| Summary: | 碩士 === 國立中央大學 === 環境工程研究所 === 103 === Mercury is a highly toxic trace element that has been recognized internationally as a global priority pollutant. Current inventories of mercury emissions indicate that anthropogenic activities are the major sources of mercury inputs to the environment, with coal combustion and solid waste incineration accounting for more than half of the total emissions. Once released, inorganic oxidized forms of mercury with relatively short atmospheric residence time would be deposited locally, then be converted by specific groups of anaerobic bacteria to methylmercury, a potent neurotoxin that can readily accumulate and magnify in biota, particularly in the aquatic food web. However, in terrestrial food chains, because lowland rice paddies display ecological functions similarly to wetlands that have been known as important sites for methylmercury formation, the paddy system can be potentially considered “hotspots” of mercury methylation. Indeed, recent studies have reported that aside from consumption of fish and seafood, high levels of methylmercury are detected in rice grown in the vicinity of anthropogenic mercury emission sources, suggesting that ingestion of rice may be another important human exposure route to methylmercury. Given that rice is a staple food in Taiwan and throughout Asia and the potential for maternal methylemrcury exposure (even at low-level) through ingestion of rice that may subsequently impact health of the offspring, it is important to conduct thorough investigation of this exposure pathway by examining why rice paddies are conductive for Hg methylation, which biogeochemical reactions may have been involved in this process, and also how additional inputs resulted from anthropogenic perturbations may eventually lead to the potential accumulation of Hg and MeHg in rice plants.
In this study, surface water, surface soil and rhizospheric soil and porewater in two rice fields near the Taichung Coal-Fired Power Plant Station were sampled. Analyses included total mercury, methylmercury and the geochemical parameters which may influence the mercury methylation cycle. In addition, microcosm, gene-probing and hydroponic experiments were carried out to investigate the primary microbes and processes that might have controlled the production of methylmercury in our study sites. Our results suggest that levels of total Hg and MeHg in paddy soil and rice grains did not exceed the current control standards set for farm land and edible rice, suggesting that the study sites are not contaminated with Hg and the air control devices employed in the coal-fired power plant may have been efficient for the control of Hg emission. However, it is observed that both bioavailability of inorganic Hg and the activity of Hg-methylating microbes were increased during the early and mid rice growing season. Results of soil incubation experiments and molecular probing revealed that sulfate-reducing bacteria may be the principal Hg-methylators in the rhizospheric zones of the study sites, suggesting that the paddy ecosystem has a great potential for enhanced Hg-methylation if elevated inputs of Hg occurred. Finally, results of hydroponic experiments implied that both passive diffusion and active transport may take place in the root uptake of MeHg in rice plants.
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