title: Investigation of biogeochemical controls on the formation, uptake and accumulation of methylmercury in rice paddies in the vicinity of a coal-fired power plant and a municipal solid waste incinerator in Taiwan.
abstract: Recent studies have shown that rice consumption is another critical route of human exposure to methylmercury (MeHg), the most toxic and accumulative form of mercury (Hg) in the food web. Yet, the mechanisms that underlie the production and accumulation of MeHg in the paddy ecosystem are still poorly understood. In 2013 and 2014, we conducted field campaigns and laboratory experiments over a rice growing season to examine Hg and MeHg cycling, as well as associated biogeochemistry in a suite of paddies close to a municipal solid waste incinerator and a coal-fired power plant station in Taiwan. Concentrations of total Hg and MeHg in paddy soil and rice grain at both sites were low and found not to exceed the control standards for farmland soil and edible rice in Taiwan. However, seasonal variations of MeHg concentrations observed in pore water samples indicate that the in situ bioavailability of inorganic Hg and activity of Hg-methylating microbes in the rhizosphere increased from the early-season and peaked at the mid-season, presumably due to the anoxia created under flooded conditions and root exudation of organic compounds. The presence of Hg-methylators was also confirmed by the hgcA gene detected in all root soil samples. Subsequent methylation tests performed by incubating the root soil with inorganic Hg and an inhibitor or stimulant specific for certain microbes further revealed that sulfate-reducers might have been the principal Hg-methylting guild at the study sites. Interestingly, results of hydroponic experiments conducted by cultivating rice in a defined nutrient solution amended with fixed MeHg and varying levels of MeHg-binding ligands suggested that chemical speciation in soil pore water may play a key role in controlling MeHg accumulation in rice, and both passive and active transport pathways seem to take place in the uptake of MeHg in rice roots.
authors.affiliation: Graduate Institute of Environmental Engineering, National Central University, Taoyuan 32001, Taiwan, ROC.
authors.forename: Yen-Bin
authors.initials: YB
authors.lastname: Su
authors.affiliation: Graduate Institute of Environmental Engineering, National Central University, Taoyuan 32001, Taiwan, ROC.
authors.forename: Wei-Chun
authors.initials: WC
authors.lastname: Chang
authors.affiliation: Institute of Environmental Engineering and Management, National Taipei University of Technology, Taipei 10608, Taiwan, ROC; Graduate Institute of Environmental Engineering, National Taiwan University, Taipei 10617, Taiwan, ROC.
authors.forename: Hsing-Cheng
authors.initials: HC
authors.lastname: Hsi
authors.affiliation: Graduate Institute of Environmental Engineering, National Central University, Taoyuan 32001, Taiwan, ROC. Electronic address: chuching@ncu.edu.tw.
authors.forename: Chu-Ching
authors.initials: CC
authors.lastname: Lin
mesh_terms: D001682:Biological Availability; D003031:Coal; D004784:Environmental Monitoring; D020387:Food Chain; D005506:Food Contamination; D006801:Humans; D017745:Incineration; D008628:Mercury / Q000032:analysis*; D008767:Methylmercury Compounds / Q000032:analysis* / Q000737:chemistry; D012275:Oryza / Q000737:chemistry* / Q000382:microbiology; D018517:Plant Roots / Q000378:metabolism; D011210:Power Plants; D058441:Rhizosphere; D012987:Soil / Q000737:chemistry*; D012988:Soil Microbiology; D012989:Soil Pollutants / Q000032:analysis*; D062611:Solid Waste; D013624:Taiwan; D018505:Waste Management; D014867:Water / Q000378:metabolism
chemical_list: D003031:Coal; D008767:Methylmercury Compounds; D012987:Soil; D012989:Soil Pollutants; D062611:Solid Waste; D014867:Water; D008628:Mercury
keywords: Biogeochemistry; Methylmercury; Paddy soil; Rice; Root uptake