Page 126 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 5. Early Holocene carbon storage and microbial activity in North Sea peats
North Sea peat layers harbour active methanogenic but no methanotrophic communities
Oxic and anoxic batch incubations were used to assess both the CH4 production and consumption potential of these basal-peat deposits. Methanogenesis was observed on methylated compounds only. In contrast to H2/CO2 and acetate, methylated compounds are a non-competitive methanogenic substrate that is metabolized by Methanosarcinales, explaining the presence of these species in these sediments (Lyimo et al. 2000).
No aerobic or anaerobic methanotrophic prokaryotes were found in these peat deposits. Like many marine sediments, the anoxic and marine nature of the environment likely led to the exclusion of an aerobic methanotrophic population (Conrad, Frenzel and Cohen 1995). In addition, the low CH4 partial pressure probably inhibited sulfate-dependent anaerobic oxidation of CH4 (Thauer 2011). Sulfate reduction in these sediments is likely linked to H2 and acetate oxidation (Oremland and Polcin 1982). Environments with methanogens but not methanotrophs are uncommon but have occasionally been identified, e.g., in coal wells and masonry (Kussmaul, Wilimzig and Bock 1998; in ‘t Zandt et al. 2018). The absence of methanotroph activity is congruent with their absence in the results of 16S rRNA gene amplicon sequencing and confirms that methanotrophic species are most likely not present or active in this environment.
Microbial communities at different submerged peat sites are diverse
We observed pronounced differences among the microbial populations at the four sampled locations (Vittorio, Max Gundelach, Senator Westphal S, and Westland sites). This heterogeneity indicates that the in situ microbial populations in these deposits are influenced by the peat forming vegetation (Gastaldo et al. 2004; Stocker 2012), in contrast to the homogenous results that would have been expected of an otherwise sedimentary-marine ecosystem. Although the sampling resolution was limited, this study provides the first insights into the microbial diversity of buried marine peat layers and confirms the relevance of the carbon source for the present-day microbial community composition (Fig. 5). Future studies with higher sampling resolution may provide a better understanding of the relationship between plant and microbial species.
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