ETBC: COLLABORATIVE RESEARCH: MASS-DEPENDENT AND INDEPENDENT MERCURY ISOTOPE FRACTIONATION DURING MICROBIAL METHYLATION AND REDOX TRANSFORMATIONS OF MERCURY IN NATURAL WATERS

知识价值：拟议的研究是基于PI和其他人的最新成就，并寻求扩大PI？微生物和非生物汞转化过程中同位素分馏的持续研究。在过去五年中，主要研究成果表明：（一）在通过几种具有汞还原酶的细菌菌株对离子汞（Hg[II]）进行微生物还原的过程中，发生了质量依赖性同位素分馏，在甲基汞（MeHg）的微生物降解过程中也发生了质量依赖性同位素分馏;（ii）Hg（II）的光还原和甲基汞的光降解导致了高达2？的质量非依赖分馏（MIF）。在淡水和海水鱼类组织中记录到的不同数量的MIF（表示为<$199 Hg和<$201 Hg）表明，甲基汞在进入水生食物网之前发生了不同程度的光降解。这些结果沿着由Blum实验室和其他人记录的天然样品中的大的变化，强烈地表明汞的同位素组成具有区分不同来源的Hg（0）排放和Hg（II）减少和MeHg降解的途径的潜力。研究主要集中在汞的氧化还原转化和甲基汞的降解。然而，甲基汞在水生食物网中的生物积累对人类和生态系统健康有着深远的影响，因此，检查汞甲基化过程中是否发生同位素分馏是一个高度优先事项。如果在甲基汞的形成过程中发生了显著的分馏，并受到不同环境条件和甲基化过程性质的调节，那么就有可能找到区分环境中甲基汞来源和途径的工具，从而加强对受汞污染的生态系统的管理。拟议的研究的第一个目标是检查汞甲基化过程中的同位素分馏硫酸盐和铁还原细菌来测试的假设，微生物汞甲基化的结果显着的汞，但不是在MIF。第二个目标是调查界定淡水和海洋环境的环境变量如何在汞氧化还原转化过程中影响MIF和MIF，以检验光化学还原、氧化和去甲基化将在反应底物和产物上留下诊断性MIF和MIF特征的假设。最后，汞同位素分馏过程中介导的一个重要组成部分的水生生态系统，光养水生生物的转化途径，迄今尚未检查。第三个目标解决了这种缺乏知识的假设，即细胞内汞（II）减少和甲基汞降解浮游植物孵育的光将导致在浮游植物和可能的MIF。更广泛的影响：拟议的研究活动将继续为新方法或确定受影响生态系统中汞转化的源、汇和途径奠定基础。这种方法有可能大大提高对汞的地球化学的时间和空间尺度的理解，从分子机制，生态系统，全球循环，地质记录。由于生态系统汞污染仍然是一个重大的公共卫生问题，该项目将支持实施健全的环境做法，以减少汞污染和接触。拟议的研究将培养一名博士后研究员，将稳定同位素方法应用于地球生物学和生态系统过程。此外，本科生和研究生将被纳入该项目，使他们接触到生物学，地质学和生态系统科学之间的前沿概念和技术。正是在这个界面上，重要的范式转变，研究进展正在取得。本科生将协助分析地球化学作为高级论文研究项目和博士论文项目的一部分。结果将予以公布和广泛传播。

Intellectual merit: The proposed research is based on recent achievements by the PIs and others, and seeks to expand the PIs? on-going studies of isotopic fractionation during microbial and abiotic Hg transformations. During the last five years the PIs have shown that: (i) Mass dependent isotopic fractionation (MDF) occurred during the microbial reduction of ionic mercury (Hg[II]) by several bacterial strains that possess the enzyme mercuric reductase and MDF also occurred during microbial methylmercury (MeHg) degradation; (ii) Photoreduction of Hg(II) and photodegradation of MeHg caused MDF as well as mass independent fractionation (MIF) of up to 2?. Varying amounts of MIF (denoted as Ä199Hg and Ä201Hg) recorded in freshwater and marine fish tissue suggested different extents of photo degradation of MeHg prior to its incorporation into the aquatic food web. These results along with large variations in natural samples documented by the Blum lab and others, strongly suggest that the isotopic composition of Hg has the potential for distinguishing between different sources of Hg(0) emissions and pathways of Hg(II) reduction and MeHg degradation.To date, the PIs? studies have focused on Hg redox transformations and MeHg degradation. Yet, the bioaccumulation of MeHg in aquatic food webs has a profound effect on human and ecosystem health and the examination of whether or not isotope fractionation occurs during methylation of Hg is, therefore, a high priority. If significant fractionation occurs during formation of MeHg and is modulated by different environmental conditions and by the nature of the methylating processes, tools for distinguishing sources and pathways of MeHg in the environment may become available and enhance the management of Hg contaminated ecosystems. The first objective of the proposed study is the examination of isotopic fractionation during Hg methylation by sulfate and iron reducing bacteria to test the hypothesis that microbial Hg methylation results in significant MDF, but not in MIF. The second objective is the investigation ofhow environmental variables, which define freshwater and marine environments, impact MIF and MDF during Hg redox transformations to test the hypothesis that photochemical reduction, oxidation and demethylation will imprint diagnostic MDF and MIF signatures on reaction substrates and products. Finally, Hg isotopic fractionation during transformation pathways mediated by an important component of aquatic ecosystems, phototrophic planktonic organisms, has not been examined to date. The third objective addresses this lack of knowledge by testing the hypothesis that intracellular Hg(II) reduction and MeHg degradation in phytoplankton incubated in the light will result in MDF and possibly MIF. Broader Impact: The proposed research activity will continue to lay the groundwork for a new approach or the identification of sources, sinks, and pathways of Hg transformations in impacted ecosystems. This approach has the potential to significantly enhance understanding of Hg biogeochemistry on temporal and spatial scales ranging from molecular mechanisms, to ecosystems, to global cycles, and to the geological record. As ecosystem Hg contamination remains a major public health concern, this project will support implementation of sound environmental practices to reduce Hg contamination and exposure. The proposed research will train a postdoctoral fellow in the application of stable isotope-based approaches in geobiology and ecosystem processes. In addition, undergraduate and graduate students will be integrated into the project, exposing them to cutting edge concepts and technologies, which are at the interface between biology, geology and ecosystem sciences. It is at this interface that importantparadigm-shifting, research advances are being made. Undergraduate students will assist with the analytical geochemistry as part of senior thesis research projects and PhD dissertation projects. Results will be published and disseminated broadly.