Demethylation─The Other Side of the Mercury Methylation Coin: A Critical Review

Abstract

The public and environmental health consequences of mercury (Hg) methylation have drawn much attention and considerable research to Hg methylation processes and their dynamics in diverse environments and under a multitude of conditions. However, the net methylmercury (MeHg) concentration that accumulates in the environment is equally determined by the rate of MeHg degradation, a complex process mediated by a variety of biotic and abiotic mechanisms, about which our knowledge is limited. Here we review the current knowledge on MeHg degradation and its potential pathways and mechanisms. We describe detoxification by resistant microorganisms that employ the Hg resistance (mer) system to reductively break the carbon− mercury (C−Hg) bond producing methane (CH 4 ) and inorganic mercuric Hg(II), which is then reduced by the mercuric reductase to elemental Hg(0). Very recent research has begun to elucidate a mechanism for the longrecognized mer-independent oxidative demethylation, likely involving some strains of anaerobic bacteria as well as aerobic methane-oxidizing bacteria, i.e., methanotrophs. In addition, photochemical and chemical demethylation processes are described, including the roles of dissolved organic matter (DOM) and free radicals as well as dark abiotic demethylation in the natural environment about which little is currently known. We focus on mechanisms and processes of demethylation and highlight the uncertainties and known effects of environmental factors leading to MeHg degradation. Finally, we suggest future research directions to further elucidate the chemical and biochemical mechanisms of biotic and abiotic demethylation and their significance in controlling net MeHg production in natural ecosystems.

Unravelling biogeochemical drivers of methylmercury production in an Arctic fen soil and a bog soil

“…2d). The decrease in MeHg concentrations could be partially attributed to potential demethylation in the soil, as previously reported (Barkay and Gu, 2021;Bridou et al, 2011;Hamelin et al, 2015;. This conclusion is supported by additional experiments incubating both the fen and bog soils with isotopically labeled Me 202 Hg.…”

Diel mercury concentration variations in a mercury-impacted stream

“…53,54 In contrast, MMHg demethylation can be mediated biotically by a broad array of aerobic and anaerobic microorganisms or by abiotic reactions. 12,55,56…”

“…53,54 In contrast, MMHg demethylation can be mediated biotically by a broad array of aerobic and anaerobic microorganisms or by abiotic reactions. 12,55,56 Hg methylation within algal biolms (periphyton) in EFPC is an important process exerting control on the diel oscillations in MMHg D concentration. Several investigators have reported net positive MMHg generation by algal biolms.…”

Prevalence of Heterotrophic Methylmercury Detoxifying Bacteria across Oceanic Regions

“…Biological MeHg demethylation and Hg 2+ reduction detoxification processes can be mediated by different biotic processes, including reductive reactions mediated by the mer operon, 30,31 but also oxidative reactions 32,33 as well as the recently described MeHg degradation performed by methanotrophic bacteria. 34 In this article, we will focus only on the study of bacteria carrying the mer operon (i.e., biological reductive MeHg degradation pathways). While the operon can be composed of different sets of genes, 30,31 the key genes are merA and merB.…”