Mercury Dynamics in the Rice Rhizosphere
Plant growth laboratory experiments
Rice, a staple food of more than four billion people, can also serve as a primary route of human exposure to methylmercury (MeHg). This bioaccumulative neurotoxicant can cause adverse effects even at trace levels, making MeHg contamination of rice a global public health concern. Methylmercury in rice grain originates from the paddy soil, where it is both microbially formed from inorganic mercury (methylated) and degraded back to inorganic mercury (demethylated). There is evidence that the release of oxygen and labile organic carbon from plant roots can affect both methylation and demethylation, suggesting that selection of rice cultivars with optimal rates of these processes could be a management tool for reducing MeHg contamination of rice grain. However, this management approach requires better understanding of how root oxygen loss and root carbon exudation affect the integrated balance of Hg methylation and demethylation.
We are growing rice in the lab and pairing two-dimensional, real-time visualizations of rhizosphere oxygen concentrations with manipulations of root exudates and soil oxygenation to 1) assess the relative importance of Hg methylation and demethylation to the total accumulation of MeHg in different areas of the rice rhizosphere, and 2) determine the relative importance of organic carbon and oxygen supplied by roots on rates of methylation and demethylation. We are manipulating root exudates by growing plants in under different atmospheric concentrations of carbon dioxide (root exudation increases under elevated CO2), we are increasing soil oxygenation by laying gas permeable tubuals along rice roots, and we are measuring rates of methylation and demethylation using mercury isotope tracers.
Our work has direct societal relevance due to its focus on rice, a staple food, and holds potential to inform agricultural strategies that minimize MeHg contamination of rice grain.