Effect of Dissolved Organic Matter Source and Character on Microbial Hg Methylation in Hg–S–DOM Solutions

• Published in: Environmental science & technology 2013-06, Vol.47 (11), p.5746-5754
• Main Authors: Graham, Andrew M, Aiken, George R, Gilmour, Cynthia C
• Format: Article
• Language: English
• Subjects: Applied sciences; Bacteria; Bioavailability; Biological and physicochemical phenomena; Biological and physicochemical properties of pollutants. Interaction in the soil; Desulfovibrio desulfuricans - metabolism; Earth sciences; Earth, ocean, space; Ecosystem; Engineering and environment geology. Geothermics; Exact sciences and technology; Humic Substances; Mercury; Mercury - metabolism; Mercury - pharmacokinetics; Methylation; Molecular weight; Nanoparticles; Nanoparticles - chemistry; Natural water pollution; Pollution; Pollution, environment geology; Soil and sediments pollution; Sulfides - chemistry; Sulfur content; Water treatment and pollution
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es400414a

Dissolved organic matter (DOM) is a key component of fate and transport models for most metals, including mercury (Hg). Utilizing a suite of diverse DOM isolates, we demonstrated that DOM character, in addition to concentration, influences inorganic Hg (Hg(II)i) bioavailability to Hg-methylating bacteria. Using a model Hg-methylating bacterium, Desulfovibrio desulfuricans ND132, we evaluated Hg-DOM-sulfide bioavailability in washed-cell assays at environmentally relevant Hg/DOM ratios (∼1–8 ng Hg/mg C) and sulfide concentrations (1–1000 μM). All tested DOM isolates significantly enhanced Hg methylation above DOM-free controls (from ∼2 to >20-fold for 20 mg C/L DOM solutions), but high molecular weight/highly aromatic DOM isolates and/or those with high sulfur content were particularly effective at enhancing Hg methylation. Because these experiments were conducted under conditions of predicted supersaturation with respect to metacinnabar (β-HgS(s)), we attribute the DOM-dependent enhancement of Hg(II)i bioavailability to steric and specific chemical (e.g., DOM thiols) inhibition of β-HgS(s) growth and aggregation by DOM. Experiments examining the role of DOM across a wide sulfide gradient revealed that DOM only enhances Hg methylation under fairly low sulfide conditions (≲30 μM), conditions that favor HgS nanoparticle/cluster formation relative to dissolved HgS species.