Environmental controls on marine methane oxidation : from deep-sea brines to shallow coastal systems

Methane is the most abundant greenhouse gas after carbon dioxide and accounts for ~25% of atmospheric warming since the onset of industrialization. Large amounts of methane are stored in the ocean seafloor as solid gas hydrates, gaseous reservoirs or dissolved in pore water. At cold seeps, various physical, chemical, and geological processes force subsurface methane to ascend along pathways of structural weakness to the sea floor. Additionally, methane can be produced in situ within organic-rich sediments. Increasing evidence suggests that ocean bottom water warming is leading to enhanced methane fluxes into the water column, for instance by dissociation of gas hydrates or by enhanced methane production in coastal ecosystems. Previous investigations showed that a large portion (~80%) of ascending methane in ocean sediments is utilised by anaerobic and aerobic methanotrophic microbes, but future elevated methane fluxes might not be counterbalanced by this sedimentary methane filter. Today, about 0.02 Gt/yr (3-3.5% of the atmospheric budget) of methane bypasses the benthic filter system on a global scale, and subsequently escapes into ocean bottom waters. In the water column, it can be oxidised aerobically (aerobic oxidation of methane - MOx), or less commonly where the water column is anoxic, anaerobically. Water column MOx is the final sink for methane before its release to the atmosphere. However, little is known on the efficiency of this pelagic microbial filter and its ability to adjust to a (rapidly) changing environment. In order to predict future changes, it is thus crucial to understand the efficiency of current water-column MOx, to identify the key organisms mediating MOx, and - most importantly - to determine environmental parameters controlling MOx. In this dissertation, I investigated the pelagic MOx filter in contrasting ocean environments using a multidisciplinary approach. Systems studied included a deep-sea brine, two gas seep systems, and a shallow, organic-rich coastal environment. The main goals were to determine hot spots of MOx, identify bacteria mediating this process, and to estimate the efficiency of the pelagic methane filter. Furthermore, an important aim was to identify environmental factors controlling the activity and distribution of MOx, which could help in predicting changes of MOx in a future (warmer) ocean. My investigations revealed the following: 1. In the water column above methane gas seeps at the West Spitsbergen margin, MOx rate measurements together with CARD-FISH analysis of the methanotrophic community revealed rapid changes in the abundance of methanotrophs. Simultaneous measurements of physico-chemical water mass properties showed that the change in methanotrophic abundance correlated with changes in the water mass present above the seep system. This water mass exchange was caused by short-term variations in the position (i.e., offshore or nearshore) of the warm-water core of the West Spitsbergen Current: In its offshore mode, methanotroph-rich bottom waters above the methane seeps showed a high MOx capacity. A shift of the warm-water core towards the shelf break during the nearshore mode of the current displaced this cold bottom water with warm surface water containing a much smaller standing stock of methanotrophs, and led to a drop in MOx capacity of ~60%. This water mass exchange, caused by short-term variations of the West Spitsbergen Current, thus constitutes an oceanographic switch severely reducing methanotrophic activity in the water column. Since fluctuating currents are widespread oceanographic features common at many methane seep systems, it follows that the variability of physical water mass transport is a globally important control on the distribution and abundance of methanotrophs and, as a consequence, on the efficiency of methane oxidation above point sources. 2. At a Blowout in the North Sea resulting from an accident during industrial drilling activities, vigorous bubble emanation from the seafloor and strongly elevated methane concentrations in the water column (up to 42 μM) indicated that a substantial fraction of methane bypassed the highly active (up to ~2920 nmol/cm3/d) AOM zone in sediments. In the water column, we measured MOx rates that were among the highest ever measured in a marine environment (up to 498 nM/d) and, under stratified conditions, have the potential to remove a significant part of the released methane prior to evasion to the atmosphere. We speculate, however, that the MOx filter is intermittently inhibited when the water column is fully mixed, so that the Blowout is a source of methane to the atmosphere. An unusual dominance of the water-column methanotrophic community by Type II methanotrophs is partially supported by recruitment of sedimentary methanotrophs, which are entrained together with sediment particles in the methane bubble plume. Hence, our study demonstrates that gas ebullition not only provides ample methane substrate to fuel MOx in the water column, it also serves as an important transport vector for sediment-borne microbial inocula that aid in the establishment/proliferation of a water-column methanotrophic community at high-flux colds seeps. 3. We investigated MOx in the water column above gassy coastal sediments on quarterly basis over a time-period of two years. At the Boknis Eck study site, which is located in the coastal inlet Eckernförde Bay in the southwestern Baltic Sea, the water column is seasonally stratified with bottom waters becoming hypoxic over the course of the stratification period. We found that MOx rates exhibited a seasonal pattern with maximum rates (up to 11.6 nmol/l/d) during the summer months when oxygen concentrations were lowest and bottom water temperatures highest. Overall, MOx consumed between 70 – 95% of methane under stratified conditions, but only 40 – 60% under mixed conditions. Additional laboratory-based experiments with adjusted oxygen concentrations in the range of 0.2 – 220 μmol/l confirmed a sub-micromolar MOx oxygen optimum. In contrast, the percentage of methane-carbon incorporation into biomass was reduced at submicromolar oxygen concentrations, suggesting a different partitioning of catabolic and anabolic processes at saturated and sub-micromolar oxygen concentrations. Additional laboratory experiments verified the above-described mesophilic behaviour of the MOx communities of both surface and bottom waters. Our results highlight the importance of MOx in mitigating methane emission from coastal waters and indicate the existence of an adaptation to hypoxic conditions on the organismic level of the water column methanotrophs. 4. Life in the deep-sea brine basin Kryos in the Eastern Mediterranean Sea faces extreme challenges since the brine is almost saturated in bischofite (MgCl2 - 3.9 mol/kg). Due to the strong density difference between the anoxic brine and the overlying Mediterranean seawater, mixing is impeded and a shallow (<3 m) interface has formed. Our ex situ measurements of microbial activity revealed highly active MOx (up to 60 nmol/kg/d) at micro-oxic conditions within the interface. In line with elevated MOx rates, the residual methane within the interface was 13C-enriched when compared to the brine, and we found diagnostic, 13C-depleted lipid biomarkers (e.g., diplopterol, -46.6‰), which can be attributed to aerobic methanotrophs. Additionally, we detected relatively δ13C-enriched fatty acids (up to -18‰) in the lower interface and in the brine, which are an indication for a different carbon fixation pathway than the Calvin Benson Cycle, such as the reverse tri-carboxylic acid carbon-fixation pathway found in sulfur-oxidizing Epsilonproteobacteria. Within the brine, we could not find evidence for AOM, despite of thermodynamically favorable conditions for this process. In contrast, we measured high rates of sulfate reduction within the brine (up to 430 μmol/kg/d) providing evidence that sulfate reducers are active under nearly Mg2+-saturated concentrations. Our results emphasize the adaptation of microbial life to the extremely harsh conditions below the chaotropicity limits of life in MgCl2-rich environments

Microbial methanogenesis in the sulfate-reducing zone of sediments in the Eckernförde Bay, SW Baltic Sea

Benthic microbial methanogenesis is a known source of methane in marine systems. In most sediments, the majority of methanogenesis is located below the sulfate-reducing zone, as sulfate reducers outcompete methanogens for the major substrates hydrogen and acetate. The coexistence of methanogenesis and sulfate reduction has been shown before and is possible through the usage of noncompetitive substrates by methanogens such as methanol or methylated amines. However, knowledge about the magnitude, seasonality, and environmental controls of this noncompetitive methane production is sparse. In the present study, the presence of methanogenesis within the sulfate reduction zone (SRZ methanogenesis) was investigated in sediments (0–30 cm below seafloor, cm b.s.f.) of the seasonally hypoxic Eckernförde Bay in the southwestern Baltic Sea. Water column parameters such as oxygen, temperature, and salinity together with porewater geochemistry and benthic methanogenesis rates were determined in the sampling area "Boknis Eck" quarterly from March 2013 to September 2014 to investigate the effect of seasonal environmental changes on the rate and distribution of SRZ methanogenesis, to estimate its potential contribution to benthic methane emissions, and to identify the potential methanogenic groups responsible for SRZ methanogenesis. The metabolic pathway of methanogenesis in the presence or absence of sulfate reducers, which after the addition of a noncompetitive substrate was studied in four experimental setups: (1) unaltered sediment batch incubations (net methanogenesis), (2) 14C-bicarbonate labeling experiments (hydrogenotrophic methanogenesis), (3) manipulated experiments with the addition of either molybdate (sulfate reducer inhibitor), 2-bromoethanesulfonate (methanogen inhibitor), or methanol (noncompetitive substrate, potential methanogenesis), and (4) the addition of 13C-labeled methanol (potential methylotrophic methanogenesis). After incubation with methanol, molecular analyses were conducted to identify key functional methanogenic groups during methylotrophic methanogenesis. To also compare the magnitudes of SRZ methanogenesis with methanogenesis below the sulfate reduction zone (> 30 cm b.s.f.), hydrogenotrophic methanogenesis was determined by 14C-bicarbonate radiotracer incubation in samples collected in September 2013. SRZ methanogenesis changed seasonally in the upper 30 cm b.s.f. with rates increasing from March (0.2 nmol cm−3 d−1) to November (1.3 nmol cm−3 d−1) 2013 and March (0.2 nmol cm−3 d−1) to September (0.4 nmol cm−3 d−1) 2014. Its magnitude and distribution appeared to be controlled by organic matter availability, C / N, temperature, and oxygen in the water column, revealing higher rates in the warm, stratified, hypoxic seasons (September–November) compared to the colder, oxygenated seasons (March–June) of each year. The majority of SRZ methanogenesis was likely driven by the usage of noncompetitive substrates (e.g., methanol and methylated compounds) to avoid competition with sulfate reducers, as was indicated by the 1000–3000-fold increase in potential methanogenesis activity observed after methanol addition. Accordingly, competitive hydrogenotrophic methanogenesis increased in the sediment only below the depth of sulfate penetration (> 30 cm b.s.f.). Members of the family Methanosarcinaceae, which are known for methylotrophic methanogenesis, were detected by PCR using Methanosarcinaceae-specific primers and are likely to be responsible for the observed SRZ methanogenesis. The present study indicates that SRZ methanogenesis is an important component of the benthic methane budget and carbon cycling in Eckernförde Bay. Although its contributions to methane emissions from the sediment into the water column are probably minor, SRZ methanogenesis could directly feed into methane oxidation above the sulfate–methane transition zone

Fluid dynamics and slope stability offshore W-Spitsbergen: Effect of bottom water warming on gas hydrates and slope stability - Cruise No. MSM21/4 - August 12 - September 11, 2012 - Reykjavik (Iceland) - Emden (Germany)

The main goal of MSM21/4 was the study of gas hydrate system off Svalbard. We addressed this through a comprehensive scientific programme comprising dives with the manned submersible JAGO, seismic and heat flow measurements, sediment coring, water column biogeochemistry and bathymetric mapping. At the interception of the Knipovich Ridge and the continental margin of Svalbard we collected seismic data and four heat flow measurements. These measurements revealed that the extent of hydrates is significantly larger than previously thought and that the gas hydrate system is influenced by heat from the oceanic spreading centre, which may promote thermogenic methane production and thus explain the large extent of hydrates. At the landward termination of the hydrate stability zone we investigated the mechanisms that lead to degassing by taking sediment cores, sampling of carbonates during dives, and measuring the methane turn-over rates in the water column. It turned out that the observed gas seepage must have been ongoing for a long time and that decadal scale warming is an unlikely explanation for the observed seeps. Instead seasonal variations in water temperatures seem to control episodic hydrate formation and dissociation explaining the location of the observed seeps. The water column above the gas flares is rich in methane and methanotrophic microorganisms turning over most of the methane that escapes from the sea floor. We also surveyed large, until then uncharted parts of the margin in the northern part of the gas hydrate province. Here, we discovered an almost 40 km wide submarine landslide complex. This slide is unusual in the sense that it is not located at the mouth of a cross shelf trough such as other submarine landslides on the glaciated continental margins around the North Atlantic. Thus, the most widely accepted explanation for the origin of such slides, i.e. overpressure development due to deposition of glacial sediments on top of water rich contourites, is not applicable. Instead we find gas-hydrate-related bottom simulating reflectors underneath the headwalls of this slide complex, possibly indicating that subsurface fluid migration plays a major role in its genesis

Shallow gas migration along hydrocarbon wells – An unconsidered, anthropogenic source of biogenic methane in the North Sea

Shallow gas migration along hydrocarbon wells constitutes a potential methane emission pathway that currently is not recognized in any regulatory framework or greenhouse gas inventory. Recently, the first methane emission measurements at three abandoned offshore wells in the Central North Sea (CNS) were conducted showing that considerable amounts of biogenic methane originating from shallow gas accumulations in the overburden of deep reservoirs were released by the boreholes. Here, we identify numerous wells poking through shallow gas pockets in 3D seismic data of the CNS indicating that about one third of the wells may leak, potentially releasing a total of 3-17 kt of methane per year into the North Sea. This poses a significant contribution to the North Sea methane budget. A large fraction of this gas (~42 %) may reach the atmosphere via direct bubble transport (0-2 kt yr-1) and via diffusive exchange of methane dissolving in the surface mixed layer (1-5 kt yr-1), as indicated by numerical modeling. In the North Sea and in other hydrocarbon-prolific provinces of the world shallow gas pockets are frequently observed in the sedimentary overburden and aggregate leakages along the numerous wells drilled in those areas may be significant

Fluxes and fate of dissolved methane released at the seafloor at the landward limit of the gas hydrate stability zone offshore western Svalbard

Widespread seepage of methane from seafloor sediments offshore Svalbard close to the landward limit of the gas hydrate stability zone (GHSZ) may, in part, be driven by hydrate destabilization due to bottom water warming. To assess whether this methane reaches the atmosphere where it may contribute to further warming, we have undertaken comprehensive surveys of methane in seawater and air on the upper slope and shelf region. Near the GHSZ limit at ?400 m water depth, methane concentrations are highest close to the seabed, reaching 825 nM. A simple box model of dissolved methane removal from bottom waters by horizontal and vertical mixing and microbially mediated oxidation indicates that ?60% of methane released at the seafloor is oxidized at depth before it mixes with overlying surface waters. Deep waters are therefore not a significant source of methane to intermediate and surface waters; rather, relatively high methane concentrations in these waters (up to 50 nM) are attributed to isopycnal turbulent mixing with shelf waters. On the shelf, extensive seafloor seepage at &lt;100 m water depth produces methane concentrations of up to 615 nM. The diffusive flux of methane from sea to air in the vicinity of the landward limit of the GHSZ is ?4–20 ?mol m?2 d?1, which is small relative to other Arctic sources. In support of this, analyses of mole fractions and the carbon isotope signature of atmospheric methane above the seeps do not indicate a significant local contribution from the seafloor source