[Show abstract][Hide abstract] ABSTRACT: In order to fully understand the cycling of elements in hydrothermal systems it is critical to understand intra-field variations in geochemical and microbiological processes in both focused, high-temperature and diffuse, low-temperature areas. To reveal important causes and effects of this variation, we performed an extensive chemical and microbiological characterization of a low-temperature venting area in the Loki's Castle Vent Field (LCVF). This area, located at the flank of the large sulfide mound, is characterized by numerous chimney-like barite (BaSO4) structures (≤ 1 m high) covered with white cotton-like microbial mats. Results from geochemical analyses, microscopy (FISH, SEM), 16S rRNA gene amplicon-sequencing and metatranscriptomics were compared to results from previous analyses of biofilms growing on black smoker chimneys at LCVF. Based on our results, we constructed a conceptual model involving the geochemistry and microbiology in the LCVF. The model suggests that CH4 and H2S are important electron donors for microorganisms in both high-temperature and low-temperature areas, whereas the utilization of H2 seems restricted to high-temperature areas. This further implies that sub-seafloor processes can affect energy-landscapes, elemental cycling, and the metabolic activity of primary producers on the seafloor. In the cotton-like microbial mats on top of the active barite chimneys, a unique network of single cells of Epsilonproteobacteria interconnected by threads of extracellular polymeric substances (EPS) was seen, differing significantly from the long filamentous Sulfurovum filaments observed in biofilms on the black smokers. This network also induced nucleation of barite crystals and is suggested to play an essential role in the formation of the microbial mats and the chimneys. Furthermore, it illustrates variations in how different genera of Epsilonproteobacteria colonize and position cells in different vent fluid mixing zones within a vent field. This may be related to niche-specific physical characteristics. Altogether, the model provides a reference for future studies and illustrates the importance of systematic comparative studies of spatially closely connected niches in order to fully understand the geomicrobiology of hydrothermal systems.
Full-text · Article · Jan 2016 · Frontiers in Microbiology
[Show abstract][Hide abstract] ABSTRACT: In order to gain insight into the lithology and crustal evolution of the northern Jan Mayen Ridge, North Atlantic, the horizontal components of an Ocean Bottom Seismometer (OBS) dataset were analyzed with regard to Vp/Vs-modelling and seismic anisotropy. The modelling suggests that the northernmost part of the ridge consists of Icelandic type oceanic crust, bordered to the north by anomalously thick oceanic crust formed at the Mohns spreading ridge. The modelled Vp/Vs-ratios suggest variations in gabbroic composition and present-day temperatures in the area. Anisotropy analysis reveals a fast S-wave component along the Jan Mayen Ridge. This pattern of anisotropy is most readily interpreted as dikes intruded along the ridge, suggesting that the magmatism can be related to the development of a leaky transform since Early Oligocene.
[Show abstract][Hide abstract] ABSTRACT: Little is known about how lithoautotrophic primary production is connected to microbial organotrophic consumption in hydrothermal systems. Using a multifaceted approach, we analyzed the structure and metabolic capabilities within a biofilm growing on the surface of a black smoker chimney in the Loki's Castle vent field. Imaging revealed the presence of rod-shaped Bacteroidetes growing as ectobionts on long, sheathed microbial filaments (> 100 um) affiliated with the Sulfurovum genus within Epsilonproteobacteria. The filaments were composed of a thick (> 200 nm) stable polysaccharide, representing a substantial fraction of organic carbon produced by primary production. An integrated -omics approach enabled us to assess the metabolic potential and in situ metabolism of individual taxonomic and morphological groups identified by imaging. Specifically, we provide evidence that organotrophic Bacteroidetes attach to and glide along the surface of Sulfurovum filaments utilizing organic polymers produced by the lithoautotrophic Sulfurovum. Furthermore, in situ expression of acetyl-CoA synthetase by Sulfurovum suggested the ability to assimilate acetate, indicating recycling of organic matter in the biofilm. This study expands our understanding of the lifestyles of Epsilonproteobacteria in hydrothermal vents, their metabolic properties and co-operative interactions in deep-sea hydrothermal vent food webs.
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[Show abstract][Hide abstract] ABSTRACT: Cambrian to middle Ordovician schists and phyllites in southwestern Baltica, now exposed in the (par-)autochthon to Lower Allochthon nappes of the Scandinavian Caledonides in southern Norway, contain previously unrecognized far-travelled detrital zircons with ages in the intervals 0.47–0.8 and 1.85–3.2 Ga and εHf in the range −27 to +18. These ages are assigned to Timanian and Fennoscandian Shield sources respectively and contrast with the locally derived detritus with zircon ages of c. 0.9–1.8
Ga and εHf values c. −13 to +10. The far-travelled zircons provide evidence that a steady, long-haul, source-to-sink drainage system existed from the northeastern fringe of Baltica to the SW passive margin across the whole palaeocontinent (c. 2000 km) since c. 521
Ma, and that the Timanian orogen shed detritus across large distances towards both
its foreland (Baltica) and hinterland (Arctica). There are several arguments against an Avalonian source for these zircons.
Recycling of the detrital zircon from the Cambrian to middle Ordovician sediments is probably responsible for the presence of Cryogenian to Middle Ordovician zircon ages in younger sedimentary sequences of southwestern Baltica. The development of an ophiolitic mélange associated with Ordovician phyllites underlying the Jotun Nappe Complex seems to mark the change to an active continental margin in the middle Ordovician, heralding the Caledonian orogeny. The study demonstrates that detrital zircon-poor fine-grained (siltstone to mudstone) sediments are an extremely valuable indicator for distal sources and favourably complement zircon-rich coarser sandstone in provenance analysis.
No preview · Article · Jun 2015 · Journal of the Geological Society
[Show abstract][Hide abstract] ABSTRACT: Deep-sea hydrothermal vents and cold seeps
are biological hot spots with chemolithotrophic bacterial
production sustaining both benthic and pelagic organisms.
Although efforts have been made to understand the diversity
and function of the bacterial composition of these
systems, first-level consumers, pelagic single cell heterotrophic
organisms, which represent an important link
between bacterial production and higher trophic levels,
remain un-described in hydrothermal vents and seeps of the
Nordic Seas. Here, we used a molecular biodiversity assay
to investigate the impact of water masses and hydrothermal
vents on the eukaryotic micro-organisms surrounding two
vents systems, Jan Mayen Vent Field and Loki‘s Castle,
and one cold seep, Ha°kon Mosby Mud Volcano. The assay
generated a total of 482 operational taxonomic units
(OTUs) based on a 99 % cut-off value, and the OTUs were
grouped according to taxonomic rank. Data analysis using
hierarchical clustering and non-metric multidimensional
scaling with class as taxonomic entries suggested that
water masses followed by depth was the dominant effect on
eukaryotic micro-organism diversity. However, in one of
the vent systems, Loki‘s Castle, the community was different
compared to the reference station. Our data suggest
that while the total production of vent systems is higher
than the surrounding waters, the biodiversity of eukaryotic
micro-organisms is more influenced by both water masses
[Show abstract][Hide abstract] ABSTRACT: Hydrogen, methane and ammonium are important chemical species for chemolithotrophic microorganisms that sustain subsurface endolithic communities. It is well known that production of H2 through serpentinization of the primary Fe(II)-containing minerals, olivine and pyroxene, in ultramafic rocks and reduction of CO2 to CH4 are processes that occur at higher temperatures. Knowledge is, however, limited about these reactions under low-temperature conditions, and how they are affected by Fe(II)-containing secondary minerals, such as brucite and serpentine, which are commonly found in these environments. In this experimental study, we explored the formation of H2, CH4 and NH4 during low-temperature (25 °C) reactions between deionised water and 1) unaltered, 2) medium altered and 3) highly altered dunites, over a period of 99 days.
No preview · Article · Nov 2014 · Chemical Geology
[Show abstract][Hide abstract] ABSTRACT: An unknown amount of the greenhouse gas methane is continuously seeping into the oceans, impacting marine life and potentially reaching the atmosphere. The introduction of subsea CO2 storage further increases the need for accurate monitoring methods to ensure that potential leaks are detected. Gas seeps in the water column appear as characteristic ``flares'' in single- and multibeam sonar images due to the high contrast in acoustic impedance between water and gas. We investigate the potential of using synthetic aperture sonar (SAS) for seep detection. SAS is known to provide high quality seafloor images with significantly improved azimuth resolution. However, standard (coherent) SAS is not suited for gas seep imaging since the required assumption of temporal stationarity is violated. We propose an alternative processing scheme where we produce two SAS images - one standard SAS image and one where
the images from individual pings have been combined noncoherently to preserve the intensity of a gas seep. The difference in mean pixel intensity reveals the presence of a seep. We collected data from two seep locations in the North Sea where shallow gas is escaping through cracked cement well casings. The seep was imaged using the HISAS 1030 sonar carried by the Hugin AUV. We show how the proposed processing scheme can be used to detect and accurately localize even a modest seep.
[Show abstract][Hide abstract] ABSTRACT: Barite chimneys are known to form in hydrothermal systems where barium-enriched fluids generated by leaching of the oceanic basement are discharged and react with seawater sulfate. They also form at cold seeps along continental margins, where marine (or pelagic) barite in the sediments is remobilized because of subseafloor microbial sulfate reduction. We test the possibility of using multiple sulfur isotopes (δ34S, Δ33S, ∆36S) of barite to identify microbial sulfate reduction in a hydrothermal system. In addition to multiple sulfur isotopes, we present oxygen (δ18O) and strontium (87Sr/86Sr) isotopes for one of numerous barite chimneys in a low-temperature (~20 °C) venting area of the Loki's Castle black smoker field at the ultraslow-spreading Arctic Mid-Ocean Ridge (AMOR). The chemistry of the venting fluids in the barite field identifies a contribution of at least 10% of high-temperature black smoker fluid, which is corroborated by 87Sr/86Sr ratios in the barite chimney that are less radiogenic than in seawater. In contrast, oxygen and multiple sulfur isotopes indicate that the fluid from which the barite precipitated contained residual sulfate that was affected by microbial sulfate reduction. A sulfate reduction zone at this site is further supported by the multiple sulfur isotopic composition of framboidal pyrite in the flow channel of the barite chimney and in the hydrothermal sediments in the barite field, as well as by low SO4 and elevated H2S concentrations in the venting fluids compared with conservative mixing values. We suggest that the mixing of ascending H2- and CH4-rich high-temperature fluids with percolating seawater fuels microbial sulfate reduction, which is subsequently recorded by barite formed at the seafloor in areas where the flow rate is sufficient. Thus, low-temperature precipitates in hydrothermal systems are promising sites to explore the interactions between the geosphere and biosphere in order to evaluate the microbial impact on these systems.
[Show abstract][Hide abstract] ABSTRACT: CorA is a copper repressible protein previously identified in the methanotrophic bacterium Methylomicrobium album BG8. In this work, we demonstrate that CorA is located on the cell surface and binds one copper ion per protein molecule, which, based on X-ray Absorption Near Edge Structure analysis, is in the reduced state (Cu(I)). The structure of endogenously expressed CorA was solved using X-ray crystallography. The 1.6 Å three-dimensional structure confirmed the binding of copper and revealed that the copper atom was coordinated in a mononuclear binding site defined by two histidines, one water molecule, and the tryptophan metabolite, kynurenine. This arrangement of the copper-binding site is similar to that of its homologous protein MopE* from Metylococcus capsulatus Bath, confirming the importance of kynurenine for copper binding in these proteins. Our findings show that CorA has an overall fold similar to MopE, including the unique copper(I)-binding site and most of the secondary structure elements. We suggest that CorA plays a role in the M. album BG8 copper acquisition.
[Show abstract][Hide abstract] ABSTRACT: Formation pathways of ancient siliceous iron formations and related Fe isotopic fractionation are still not completely understood. Investigating these processes, however, is difficult as good modern analogues to ancient iron formations are scarce. Modern siliceous Fe oxyhydroxide deposits are found at marine hydrothermal vent sites, where they precipitate from diffuse, low temperature fluids along faults and fissures on the seafloor. These deposits exhibit textural and chemical features that are similar to some Phanerozoic iron formations, raising the question as to whether the latter could have precipitated from diffuse hydrothermal fluids rather than from hydrothermal plumes.
In this study, we present the first data on modern Fe oxyhydroxide deposits from the Jan Mayen hydrothermal vent fields, Norwegian-Greenland Sea. The samples we investigated exhibited very low δ56Fe values between −2.09‰ and −0.66‰. Due to various degrees of partial oxidation, the Fe oxyhydroxides are with one exception either indistinguishable from low-temperature hydrothermal fluids from which they precipitated (−1.84‰ and −1.53‰ in δ56Fe) or are enriched in the heavy Fe isotopes. In addition, we investigated Fe isotope variations in Ordovician jasper beds from the Løkken ophiolite complex, Norway, which have been interpreted to represent diagenetic products of siliceous ferrihydrite precursors that precipitated in a hydrothermal plume, in order to compare different formation pathways of Fe oxyhydroxide deposits. Iron isotopes in the jasper samples have higher δ56Fe values (−0.38‰ to +0.89‰) relative to modern, high-temperature hydrothermal vent fluids (ca. −0.40‰ on average), supporting the fallout model. However, formation of the Ordovician jaspers by diffuse venting cannot be excluded, due to lithological differences of the subsurface of the two investigated vent systems.
Our study shows that reliable interpretation of Fe isotope variations in modern and ancient marine Fe oxyhydroxide deposits depends on comprehensive knowledge of the geological context. Furthermore, we demonstrate that very negative δ56Fe values in such samples might not be the result of microbial dissimilatory iron reduction, but could be caused instead by inorganic reactions.
Full-text · Article · Feb 2014 · Geochimica et Cosmochimica Acta
[Show abstract][Hide abstract] ABSTRACT: We have discovered a 3 km long seafloor fracture system in the Central North Sea (block 16/4). The discovery was made using a synthetic aperture sonar (HISAS) mounted on a Kongsberg Hugin AUV. The surface expression of the structure – named the Hugin Fracture - changes along the strike and it is characterized by: 1) linear; 2) en echelon; and 3) branching segments. Ring-structures, that typically are 5-10 meters across, are common along the feature. Micro-bathymetry acquired using the HISAS system demonstrates that sub-meter scale elevation changes occur across the fracture. Microbial mats occur along different parts of the structure showing that active seepage is taking place. AUV based photo-imaging of parts of the structure shows that the microbial mats predominantly are associated with ring structures and some of the linear fracture segments. Sediment pore waters extracted from push cores show Na, Cl and Mg contents that are 10-15% lower compared to background pore fluid concentrations. This points towards a fresh water input. The fracture pore fluid compositions are also characterized by elevated methane, ammonium and hydrogen sulphide contents compared to the background seawater. The presence of these volatiles is likely caused by subsurface microbial activity, and carbon isotope analyses confirm a biological source of the detected methane. However, the presence of ethane (CH4/C2H6 of 126) indicates a small input of a thermogenic carbon to these fluids. Subsurface imaging using a hull mounted parametric sub bottom profiler reveal Holocene stratified sediments overlying quaternary moraine in the area. The sub bottom profiler data show sub-meter scale vertical movements along the fracture. Associated with the structures are small bright spots that may reflect gas accumulations. No deep-seated fault system is apparent below the fracture in 3D seismic data from the area. However, the 3D seismic data show that the structure is located above the boundary of a channel-like feature present around 150m subseafloor. This subsurface feature probably represents a shallow fluvial channel that formed during glaciation/deglaciation. A structural connection between the seafloor seepage structure and a fluvial channel is supported by the pore water chemistry at the investigated seepage sites. High undrained shear strengths (up to 180 kN/m2) have been measured in the shallow clay-rich sediments that are present in this area (Sejrup et al. 1987). We therefore propose that the Hugin Fracture formed by brittle failure of such stiff sediments as a result of differential compaction located to the margin of the shallow fluvial channel. The discovery of the Hugin Fracture demonstrates that the clay-rich and impermeable sediments that represent the uppermost seal of the Utsira Formation may be broken as a result of brittle failure, and that km-scale fracture systems may form as a result. The fracturing of this impermeable top layer may connect permeable fluvial channels to the seafloor. The presence in the fluids of hydrocarbons (ethane) that typically form by deep thermogenic processes suggests that this shallow fluid flow system is connected, or has been connected, to sedimentary sequences that are present below the Utsira Formation.
[Show abstract][Hide abstract] ABSTRACT: In marine sediments archaea often constitute a considerable part of the microbial community, of which the Deep Sea Archaeal Group (DSAG) is one of the most predominant. Despite their high abundance no members from this archaeal group have so far been characterized and thus their metabolism is unknown. Here we show that the relative abundance of DSAG marker genes can be correlated with geochemical parameters, allowing prediction of both the potential electron donors and acceptors of these organisms. We estimated the abundance of 16S rRNA genes from Archaea, Bacteria, and DSAG in 52 sediment horizons from two cores collected at the slow-spreading Arctic Mid-Ocean Ridge, using qPCR. The results indicate that members of the DSAG make up the entire archaeal population in certain horizons and constitute up to ~50% of the total microbial community. The quantitative data were correlated to 30 different geophysical and geochemical parameters obtained from the same sediment horizons. We observed a significant correlation between the relative abundance of DSAG 16S rRNA genes and the content of organic carbon (p < 0.0001). Further, significant co-variation with iron oxide, and dissolved iron and manganese (all p < 0.0000), indicated a direct or indirect link to iron and manganese cycling. Neither of these parameters correlated with the relative abundance of archaeal or bacterial 16S rRNA genes, nor did any other major electron donor or acceptor measured. Phylogenetic analysis of DSAG 16S rRNA gene sequences reveals three monophyletic lineages with no apparent habitat-specific distribution. In this study we support the hypothesis that members of the DSAG are tightly linked to the content of organic carbon and directly or indirectly involved in the cycling of iron and/or manganese compounds. Further, we provide a molecular tool to assess their abundance in environmental samples and enrichment cultures.
Full-text · Article · Oct 2013 · Frontiers in Microbiology
[Show abstract][Hide abstract] ABSTRACT: Deep-sea hydrothermal vents are unique environments on Earth, as they host chemosynthetic ecosystems fuelled by geochemical energy with chemolithoautotrophic microorganisms at the basis of the foodwebs. Whereas discrete high-temperature venting systems have been studied extensively, the microbiotas associated with low temperature diffuse venting are not well understood. We analysed the structure and functioning of microbial communities in two diffuse venting sediments from the Jan Mayen vent fields in the Norwegian-Greenland Sea, applying an integrated 'omics' approach combining meta-transcriptomics, -proteomics and -genomics. PCR-independent three-domain community profiling showed that the two sediments hosted highly similar communities dominated by Epsilon-, Delta- and Gamma-Proteobacteria, besides ciliates, nematodes and various archaeal taxa. Active metabolic pathways were identified through transcripts and peptides, with genes of sulfur and methane oxidation, and carbon fixation pathways highly expressed, in addition to genes of aerobic and anaerobic (nitrate and sulfate) respiratory chains. High expression of chemotaxis and flagella genes reflected a lifestyle in a dynamic habitat rich in physico-chemical gradients. The major metabolic pathways could be assigned to distinct taxonomic groups, thus enabling hypotheses about the function of the different pro- and eukaryotic taxa. This study advances our understanding of the functioning of microbial communities in diffuse hydrothermal venting sediments.
[Show abstract][Hide abstract] ABSTRACT: The use of DNA as a marker for prey inside the gut of predators has been instrumental in further understanding of known and unknown interactions. Molecular approaches are in particular useful in unavailable environments like the deep sea. Trophic interactions in the deep sea are difficult to observe in situ, correct deep-sea experimental laboratory conditions are difficult to obtain, animals rarely survive the sampling, or the study organisms feed during the sampling due to long hauls. Preliminary studies of vent and seep systems in the Nordic Seas have identified the temperate-cold-water pelagic amphipod Themisto abyssorum as a potentially important predator in these chemosynthetic habitats. However, the prey of this deep-sea predator is poorly known, and we applied denaturing high performance liquid chromatography (DHPLC) to investigate the predator-prey interactions of T. abyssorum in deep-water vent and seep systems. Two deep-water hydrothermally active localities (The Jan Mayen and Loki's Castle vent fields) and one cold seep locality (The Håkon Mosby mud volcano) in the Nordic Seas were sampled, genomic DNA of the stomachs of T. abyssorum was extracted, and 18S rDNA gene was amplified and used to map the stomach content. We found a wide range of organisms including micro-eukaryotes, metazoans and detritus. Themisto abyssorum specimens from Loki's Castle had the highest diversity of prey. The wide range of prey items found suggests that T. abyssorum might be involved in more than one trophic level and should be regarded as an omnivore and not a strict carnivore as have previously been suggested.
[Show abstract][Hide abstract] ABSTRACT: Recent studies explored the potential of stable Cr isotopic
variations in oceanic sedimentary archives as tracer for
atmospheric oxygen levels through Earth’s history [1,2].
Thereby, the stable Cr isotopic variations in up to 2.75 Ga old
BIFs were interpreted to indicate oxidative chromium
weathering on the continents initiated by an accumulation of
small levels of free atmospheric oxygen some 350 Ma before
the ca. 2.4 to 2.32 Ga great oxidation event (GOE) . This
interpretation, however, was challenged by others , who
propose that the Cr isotopic variations in these predominantly
Algoma type BIFs are due to non-redox isotopic effects
caused by rapid precipitation from their deep-water
In order to shed light on the applicability of stable Cr
isotopes as paleo-redox tracer we investigated carbonates from
well-defined supra-, intra- and subtidal depositional
environments from the ca. 2.55-2.48 Ga old Malmani
Subgroup of the Transvaal Basin in South Africa and 2.06 Ga
old lacustrine carbonates, marine stromatolites and near-shore
jaspilites from the Pechenga Greenstone Belt, as well as 2.0
Ga organic-rich, siliceous deposits from the Onega Basin, both
situated in the NW Fennoscandian Shield. The relatively large
variations (ca. +1.4‰ to -1.2‰ in #53/52Cr) found in post-GOE
sedimentary archives from the Fennoscandian Shield support
chromium redox-cycling associated with oxidative chromium
weathering on the continental surface. Compared to the large
Cr isotope variations of these post-GOE deposits, the range in
#53/52Cr values of late Archean sedimentary archives appears
to be much smaller. Possible scenarios to explain the observed
Cr isotopic variations in these sedimentary archives and their
implications to the presence of free atmospheric oxygen will
 Frei et al. (2009) Nature 461, 250-253.  Frei et al.
(2011) EPSL 312, 114-125.  Konhauser et al. (2011)
Nature 478, 369-373.
[Show abstract][Hide abstract] ABSTRACT: The Epsilonproteobacteria, including members of the genus Sulfurovum, are regarded as important primary producers in hydrothermal systems. However, their in situ gene expression in this habitat has so far not been investigated. We report a metatranscriptomic analysis of a Sulfurovum-dominated biofilm from one of the chimneys at the Loki's Castle hydrothermal system, located at the Arctic Mid Ocean Ridge. Transcripts involved in hydrogen oxidation, oxidation of sulfur species, aerobic respiration and denitrification were abundant and mostly assigned to Sulfurovum, indicating that members of this genus utilize multiple chemical energy sources simultaneously for primary production. Sulfurovum also seemed to have a diverse expression of transposases, potentially involved in horizontal gene transfer. Other transcripts were involved in CO2 fixation by the reverse TCA cycle, the CRISPR-Cas system, heavy metal resistance, and sensing and responding to changing environmental conditions. Through pyrosequencing of PCR amplified 16S rRNA genes, the Sulfurovum-dominated biofilm was compared with another biofilm from the same chimney, revealing a large shift in the community structure of Epsilonproteobacteria-dominated biofilms over a few metres.
[Show abstract][Hide abstract] ABSTRACT: Water-rock interactions in ultramafic lithosphere generate reduced chemical species such as hydrogen that can fuel subsurface microbial communities. Sampling of this environment is expensive and technically demanding. However, highly accessible, uplifted oceanic lithospheres emplaced onto continental margins (ophiolites) are potential model systems for studies of the subsurface biosphere in ultramafic rocks. Here, we describe a microbiological investigation of partially serpentinized dunite from the Leka ophiolite (Norway). We analysed samples of mineral coatings on subsurface fracture surfaces from different depths (10-160 cm) and groundwater from a 50-m-deep borehole that penetrates several major fracture zones in the rock. The samples are suggested to represent subsurface habitats ranging from highly anaerobic to aerobic conditions. Water from a surface pond was analysed for comparison. To explore the microbial diversity and to make assessments about potential metabolisms, the samples were analysed by microscopy, construction of small subunit ribosomal RNA gene clone libraries, culturing and quantitative-PCR. Different microbial communities were observed in the groundwater, the fracture-coating material and the surface water, indicating that distinct microbial ecosystems exist in the rock. Close relatives of hydrogen-oxidizing Hydrogenophaga dominated (30% of the bacterial clones) in the oxic groundwater, indicating that microbial communities in ultramafic rocks at Leka could partially be driven by H2 produced by low-temperature water-rock reactions. Heterotrophic organisms, including close relatives of hydrocarbon degraders possibly feeding on products from Fischer-Tropsch-type reactions, dominated in the fracture-coating material. Putative hydrogen-, ammonia-, manganese- and iron-oxidizers were also detected in fracture coatings and the groundwater. The microbial communities reflect the existence of different subsurface redox conditions generated by differences in fracture size and distribution, and mixing of fluids. The particularly dense microbial communities in the shallow fracture coatings seem to be fuelled by both photosynthesis and oxidation of reduced chemical species produced by water-rock reactions.