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

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-rnethylating 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 (similar to 1-8 ng Hg/mg C) and sulfide concentrations (1-1000 mu M). All tested DOM isolates significantly enhanced Hg methylation above DOM-free controls (from similar to 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 (beta-HgS(s)), we attribute the DOM-dependent enhancement of Hg(II)(i) bioavailability to steric and specific chemical (e.g., DOM thiols) inhibition of beta-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 (less than or similar to 30 mu M), conditions that favor HgS nanoparticle/cluster formation relative to dissolved HgS species.