Spatial Location and Biogeochemical Drivers of Mercury Methylation and Demethylation in the Rice Rhizosphere

大米是40多亿人的主食，可能是人类接触甲基汞（MeHg）的主要途径。甲基汞在生物体中具有生物蓄积性，具有高度神经毒性。即使在微量水平，甲基汞也会对人体健康产生不利影响。相比之下，无机汞的生物可利用性较低，不会进行生物积累。因此，水稻的甲基汞污染是一个全球性的公共卫生问题。稻谷中甲基汞的含量来源于水稻土，被水稻根系吸收并输送到发育中的稻谷中。所有土壤都含有来自自然以及现代和历史上人为来源的无机汞。水稻土中的甲基汞是由水生微生物从这种无机汞中产生的，这一过程被称为甲基化。另一组微生物将甲基汞转化为无机汞，称为去甲基化。植物吸收甲基汞的有效性取决于水稻根系周围土壤区（称为水稻根际）甲基化和去甲基化的相对速率。水稻根系向根际土壤释放氧气和有机碳。有机碳和氧的供应都可以调节甲基化和去甲基化。该项目旨在阐明植物输入的氧和有机碳如何影响甲基化和去甲基化速率，从而改变根际甲基汞浓度。研究结果可以为减少水稻中甲基汞污染的农业策略提供信息。例如，这些发现可以指导水稻品种的选择，这些品种释放的氧和碳的数量和比例有利于去甲基化而不是甲基化。项目人员将制作一段艺术性高质量的视频，向K-12学生、非专业人士、政策制定者和水稻种植者传播研究结果，展示水稻和其他食品中甲基汞的健康危害，介绍复杂的甲基汞循环，并讨论减少稻田中甲基汞产量的策略。该项目将根渗液和土壤氧合的实验操作与汞同位素物种形成方法和根际氧浓度的二维实时可视化相结合，以验证碳和氧如何影响水稻土中甲基化和去甲基化的速率和空间发生的假设。在营养生长高峰期，在氧可视化的指导下，以毫米尺度向根际土壤注入富集同位素MeHg和无机汞示踪剂，以评估不同氧化还原区的甲基化和去甲基化速率。在植物完成其生命周期后，收集稻米，以确定改变的根分泌物或土壤氧合对稻米中甲基汞浓度的净综合影响，这是与人类健康最相关的终点。这种独特的方法可以在一个完整的、有生命的植物-土壤系统中，在一个精细的尺度上检查甲基汞生产的空间分布和生物地球化学驱动因素。这种空间分辨率是表征有机碳和氧如何增加或减少水稻根际甲基汞净产量的必要条件，从而有助于控制水稻籽粒的甲基汞污染。重要的是，该项目直接研究汞去甲基化，这是土壤中甲基汞降解的重要途径，但与汞甲基化相比，人们对这一过程知之甚少。

Rice, a staple food of more than four billion people, can serve as a primary route of human exposure to methylmercury (MeHg). Methylmercury bioaccumulates in organisms and is highly neurotoxic. Even at trace levels, MeHg adversely affects human health. By contrast, inorganic mercury is less bioavailable and does not bioaccumulate. Thus, MeHg contamination of rice is a global public health concern. The MeHg content of rice grain originates from the paddy soil, where it is taken up by rice roots and transported to the developing grain. All soils contain inorganic mercury derived from natural, as well as modern and historical anthropogenic sources. The MeHg in paddy soil is produced by aquatic microorganisms from this inorganic mercury, a process known as methylation. Another suite of microorganisms transform MeHg back into inorganic mercury, termed demethylation. The availability of MeHg for plant uptake depends on the relative rates of methylation and demethylation in the soil zone surrounding rice roots, an area called the rice rhizosphere. The roots of rice plants release both oxygen and organic carbon into rhizosphere soil. The supply of both organic carbon and oxygen can modulate methylation and demethylation. This project is clarifying how plant inputs of oxygen and organic carbon affect rates of methylation and demethylation and thus alter rhizosphere MeHg concentrations. Results can inform agricultural strategies to minimize MeHg contamination of rice grain. For example, findings may guide selection of rice cultivars that release oxygen and carbon in amounts and proportions that favor demethylation over methylation. Project personnel will disseminate results to K-12 students, laypeople, policymakers, and rice growers by creating an artistic, high-quality video that presents health hazards of MeHg in rice and other foods, introduces the complex MeHg cycle, and discusses strategies for minimizing MeHg production in rice paddies. The project combines experimental manipulations of root exudates and soil oxygenation with mercury isotope speciation methods and two-dimensional, real-time visualizations of rhizosphere oxygen concentrations to test hypotheses about how carbon and oxygen influence rates and spatial occurrence of methylation and demethylation within paddy soil. During peak vegetative growth, enriched isotope MeHg and inorganic mercury tracers are injected at the mm scale into rhizosphere soil, guided by oxygen visualizations, to assess rates of methylation and demethylation in different redox zones. After plants complete their lifecycle, grain is collected to identify the net, integrated effect of altered root exudation or soil oxygenation on MeHg concentrations in rice grain, the endpoint most relevant to human health. This unique approach allows examination of the spatial distribution and biogeochemical drivers of MeHg production at a fine scale in an intact, living, plant-soil system. Such spatial resolution is necessary to characterize how organic carbon and oxygen enhance or diminish the net production of MeHg in the rhizosphere of rice, and thus help control MeHg contamination of rice grain. Importantly, the project directly studies mercury demethylation, an important pathway for MeHg degradation in soils, but a process that is poorly understood in comparison to mercury methylation.