Reducing
Bacteria to Methylmercury Production in Freshwater River Sediments
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Abstract
We investigated microbial methylmercury (CH<sub>3</sub>Hg) production
in sediments from the South River (SR), VA, an ecosystem contaminated
with industrial mercury (Hg). Potential Hg methylation rates in samples
collected at nine sites were low in late spring and significantly
higher in late summer. Demethylation of <sup>14</sup>CH<sub>3</sub>Hg was dominated by <sup>14</sup>CH<sub>4</sub> production in spring,
but switched to producing mostly <sup>14</sup>CO<sub>2</sub> in the
summer. Fine-grained sediments originating from the erosion of river
banks had the highest CH<sub>3</sub>Hg concentrations and were potential
hot spots for both methylation and demethylation activities. Sequencing
of 16S rRNA genes of cDNA recovered from sediment RNA extracts indicated
that at least three groups of sulfate-reducing bacteria (SRB) and
one group of iron-reducing bacteria (IRB), potential Hg methylators,
were active in SR sediments. SRB were confirmed as a methylating guild
by amendment experiments showing significant sulfate stimulation and
molybdate inhibition of methylation in SR sediments. The addition
of low levels of amorphous iron(III) oxyhydroxide significantly stimulated
methylation rates, suggesting a role for IRB in CH<sub>3</sub>Hg synthesis.
Overall, our studies suggest that coexisting SRB and IRB populations
in river sediments contribute to Hg methylation, possibly by temporally
and spatially separated processes