Barkay Tamar, Gu Baohua
Department of Biochemistry and Microbiology, School of Environmental and Biological Sciences, Rutgers University, New Brunswick, New Jersey 08901, United States.
Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
ACS Environ Au. 2021 Nov 2;2(2):77-97. doi: 10.1021/acsenvironau.1c00022. eCollection 2022 Mar 16.
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 () system to reductively break the carbon-mercury (C-Hg) bond producing methane (CH) 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 long-recognized -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.
汞（Hg）甲基化对公众健康和环境造成的影响已引起广泛关注，针对汞甲基化过程及其在不同环境和多种条件下的动态变化，已有大量研究。然而，环境中甲基汞（MeHg）的净积累浓度同样取决于MeHg的降解速率，这是一个由多种生物和非生物机制介导的复杂过程，目前我们对此了解有限。在此，我们综述了当前关于MeHg降解及其潜在途径和机制的知识。我们描述了抗性微生物的解毒作用，这些微生物利用汞抗性（）系统还原断裂碳 - 汞（C - Hg）键，生成甲烷（CH）和无机汞Hg（II），然后Hg（II）由汞还原酶还原为元素汞Hg（0）。最近的研究已开始阐明一种长期以来被认可的非依赖性氧化脱甲基作用机制，可能涉及一些厌氧细菌菌株以及好氧甲烷氧化细菌，即甲烷营养菌。此外，还描述了光化学和化学脱甲基过程，包括溶解有机物（DOM）和自由基的作用以及自然环境中黑暗非生物脱甲基作用，目前我们对此了解甚少。我们重点关注脱甲基作用的机制和过程，并强调导致MeHg降解的环境因素的不确定性和已知影响。最后，我们提出了未来的研究方向，以进一步阐明生物和非生物脱甲基作用的化学和生化机制及其在控制自然生态系统中MeHg净生成方面的意义。
