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

水稻根际汞甲基化和去甲基化的空间位置和生物地球化学驱动因素

• 批准号: 1740839
• 负责人: Rebecca Neumann
• 金额: $ 35.04万
• 依托单位: University of Washington
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-11-01 至 2022-10-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1740839&HistoricalAwards=false
• 关键词: Spatial; Location; Biogeochemical; Drivers; Mercury

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.

大米是40多亿人的主食，也是人类接触甲基汞（MeHg）的主要途径。甲基汞在生物体内生物累积，具有高度神经毒性。即使是微量的甲基汞，也会对人类健康产生不利影响。相比之下，无机汞的生物利用度较低，不会发生生物累积。因此，稻米中的甲基汞污染是一个全球性的公共卫生问题。水稻籽粒中的甲基汞含量来源于水稻土，在水稻土中被水稻根系吸收并运输到发育中的籽粒中。所有土壤都含有来自自然以及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转化为无机汞，称为脱甲基作用。植物吸收甲基汞的有效性取决于水稻根周围土壤区域（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和有机碳。有机碳和氧的供应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的速率，从而改变根际甲基汞浓度。研究结果可以为农业战略提供信息，以尽量减少稻米中的甲基汞污染。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。项目工作人员将制作一部艺术性的高质量视频，介绍稻米和其他食品中的甲基汞对健康的危害，介绍甲基汞的复杂循环，并讨论尽量减少稻田中甲基汞生产的战略，从而向K-12学生、非专业人士、决策者和稻米种植者传播研究成果。该项目结合实验操作的根分泌物和土壤氧化与汞同位素形态的方法和二维，根际氧浓度的实时可视化测试假设碳和氧如何影响率和空间发生的甲基化和去甲基化水稻土。在高峰期的营养生长，富集同位素甲基汞和无机汞示踪剂注入根际土壤中的毫米尺度，氧可视化的指导下，在不同的氧化还原区的甲基化和去甲基化率进行评估。在植物完成其生命周期后，收集谷物，以确定改变根系分泌物或土壤氧化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-土壤系统中以精细的尺度检查甲基汞生产的空间分布和生物地球化学驱动因素。这样的空间分辨率是必要的，以表征有机碳和氧如何增强或减少甲基汞在水稻根际的净生产，从而帮助控制甲基汞污染的稻米。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一个重要途径，但与汞甲基化相比，对这一过程的了解甚少。