Collaborative Research: Methanotrophic-Mediated Methylmercury Transformation: An Important Yet Poorly Understood Biogeochemical Process

甲基汞是一种由某些微生物产生的非常有效的神经毒素。一旦形成，甲基汞很容易被生物放大，也就是说，生物体中的甲基汞浓度随着食物链的上升而增加。其他微生物有能力降解甲基汞，从而限制了这一生物放大过程。然而，这些已知的甲基汞降解系统在大多数环境中似乎并不重要。然而，最近的研究发现，不同的微生物通过一个尚未完全阐明的过程，在与环境更相关的条件下降解甲基汞。因此，这一过程可能对控制甲基汞的生物积累及其毒性非常重要。在这个项目中，我们将描述这个过程，并确定它在环境中的传播范围有多广。甲烷氧化菌，即甲烷氧化菌，在环境中广泛存在，但它们对汞的生物地球化学循环的影响才刚刚被研究。研究人员最近发现，甲烷氧化菌结合并去甲基化大量的甲基汞(MeHg)，这是一种神经毒性形式的汞，由厌氧微生物活动产生。值得注意的是，甲烷氧化菌没有编码具有良好特性的有机汞裂解酶的merB，这表明甲烷氧化菌使用一种尚不清楚的机制来去甲基化甲基汞。此外，与有机汞裂解酶不同，甲烷营养介导的甲基汞降解是在与环境相关的条件下(即汞的纳摩尔浓度和近中性的pH)观察到的，有机汞裂解酶只有在环境中罕见的条件下才能发挥作用。因此，似乎甲烷营养介导的甲基汞降解比典型的merB介导的途径更具环境意义。因此，这项提案的目标是确定甲烷氧化菌如何吸收和降解甲基汞。研究人员将研究一套跨越这些微生物的系统发育和生理多样性的甲烷氧化菌，以及其中一个物种的几个突变体，以确定这些微生物如何吸收甲基汞并使其去甲基化，以及甲基汞的吸收和降解对生长、活动和转录组的影响。

Methylmercury is a very potent neurotoxin produced by some microbes. Once formed, methylmercury can easily bio-magnify, that is, the concentrations of methylmercury in organisms increase as one goes up the food chain. Other microbes have the ability to degrade methylmercury, thus limiting this process of bio-magnification. These known systems of methylmercury degradation, however, do not appear to be significant in most environments. Recent work, however, has found that different microbes, through a process yet to be full elucidated, degrade methylmercury under more environmentally relevant conditions. This process may thus be very important in controlling methylmercury bioaccumulation, and its toxicity. In this project we will delineate this process and determine how wide-spread it may be in the environment. Methane-oxidizing bacteria, i.e., methanotrophs, are widespread in the environment, but their impact on biogeochemical cycling of mercury, has only just been investigated. The investigators have recently found that methanotrophs bind and demethylate substantial amounts of methylmercury (MeHg), a neurotoxic form of mercury that is generated via anaerobic microbial activity. What is remarkable is that methanotrophs do not have merB, encoding for the well-characterized organomercurial lyase, indicating that methanotrophs use an as yet unknown mechanism to demethylate MeHg. Further, methanotrophic-mediated MeHg degradation was observed under environmentally relevant conditions (i.e., nanomolar concentrations of mercury and circumneutral pH), unlike the organomercurial lyase, which is operative only under conditions rarely seen in the environment. As such, it appears that the methanotrophic-mediated MeHg degradation is much more environmentally significant than the canonical merB-mediated pathway. The objectives of this proposal are thus to determine how methanotrophs take up and degrade MeHg. Investigators will examine a suite of methanotrophs that span the phylogenetic and physiological diversity of these microbes as well as several mutants of one of these species to determine how these microorganisms take up and demethylate MeHg, and the impact of MeHg uptake and degradation on growth, activity and transcriptome.