Methylmercury (MeHg) is a neurotoxin mainly produced by microorganisms in suboxic and anoxic environments such as peatlands. Peatlands are an important source of MeHg in adjacent aquatic ecosystems, thus increasing the risk of human and wildlife exposure to this toxic compound. An improved understanding of factors limiting microbial net MeHg formation in peatlands could benefit the management and mitigation of this toxic compound particularly in peatland-rich landscapes.
In this thesis, a chronosequence trophic gradient of peatlands within the space of a few kilometers, all subjected to similar atmospheric deposition, underlying geology and climate patterns, was studied to determine the influence of biogeochemical factors on net MeHg formation in peat. Along the peatland chronosequence, higher net MeHg formation in peat soil of the younger peatlands was attributed to more nutrient rich conditions (Paper I). The same trend in net MeHg formation was observed in porewater, which was deemed more related to the shifts in the availability of electron acceptors for methylating microorganisms than to the abundance of electron donors (Paper II). The results of modeling the solubility of Hg(II) suggest that the net MeHg formation along the chronosequence could also be influenced by the supply of bioavailable Hg(II) to methylating organisms (Paper II).
The microbial community composition was significantly correlated to net MeHg formation along the chronosequence, with spatial patterns driven by environmental factors (Paper IV). Laboratory incubations with a combination of amended inhibitors/stimulators revealed the presence of different microbial processes in relation to the biogeochemistry. These differences are suggested to contribute to net MeHg formation along the chronosequence (Paper III). Quantitative gene expressions of specific microbial functional groups suggest that the role of SRB in net MeHg formation varied across the chronosequence, while methanogenic archaea were important for this across all the peatlands (Paper IV)
