UC Santa Barbara

The ocean is a dynamic environment for methane’s biogeochemical processes. Three major processes take center stage: biological methanogenesis, anaerobic methane oxidation, and aerobic methane oxidation. These three key methane biogeochemical processes are intricately balanced within the oceanic environment, resulting in minimal methane escaping into the atmosphere. This dissertation aims to integrate fieldwork with laboratory research, combining investigative tools such as chemical measurements, radiotracer incubations, anaerobic cultivation, and stable isotope probing. The objective is to probe the behaviors of aerobic methane oxidation in relation to the ocean’s oxygen availability and isotopic signatures during methanogenesis from methyl-based compounds.Chapters 1 and 2 of my dissertation delve into the effectiveness and efficiency of aerobic methane oxidation in the ocean’s water column. The Santa Barbara Basin (SBB) is known for experiencing seasonal deoxygenation and reoxygenation cycles, resulting in fluctuating methane concentrations in the deep water column. In chapter 1, my study comprehensively investigated this seasonal cycle through a nine-month period of repeated sampling and measurement of key parameters associated with methane biogeochemistry in the deep waters of the SBB. These parameters include oxygen, nitrate, methane concentrations, and the rate of aerobic methane oxidation. My findings revealed a sequential pattern. First, a decline in oxygen concentration was observed to precede a decrease in nitrate concentration. Second, the accumulation of methane followed, with a marked decline in both oxygen and nitrate levels. Finally, changes in the methane oxidation rate, which reflects the activity of the methanotroph community, occurred subsequently, albeit with a slight time lag. I also discovered that the rate of methane oxidation is primarily dependent on the availability of methane within the water column. Furthermore, my research uncovered that the transient methane pulse accompanying the observed oxygen depletion in the SBB triggered the development of a persistent methanotrophic community, even after methane concentrations had returned to normal levels – an ecological memory effect. In Chapter 2, beyond the analysis of observed trends, I utilized methane concentrations, methane oxidation rates, and vertical methane diffusion to calculate the minimum methane source required for the deep water column. This computation was conducted across a range of contrasting environmental conditions, ultimately revealing that anoxic conditions demand a greater influx of methane into the water column. A comparison to data collected during a 2023 oceanic research expedition revealed an even greater demand for methane input, in the presence of well-established and persistent anoxic conditions. Chapter 3 of my dissertation is dedicated to investigating mechanistic underpinnings of stable carbon isotope fractionations during methylotrophic methanogenesis by marine archaea. Understanding these mechanistic underpinnings enables the inclusion of methylotrophic methanogenesis in isotopically informed biogeochemical reaction networks for anaerobic environments. This study included wild type methylotrophic methanogens as well as a mutant strain in which the reduction of methylated substrates is coupled to hydrogen oxidation or acetate oxidation, providing further mechanistic insight of isotopic variations originating at the reaction branch point.