Page 185 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Pelobacter species that acetate is required as carbon source for growth, but that it is not used in central energy metabolism (Schink 1985; Lovley et al. 1995; Richter et al. 2007). The MAG reported here contains several genes encoding cation acetate symporters and an acetate kinase (Supplementary Table S7). The increase in coverage of the Pelobacteraceae MAG upon acetate amendment indicates the supportive role of acetate for growth. The absence of a temperature response in the Pelobacteraceae MAG indicates a psychrophilic nature of the species.
The central metabolism of Pelobacteraceae revolves around the metabolism of a wide
variety of organic compounds, including complex compounds like trihydroxybenzenes and polyethyleneglycol, linked to the reduction of alternative electron acceptors (Schink and
Pfennig 1982; Schink and Stieb 1983). For Pelobacter carbinolicus it was reported that Fe(III)
and elemental sulfur can be used as alternative electron acceptors (Lovley et al. 1995). In permafrost environments, Desulfuromonadales sequences have been linked to sulfur and metal
reduction (Gittel et al. 2014; Dao et al. 2018). The Pelobacteraceae MAG described in our
study contains several genes encoding c-type cytochromes, including a cytochrome c3 that is potentially involved in metal reduction (Lovley et al. 1995). However, in the long-term incubations oxidized metals and elemental sulfur are likely rapidly depleted.
Specific co-occurrences with methanogenic archaea have only been described in a few studies.
The early work by Bryant et al. describes a culture of “Methanobacillus omelianskii”, a co- 8 culture of Pelobacter with hydrogenotrophic methanogens (Bryant et al. 1967). A study by
Timmers et al. on methanogenic, sulfate-reducing sludge found a co-existence of methanogens and Pelobacter-related Desulfuromonadales (Timmers et al. 2015). Here we also observed their co-occurrence with methanogenic archaea in both substrate-amended and unamended sediments which highlights their potential role in organic-rich methanogenic ecosystems.
The two Bacteroidales MAGs were present in all conditions (Table 1). Bacteroidales are dominant players in organic-rich lake sediments where they can play an important role in polysaccharide degradation (Schwarz, Eckert and Conrad 2007; Thomas et al. 2011; He et al. 2015; Wang et al. 2016). A 16S rRNA gene amplicon study by Wang et al. detected Bacteroidetes amongst the key players in soil and lake sediments from London Island, Svalbard (Wang et al. 2016). In addition, a study on the CH4 food web in Arctic sediments found Bacteroidetes amongst the dominant microorganisms, based on 16S rRNA gene pyrosequencing data (He et al. 2015). Our observations are in line with previous studies and
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