Water column methane, methane oxidation, and pmoA gene copies above southern California methane seeps determined from samples collected on R/V Atlantis cruise AT50-12 in July 2023

Marine methane (CH₄) seeps are dynamic biogeochemical systems that modulate carbon cycling and support high-biomass communities through microbial methane (CH₄) oxidation. While most CH₄ is consumed anaerobically in sediments, a fraction enters the water column, where aerobic methanotrophs form a biological filter limiting CH₄ flux to the atmosphere. However, the extent to which this microbial activity and CH₄ influence extend beyond visibly active seep zones remains poorly constrained, with implications for deep-sea food webs, biogeochemical gradients, and carbon cycling. We investigated CH₄ dynamics and methanotroph distribution across three seep sites on the Southern California margin (Del Mar, Santa Monica (800 meters) Mound, Lasuen Knoll; ~400–1200 meters depth). Using radiotracer (³H-CH₄) incubations, CH₄ concentration profiling, and particulate methane monooxygenase (pmoA) gene quantification, we sampled vertical and horizontal transects and near-bottom waters via the HOV Alvin. CH₄ oxidation was active not only within seep plumes but also in off-seep regions, with the highest rate (454 nanomoles per liter per day) observed within a CH₄-rich bubble plume. pmoA gene abundances remained relatively stable across both seep and off-seep waters, suggesting a persistent CH₄-oxidizing potential. These findings support an expanded "methanosphere," a CH₄-influenced microbial zone shaped by physical transport and environmental gradients. This dataset is part of a PhD thesis: Klonicki-Ference, Emily F. "Microbial Regulation of Methane and Redox Dynamics in the Water Column: From a Proterozoic Ocean Analog to Modern Marine Seeps." PhD diss., University of California, Los Angeles, 2025.