;Since the industrial revolution, the release of mercury (Hg) from emission sources to the environment has been predominantly resulted from human activities, with burning of fossil fuels and waste being the leading contributors. Once released, a partial amount of Hg (mostly in its divalent inorganic forms) would return to the Earth’s surface by wet or dry deposition and then be converted in situ by certain heterogeneous anaerobic bacteria to methylmercury (MeHg), the most toxic form of Hg and known for its great bioaccumulation tendency . While consumption of predatory fish and seafood has been considered the primary route for human exposed to MeHg, recent studies have reported high levels of MeHg in rice grown in the vicinity of anthropogenic Hg releasing sources, suggesting that ingestion of crops from the terrestrial food chain may be another critical route of human exposure to MeHg. Given that (i) rice is a staple food in Taiwan and throughout Asia and (ii) the potential for maternal MeHg 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 route 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, four paddy sites within the agricultural area of the Beitou municipal solid waste (MSW) incinerator were chosen to sample surface water, topsoil and root soil. Total Hg, MeHg, as well as ancillary geochemical/microbiological parameters in soil, porewater, and rice grains were analyzed. In addition, microcosm and hydroponic experiments were carried out to probe (i) the primary Hg methylators in the root soil of the study sites and (ii) the influence of coordination chemistry on the uptake of MeHg by roots of rice plants. Results showed that the levels of total Hg and MeHg in paddy soil and rice grains did not exceed the current standards set for farmland soil and edible rice, suggesting that our study sites are not contaminated with Hg and the air control devices employed in the Beitou MSW incinerator may have been efficient for the control of Hg emission. However, it is observed that both the bioavailability of inorganic Hg and the activity of Hg-methylating microbes increased during the early and mid rice growing season, presumably due to the anoxia created under flooded conditions. This suggested that the paddy ecosystem has a great potential for enhanced Hg-methylation if elevated inputs of Hg occurred, and hence there is a need for constant monitoring of the Hg level in this area. Results of microcosm experiments revealed that sulfate-reducing bacteria may be the principal Hg-methylators in the rhizospheric zones of the study sites. Molecular identification of the hgcA gene in the root soil samples further confirmed the existence of Hg-methylating microbes. Lastly, using different forms of ligands to alter MeHg speciation in the growing medium, preliminary results from the hydroponic culturing of rice implied that both passive diffusion and active transport mechanisms may all take place in the root uptake of MeHg in rice.