Microbial community dynamics involved in organic carbon degradation and mercury methylation in subarctic peatlands
Creators: name: Roth, Spencer W.
Publisher: name: No Publisher Supplied
Year: 2021

description: 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. Taxonomical analysis of hgcA homologs shows a dominance of syntrophic families in rich fen sites, and my results show that pore water sulfate and the sulfate-reducing bacterial community are significant predictor variables of MeHg accumulation, furthering our understanding of the factors controlling Hg cycling in the environment.My results in Chapter 3 show that inputs of nutrients to a fen wetland from a groundwater source promoted robust and diverse Hg methylating communities. Incubation studies demonstrated that proximity to the groundwater source resulted in significantly higher Hg methylation rates, driven by microorganisms growing in syntrophy. These results indicate that transient changes in fen hydrology may promote MeHg production.Collectively, my dissertation results suggest that both transient changes in geochemistry as well as longer term shifts in wetland trophic status significantly impact microbial community structure and function. As permafrost is lost in Arctic and subarctic regions, inputs of nutrients through groundwater and surface water are expected to increase and shifts from bogs to fens have been documented. My results indicate that these changes will promote robust methanogenic and Hg methylating communities, leading to increased methane production and Hg methylation. These conclusions have important implications, as methane is a potent greenhouse gas, and MeHg is a neurotoxic compound that bioaccumulates in aquatic food webs.