1. Max Planck Institute for Marine Microbiology, 28359 Bremen, Germany
2. Alfred Wegener Institute for Polar and Marine Research, 27515 Bremerhaven, Germany
3. Centre Ifremer de Brest, BP70, 29280 Plouzane, France
4. International University Bremen, 28759 Bremen, Germany
5. These authors contributed equally to this work.
6. Present addresses: DFG Research Center Ocean Margins, University of Bremen, 28334 Bremen, Germany (M.E.); UMR 7156 Université Louis-Pasteur/CNRS, Département Microorganismes, Génomes, Environnement, 67083 Strasbourg Cedex, France (T.N.).

Correspondence to: Antje Boetius^1,2,4 Correspondence and requests for materials should be addressed to A.B. (Email: aboetius@mpi-bremen.de). The nucleotide sequence data have been deposited in EMBL, GenBank and the DDBJ nucleotide sequence database under accession numbers AJ704650–AJ704653, AJ704631 and AM287206–AM287207.

Abstract

Mud volcanism is an important natural source of the greenhouse gas methane to the hydrosphere and atmosphere^{1, 2}. 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^{6, 7, 8}. Unfortunately, global methane emission from active submarine mud volcanoes cannot be quantified because their number and gas release are unknown⁹. 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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