稻田生态系统中硫生物地球化学循环对汞形态和生物可利用性的影响及其机制

Because of coal combustion, mining and smelting of mercury and other nonferrous metals, together with industrial and agricultural activities, soils have been polluted seriously with mercury in some areas of China. For instance, methylmercury has been detected in rice grains from mercury mining sites in Wanshan district,which poses a potential risk to human health.The study of mercury to methylmercury transformation will provide important clues on how to prevent methylmercury accumulation in rice grains. A number of studies in aquatic ecosystems have shown that sulfur biogeochemical cycle can profoundly affect mercury speciation and methylation.Which factors dominate the mercury transformation is likely to differ from ecosystem to ecosystem.In this study,rice will be cultured in root boxes filled with mercury contaminated paddy soil,together with different dosages of sulfur-containing materials.By taking advantages of synchrotron radiation, pedology, and molecular ecology methods,we will examine the content, distribution, speciation of sulfur and mercury (especially methylmercury) in the bulk soil and rhizosphere at different growth phases of rice.In addition, the distribution of sulfate reducing bacteria and the methylmercury migration process in the rhizosphere and bulk soil will be studied to elucidate how soil ecosystem affects mercury transformation. The effect of water management on mercury speciation, migration, and mercury-sulfur synergistic transformation will also be investigated. Taken together, the influence of sulfur biogeochemistry cycle on mercury speciation, bioavailability and its potential mechanism in rice paddy ecosystem will be explored. This study will provide a scientific basis for how to control mercury absorption and methylmercury accumulation in rice by sulfur fertilization and other agronomic management practices.

燃煤、汞等有色金属的开采冶炼、工农业生产使我国部分地区土壤受到了汞污染，万山汞矿区所产稻米中含有甲基汞，对人体健康构成威胁，研究稻田土壤中汞的形态转化机制对调控水稻汞和甲基汞的吸收有重要意义。对水生态系统的大量研究表明硫生物地球化学循环对汞的形态转化有重要影响。本项目针对稻田生态系统的特点，在不同硫状态下的汞污染稻田土壤中培植水稻，结合同步辐射技术与土壤重金属污染化学和分子生态学研究技术，调查水稻生长各时期根际土壤、非根际土壤、水稻植株中硫、汞（特别是甲基汞）的含量、微区分布和结合形态、硫酸盐还原菌的种群结构和根际空隙水内甲基汞的迁移过程，研究稻田土壤干湿交替变化条件下硫状态对汞形态和迁移性的影响，在此基础上探究稻田生态系统中硫的生物地球化学循环对汞形态及生物可利用性的影响及可能机制。研究的开展可为开发通过硫素施用并结合其它农艺措施原位阻控水稻汞（甲基汞）吸收的有效技术提供科学依据。

用贵州和海南稻田土壤培植水稻，通过测定不同硫、汞暴露条件下稻田土壤、根际、根表铁膜、稻米及其他组织的汞含量、微区分布、形态以及微生物群落结构、甲基化相关基因hgcA拷贝数，研究了硫对土壤汞形态及稻米汞蓄积的影响及可能机制。取得以下结果：. 稻田土壤中S、S2O32-、SO42-均可转化成S(-II)。后者使土壤可交换态和有机结合态汞转化为残渣态；使土壤残渣态汞（HgS）形成配合物溶解。. 硫酸盐添加对贵州稻田土壤微生物群落结构无规律性影响，对hgcA基因拷贝数无显著影响，硫酸盐加入引起的HgS溶解增加了Hg的生物可利用性，另一方面也使硫酸盐还原菌活性增加，造成土壤甲基汞含量升高。. 根表铁膜可以吸附无机汞和甲基汞，抑制它们的吸收及向上转运，对甲基汞的影响更显著，S可以促进根表铁膜形成。. 水稻体内存在显著的甲基汞的去甲基化，没有明显的汞甲基化过程。S，Se，铁膜对水稻体内汞的转化无明显影响。 . 硫化汞暴露条件下稻米汞含量与硫化汞粒径、晶型有关。粒径越小，蓄积越多，纳米硫化汞暴露使稻米汞蓄积水平与HgCl2基本相当；β-HgS暴露稻米汞蓄积水平高于α-HgS；稻米蓄积汞的主要形式是甲基汞；硫酸盐使各种不同形态HgS暴露的稻米汞蓄积水平提高；硫化汞暴露条件下水稻汞蓄积的原因是微生物将HgS转化成甲基汞，另一方面水稻可以直接吸收纳米HgS。. 水稻根部汞主要结合于15-25 kDa蛋白条带。IHg暴露下该条带汞含量显著高于MeHg暴露；加Se使部分汞与分子量为55-70 kDa蛋白结合，与15-25 kDa结合的汞含量减少；硒同时存在于这些含汞条带中，提示硒诱导了汞与大分子蛋白的结合，硒汞间可能有直接结合。这可能是硒抑制汞向上转运的原因。. 这些结果为深入研究硫、汞生物地球化学循环的相互影响提供了基础，也为合理施用含硫肥料调控稻米汞蓄积提供了科学依据。.