nature05227 Niemann et al_2006

nature05227 Niemann et al_2006 - Vol 443 | 19 October 2006 |

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LETTERS Novel microbial communities of the Haakon Mosby mud volcano and their role as a methane sink Helge Niemann 1,2 *, Tina Lo ¨sekann 1 *, Dirk de Beer 1 , Marcus Elvert 1 { , Thierry Nadalig 3 { , Katrin Knittel 1 , Rudolf Amann 1 , Eberhard J. Sauter 2 , Michael Schlu ¨ter 2 , Michael Klages 2 , Jean Paul Foucher 3 & Antje Boetius 1,2,4 Mud volcanism is an important natural source of the greenhouse gas methane to the hydrosphere and atmosphere 1,2 . Recent investi- gations 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 6–8 . Unfortunately, global methane emission from active submarine mud volcanoes cannot be quantified because their number and gas release are unknown 9 . It is also unclear how efficiently methane-oxidizing microorganisms remove meth- ane. Here we investigate the methane-emitting Haakon Mosby Mud Volcano (HMMV, Barents Sea, 72 6 N, 14 6 44 9 E; 1,250 m water depth) to provide quantitative estimates of the in situ com- position, distribution and activity of methanotrophs in relation to gas emission. The HMMV hosts three key communities: aerobic methanotrophic bacteria ( Methylococcales ), anaerobic methano- trophic archaea (ANME-2) thriving below siboglinid tubeworms, and a previously undescribed clade of archaea (ANME-3) asso- ciated with bacterial mats. We found that the upward flow of sulphate- and oxygen-free mud volcano fluids restricts the avail- ability 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. The HMMV (Fig. 1), a circular structure of 1 km diameter and , 10 m elevation above the adjacent sea floor, has been studied since the 1990s as a typical example of an active mud volcano 9 . Its forma- tion might have coincided with a submarine landslide during the late Pleistocene, 330,000–200,000 years ago 10 . Today, fluids, gas and muds rise from a depth of 2–3 km through a conduit below the HMMV 10,11 . The emitted gas is of a mixed microbial/thermogenic origin and consists of . 99% CH 4 with a d 13 C-isotope signature of –60 % (refs 12, 13). The rising fluids are depleted in sulphate, chlo- ride and magnesium as a result of subsurface clay dewatering 11 . Investigation of the HMMV with RV Polarstern and ROV Victor 6000 in 2003 showed extensive outcroppings of fresh subsurface muds associated with steep thermal gradients 14 , gas and fluid vents, and a large gas plume reaching the mixed upper water column above the HMMV 8,12 . Seafloor videography in combination with geochem- ical measurements provided in situ estimates of gas flux 8 , fluid flow 15 and habitat distribution 16 . We focused on the three main concentric habitats above the gassy muds (Fig. 2): the centre of the HMMV, which was devoid of visible epifauna; thiotrophic bacterial mats dominated by a Beggiatoa species; and surrounding fields of sibogli- nid tubeworms. The concentrations of gases in sediments and bot-
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nature05227 Niemann et al_2006 - Vol 443 | 19 October 2006 |

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