Abstract
Methylation of tracer and ambient mercury (200Hg and 202Hg, respectively) equilibrated with four different natural organic matter (NOM) isolates was investigated in vivo using the Hg-methylating sulfate-reducing bacterium Desulfobulbus propionicus 1pr3. Desulfobulbus cultures grown fermentatively with environmentally representative concentrations of dissolved NOM isolates, Hg[II], and HS− were assayed for absolute methylmercury (MeHg) concentration and conversion of Hg(II) to MeHg relative to total unfiltered Hg(II). Results showed the 200Hg tracer was methylated more efficiently in the presence of hydrophobic NOM isolates than in the presence of transphilic NOM, or in the absence of NOM. Different NOM isolates were associated with variable methylation efficiencies for either the 202Hg tracer or ambient 200Hg. One hydrophobic NOM, F1 HpoA derived from dissolved organic matter from the Florida Everglades, was equilibrated for different times with Hg tracer, which resulted in different methylation rates. A 5 day equilibration with F1 HpoA resulted in more MeHg production than either the 4 h or 30 day equilibration periods, suggesting a time dependence for NOM-enhanced Hg bioavailability for methylation.
Highlights
The organometallic neurotoxin methylmercury (MeHg) inhibits human fetal and adult neurological development and cellular functionality (e.g., Marsh et al, 1980; Mergler et al, 2007; Newland et al, 2008; Ceccatelli et al, 2010)
For the later timepoints (72 and 120 h), cells cultured with natural organic matter (NOM) isolates F1 Hydrophobic Acid (F1 HpoA) 4hr, F1 HpoA 5dy, F1 HpoA 30dy, and Williams Lake Hydrophobic Acid (WL HpoA) (30 day) showed more growth than did cells cultured with Suwannee River Humic Acid (SRHA) or no NOM
D. propionicus was provided with propionate as the carbon substrate and sulfate as the terminal electron acceptor
Summary
The organometallic neurotoxin methylmercury (MeHg) inhibits human fetal and adult neurological development and cellular functionality (e.g., Marsh et al, 1980; Mergler et al, 2007; Newland et al, 2008; Ceccatelli et al, 2010). Recent studies have identified and NOM Effect on Bacterial Hg Methylation characterized a functional gene cluster (hgcAB) for Hg methylation in microorganisms (Parks et al, 2013; Poulain and Barkay, 2013). The role of this gene in biogeochemical Hg cycling, as well as its evolutionary origin and ecological distribution, require further study. Whatever the methylating mechanism(s) and biogeochemical pathway(s), microbial Hg methylation has been an active area of research for over 30 years because of its complexity and central importance to the global Hg cycle (Fitzgerald et al, 2007; Selin, 2009; Sonke et al, 2013)
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