Mud volcanism is an important natural source of the greenhouse gas methane to the hydrosphere and atmosphere. Recent investigations show that the number of active submarine mud volcanoes might be much higher than anticipated (for example, see refs 3-5), and that gas emitted from deep-sea seeps might reach the upper mixed ocean. Unfortunately, global methane emission from active submarine mud volcanoes cannot be quantified because their number and gas release are unknown. It is also unclear how efficiently methane-oxidizing microorganisms remove methane. Here we investigate the methane-emitting Haakon Mosby Mud Volcano (HMMV, Barents Sea, 72° N, 14° 44′ E; 1,250 m water depth) to provide quantitative estimates of the in situ composition, distribution and activity of methanotrophs in relation to gas emission. The HMMV hosts three key communities: aerobic methanotrophic bacteria (Methylococcales), anaerobic methanotrophic archaea (ANME-2) thriving below siboglinid tubeworms, and a previously undescribed clade of archaea (ANME-3) associated with bacterial mats. We found that the upward flow of sulphate- and oxygen-free mud volcano fluids restricts the availability of these electron acceptors for methane oxidation, and hence the habitat range of methanotrophs. This mechanism limits the capacity of the microbial methane filter at active marine mud volcanoes to <40% of the total flux.
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Acknowledgements The expeditions ‘AWI’ on RV L’Atalante in 2001 and ‘ARK XIX3b’ on RV Polarstern in 2003, both with ROV Victor 6000, were jointly planned, coordinated and carried out by the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany, and the French Research Institute for Exploitation of the Sea (IFREMER) in Brest, France. We thank the captain and crew, the team of the ROV Victor 6000 and the shipboard scientific community of the RV Polarstern and L’Atalante for their help at sea. We thank S. Lüdeling, L. Baumann, V. Beier, I. Busse, F. Heinrich, G. Eickert, A. Nordhausen, J. Rogenhagen and R. Usbeck for technical assistance, C. Edy for help with georeferencing, U. Witte for providing the benthic lander technology, and S. Joye for comments on the manuscript. This is publication no. GEOTECH-235 of the R&D-Programme GEOTECHNOLOGIEN, Project MUMM, funded by the German Ministry of Education and Research (BMBF) and German Research Foundation (DFG).