Dissolved Organic Matter Enhances Microbial Mercury Methylation Under Sulfidic Conditions

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• 作者：Andrew M. Graham ; George R. Aiken ; Cynthia C. Gilmour
• 刊名：Environmental Science & Technology (ES&T)
• 出版年：2012
• 出版时间：March 6, 2012
• 年：2012
• 卷：46
• 期：5
• 页码：2715-2723
• 全文大小：485K
• 年卷期：v.46,no.5(March 6, 2012)
• ISSN：1520-5851

文摘

Dissolved organic matter (DOM) is generally thought to lower metal bioavailability in aquatic systems due to the formation of metal鈥揇OM 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鈥?.0 nM Hg and 2鈥?0 渭M sulfide) that favor the formation of 尾-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鈥揇OM鈥搒ulfide solutions amended with sufficient l-cysteine to prevent 尾-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.