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Research article2018Peer reviewed

Mechanisms of Methyl Mercury Net Degradation in Alder Swamps: The Role of Methanogens and Abiotic Processes

Kronberg, Rose-Marie; Schaefer, Jeffra K.; Bjorn, Erik; Skyllberg, Ulf

Abstract

Wetlands are common net producers of the neurotoxin monomethylmercury (MeHg) and are largely responsible for MeHg bioaccumulation in aquatic food-webs. However, not all wetlands net produce MeHg; notable exceptions are black alder (Alnus glutinosa) swamps, which net degrade MeHg. Here we report the mechanisms of MeHg demethylation in one such swamp (EHT), shown to be a sink for MeHg during four consecutive years. The potential demethylation rate constant (k(d) ) in soil incubations was similar to 3 times higher in the downstream (EHT-D: k(d) similar to 0.14 d(-1)) as compared to the upstream part of the swamp (EHT-U: k(d) 0.05 d(-1)). This difference concurred with increased stream and soil pH, and a change in plant community composition. Electron acceptor and inhibitor addition experiments revealed that abiotic demethylation dominated at EHT-U while an additional and equally large contribution from biotic degradation was observed at EHT-D, explaining the increase in MeHg degradation. Biotic demethylation (EHT-D) was primarily due to methanogens, inferred by a decrease in k(d) to autoclaved levels following selective inhibition of methanogens. Though methanogen-specific transcripts (mcrA) were found throughout the wetland, transcripts clustering with Methanosaetaceae were exclusive to EHT-D, suggesting a possible role for these acetoclastic methanogens in the degradation of MeHg.

Published in

Environmental Science and Technology Letters
2018, Volume: 5, number: 4, pages: 220-225 Publisher: AMER CHEMICAL SOC