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

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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.