Geochemical and Microbiological Controls on Mercury Methylation in Natural Waters

• Main Author: Agather, Alison M.
• Format: Text
• Language: unknown
• Published: CORE Scholar 2018
• Subjects: Department of Earth and Environmental Sciences; Biogeochemistry; Environmental Science; Limnology; Oceanography; Monomethylmercury; MMHg; Mercury Methylation; Hg methylation; Environmental Sciences; Physical Sciences and Mathematics; Arctic; Arctic Ocean; Bering Strait; Canada; Chukchi Shelf; Dayton; Fitzgerald; Pacific; Chukchi; Northwest Atlantic
• Online Access: https://corescholar.libraries.wright.edu/etd_all/2201; https://corescholar.libraries.wright.edu/cgi/viewcontent.cgi?article=3342&context=etd_all

Mercury (Hg) is a global pollutant toxic to humans and wildlife. Monomethylmercury (MMHg) is a bioavailable compound that bioaccumulates and biomagnifies in food webs. Humans are primarily exposed to MMHg from seafood consumption (Sunderland 2007), and high quantities of the neurotoxin lead to reduced neurocognitive functioning in adults and the children of exposed mothers (Cohen et al. 2005, Yokoo et al. 2003). Negative effects from MMHg accumulation on the health of humans and wildlife requires a more complete understanding of the chemistry and microbiology driving Hg methylation in both marine and freshwater systems. This work focuses on water column distribution, speciation, and methylation of Hg. The aims of this dissertation are three-fold: (1) characterize the speciation and distribution of Hg in the western Arctic Ocean; (2) examine seasonal variations in Hg speciation, methylation, and demethylation, and the microbial communities of Hg methylators in Crystal Lake, Ohio; and (3) quantify Hg methylation rates and characterize methylating microbial communities in waters on the continental shelf of the northwest Atlantic Ocean. While Hg methylation has been studied for decades, this work is built upon recent improvements in Hg detection limits, and newly discovered genes responsible for Hg methylation. In conjunction with U.S. Arctic GEOTRACES (GN01), the western Arctic Ocean was sampled in the summer of 2015. Although Hg concentrations in the Canada and Eurasian Basins were low relative to the Atlantic and Pacific Oceans, higher MMHg concentrations were observed in Arctic seawater that recently interacted with continental margins. We estimate that the Arctic Ocean receives 4-71 kmol Hg yr−1 from the Bering Strait, which is likely to interact with sediments of the shallow continental shelves before entering into the Arctic Ocean. This is potentially important, because while the estimated atmospheric input to the Arctic Ocean is ~400 kmol Hg yr−1, inflowing Hg from the Bering Strait may still be an important source of Hg that can be methylated on the Chukchi Shelf. Mercury methylation potentials were measured in a stratified freshwater system, Crystal Lake, in Dayton, Ohio (objective #2). Mercury methylation occurred in both oxic and anoxic portions of the water column, but methylation potentials were greatest at the oxic/anoxic boundary layer. Mercury methylating genes were found throughout the water column and had the greatest copy number in the hypolimnion. Similarly, previous marine work showed that sediments and the microbial communities therein are large sources of MMHg to near shore marine systems (Fitzgerald et al. 2007), which led to methylation and demethylation studies along the northwest Atlantic shelf (objective #3). Greater abundance of Hg methylating microbes were observed in water overlying sediment as opposed to shallower waters, but methylation potentials did not significantly differ. Together, these results suggested that (1) Archaea may be responsible for Hg methylation in oxic waters; and (2) redox transition zones in the water column and the sediment-water interface are important sources of bioavailable MMHg. These studies improve our understanding of Hg cycling in natural waters and suggest possible conditions and organisms that stimulate Hg methylation.