Methylmercury production in a chronically sulfate-impacted sub-boreal wetland

Abstract

Increased deposition of atmospheric sulfate exacerbates methylmercury (MeHg) production in freshwater wetlands by stimulating methylating bacteria, but it is unclear how methylation in sub-boreal wetlands is impacted by chronically elevated sulfate inputs, such as through mine discharges. The purpose of our study is to determine how sulfate discharges to wetlands from iron mining activities impact MeHg production. In this study, we compare spatial and temporal patterns in MeHg and associated geochemistry in two wetlands receiving contrasting loads of sulfate. Two orders of magnitude less sulfate in the un-impacted wetland create significant differences in acid-volatile sulfide and porewater sulfide; however, dissolved and solid-phase MeHg concentrations and methylation rate potentials (Kmeth) are statistically similar in both wetlands. Permitted mine pumping events flood the sulfate-impacted wetland with very high sulfate waters during the fall. In contrast to observations in sulfate-limited systems, this large input of sulfate to a chronically sulfate-impacted system led to significantly lower potential relative methylation rates, suggesting a predominance of demethylation processes over methylation processes during the sulfate loading. Overall, short-term measurements of methylation and demethylation potential are unrelated to gross measures of long-term MeHg accumulation, indicating a decoupling of short- and long-term process measurements and an overall disequilibrium in the systems. High sulfide accumulation, above ∼600-800 μg l(-1) sulfide, in the sulfate-impacted system lowers long-term MeHg accumulation, perhaps as a result of less bioavailable Hg-S complexes. Although continued research is required to determine how sulfate-limited freshwater wetlands might respond to new, large inputs of high-sulfate runoff from mining operations, chronically impacted wetlands do not appear to continually accumulate or produce MeHg at rates different from wetlands unimpacted by mining.

Recent advances in the study of mercury methylation in aquatic systems

“…Previous research has suggested that warmer water temperatures may promote bacterial methylation (Bacci 1989;Bodaly et al 1993;Ramlal et al 1993), which is supported by recent studies that identified a positive correlation for MeHg production rates and temperature (Monperrus et al 2007a;Johnson et al 2016). In some cases, methylation could only be detected in lake hypolimnia (which are perpetually cold due to a lack of thermal circulation) (Eckley and Hintelmann 2006), but this also is likely due to the anoxic nature of hypolimnia; the link between methylation and cool temperatures is ultimately unclear.…”

“…These short-term measurements of potential rates of methylation and demethylation have been found to be unrelated to gross measures of long-term MeHg accumulation (Drott et al 2008;Johnson et al 2016). This suggests that these snapshot measurements may not be valuable in determining long-term process measurements.…”

Molecular evidence for novel mercury methylating microorganisms in sulfate-impacted lakes

“…The origins of MeHg are of interest in the watershed due to restrictive fish consumption advisories, and previous studies have addressed the complex relationship between MeHg accumulation and sulfate loading (e.g., refs. [34][35][36][37] and references therein). These studies show that, across the watershed, methylation rates and MeHg accumulation are not always clearly related to sulfate concentration.…”

Increase in Nutrients, Mercury, and Methylmercury as a Consequence of Elevated Sulfate Reduction to Sulfide in Experimental Wetland Mesocosms

“…At low SO 4 concentrations, the addition of SO 4 can stimulate MSR and Hg methylation (Jeremiason et al, ). At higher SO 4 concentrations, a greater abundance of inorganic sulfide appears to decrease the availability of inorganic Hg for Hg methylation (Hsu‐Kim et al, ; Johnson et al, ). Because it has been observed that low SO 4 additions often increase Hg methylation and higher SO 4 concentrations decrease methylation, it has been proposed that there is a range of SO 4 and sulfide concentrations are optimal for Hg methylation, above which methylation is inhibited (Hsu‐Kim et al, ).…”