Mechanistic understanding of MeHg-Se antagonism in soil-rice systems: the key role of antagonism in soil.

Yongjie Wang,Dongmei Zhou,Huan Zhong,Jiating Zhao,Fei Dang,R Douglas Evans

doi:10.1038/srep19477

Abstract

Methylmercury (MeHg) accumulation in rice has great implications for human health. Here, effects of selenium (Se) on MeHg availability to rice are explored by growing rice under soil or foliar fertilization with Se. Results indicate that soil amendment with Se could reduce MeHg levels in soil and grain (maximally 73%). In contrast, foliar fertilization with Se enhanced plant Se levels (3–12 folds) without affecting grain MeHg concentrations. This evidence, along with the distinct distribution of MeHg and Se within the plant, demonstrate for the first time that Se-induced reduction in soil MeHg levels (i.e., MeHg-Se antagonism in soil) rather than MeHg-Se interactions within the plant might be the key process triggering the decreased grain MeHg levels under Se amendment. The reduction in soil MeHg concentrations could be mainly attributed to the formation of Hg-Se complexes (detected by TEM-EDX and XANES) and thus reduced microbial MeHg production. Moreover, selenite and selenate were equally effective in reducing soil MeHg concentrations, possibly because of rapid changes in Se speciation. The dominant role of Se-induced reduction in soil MeHg levels, which has been largely underestimated previously, together with the possible mechanisms advance our mechanistic understanding about MeHg dynamics in soil-rice systems.

Highlights

Concerns about methylmercury (MeHg) accumulation in rice grain have been raised, mainly because consumption of mercury contaminated rice[1], in addition to fish, is considered to be an important pathway of human exposure to MeHg2
Significant differences in soil MeHg levels were noted among treatments
MeHg levels decreased by 13–46% on day 20 and were less variable on day 125 (Fig. 1B), with Se dose having a significant effect on soil MeHg but not Se species (F1, 22 = 2.663, p = 0.122) on day 20

Summary

Introduction

Concerns about methylmercury (MeHg) accumulation in rice grain have been raised, mainly because consumption of mercury contaminated rice (up to 145 μ g MeHg kg–1)[1], in addition to fish, is considered to be an important pathway of human exposure to MeHg2. Little information has been available about the potential effects of Se on MeHg accumulation in plants until recently when a pilot field survey demonstrated a downward trend in brown rice MeHg levels with increasing soil Se levels in field-collected samples from a mining-contaminated area[16]. Of MeHg translocation within plant and/or possible complexation between IHg and Se in soil were hypothesized to account for the reduced MeHg accumulation in rice following Se addition, i.e., MeHg-Se antagonism[16,17]. These data are important and valuable in the evaluation of MeHg-Se antagonism. The effect of both selenite and selenate (the main Se species for plant uptake from soil)[25], in varying concentrations, as well as soil versus foliar fertilization with Se, on MeHg bioaccumulation will be explored using two soils with contrasting ambient Se levels (i.e., Low-Se and High-Se soils)

Methods

Results

Discussion

Conclusion