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

大米是超过40亿人的主食，可以作为人类接触甲基汞(MeHg)的主要途径。甲基汞在生物体内积累，具有高度的神经毒性。即使在微量水平上，甲基汞也会对人类健康造成不利影响。相比之下，无机汞的生物利用率较低，不会生物积累。因此，大米的甲基汞污染是一个全球公共卫生问题。稻谷中甲基汞的含量来源于稻田土壤，在稻田土壤中被水稻根部吸收并输送到发育中的稻谷中。所有土壤都含有来自自然、现代和历史人为来源的无机汞。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重新转化为无机汞，称为去甲基化。植物吸收甲基汞的有效性取决于水稻根部周围土壤区域甲基化和去甲基化的相对速率，这一区域被称为水稻根际。水稻根部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。该项目正在阐明植物输入的氧气和有机碳如何影响甲基化和去甲基化的速率，从而改变根际甲基汞的浓度。这一结果可以为农业战略提供参考，以最大限度地减少稻谷中的甲基汞污染。例如，研究结果可能会指导水稻品种的选择，这些品种释放的氧气和碳的数量和比例有利于去甲基化而不是甲基化。项目人员将通过制作一个艺术性的高质量视频向K-12学生、普通人、政策制定者和稻米种植者传播成果，展示大米和其他食品中甲基汞对健康的危害，介绍复杂的甲基汞循环，并讨论将稻田中的甲基汞产量降至最低的策略。该项目将根分泌物和土壤氧化的实验操作与汞同位素形态形成方法和根际氧浓度的二维实时可视化相结合，以测试关于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说。在营养生长高峰期，在氧气可视化的引导下，向根际土壤注入毫米尺度的浓缩同位素甲基汞和无机汞示踪剂，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，收集谷物以确定根部分泌物或土壤氧化改变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。这种独特的方法允许在一个完整的、活的植物-土壤系统中以精细的规模检查甲基汞生产的空间分布和生物地球化学驱动因素。这种空间分辨率对于表征有机碳和有机氧如何增强或减少水稻根际甲基汞的净生产量，从而帮助控制水稻籽粒中的甲基汞污染是必要的。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.