汞在双穗雀稗中的富集及形态转化机理研究

Health and ecosystem issues derived from mercury (Hg) contamination is one of the top concerns of the society. Efforts in controlling and mitigating the risk of Hg contamination in environment therefore may significantly contribute to securing the safety of the agricultural products. Phytoextraction using hyperaccumulating plants is regarded as the most promising remediation approach. However, no Hg hyperaccumulator has been reported yet. In order to promote the development of phytoextraction of Hg contaminated environmental media, there is an urgent need to seek and screen the suitable plant species which have a super ability to uptake and accumulate Hg. Recently we found out an aquatic plant, named Paspalum paspaloides, was found in a Hg industrial zone. The total concentrations of Hg (THg) and as methylmercury (MeHg) in the plant tissues are higher than those in other reported plants. Little information is available about this plant in terms of its uptake, accumulation, translocation and tolerance of Hg and relevant mechanism as well. In this project, such promising techniques as greenhouse trial, stable isotope tracer, X-ray absorption near-edge structure analysis and proteomics will be employed to address i) the kinetics related to Hg uptake by the root of P. Paspaloides in both inorganic and organic forms; ii) influence of iron-plaque on the Hg influx through soil-root interface; iii) the existing forms and distribution of Hg at the tissue, cellular and subcellular levels as well as any possible transformations; iv) the relationship between protein syntheses and different Hg stresses; v) emission flux of Hg via plant-air interface, in an attempt to reveal the mechanism related to high level Hg accumulation occurring in P. Paspaloides; and vi) the pot trial concerning the phytoextraction efficient using this plant on two Hg-contaminated soils. It is hoped that the outcomes from this study may provide a solid theoretical basis for scientific evaluation to the potential and feasibility if apply this plant in the remediation practice.

汞污染的健康和生态风险是当前社会关注的热点。加强汞污染环境治理，对于保障农产品的食用安全性具有极其重要的意义。植物提取技术被认为是最具发展前景的汞污染环境修复手段。但是目前汞超富集植物尚未见报道，发掘具有汞高富集能力的植物种质资源实乃当务之急。新近在汞工业区发现的水生植物双穗雀稗能高度富集总汞和甲基汞，具有应用于汞污染植物修复的潜力。关于双穗雀稗对于汞的吸收、累积、迁移和耐性的研究鲜有报道。本项目拟通过培养实验、稳定同位素示踪、X射线吸收近边结构分析和蛋白组学技术，以阐明①双穗雀稗对不同形态汞的吸收动力学；②铁膜对根-土界面汞吸收通量的影响；③汞在组织水平、细胞水平和亚细胞水平汞赋存形态及分布特征和可能的形态转化；④植物体内蛋白表达与汞浓度的关联性；⑤汞在植物-大气界面的释放通量；⑥污染土壤修复实际效果，从而揭示双穗雀稗汞富集的内在机理，并为科学地评估其汞污染修复潜力及可行性提供理论依据。

本研究以双穗雀稗为研究对象，通过室内水培及土培模拟实验，探讨双穗雀稗对汞的富集特征。研究结果表明：（1）双穗雀稗对Hg2+和MeHg+的吸附过程均符合伪二级动力学模型，线性系数R2的数值范围均达到0.996-0.999。粒子内扩散模型的线性系数R2的数值范围是0.912-0.996和0.947-0.988，并且速率常数k3与伪二级动力学速率常数k2随Hg2+和MeHg+初始浓度的变化是相反的，这暗示粒子内扩散也影响这两个吸附过程；（2）根部铁膜的存在提高了双穗雀稗对THg 和MeHg的富集系数，但显著降低了其转运系数，并且铁膜促进了Hg的甲基化过程；（3）半纤维素1类中的羧基是双穗雀稗细胞壁中Hg的主要结合位点。当有铁膜覆盖后，主要结合位点变为果胶，果胶中的氨基和叁键或者累积双键为Hg的主要结合位点，（4）在土培试验中，根际距离和生长周期对土壤中Hg的变化趋势有较强的影响。在60天的种植过程中，高浓度Hg污染地区土壤中THg的值在最接近根部(0-2 cm)的区域降低了45%，MeHg下降了64%，但在20天时，土壤0-4cm处MeHg浓度受到根系分泌物的影响会出现增高的表现。土壤中Hg的不同形态分析结果显示，元素态与残渣态Hg为土壤中Hg最主要的表现形态，但在高浓度Hg汞污染土壤中，有机结合态Hg也是最重要的表现形态之一，其代表了强烈的生物利用性，也代表了环境风险；（5）蛋白质组学技术研究结果显示，在汞胁迫下，双穗雀稗总共有49种丰度差异蛋白，其中有32个上调表达与17个下调表达，这些差异蛋白在经过基因本体论分析后表现出物种分子功能，分别是光合作用与能量代谢(31%), 氧化应激反应(14%), 蛋白质折叠(16%), 硫化合物代谢(10%)，金属结合与离子转运(29%)。此外, 蛋白质表达情况主要与代谢通路相关，占60%以上。双穗雀稗响应Hg胁迫的蛋白质组学分析结果表明光合作用与能量代谢及大量的代谢通路是该过程最主要的变化形式，但硫化合物代谢中5'-Adenylyl sulfate 还原酶的下调表达是双穗雀稗响应Hg胁迫与其他植物相比表达趋势相反的蛋白，主要是由于硫化汞的形成。