Page 129 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

mainly dominated by Delta- and Gammaproteobacteria, whereas deeper parts of the subseafloor were dominated by the JS1 lineage and Chloroflexi (Wilms et al. 2006; Parkes et al. 2007).
This distribution is in line with our findings of high relative abundances of JS1 lineage bacteria in the peat deposits (Fig. 5). A 16S rRNA PCR-DGGE study of two Wadden Sea tidal flats (Neuharlingersieler Nacken and Gröninger Plate) found that JS1 lineage bacteria were most abundant in the Neuharlingersieler Nacken samples with the highest total organic carbon contents (1-2%) (Webster et al. 2007). Considering these previous findings of JS1 lineage bacteria in organic-rich environments, it is not unexpected that JS1 are dominant bacteria in these deep organic-rich peat deposits.
Peat structure, CH4 storage, and microbial community structure
The Vittorio site contained the maximum CH4 concentrations observed in this study, both above and below the peat layer. Previous studies in non-peat marine environments have found CH4 bubble emanations and dissolved CH4 seepage vary strongly depending on sediment characteristics (Schneider von Deimling et al. 2015a; Steinle et al. 2016). It is likely that the compacted nature of basal-peat deposits impacts the diffusion of CH4 through the sediment (Grunwald et al. 2009).
Pore water analysis indicated conversion of the peat deposits into marine systems (Table S4); that is, marine microbes have been introduced into sediments that previously harboured freshwater microbial communities. This is reflected by the occurrence of Dehalococcoidia and candidate phylum JS1 bacteria (labelled “JS1” in Fig. 5), which are characteristic of marine sediments. These species showed the highest abundances in the two sites located nearest to each other in the study area, Vittorio Z and Max Gundelach SW (Wasmund et al. 2014; Nobu et al. 2016a). Candidate phylum JS1 bacteria were omnipresent, whereas Dehalococcoidia were more abundant in three of the 12 layers, without a clear link to depth or local conditions. Future research is needed to unravel the mechanism underlying this pattern of abundance.
The high CH4 concentrations observed at the Vittorio and Darci’s sites occurred in the presence of nitrate and ammonium, previously shown to be indicative of increased rates of biological mineralization (Burdige 1991). However, the Westland site displayed comparably high ammonium concentrations without a link to CH4 concentrations.
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