Microbial methanogenesis in the subsurface seafloor is responsible for the formation of large and dynamic gas reservoirs like the recently discovered gas hydrate deposits. Gas seepage occurs wherever methane builds up an overpressure outside the hydrate stability field, illustrating the potential importance of ocean margins for the global methane budget. However, a variety of bacteria and archaea are capable of methane consumption, and control the emission of methane to the hydrosphere. Unfortunately, much less is known about the microbial methane turnover in the ocean than about methane turnover in freshwater or terrestrial habitats. This investigation compares rates of methane production, anaerobic and aerobic methane oxidation at different marine sites, combining radiotracer (on-site) and in vitro measurements. Samples were obtained from gas hydrate bearing sediments, cold seeps, organic-rich and organic-poor subsurface sediments. All investigated subsurface sediments had the potential for methanogenesis as well as for methanotrophy. The anaerobic oxidation of methane (AOM) was highest in samples from gas hydrate areas and cold seeps. AOM was strongly influenced by methane partial pressure and temperature, indicating a substantial underestimation of in situ activity with current ex situ measuring techniques. A potential for aerobic methane oxidation was detected at all sites where the sediment had contact with oxic bottom water. A first comparison of methane turnover rates in diverse marine habitats showed that microbial methane oxidation provides a very effective barrier for methane emissions from the subsurface seafloor. © 2005 Elsevier B.V. All rights reserved.