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Abstract Title: Carbon and oxygen alter mercury methylation in rice paddy soil and MeHg accumulation in rice grain
Presenter Name: Rachel Strickman
Session: Mercury in Freshwater Ecosystems
Co-Authors: Rachel Strickman

Abstract Information :

Contamination of rice grain with methylmercury (MeHg) is a public health threat. Biotic methylation and demethylation dominate MeHg dynamics in rice paddies. Both processes may be up- or down-regulated by the supply of organic carbon and oxygen, but existing studies largely have neglected the role of oxygen and demethylation on regulating soil MeHg concentration. To address this gap, we manipulated carbon and oxygen levels in the soil of experimental microcosms (N=9) planted with rice. Oxygen was delivered from a reservoir of oxygen gas using permeable silicone tubing. Carbon was supplied as glucose in hydroponically-collected root exudate solution. At tillering, we assessed methylation and demethylation potential rate constants (Kmeth and Kdemeth), and soil MeHg. Upon maturity, grain was also analyzed for MeHg. Carbon increased soil MeHg in the carbon-only treatment (0.26ñ0.13 ng/g) relative to the control (0.13ñ0.01 ng/g; p=0.019), and in the carbon/oxygen treatment (0.19ñ0.13ng/g) relative to oxygen-only treatment (0.06ñ0.02ng/g; p=0.015). Soil MeHg was lower in the oxygen-only treatment relative to control, and lower in the carbon/oxygen treatment relative to the carbon-only treatment, suggesting that oxygen reduced soil MeHg, but these differences were not significant (p=0.34,0.38). Kmeth displayed a pattern of treatment-level effects identical to those for MeHg concentration and correlated strongly with soil MeHg (r=0.70 p=0.01). Our findings indicate the carbon supply stimulated soil Kmeth and thus soil MeHg concentrations. This process occurred even in the presence of supplemental oxygen. Demethylation (0.14?0.54 per day) did not differ among experimental treatments (p=0.183). MeHg concentrations in grain showed a significant interaction with carbon and oxygen, with less (p=0.005) MeHg in carbon-oxygen treatment (8.08ñ2.54ng/g) compared to the carbon-only treatment (12.34ñ3.4ng/g). We infer that the presence of oxygen partially offsets the stimulatory effect of carbon on rice grain MeHg burdens, suggesting that rice varieties with high radial oxygen loss may accumulate less MeHg.

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