Collaborative Research: Mercury isotope fractionation during microbial and abiotic redox transformations

本文提出了利用微生物和非生物氧化还原转化对汞同位素分馏的研究。据信，这项工作适合地球科学项目：地球微生物过程中的生物地球科学机会研究。具体来说，这项研究符合该计划的第二重点，强调发展新的研究技术（包括同位素代理），以解决生物学和地质学交叉产生的问题。两位pi （Blum和Klaue）在过去五年中一直致力于开发一种具有足够高精度的分析方法，以测量汞同位素组成的自然变化。该小组已经证明，他们可以常规测量汞同位素比率，精度达到每原子质量单位（amu） 0.02 permil，该小组最近的研究已经探索了陨石和矿床中汞同位素的变化。汞同位素应用于环境中汞研究和作为生物地球化学代理的一个核心问题是微生物在各种氧化还原过程中分馏汞的程度。要定量地回答这个问题，需要一个专门研究汞转化微生物学的团队（罗格斯大学的pi Barkay和Reinfelder），以及专门研究汞同位素地球化学的团队（密歇根大学的pi Blum和Klaue）。在初步实验中，该团队已经证明，一种产生汞还原酶（MR）的抗汞大肠杆菌菌株在将汞（II）还原为汞(0)的过程中会将汞同位素分离。这种分馏的幅度高达1.5毫微克/ μ m，（50倍的不确定度），它遵循瑞利定律，分馏系数为1.0006。这些初步结果使PIs非常乐观，认为汞同位素将为环境中汞氧化还原转化提供强有力的新示踪剂，并作为地质时期汞氧化还原过程变化的代用记录。本研究的目的是进行一系列精心控制的实验，以研究在每个主要的微生物和非生物氧化还原转化过程中汞同位素的分馏。将被研究的具体过程包括：1)通过细菌MR，通过依赖光和独立的藻类过程，通过硫杆菌中依赖铁（II）的还原，和光还原，将Hg（II）还原为Hg(0)。2)细菌酶将Hg(0)氧化为Hg(II)，其主要作用是保护细胞免受氧损伤。3)硫酸盐还原菌（SRB）将Hg （II）甲基化为MeHg。4) MeHg分别通过还原和氧化途径降解为Hg （II）、CH4和CO2。该项目将为汞同位素系统学以及汞同位素在地质和生物地球化学中的应用奠定基础。一旦开发出来，汞同位素工具将使未来的研究能够解决以下问题：1)跟踪环境和地质沉积物中各种汞的来源、途径和汇；2)在湖泊和海洋沉积物中使用汞同位素作为古氧化还原代理；3)研究沉积物和沉积岩中汞与微生物相互作用的演化。知识价值。拟议的研究活动将为一种全新的方法奠定基础，这种方法有可能大大提高对汞生物地球化学的理解，范围从微生物机制到单个湖泊到全球循环，最后到地质记录。研究团队成员在研究Hg-微生物相互作用（Rutgers PIs）和Hg的同位素地球化学和水生生态学（Michigan PIs）方面已经证明了自己的记录，并准备在这一联合研究领域取得快速进展。更广泛的影响。这项拟议中的研究将整合罗格斯大学和密歇根大学的学生，让他们在先进的实验室技术和生物科学与地质科学之间的知识交叉领域获得经验，这些领域正在取得重要的、有时是范式转变的研究进展。罗格斯大学的一名女博士生已经完成了初步实验，她将在她的论文工作中进行微生物实验，并前往密歇根参加质谱和理论同位素地球化学的研究。密歇根大学的本科生将协助分析地球化学作为高级论文研究项目的一部分。方法和科学方法的相互融合将有利于所有研究参与者，并可能导致更多的合作。结果将被广泛出版和传播。由于接触汞仍然是一个主要的公共卫生问题，该项目将协助实施无害环境做法，以减少汞污染和接触。

EAR 0433772BLUM An investigation of the isotope fractionation of mercury (Hg) by microbial and abiotic redox transformations is proposed herein. It is believed that this work fits appropriately within the Geosciences program: Research in Biogeosciences Opportunities in Geomicrobial Processes. Specifically, this research fits into Focus II of this program, which emphasizes developing new research techniques (including isotope proxies) to address questions generated at the intersection between biology and geology. Two of the PIs (Blum and Klaue) have worked for the past five years to develop an analytical method with high enough precision to allow measurement of natural variations in the isotopic composition of Hg. This group has demonstrated that they can routinely measure Hg isotope ratios to an accuracy of 0.02 permil per atomic mass unit (amu) and recent research by this group has explored Hg isotope variation in meteorites and ore deposits. A central question in the application of Hg isotopes to the study of Hg in the environment and as a biogeochemical proxy is the degree to which microbes fractionate Hg during various redox processes. To answer this question quantitatively requires a team with specialization in the microbiology of Hg transformations (PIs Barkay and Reinfelder at Rutgers), and specialization in the isotope geochemistry of Hg (PIs Blum and Klaue at Michigan). In preliminary experiments this team has demonstrated that a strain of Hg resistant Escherichia coli, which produce the mercuric reductase enzyme (MR), fractionate Hg isotopes during the reduction of Hg (II) to Hg(0). The magnitude of this fractionation is up to 1.5 permil per amu, (50 times uncertainty) and it follows a Rayleigh law with a fractionation factor of 1.0006.These initial results give the PIs great optimism that Hg isotopes will provide a powerful new tracer of Hg redox transformation in the environment, and a proxy record of changes in Hg redox processes through geological time. The objective of this study is to conduct a series of carefully controlled experiments to investigate Hg isotope fractionation during each of the major microbial and abiotic redox transformations. The specific processes that will be investigated include: 1) The reduction of Hg (II) to Hg(0) by the bacterial MR, by light-dependent and independent algal processes, by Fe(II)- dependent reduction in thiobacilli, and by photoreduction. 2) The oxidation of Hg (0) to Hg(II) by bacterial enzymes whose primary cellular role is protection against oxygen damage. 3) The methylation of Hg (II) to MeHg by sulfate reducing bacteria (SRB). 4) The degradation of MeHg to Hg (II) and CH4 and CO2 by the reductive and oxidative pathways, respectively. This project will set the groundwork for Hg isotope systematics and the use of Hg isotopes in geology and biogeochemistry. Once developed, the Hg isotope tool will allow future studies that address issues such as: 1) the tracking of sources, pathways and sinks of various Hg species in the environmental and in geological deposits, 2) the use of Hg isotopes as a paleo-redox proxy in lacustrine and marine deposits, and 3) the study of the evolution of Hg-microbe interactions in sediments and sedimentary rocks.Intellectual Merit. The proposed research activity will lay the groundwork for a completely new methodology that has the potential to significantly enhance understanding of mercury biogeochemistry on scales ranging from microbial mechanisms to individual lakes to global cycles and finally to the geological record. The research team members have proven track records in studying Hg-microbe interactions (Rutgers PIs) and the isotope geochemistry and aquatic ecology of Hg (Michigan PIs) and are poised to make rapid advancements in this combined research area.Broader Impacts. The proposed research will integrate students from both Rutgers and theUniversity of Michigan, giving them experience in advanced laboratory techniques and at the intellectual intersection between the biological and geological sciences where important, and sometimes paradigm- shifting, research advances are being made. A female PhD student at Rutgers, who has already completed the preliminary experiments, will work on the microbial experiments for her dissertation work and travel to Michigan to participate in the mass spectrometry and theoretical isotope geochemistry. An undergraduate student at Michigan will assist with the analytical geochemistry as part of a senior thesis research project. The cross-fertilization of methods and scientific approaches will be beneficial to all of the research participants and will likely lead to additional collaborations. Results will be published and disseminated broadly. As exposure to Hg remains a major public health concern this project will assist in the implementation of sound environmental practices to reduce Hg contamination and exposure.