BIOTIC AND ABIOTIC MERCURY METHYLATION IN YELLOWSTONE THERMAL WATERS

Mercury (Hg) is pervasive in Yellowstone’s thermal waters, with dissolved (<0.45µm filter) Hg concentrations across five orders of magnitude, from 0.2 ng/L Hg at Mammoth Hot Springs to 27 µg/L Hg at Washburn Hot Springs. Methylmercury (MeHg), a bioaccumulating neurotoxin, has been determined through previous studies to be associated with methylating microbial communities in hot spring discharge channels and wetlands. The work presented here will discuss biotic and abiotic processes that result in methylmercury concentrations of 0.04 to 10.3 ng/L measured directly in thermal groundwater collected at the discharge point. While many of these MeHg concentrations are elevated, the percentage of MeHg in the total dissolved mercury pool is 0.05 to 15 %. Springs with greater MeHg, both in terms of concentration and percentage of MeHg, are in Mud Volcano and Crater Hills, both vapor-dominated areas. Sulfate, ammonium, Hg, and MeHg, are magnified through steam distillation in these vapor-dominated systems, with the oxidation of H₂S to SO₄²⁺ generating H⁺. Thus, there is a tendency of greater MeHg concentrations to be associated with lower pH waters. However, the relationship between pH and dissolved Hg concentrations is not statistically significant. The MeHg in thermal groundwater is thought to be in part abiotic based on the absence of enzymes associated with microbial mercury methylation. While the microbially-produced enzymes catalyze the formation of MeHg, it is plausible that MeHg is formed abiotically in the subsurface at lower concentrations (< 0.1 ng/L), with concentrations magnified through steam distillation.