Abstract:Mud volcanism is an important natural source of the greenhouse gas methane to hydrosphere and atmosphere 1,2 . Recent investigations show that the number of active submarine mud volcanoes may be much higher than anticipated (eg. 3-5), and that gas emitted from deep-sea seeps may 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 is unknown 9 . Another uncertainty is the efficiency of methane oxidizing microorganisms in methane removal. Here we investigated the methane-emitting Haakon Mosby Mud Volcano (HMMV, Barents Sea, 72°N, 14°44'E; 1250 m water depth), to provide quantitative estimates of 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 novel clade of archaea (ANME-3) associated with bacterial mats. We found that 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.The HMMV (Fig. 1), a circular structure of 1 km diameter and <10 m elevation above the adjacent seafloor, has been studied since the 1990s as a typical example of an active mud volcano 9 . Its formation may have coincided with a submarine landslide during the late Pleistocene, 330-200 ka before present 10 . Today, fluids, gas and muds rise from 2-3 km depth through a conduit below the HMMV 11,10 . The emitted gas is of a mixed microbial/thermogenic origin and consists of >99% CH4 with a δ 13 C-isotope signature of -60‰ 12,13 . The rising fluids are depleted in sulphate, chloride and magnesium caused by subsurface clay dewatering 11 . Investigation of the HMMV with RV POLARSTERN and ROV VICTOR 6000 in 2003 showed extensive outcroppings 1 2006-01-01028b_Boetius_MS 3 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 12,8 .Seafloor videography in combination with geochemical 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 HMMV, which was devoid of epifauna; thiotrophic bacterial mats dominated by a Beggiatoa species; and surrounding fields of siboglinid tubeworms. Gas concentrations in sediments and bottom water were elevated in all three habitats (Tab. (Fig. 3a). Only minor amounts of methanotroph lipids (<0.1 µg gdw -1 ) and very low cell numbers (~10 7 cells cm -3 ) were found below 5 cm sediment depth ( Fig. 3a3-3a4). ANME cells were not microscopically detectable in the centre cores using all know...

Submarine mud volcanoes are formed by expulsions of mud, fluids, and gases from deeply buried subsurface sources. They are highly reduced benthic habitats and often associated with intensive methane seepage. In this study, the microbial diversity and community structure in methane-rich sediments of the Haakon Mosby Mud Volcano (HMMV) were investigated by comparative sequence analysis of 16S rRNA genes and fluorescence in situ hybridization. In the active volcano center, which has a diameter of about 500 m, the main methaneconsuming process was bacterial aerobic oxidation. In this zone, aerobic methanotrophs belonging to three bacterial clades closely affiliated with Methylobacter and Methylophaga species accounted for 56% ؎ 8% of total cells. In sediments below Beggiatoa mats encircling the center of the HMMV, methanotrophic archaea of the ANME-3 clade dominated the zone of anaerobic methane oxidation. ANME-3 archaea form cell aggregates mostly associated with sulfate-reducing bacteria of the Desulfobulbus (DBB) branch. These ANME-3/DBB aggregates were highly abundant and accounted for up to 94% ؎ 2% of total microbial biomass at 2 to 3 cm below the surface. ANME-3/DBB aggregates could be further enriched by flow cytometry to identify their phylogenetic relationships. At the outer rim of the mud volcano, the seafloor was colonized by tubeworms (Siboglinidae, formerly known as Pogonophora). Here, both aerobic and anaerobic methane oxidizers were found, however, in lower abundances. The level of microbial diversity at this site was higher than that at the central and Beggiatoa species-covered part of the HMMV. Analysis of methyl-coenzyme M-reductase alpha subunit (mcrA) genes showed a strong dominance of a novel lineage, mcrA group f, which could be assigned to ANME-3 archaea. Our results further support the hypothesis of Niemann et al. (54), that high methane availability and different fluid flow regimens at the HMMV provide distinct niches for aerobic and anaerobic methanotrophs.Large amounts of methane are stored in the subsurface ocean as crystalline gas hydrate, dissolved in porewater, and as free gas. Most of the methane which rises from subsurface reservoirs is consumed by anaerobic microorganisms inhabiting sulfate-penetrated sediment layers before it reaches the seafloor (65). This microbially mediated anaerobic oxidation of methane (AOM) controls the emission of the greenhouse gas methane from the ocean to the atmosphere (see reference 23 and references therein).In marine habitats, the metabolic process of AOM is proposed to be a reversed methanogenesis coupled to the reduction of sulfate involving methanotrophic archaea (ANME archaea) and sulfate-reducing bacteria (SRB). ANME archaea and SRB are assumed to interact syntrophically (26) and form microbial consortia which oxidize methane with equimolar amounts of sulfate, yielding bicarbonate and sulfide (26, 51, 52). Sulfide produced as a by-product of AOM at cold seeps often supports chemosynthetic communities which derive energy from sulfide oxidation. ...

Methane hydrate is an icelike substance that is stable at high pressure and low temperature in continental margin sediments. Since the discovery of a large number of gas flares at the landward termination of the gas hydrate stability zone off Svalbard, there has been concern that warming bottom waters have started to dissociate large amounts of gas hydrate and that the resulting methane release may possibly accelerate global warming. Here, we corroborate that hydrates play a role in the observed seepage of gas, but we present evidence that seepage off Svalbard has been ongoing for at least 3000 years and that seasonal fluctuations of 1° to 2°C in the bottom-water temperature cause periodic gas hydrate formation and dissociation, which focus seepage at the observed sites.