UCLA

Microbial methane oxidation modulates redox structure, regulates methane flux, and drives carbon cycling in marine environments, with implications for both Earth’s biospheric evolution and the functioning of modern ecosystems. This thesis examines water column methanotrophy across environmental settings, from a Proterozoic Ocean analog to modern cold seeps, to address two central questions: (1) What role might methane oxidation have played in influencing oxygen levels and redox balance in Mesoproterozoic oceans? (2) How do environmental regimes, physical transport, and seep dynamics shape water column methanotrophy and its influence on the fate and footprint of methane in modern oceans?Addressing Question 1, we studied methane oxidation across a steep geochemical gradient in Green Lake, a meromictic, euxinic lake in Fayetteville, New York, used as a Mesoproterozoic Ocean analog. High rates of methanotrophy and sulfate reduction co-occurred below the chemocline, alongside monomethylamine-based methanogenesis. Aerobic methanotrophs, including Crenothrix and Methylacidiphilales, were likely active in the euxinic waters forming an efficient methane filter, potentially via denitrification or fermentation. A sulfide-dominated redox imbalance limited oxygen intrusion and fueled microbial activity, creating a model for how microbial feedbacks may have modulated oxygen availability and redox stability in early oceans.For Question 2, contrasting seep environments were explored: low-oxygen sites along the Southern California margin (~400–1000 m) and fully oxygenated seeps in the Gulf of Alaska (~2000–5000 m). In Southern California, aerobic methanotrophs were vertically structured and widespread, with consistent near-seafloor abundance of the particulate methane monooxygenase (pmoA) gene. Oxidation was closely coupled to methane and pmoA at Santa Monica Mound but showed weaker associations at Del Mar and Lasuen Knoll. Alaskan sites revealed diverse controls: oxidation tracked methane at Edge, aligned with both methane and pmoA at Shumagin, and persisted at Sanak despite low pmoA abundance, possibly reflecting lateral methane transport and tidal influence. Lateral transects across Sanak, Santa Monica, and Del Mar showed oxidation extending beyond seep boundaries, likely shaped by bottom currents. These patterns suggest water column methanotrophy forms a spatially dynamic, functionally cohesive microbial envelope structured by oxygen, methane gradients, and physical transport, mediating carbon flow to support ecosystems across seep and off-seep environments.