Date of Award
Spring 2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemical and Environmental Engineering (ENAS)
First Advisor
Peccia, Jordan
Abstract
Methane (CH4), like carbon dioxide (CO2), is a potent greenhouse gas that has played a critical role in the warming of the planet. Yet unlike CO2, virtually all of the methane emitted into the atmosphere is the product of biological activity. Specifically, it is the metabolic byproduct of a group of archaea that are collectively referred to as methanogens. These methanogens thrive in a diversity of habitats—from landfills, to ruminant guts, to the wood of trees—and excel at converting carbon into methane. Their vast presence and activity in natural environments, like wetlands and inland waters, accounts for more than half of all globally emitted methane. Although this production is inherently natural, it is feared that climate change may inadvertently enhance methanogenesis on a global scale, as warming temperatures could not only increase the rate of methane production from existing sources, but also transform vast swaths of land into new habitat favorable for methanogen growth. If realized, this process could further accelerate the emission of methane to the atmosphere, which in-turn would further advance the pace at which the planet warms. To avoid this runaway scenario, we may need to consider possible engineered controls to reasonably constrain the emission of methane from natural environments. As any geoengineering effort can pose significant risks, it stands to reason that the sources displaying the highest, or fastest growing, emission rates should be targeted first for intervention. Yet despite decades of research into the emission of methane from biogenic sources, much remains unresolved, evidenced by the fact that gaps of 100-200 Tg CH4 yr-1 still persist in global emissions models. To that end, the goal of this research was to improve our understanding of the drivers, controls, and variability of methane emissions from wetlands (the largest natural source) and trees (a ubiquitous, but poorly resolved source). To explain dramatic, localized spatial variance of methane fluxes from the surface of a wetland, we leveraged depth-stratified amplicon sequencing and gene abundance measurements of methanogenesis and methanotrophy. Our results showed that the majority (>75%) of flux variance was attributable to shifts in methanogen abundance (at depths ≥15 cm), which, in-turn, was strongly modulated by underlying peat depth. When accounting for the variance due to sampling and refusal depth, community composition also proved to be significantly associated with methane flux. These findings suggest that microbial factors likely underlie localized variance in wetland CH4 flux, and that a greater reliance on biological predictors could transform our ability to understand and model wetland methane fluxes at finer scales than is currently possible. To explore the origin of methane fluxes observed from the trunk of living trees, we first developed a method for high-throughput sampling of microbial communities within wood and determined a limit-of-detection of approximately 500 cells per 100 mg of (dry) wood. We then employed this method to survey the microbial communities, including methane cycling taxa, within the wood of over 150 living trees across 16 different species. We found a diverse and distinct microbial community within the wood of these trees, with different niches of specialization dominating in heartwood and sapwood, and observed that these woodborne microbiomes vary significantly across tree species. Moreover, in over 90% of trees sampled, we observed the presence of methanogenic archaea, suggesting that internal production of methane likely plays a foremost role in the emission of methane from tree stems. Overall, this work demonstrates that improving our means to monitor the abundance, activity, and diversity of methane cycling communities in natural environments offers us a promising pathway to improve our understanding of biogenic methane emissions, which may aid in future efforts to better model and constrain these fluxes.
Recommended Citation
Arnold, Wyatt, "Microbial Controls on Globally Significant Methane Cycling in Wetlands and Forests" (2024). Yale Graduate School of Arts and Sciences Dissertations. 1287.
https://elischolar.library.yale.edu/gsas_dissertations/1287
