| Summary: | Northern peat-forming wetlands are anoxic environments in which anaerobic biogeochemical processes dominate. Specifically, peatlands have been identified as hot spots for methanogenesis and mercury (Hg) methylation. Arctic and subarctic peatlands store substantial amounts of organic carbon and Hg, and approximately 50% of northern peatlands are underlain by permafrost. As permafrost melts in response to climate change, hydrological changes are causing ombrotrophic bogs to transition towards minerotrophic fens. To understand how these shifts will impact soil microbial communities and carbon and Hg cycling, I assessed peat microbial communities through shotgun metagenomics, characterized pore water geochemistry, and measured gas production, volatile fatty acid degradation and mercury methylation in incubations from sites along a wetland trophic gradient ranging from ombrotrophic bogs to nutrient rich fens near Fairbanks, AK, USA. Results from Chapter 1 show that bog sites had rates of methane production 78-times lower than fens, corresponding to significantly lower average abundance of methanogenesis genes and taxa when compared to fens. Sulfate was a major factor delineating fens into two categories, and fens with detectable sulfate (rich fens) had the significantly highest abundance of methanogenesis genes and taxa compared to sites with no detectable sulfate (poor fens). Incubation studies paired with metagenomics indicate that the bottleneck step in organic carbon mineralization in peatlands shifts with shifting trophic status. Methane production in bogs was limited by low abundance and activity of methanogens, whereas substrate availability limited methane production in rich fens. In Chapter 2, I show that rich fens were ecosystems of high methylmercury (MeHg) accumulation, corresponding to abundant and diverse Hg methylating communities. In contrast to fens, bogs had low concentrations of pore water MeHg and 50% of bog metagenomes had no detectable homologs of hgcA, the gene involved in Hg methylation. ...
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