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强效的神经毒素。甲基汞一旦形成，就很容易被生物放大，也就是说，随着食物链的上升，生物体中甲基汞的浓度会增加。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显著。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，在更环境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。甲烷氧化细菌，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只是刚刚被研究过。研究人员最近发现，甲烷氧化菌结合并使大量甲基汞（MeHg）去甲基化，这是一种通过厌氧微生物活动产生的汞的神经毒性形式。值得注意的是，甲烷氧化菌不含merB，而merB编码具有良好特征的有机聚合物裂解酶，这表明甲烷氧化菌使用一种尚不清楚的机制来脱甲基化甲基汞。此外，在与环境相关的条件下（即汞的纳摩尔浓度和环中性pH值），可以观察到甲烷氧化介导的甲基汞降解，而不像有机汞裂解酶，它只能在环境中很少见到的条件下起作用。因此，甲烷化介导的甲基汞降解似乎比典型的merb介导途径对环境的影响更大。因此，本提案的目标是确定甲烷氧化菌如何吸收和降解甲基汞。研究人员将研究一系列甲烷化营养菌，这些微生物跨越了这些微生物的系统发育和生理多样性，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和去甲基化甲基汞，以及甲基汞的摄取和降解对生长、活性和转录组的影响。

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.