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

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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.