Environmental Science and Engineering Seminar
Methane, a potent greenhouse gas, is the second most important greenhouse gas contributor to anthropogenic climate change. Wetlands constitute the largest natural methane source and are predicted to play a critical role in future carbon-climate feedbacks. A mechanistic understanding of the complex and dynamic interplay between microbial, hydrological, and plant-associated processes in wetlands is critical to deciphering current atmospheric trends in methane and predicting future emissions. Recent studies suggest that a large fraction of methane emitted from wetlands actually originates from oxygenated soils and peats that lie above or near the water table, challenging the paradigm that methane production by methanogenic Archaea is confined to oxygen-free, water-logged habitats. I will discuss research in my group aimed at resolving this apparent paradox by elucidating how O2 stimulates wetland methane production. We propose a critical role for redox transitions because natural wetting and drying cycles generate spatial and temporal gradients between oxygen-rich and oxygen-poor conditions. Using bottle incubations of peat, we find that oxygen exposure during a temporal redox oscillation dramatically enhances methane production by orders of magnitude compared to peat incubated under continuously anoxic conditions. Analyses of headspace gases, microbial community, peat chemistry indicate that oxygen exposure enables efficient degradation of complex, recalcitrant, and inhibitory forms of carbon into simpler energy and carbon substrates that fuel methanogenic microbes, resulting in enhanced methane production. Our findings that oxygen variability can promote biogeochemical cascades leading to significantly increased methane yields provide a mechanism for the wetland methane paradox. They also suggest a possible climate feedback between increasing temperature, hydrologically-driven oxygen variability, and methane emissions that requires further evaluation in future experimental and modeling studies. Ongoing and future research in the group focuses on characterizing key microbiological and hydrological determinants on the strength of oxygen-enhanced methanogenesis.