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

The public and environmentalhealth consequences of mercury (Hg)methylation have drawn much attention and considerable research toHg methylation processes and their dynamics in diverse environmentsand under a multitude of conditions. However, the net methylmercury(MeHg) concentration that accumulates in the environment is equallydetermined by the rate of MeHg degradation, a complex process mediatedby a variety of biotic and abiotic mechanisms, about which our knowledgeis limited. Here we review the current knowledge on MeHg degradationand its potential pathways and mechanisms. We describe detoxificationby resistant microorganisms that employ the Hg resistance (mer) system to reductively break the carbon-mercury(C-Hg) bond producing methane (CH4) and inorganicmercuric Hg-(II), which is then reduced by the mercuric reductase toelemental Hg(0). Very recent research has begun to elucidate a mechanismfor the long-recognized mer-independent oxidativedemethylation, likely involving some strains of anaerobic bacteriaas well as aerobic methane-oxidizing bacteria, i.e., methanotrophs.In addition, photochemical and chemical demethylation processes aredescribed, including the roles of dissolved organic matter (DOM) andfree radicals as well as dark abiotic demethylation in the naturalenvironment about which little is currently known. We focus on mechanismsand processes of demethylation and highlight the uncertainties andknown effects of environmental factors leading to MeHg degradation.Finally, we suggest future research directions to further elucidatethe chemical and biochemical mechanisms of biotic and abiotic demethylationand their significance in controlling net MeHg production in naturalecosystems.

Automated summary

This article reviews the current knowledge on MeHg degradation and its potential pathways and mechanisms, highlighting detoxification processes by resistant microorganisms using the mer system and other demethylation processes. It emphasizes the uncertainties and known effects of environmental factors on MeHg degradation.