| 30 Jul 2026
An Expanded Methanosphere: Methane Oxidation and Methanotrophs Surrounding Cold Seeps Across Oxygen Gradients on the Southern California Margin
Abstract. Marine methane (CH4) seeps are dynamic biogeochemical systems that regulate carbon and sustain high-biomass communities through microbial CH4 oxidation. While most CH4 is consumed anaerobically in sediments, a fraction escapes into the water column, where aerobic methanotrophs act as a biological filter limiting atmospheric flux. However, the spatial extent of CH4 influence beyond active seep zones remains poorly constrained, with implications for deep-sea food webs and carbon cycling. We investigated microbial CH4 turnover and methanotroph distribution across three seep sites on the Southern California margin (Del Mar (1020 m), Santa Monica Mound (800 m), and Lasuen Knoll (400 m)) focusing on extent of horizontal transport, presence of vertical gradients, and oxygen controls. Using radiotracer (3H-CH4) incubations, CH4 concentration profiles, 16S rRNA gene sequencing, and particulate methane monooxygenase (pmoA) gene quantification, we characterized CH4-fueled processes along vertical and lateral transects, including near-bottom waters sampled via HOV Alvin. CH4 oxidation was active both within seep plumes and in off-seep waters extending hundreds of meters from the source, with maximum rates reaching 454 nmol L⁻¹ d⁻¹ in a CH4-rich bubble plume. Methanotrophic communities showed vertical structuring, with higher diversity near the seafloor. pmoA gene abundances remained consistent across seep and seep-adjacent environments, indicating widespread oxidation potential. Environmental controls were site-specific: oxidation correlated positively with CH4 and negatively with oxygen at some sites, whereas oxygen enhanced oxidation at others. These findings support an expanded “methanosphere,” in which CH4-driven microbial processes extend beyond seep boundaries, linking local seep activity to broader deep-sea biogeochemical dynamics.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Biogeosciences.
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