Methylmercury in Managed Wetlands

Methylmercury (MeHg), a bioaccumulative neurotoxin, is microbially produced in anoxic wetland environments.  The direct or indirect management of wetlands is pervasive, but many questions remain regarding the impact of wetland management on MeHg biogeochemistry.  To address this, I investigated the extent, drivers, and consequences of MeHg production in important types of managed wetlands, as well as the response of MeHg to specific management interventions.  In a field study which simulated industrial increases and legislated decreases in sulfate deposition to peatlands, I found evidence that MeHg production in peatlands is quantitatively related to sulfate-mediated changes in the community structure of the Deltaproteobacteria, rather than simple metabolic stimulation of sulfate-reducing bacteria (SRB), one in the best studied groups of mercury methylators.  Furthermore, the structure of the bacterial community, as well as MeHg accumulation, was resilient to the end of sulfate amendments, demonstrating the value of this management intervention.  In a series of observational biogeochemical field studies in artificial wetlands, I found similar evidence that the wetland microflora was resilient to the disturbance of wetland dredging, which only temporarily reduced MeHg production and concentrations.  Methylmercury production in stormwater wetlands was carried out by SRB, with their activity modulated by availability of inorganic mercury, labile and total organic carbon, and competition with nitrate reducers.  MeHg production in stormwater wetlands was overall dampened in comparison to those managed for habitat provision.  Finally, I used a controlled greenhouse experiment to study the source of MeHg and IHg to paddy grown rice, a staple food recently shown to be a significant dietary source of MeHg to some populations.  I confirmed the soil origin of MeHg to rice grains, and identified a window of elevated MeHg uptake during vegetated growth. Inorganic mercury, by contrast, appears to derive from the atmosphere alone, presenting challenges in controlling its concentrations in rice.