Samples of thermogenic hydrocarbon gases, from vents and gas hydrate mounds within a sea-floor chemosynthetic community on the Gulf of Mexico continental slope at about 540 m depth, were collected by research submersible. Our study area is characterized by low water temperature (mean =7 °C), high pressure (about 5400 kPa), and abundant structure II gas hydrate. Bacterial oxidation of hydrate-bound methane (CH4) is indicated by three isotopic properties of gas hydrate samples. Relative to the vent gas from which the gas hydrate formed, (1) methane-bound methane is enriched in 13C by as much as 3.8‰ PDB (Peedee belemnite), (2) hydrate-bound methane is enriched in deuterium (D) by as much as 37‰ SMOW (standard mean ocean water), and (3) hydrate-bound carbon dioxide (CO2) is depleted in 13C by as much as 22.4‰ PDB. Hydrate-associated authigenic carbonate rock is also depleted in 13C. Bacterial oxidation of methane is a driving force in chemosynthetic communities, and in the concomitant precipitation of authigenic carbonate rock that modifies sea-floor geology. Bacterial oxidation of hydrate-bound methane expands the potential boundaries of life in extreme environments.