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Dissolved Organic Matter Enhances Microbial Mercury Methylation Under Sulfidic Conditions

Andrew M. Graham,*,† George R. Aiken, and Cynthia C. Gilmour

†Smithsonian Environmental Research Center, 647 Contees Wharf Road, Edgewater, Maryland 21037, United States
‡U.S. Geological Survey, 3215 Marine Street, Suite E127, Boulder, Colorado 80303, United States
*Corresponding author. E-mail: grahaman@si.edu; phone: (443) 482-2472.
Notes: The authors declare no competing financial interest.

Reprinted with permission from Environ. Sci. Technol., 2012, 46 (5), pp 2715-2723, DOI: 10.1021/es203658f. Copyright 2012 American Chemical Society.


Experimental Section
Results and Discussion
Figures, Table
& Supporting Information

Dissolved organic matter (DOM) is generally thought to lower metal bioavailability in aquatic systems due to the formation of metal-DOM complexes that reduce free metal ion concentrations. However, this model may not be pertinent for metal nanoparticles, which are now understood to be ubiquitous, sometimes dominant, metal species in the environment. The influence of DOM on Hg bioavailability to microorganisms was examined under conditions (0.5-5.0 nM Hg and 2-10 µM sulfide) that favor the formation of β-HgS(s) (metacinnabar) nanoparticles. We used the methylation of stable-isotope enriched 201HgCl2 by Desulfovibrio desulfuricans ND132 in short-term washed cell assays as a sensitive, environmentally significant proxy for Hg uptake. Suwannee River humic acid (SRHA) and Williams Lake hydrophobic acid (WLHPoA) substantially enhanced (2- to 38-fold) the bioavailability of Hg to ND132 over a wide range of Hg/DOM ratios (9.4 pmol/mg DOM to 9.4 nmol/mg DOM), including environmentally relevant ratios. Methylmercury (MeHg) production by ND132 increased linearly with either SRHA or WLHPoA concentration, but SRHA, a terrestrially derived DOM, was far more effective at enhancing Hg-methylation than WLHPoA, an aquatic DOM dominated by autochthonous sources. No DOM-dependent enhancement in Hg methylation was observed in Hg-DOM- sulfide solutions amended with sufficient L-cysteine to prevent β-HgS(s) formation. We hypothesize that small HgS particles, stabilized against aggregation by DOM, are bioavailable to Hg-methylating bacteria. Our laboratory experiments provide a mechanism for the positive correlations between DOC and MeHg production observed in many aquatic sediments and wetland soils.

diagram showing the influence of dissolved organic matter on mercury bioavailability
[larger image]

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