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

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 ∼3 times higher in the downstream (EHT-D: k d ∼ 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.

Shining light on recent advances in microbial mercury cycling

“…To the best of our knowledge, only one recent study has been published that fits this description. In this work, Kronberg et al 2018 showed that methanogens are important contributors to OD in a wetland ecosystem known to be a FACETS | 2018 | 3: 858-879 | DOI: 10.1139/facets-2018-0015 862 facetsjournal.com sink for MeHg. Together with previous work on methanogens, this finding frames methane cycling microbes as key players in controlling MeHg levels in the environment.…”

Periphyton and Flocculent Materials Are Important Ecological Compartments Supporting Abundant and Diverse Mercury Methylator Assemblages in the Florida Everglades

“…Along with Hg methylation, MeHg demethylation is likely to be a crucial determinant of net concentrations of MeHg, which had been well documented in the three compartments of the Everglades (31,34). Two pathways have been proposed for microbial MeHg demethylation: the reductive pathway that first cleaves MeHg to Hg(II) and CH 4 by an organomercurial-lyase enzyme encoded by merB and then to Hg(0) by mercuric reductase encoded by merA of Hg-resistant bacteria (46)(47)(48)(49)(50), as well as by the oxidative pathway that completely oxidizes MeHg to CO 2 and Hg(II) by SRP and methanogens (51)(52)(53)(54). More recently, a methanotroph was also reported to degrade MeHg via the potential activity of methanol dehydrogenase (55).…”

Anaerobic guilds responsible for mercury methylation in boreal wetlands of varied trophic status serving as either a methylmercury source or sink

“…While many wetlands, including boreal peatlands, net‐produce and export MeHg to downstream environments (Tjerngren et al ., 2012b), it was recently established by input–output budgets that black alder ( Alnus glutinosa ) swamps are consistent MeHg sinks (Kronberg et al ., 2012; Tjerngren et al ., 2012a; Kronberg et al ., 2018). Because alder swamps are nutrient‐rich environments, they typically form in depressions and along edges of lakes and streams in the lower sections of boreal landscapes.…”

“…Thus, alder swamps are most often situated downstream of major sources of MeHg, such as upland forest clear‐cuts (Kronberg et al ., 2016b) and nutrient poor peatlands (Kronberg et al ., 2012), but still upstream of most aquatic ecosystems, such as lakes and rivers. This means that the net MeHg degradation in alder swamps (Kronberg et al ., 2018) has the potential to play an important role in protecting aquatic biota from MeHg accumulation. While alder swamps support high MeHg demethylation rates, these wetlands also support high rates of MeHg production.…”