Microbial methane oxidation plays a critical role curbing methane emissions from the seafloor into the atmosphere and is widely considered to be the largest marine methane “sink”. Hydrothermal vent systems are ubiquitous, geologically diverse, and produce >70% of the abiotic mid-ocean methane flux to the hydrosphere. However, their methane oxidizing capacity is underexplored. This proposal aims to better understand the potentially significant role of mid-ocean ridges and their associated microbial communities in the marine methane cycle. There are three objectives: 1) Quantify rates of methane oxidation (aerobic and anaerobic) in sulfides, sediments, and plumes across a range of environmentally-relevant conditions; 2) Investigate the influence of geochemistry and temperature on methanotroph ecophysiology; and 3) Constrain net methane fluxes at hydrothermal vents relative to local and global methane budgets. Our approach includes A) methane oxidation laboratory rate measurements at simulated in situ pressures and temperatures; B) In situpreservation of sulfides and plume water for metagenomics and metatranscriptomics; C) In situ mass spectrometry and geochemical characterization to produce a geo-referenced map of methane at the vents; and D) Process-based modeling to consider the influence of these factors on the magnitude of the methanotrophic sink. We anticipate the results from this effort will transform our understanding of the diverse and critical role of seafloor systems in mitigating methane emissions.