Inhibition of Methylmercury and Methane Formation by Nitrous Oxide in Arctic Tundra Soil Microcosms

Climate warming causes permafrost thaw predicted to increase toxic methylmercury (MeHg) and greenhouse gas [i.e., methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O)] formation. A microcosm incubation study with Arctic tundra soil over 145 days demonstrates that N2O at 0.1 and 1 mM markedly inhibited microbial MeHg formation, methanogenesis, and sulfate reduction, while it slightly promoted CO2 production. Microbial community analyses indicate that N2O decreased the relative abundances of methanogenic archaea and microbial clades implicated in sulfate reduction and MeHg formation. Following depletion of N2O, both MeHg formation and sulfate reduction rapidly resumed, whereas CH4 production remained low, suggesting that N2O affected susceptible microbial guilds differently. MeHg formation strongly coincided with sulfate reduction, supporting prior reports linking sulfate-reducing bacteria to MeHg formation in the Arctic soil. This research highlights complex biogeochemical interactions in governing MeHg and CH4 formation and lays the foundation for future mechanistic studies for improved predictive understanding of MeHg and greenhouse gas fluxes from thawing permafrost ecosystems.

Automated summary

Climate warming leads to permafrost thaw, which is predicted to increase the formation of toxic methylmercury and greenhouse gases (methane, carbon dioxide, and nitrous oxide). A 145-day microcosm incubation study with Arctic tundra soil shows that nitrous oxide significantly inhibits microbial methylmercury formation, methanogenesis, and sulfate reduction, while slightly promoting carbon dioxide production. Analysis of microbial communities reveals that nitrous oxide decreases the abundance of methanogenic archaea and microbial clades associated with sulfate reduction and methylmercury formation. Depletion of nitrous oxide leads to rapid resumption of methylmercury formation and sulfate reduction, while methane production remains low, indicating differential effects on susceptible microbial guilds. The formation of methylmercury strongly coincides with sulfate reduction, supporting previous reports linking sulfate-reducing bacteria to methylmercury formation in Arctic soil. This research highlights the complex biogeochemical interactions governing methylmercury and methane formation, providing a basis for future mechanistic studies on the understanding of these processes in thawing permafrost ecosystems.