The relative importance of mercury methylation and demethylation in rice paddy soil varies depending on the presence of rice plants.

Neurotoxic methylmercury (MeHg) accumulates in rice grain from paddy soil, where its concentration is controlled by microbial mercury methylation and demethylation. Both up- and down-regulation of methylation is known to occur in the presence of rice plants in comparison to non-vegetated paddy soils; the influence of rice plant presence/absence on demethylation is unknown. To assess the concurrent influence of rice plant presence/absence on methylation and demethylation, and to determine which process was more dominant in controlling soil MeHg concentrations, we maintained six rhizoboxes of paddy soil with and without rice plants. At the peak of plant growth, we simultaneously measured ambient MeHg, ambient inorganic mercury (IHg), and potential rate constants of methylation and demethylation (Kmeth and Kdemeth) in soil using stable isotope tracers and ID-GC-ICPMS. We also measured organic matter content, elemental S, and water-extractable sulfate. We found MeHg concentrations were differentially controlled by MeHg production and degradation processes, depending on whether plants were present. In non-vegetated boxes, MeHg concentration was controlled by Kmeth, as evidenced by a strong and positive correlation, while Kdemeth had no relation to MeHg concentration. These results indicate methylation was the dominant driver of MeHg concentration in non-vegetated soil. In vegetated boxes, Kdemeth strongly and negatively predicted MeHg concentration, indicating that demethylation was the dominant control in soil with plants. MeHg concentration, Kmeth, and % MeHg all had significantly less variance in vegetated than in non-vegetated soils due to a consistent elimination of greater values. This pattern suggests that reduced MeHg production capacity was a secondary control on MeHg concentrations in vegetated soils. We observed no difference in the magnitude or variance of Kdemeth between treatments, suggesting that demethylation was robust to soil chemical conditions influenced by the plant, perhaps because of a wider taxonomic diversity of demethylators. Our results suggest that methylation and demethylation processes could both be leveraged to alter MeHg concentrations in rice paddy soil.

Mercury methylation in stormwater retention ponds at different stages in the management lifecycle.

Stormwater retention ponds effectively manage erosion, flooding, and pollutant loadings, but are also sources of methylmercury (MeHg), a bioaccumulative neurotoxin which is produced by anaerobic aquatic microorganisms. Stormwater retention ponds have a 10-15 year working life, after which they are dredged and reflooded. In this study, we related MeHg biogeochemistry to the different stages of the management lifecycle. In a new, a dredged, and a mature stormwater retention pond, we measured MeHg and inorganic mercury (IHg) concentrations, and the potential for MeHg formation (Kmeth), during the early summer, peak summer, and fall of 2013. In our study sites, MeHg concentrations appear to be driven by mercury (Hg) methylation, indicated by significant correlations between Kmeth values and MeHg concentrations and the percent of Hg present as MeHg. Relationships between Hg variables and ancillary biogeochemistry suggest that Hg methylation is carried out by sulfate reducing bacteria, but that the process is modulated by the supply of IHg substrate, sediment total and labile organic carbon, and possibly competition with nitrate reducers. Wetlands at different points in the management lifecycle differ in terms of their MeHg biogeochemistry. The organic matter-poor new wetland had low MeHg production (mean Kmeth 0.014 per day) and sediment concentrations (mean 0.015 ng g-1), while the mature wetland both produced and accumulated MeHg about five times more actively. Methylmercury production capacity was only temporarily reduced in the reflooded sediments of the dredged wetland, which experienced rapid increases in Kmeth values from low (mean 0.015 per day) immediately after dredging, to values similar to those in the mature wetland after five months. This pattern may have been related to recolonization of the sediments with mercury methylators or increased microbial activities in response to the addition of fresh organic matter. Additional studies should focus on the applicability of these patterns to stormwater retention ponds in other areas, and particularly investigate the effects of stormwater pond dredging on their microbial ecology and MeHg biogeochemistry.

Accumulation and translocation of methylmercury and inorganic mercury in Oryza sativa: An enriched isotope tracer study.

Methylmercury (MeHg) accumulation in rice is an emerging human health issue, but uptake pathways and translocation into the grain remain poorly understood. We grew Oryza sativa plants in pots of wetland soil amended with an enriched mercury isotope (94.3% 200Hg) tracer, alongside unvegetated control pots, and assessed both ambient and tracer MeHg and inorganic Hg (IHg) concentrations in soil and plant tissues at three growth stages. Based on similar ratios of ambient:tracer MeHg concentrations in soil and plant tissues, we provide the first direct evidence that MeHg is first synthesized in saturated soil and subsequently translocated to rice grains. There is no evidence of in planta methylation of IHg, but significant losses of MeHg from plant tissues between flowering and maturity indicates likely in planta demethylation. In this greenhouse experiment, lower percent of tracer MeHg in vegetated soils at late growth stages suggests that rice plants reduce the net MeHg accumulation capacity of soils, although the mechanism remains unclear. For IHg, roots accumulated Hg from the soil, straw from the soil and the atmosphere, and grain almost entirely from the atmosphere. Management strategies that aim to reduce MeHg accumulation in rice should focus on mercury methylation in paddy soils, but IHg reductions will depend on regional controls of atmospheric Hg.

Methylmercury production and accumulation in urban stormwater ponds and habitat wetlands.

Stormwater management ponds and created habitat wetlands effectively manage erosion, flooding, and pollutant loadings while providing biodiversity and aesthetic benefits, but these structures are also potential sources of methylmercury (MeHg), a bioaccumulative neurotoxin. While MeHg accumulation has been confirmed in habitat wetlands, the extent of MeHg production and accumulation in stormwater ponds is unknown. Additionally, the fine-scale spatial variation in MeHg in these wetlands has never been explored despite the possibility that cycles of wetting and drying, and the presence of aquatic plants may stimulate methylation at their margins. To address these knowledge gaps, we compared MeHg and inorganic mercury concentrations, the percent of total mercury present as MeHg (%MeHg), and potential mercury methylation rate constants (Kmeth) in the sediments of terrestrial-aquatic transects through several stormwater and habitat wetlands. We present novel evidence confirming the in situ production of MeHg in both stormwater ponds and habitat wetlands, but observe no systematic differences across the terrestrial-aquatic gradient, suggesting that routine variations in water level do not alter MeHg production and accumulation. Stormwater ponds effectively trap mercury while converting relatively little to MeHg, as evidenced by lower MeHg concentrations, %-MeHg, and Kmeth values than habitat wetlands, but often greater inorganic Hg concentrations. The relationship of aquatic vegetation to MeHg accumulation is weak and ambiguous, suggesting plants are not strong drivers of MeHg biogeochemistry in these systems. Although the MeHg hazard associated with individual artificial wetlands is low, they may be important sources of MeHg at the landscape level.

Experimental sulfate amendment alters peatland bacterial community structure.

As part of a long-term, peatland-scale sulfate addition experiment, the impact of varying sulfate deposition on bacterial community responses was assessed using 16S tag encoded pyrosequencing. In three separate areas of the peatland, sulfate manipulations included an eight year quadrupling of atmospheric sulfate deposition (experimental), a 3-year recovery to background deposition following 5years of elevated deposition (recovery), and a control area. Peat concentrations of methylmercury (MeHg), a bioaccumulative neurotoxin, were measured, the production of which is attributable to a growing list of microorganisms, including many sulfate-reducing Deltaproteobacteria. The total bacterial and Deltaproteobacterial community structures in the experimental treatment differed significantly from those in the control and recovery treatments that were either indistinguishable or very similar to one another. Notably, the relatively rapid return (within three years) of bacterial community structure in the recovery treatment to a state similar to the control, demonstrates significant resilience of the peatland bacterial community to changes in atmospheric sulfate deposition. Changes in MeHg accumulation between sulfate treatments correlated with changes in the Deltaproteobacterial community, suggesting that sulfate may affect MeHg production through changes in the community structure of this group.