Methane isotopologue fractionation during microbial methanogenesis and methonotrophy by pure and mixed laboratory cultures

甲烷既是一种替代能源，也是一种强大的温室气体。了解它的来源和汇对于我们能源和环境的未来是重要的。一组称为产甲烷菌的微生物是大气中甲烷的主要来源，也是深层沉积物中甲烷水合物的主要来源。甲烷由一个C原子和四个H原子组成。每一个都有两个同位素，分别是13C/12C和D/H。由于同位素取代分子的反应性不同，微生物甲烷中的重同位素碳和氢以及双取代同位素(13CH3D)往往是枯竭的。在这个项目中，研究人员将进行一系列实验室实验，包括生物电化学系统和共培养，以检查产甲烷微生物对甲烷同位素分馏的主要控制。其目标是建立一个统一的模型，将微生物甲烷的同位素比率与环境和生理参数联系起来，以便开发甲烷同位素比率作为其来源的诊断工具。该项目将支持一名研究生，并促进麻省理工学院和Aerodyne Research Inc.之间的合作伙伴关系。研究人员的实验室将参与K-12推广计划，以演示微生物燃料电池和在生物地球化学中使用同位素。碳和氢同位素比率已被广泛用于追踪环境中甲烷的来源。除了体积同位素比率，双取代甲烷同位素的丰度(13CH3D)也被建议作为甲烷形成温度的量规。虽然这些替代物被用作确定甲烷来源(S)的工具，但它们往往得出的数据与其他地球化学参数不一致。本研究旨在通过一系列实验室培养实验确定微生物产甲烷和甲烷营养过程中甲烷同位素分馏的主要控制因素(S)。这些实验包括：1)氧化还原电位下的生物电化学系统；2)硫酸盐还原细菌和产甲烷菌的共培养；3)好氧和厌氧甲烷氧化细菌的培养。用新型可调谐红外激光直接吸收光谱仪(Tildas)测量甲烷的四种同位素组成(12CH4、13CH4、12CH3D和13CH3D)。调查人员将测试这些假设，即甲烷生成或甲烷营养的速率和环境的氧化还原潜力，而不是途径，给予同位素分馏的主要控制，以及同位素平衡是否需要甲烷的厌氧氧化或其他特定的微生物群落结构。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Methane is both an alternative energy source and a strong greenhouse gas. It is important to understand its sources and sinks for the future of our energy and environment. A group of microbes, called methanogens, is the major source of methane to the atmosphere as well as to methane hydrates in deep sediments. Methane is composed of one C and four H atoms. Each has two isotopes, 13C/12C and D/H, respectively. Microbial methane is often depleted in heavy isotopes of carbon and hydrogen as well as a doubly substituted isotopologue (13CH3D) because of the different reactivity of isotopologues (isotope substituted molecules). In this project, investigators will carry out a series of laboratory experiments, including bioelectrochemical systems and co-cultures, to examine the major control of methane isotopologue fractionations by methanogenic microbes. The goal is to construct a unified model that links isotopologue ratios of microbial methane to environmental and physiological parameters, in order to develop methane isotopologue ratios as a diagnostic tool for its source. The project will support a graduate student, and promotes partnership between the Massachusetts Institute of Technology and Aerodyne Research Inc. The laboratory of the investigator will participate in K-12 outreach programs to demonstrate microbial fuel cells and the use of isotopes in biogeochemistry. Carbon and hydrogen isotope ratios have been widely used to trace the origin of methane in the environment. In addition to bulk isotope ratios, the abundance of doubly substituted methane isotopologues (13CH3D) has been proposed as a gauge of methane formation temperature. While these proxies are used as a tool to fingerprint the source(s) of methane, they often yield data that are inconsistent with other geochemical parameters. This proposed research aims to identify the major control(s) of methane isotopologue fractionation during microbial methanogenesis and methanotrophy from a series of controlled laboratory culture experiments using 1) bioelectrochemical systems under well-defined redox potentials, 2) co-cultures of sulfate reducing bacteria and methanogens, and 3) cultures of aerobic and anaerobic methanotrophs. Four isotopologue compositions of methane (12CH4, 13CH4, 12CH3D, & 13CH3D) will be measured by a novel tunable infrared laser direct absorption spectroscopy (TILDAS) instrument. Investigators will test the hypotheses that the rate of methanogenesis or methanotrophy and the redox potential of the environment, rather than the pathway, impart the major control on isotopologue fractionation, and whether isotopologue equilibrium requires anaerobic oxidation of methane or other specific microbial community structures.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.