Published: March 02, 2011 r2011 American Chemical Society 2711 dx.doi.org/10.1021/es103384v | Environ. Sci. Technol. 2011, 45, 2711–2717 ARTICLE pubs.acs.org/est The Process of Methylmercury Accumulation in Rice (Oryza sativa L.) Bo Meng, †,‡ Xinbin Feng,* ,† Guangle Qiu, † Peng Liang, † Ping Li, † Chunxiao Chen, † and Lihai Shang † † State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, P.R. China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100049, P.R. China b S Supporting Information 1. INTRODUCTION The organic species methylmercury (MeHg) is of great concern due to its developmental neurotoxicity. 1 Because of its tendency to biomagnify in the food chain, MeHg concentrations in certain fish species can reach a level 10 6 times higher than ambient water Hg concentrations. 2 Consumption of fish, fish products, and marine mammals is currently considered as the main pathway of human exposure to MeHg, posing a worldwide human health threat. 3 However, recent studies have elucidated that rice consumption can be the main pathway of MeHg exposure to humans in Hg mining areas and also in certain inland areas in Southwestern China. 4-7 Generally, Hg concen- trations in most foodstuffs (except for fish) are below 20 μg kg -1 and mainly present in inorganic forms. 8,9 Regarding MeHg in rice, Horvat et al. 10 reported an elevated concentration of 140 μg kg -1 in rice collected at the Wanshan Hg mining site, Guizhou province. Qiu et al. 7 reported that MeHg in rice grown at abandoned Hg mining areas contained levels >100 μg kg -1 in the edible portion, which is 10-100 fold higher than that of other locally grown crop plants, such as corn (Zea mays L.), rape (Brassica campestris), tobacco (Nicotiana tabacum), and cabbage (Brassica oleracea). At Wuchuan, another large abandoned Hg mining area in NE Guizhou, similarly high levels of MeHg in rice plants have been reported. 11 More recently, Zhang et al. 5 deter- mined that on average, the bioaccumulation factors (BAFs) for MeHg in rice were more than 800 times (with the maximum of 40 000 times) higher than those for inorganic Hg. Meng et al. 12 demonstrated that the rice seed (brown rice) has the highest ability to accumulate MeHg compared to other tissues, and that paddy soil could be a potential MeHg source to tissues of rice plants. The rice paddy is one of the most prevalent land uses throughout South and East Asia, where rice provides the dominant staple food. The physicochemical conditions present in the paddy environment facilitate Hg-methylation due to the presence of a flora of sulfur-reducing bacteria (SRB). 13,14 Gouxi as a rural artisanal Hg production site and Huaxi as a regional background control site were selected for this study. Detailed information concerning each sampling location is shown in the Supporting Information. Received: October 6, 2010 Accepted: February 17, 2011 Revised: February 17, 2011 ABSTRACT: Recent studies have shown that rice consumption can be an important pathway of methylmercury (MeHg) exposure to humans in Hg mining areas and also in certain inland areas of Southwestern China. The seed of rice has the highest ability to accumulate MeHg compared to other tissues. The main objective of this study was to investigate the process of (MeHg) accumulation in rice seed (Oryza sativa L.) by monitoring MeHg levels in specific tissues of rice plants experiencing various levels of Hg multisource pollution during a full rice growing season. Four groups of experimental planta- tions were utilized, distributed among a rural artisanal Hg production site and a regional background control site. Our results suggest that the newly deposited Hg is more readily transformed to MeHg and accumulated in rice plants than Hg forms with an extended residence time in soil, and soil is the potential source of MeHg in the tissues of rice plants. MeHg in soil was first absorbed by roots and then translocated to the above-ground parts (leaf and stalk). During the full rice growing season only a very small amount of MeHg was retained in the root section. In the premature plant, the majority of MeHg is located in the leaf and stalk; however, most of this MeHg was transferred to seed during the ripening period.