Water column methane, methane oxidation, and pmoA gene copies above methane seeps determined from samples collected off the Aleutian Islands, Gulf of Alaska on R/V Atlantis cruise AT50-24 in May to June 2024

Cold seeps along the eastern Aleutian subduction zone in the Gulf of Alaska fuel benthic ecosystems through microbial methane (CH₄) consumption, yet the structure and controls of water column CH₄ oxidation in these deep, cold waters remain poorly resolved. During a May–June 2024 expedition with the R/V Atlantis and HOV Alvin, we studied CH₄ and its bacterial oxidation from surface to seafloor above three deep seep sites (2000 to 5000 meters): Edge, Shumagin, and Sanak, by combining radiotracer incubations with pmoA gene  profiling. CH₄ oxidation occurred throughout the water column, with peak rates (1 to 242 nanomoles per liter per day) in near-seafloor Alvin samples and 0.1 to 0.25 nanomoles per liter per day in CTD rosette samples 10 to 30 meters above bottom. Rates varied by site and depth. CH₄ oxidation in surface waters, coinciding with an algal bloom, suggests cryptic cycling via in situ production and consumption. A ~325-meter near-bottom CTD transect at Sanak revealed lateral gradients in CH₄ and oxidation aligned with bottom currents, with oxidation highest near hydrate-bearing gas vents and at the off-seep distal end. These findings show that aerobic CH₄ oxidation peaks near the seafloor to ~30 meters above but extends laterally and vertically beyond active seepage. Oxidation was detected even where methanotroph gene abundance was low, potentially indicating the influence of lateral CH₄ transport and tidal currents. The methanosphere thus emerges as a dynamic and spatially diffuse microbial system shaped by CH₄ availability and physical transport processes.