Clumped Isotopes and Cryptic Cycling of Methane: Tracking the Microbial Footprint
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Clumped Isotopes and Cryptic Cycling of Methane: Tracking the Microbial Footprint


Methane is the simplest form of organic matter on Earth, the most abundant organic molecule in the atmosphere where it is a potent greenhouse gas, an important source of energy, and considered to be a signature of life on other worlds. Methane’s genesis and its fate can occur abiotically and biotically in various atmospheric, geologic, and geographic settings. Investigations into the porewater and solid phase geochemistry, metabolic ex-situ activity and stable isotope compositions have brought considerable insight into distinguishing and tracking the microbial footprints of methane production and consumption. However, questions remain pertaining to, for example, 1) the potential for geomicrobiological communities to simultaneously produce and consume methane in sedimentary environments and, 2) the biological signatures of rare methane clumped isotopologues that indicate whether methane has been altered by oxidation.ii The first question will be addressed in this dissertation, using a newly adapted radiotracer method to show the cryptic, simultaneous methane production and consumption by biological activity in coastal wetland and marine sediment. Radiotracer analysis of the sediment from within the top 15-20 cm, collected from a coastal wetland and a marine environment revealed the cryptic, simultaneous methane production and consumption by biological activity. The findings strongly suggest that this cryptic methane cycling was overlooked by previous studies and must be considered in sediment within other wetland and marine settings. The second issue will be addressed by using the latest technological advancements in high resolution mass spectrometry to elucidate the rare methane isotopologue signatures of microbial methane oxidation in bacterial pure cultures, enzyme extracts, in environmental methane-bearing natural fluids, and in sediment slurries. Laboratory experiments that induced methane oxidation by biological activity revealed that the residual methane have distinct methane clumped isotopologue signatures in a two-dimensional space. The experimental data were applied to open and closed system models, which not only revealed that methane clumped isotopologue signatures of various microbial methane oxidation pathways are distinct from each other but also distinct from methane sources. The findings have furthered the utility of the methane clumped isotopologues in a two-dimensional space to distinguish source and process.

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