Evidence from lipid biomarkers and the methane isotope fraction factor for the methane-degrading community in a peatland with abandoned oil wells (Northern Germany)

Abandoned oil and gas wells (O&G) are typically buried in Germany and thus a characterization of potential leakages is challenging. An industrial peat site in Steimbke (Northern Germany), which includes numerous buried oil wells, is an excellent area for studying the effects and challenges. In these peats, methane with an isotopic signature characteristic for a biogenic formation is highly abundant and likely consumed in large quantities by aerobic methanotrophic bacteria (MOB). These data underline the high complexity of characterising potential O&G well leakages (see also presentation of Sebastian Jordan et al.), but also indicate a high methane oxidizing capacity at peat sites in Germany and other comparable areas in Northern Europe.

Here, we present soil gas and lipid-biomarker data on the aerobic methane oxidizing community at Steimbke. We found that methane oxidation rates (MOx), as well as lipids typical for MOB, were enhanced at places, where high natural methane concentrations occurred. As typical lipids for MOB we studied phospholipid fatty acids (PLFA) and observed an increase of unsaturated PLFA with 16 and 18 carbon atoms with a likely unsaturation at the Δ8 position. These PLFA are typical for MOB-clusters of the Type II (α-proteobacteria) and Type I (γ-proteobacteria), respectively (e.g., Bowman et al., 1991; Nichols et al., 1985). Without PLFA concentrations being conclusive on the relative abundance of bacterial groups, our data argue for that both types are actively oxidizing methane in the Steimbke peats.

We also analysed the carbon isotopic fractionation factor of methane oxidation in the lab and modelled the isotopic behaviour of methane and CO₂. Both were directly linked and demonstrate an ε of ~ -31 ‰ for methane (acc. Feisthauer et al., 2011). This is at the upper end of the known range of ε, which were previously reported between -3 and -39 ‰ for MOx (Templeton et al., 2006). Transferring the assessed fractionation factor to our field data and assuming MOx being responsible for the low methane emissions in the field, methane passing the MOB-layer should be substantially enriched in ¹³C during methane oxidation. Methane concentrations at the soil-atmosphere interface, however, were mostly too low for isotopic measurements. Only at a few sampling site, methane δ¹³C values could be determined, not showing a significant ¹³C enrichment. Thus, we assume this methane has likely bypassed the MOB layer. All in all, our data on the positive correlation of potential MOx and methane concentrations suggest that the microbial community can adapt, e.g., to a leaking O&G well. Thus, a shifting microbial community would help to mitigate methane emissions to the atmosphere. However, there were indications that high fluxes could bypass this biological methane filter.

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