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.