The methane-rich areas, the Loki's Castle vent field and the Jan Mayen vent field at the Arctic Mid Ocean Ridge (AMOR), host abundant niches for anaerobic methane-oxidizers, which are predominantly filled by members of the ANME-1. In this study, we used a metagenomic-based approach that revealed the presence of phylogenetic and functional different ANME-1 subgroups at AMOR, with heterogeneous distribution. Based on a common analysis of ANME-1 genomes from AMOR and other geographic locations, we observed that AMOR subgroups clustered with a vent-specific ANME-1 group that occurs solely at vents, and with a generalist ANME-1 group, with a mixed environmental origin. Generalist ANME-1 are enriched in genes coding for stress response and defense strategies, suggesting functional diversity among AMOR subgroups. ANME-1 encode a conserved energy metabolism, indicating strong adaptation to sulfate-methane-rich sediments in marine systems, which does not however prevent global dispersion. A deep branching family named Ca. Veteromethanophagaceae was identified. The basal position of vent-related ANME-1 in phylogenomic trees suggests that ANME-1 originated at hydrothermal vents. The heterogeneous and variable physicochemical conditions present in diffuse venting areas of hydrothermal fields could have favored the diversification of ANME-1 into lineages that can tolerate geochemical and environmental variations.

Summary Recent studies suggest an unforeseen role of archaea in the anaerobic oxidation of non-methane alkanes using enzymes of the methanogenesis pathway and requiring partnerships with bacteria that have not yet been cultured. Stable isotope probing (SIP) approaches provide an important culture-independent tool to track activity and carbon assimilation of these microorganisms in situ. However, the established stable isotope probing of lipids requires labor-intense purification of individual molecules and chemical derivatization or cleavage reactions such as ether-cleavage or saponification to produce GC-irmMS-amenable derivatives. A recently developed promising technique is isotope pattern matching (IPM) which uses isotopomer patterns of intact lipids analyzed by HPLC-MS to determine uptake of stable isotopes. We have incubated methane- and other short chain alkane-oxidizing enrichment cultures from hydrothermal sites in the Guaymas Basin with 13C-labeled bicarbonate and 13C-labeled alkane substrates at 50°C and monitored label-uptake into the lipid pool using traditional SIP and IPM methods. This presentation will show data of diagnostic lipids, including intact polar archaeal and bacterial lipids and compare IPM with traditional SIP experiments of selected biomarkers with the goal to understand carbon substrate affinity of alkane-oxidizing enrichment cultures.

Summary Here, we introduce two members of the new archaeal group Alkanophagales. Archaea belonging to this group are closely related to ANME-1 and Syntrophoarchaeales. We have enriched these thermophilic archaea using anoxic slurries from Guaymas Basin sediments, sulfate as electron acceptor, and liquid medium-chain alkanes as electron donors. The Alkanophagales representatives oxidize these alkanes anaerobically using divergent methyl-coenzyme M reductases. They shuttle the electrons released during alkane oxidation to Thermodesulfobacteria, which use them to reduce sulfate to sulfide. Thermodesulfobacteria constitute a new partner bacterium for anaerobic alkane-oxidizing archaea.

This study focused on biogeochemical processes and microbial activity in sediments of a natural deep-sea CO2 seepage area (Yonaguni Knoll IV hydrothermal system, Japan). The aim was to assess the influence of the geochemical conditions occurring in highly acidic and CO 2 saturated sediments on sulfate reduction (SR) and anaerobic methane oxidation (AOM). Porewater chemistry was investigated from retrieved sediment cores and in situ by microsensor profiling. The sites sampled around a sediment-hosted hydrothermal CO2 vent were very heterogeneous in porewater chemistry, indicating a complex leakage pattern. Near the vents, droplets of liquid CO2 were observed emanating from the sediments, and the pH reached approximately 4.5 in a sediment depth> 6 cm, as determined in situ by microsensors. Methane and sulfate co-occurred in most sediment samples from the vicinity of the vents down to a depth of 3 m. However, SR and AOM were restricted to the upper 7–15 cm below seafloor, although neither temperature, low pH, nor the availability of methane and sulfate could be limiting microbial activity. We argue that the extremely high subsurface concentrations of dissolved CO 2 (1000–1700 mM), which disrupt the cellular pH homeostasis, and lead to end-product inhibition. This limits life to the surface sediment horizons above the liquid CO2 phase, where less extreme conditions prevail. Our results may have to be taken into consideration in assessing the consequences of deep-sea CO 2 sequestration on benthic element cycling and on the local ecosystem state.

This study combines sediment geochemical analysis, in situ benthic lander deployments and numerical modeling to quantify the biogeochemical cycles of carbon and sulfur and the associated rates of Gibbs energy production at a novel methane seep. The benthic ecosystem is dominated by a dense population of tube-building ampharetid polychaetes and conspicuous microbial mats were unusually absent. A 1D numerical reaction-transport model, which allows for the explicit growth of sulfide and methane oxidizing microorganisms, was tuned to the geochemical data using a fluid advection velocity of 14cmyr−1. The fluids provide a deep source of dissolved hydrogen sulfide and methane to the sediment with fluxes equal to 4.1 and 18.2mmolm−2d−1, respectively. Chemosynthetic biomass production in the subsurface sediment is estimated to be 2.8mmolm−2d−1 of C biomass. However, carbon and oxygen budgets indicate that chemosynthetic organisms living directly above or on the surface sediment have the potential to produce 12.3mmolm−2d−1 of C biomass. This autochthonous carbon source meets the ampharetid respiratory carbon demand of 23.2mmolm−2d−1 to within a factor of 2. By contrast, the contribution of photosynthetically-fixed carbon sources to ampharetid nutrition is minor (3.3mmolm−2d−1 of C). The data strongly suggest that mixing of labile autochthonous microbial detritus below the oxic layer sustains high measured rates of sulfate reduction in the uppermost 2cm of the sulfidic sediment (100–200nmolcm−3d−1). Similar rates have been reported in the literature for other seeps, from which we conclude that autochthonous organic matter is an important substrate for sulfate reducing bacteria in these sediment layers. A system-scale energy budget based on the chemosynthetic reaction pathways reveals that up to 8.3kJm−2d−1 or 96mWm−2 of catabolic (Gibbs) energy is dissipated at the seep through oxidation reactions. The microorganisms mediating sulfide oxidation and anaerobic oxidation of methane (AOM) produce 95% and 2% of this energy flux, respectively. The low power output by AOM is due to strong bioenergetic constraints imposed on the reaction rate by the composition of the chemical environment. These constraints provide a high potential for dissolved methane efflux from the sediment (12.0mmolm−2d−1) and indicates a much lower efficiency of (dissolved) methane sequestration by AOM at seeps than considered previously. Nonetheless, AOM is able to consume a third of the ascending methane flux (5.9mmolm−2d−1 of CH4) with a high efficiency of energy expenditure (35mmolCH4kJ−1). It is further proposed that bioenergetic limitation of AOM provides an explanation for the non-zero sulfate concentrations below the AOM zone observed here and in other active and passive margin sediments.

Abstract. Fluid flow related seafloor structures and gas seeps were detected in the North Sea in the 1970s and 1980s by acoustic sub-bottom profiling and oil rig surveys. A variety of features like pockmarks, gas vents and authigenic carbonate cements were found to be associated with sites of oil and gas exploration, indicating a link between these surface structures and the underlying, deep hydrocarbon reservoirs. In this study we performed acoustic surveys and videographic observation at Gullfaks, Holene Trench, Tommeliten, Witch's Hole and the giant pockmarks of the UK Block 15/25, to investigate the occurrence and distribution of cold seep ecosystems in the Northern North Sea. The most active gas seep sites, i.e. Gullfaks and Tommeliten, were investigated in detail. At both sites, gas bubbles escaped continuously from small holes in the seabed to the water column, reaching the upper mixed surface layer. At Gullfaks a gas emitting, flat area of 0.1 km2 of sandy seabed covered by filamentous sulfur-oxidizing bacteria was detected. At Tommeliten, we found a patchy distribution of small bacterial mats indicating sites of gas seepage. Below the patches the seafloor consisted of sand from which gas emissions were observed. At both sites, the anaerobic oxidation of methane (AOM) coupled to sulfate reduction (SR) was the major source of sulfide. Molecular analyses targeting specific lipid biomarkers and 16S rRNA gene sequences identified an active microbial community dominated by sulfur-oxidizing and sulfate-reducing bacteria (SRB) as well as methanotrophic bacteria and archaea. Stable carbon isotope values of specific, microbial fatty acids and alcohols from both sites were highly depleted in the heavy isotope 13C, indicating that the microbial community incorporates methane or its metabolites. The microbial community composition of both shallow seeps shows high similarities to the deep water seeps associated with gas hydrates such as Hydrate Ridge or the Eel River basin.

General remarks and recommendations: Improve description of experimental setup: What is the source of methane in the reservoir? AC: As a methane source we have used commercially available methane:CO2-mixture, (90:10) kept in the reservoir headspace at 1.5 atmospheres. The details are now given in the materials and methods section (line 171 ff).and in table 1 The authors should analyze the composition with respect CO and higher hydrocarbons instead of speculating about an additional electron donor for sulfate reduction (p. 3076,