Page 186 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 8. Species-specific responses of enriched thermokarst lake sediments
highlight their potential role in supporting methanogenic communities. Interestingly, only minor responses to substrate amendment were observed, which indicates that the two Bacteroidales MAGs are probably mainly controlled by the availability of in situ polymeric substrates.
The Peptostreptococcales MAG was present in all samples and showed a minor response to substrate amendment (Table 1). Peptostreptococcales/Clostridia possess a wide array of fermentation pathways and are common inhabitants of soils and sediments (Tracy et al. 2012). They are common inhabitants of permafrost soils and sediments, where they can play important roles in the production of methanogenic substrates, including acetate, formate and H2 (Lipson et al. 2013; Tveit et al. 2015; Heslop et al. 2019). Specifically for Peptostreptococcaceae, their fermentative metabolism is often linked to acetate production (Slobodkin 2014). The Peptostreptococcales MAG described here contained genes encoding acetate kinase and acetyl- CoA synthase (Supplementary Table S7). Several members of the Clostridia have been linked to the degradation of cellulose and humic substances, but this metabolic potential was not detected in the Peptostreptococcales MAG (Lynd et al. 2002; Ueno et al. 2016).
The 10 most abundant MAGs included three MAGs of the less well studied Orders Elusimicrobiales and Anaerolineales. The Phylum Elusimicrobia (formerly “Termite Group 1”) is widespread in soils and sediments, including Arctic lakes (Herlemann, Geissinger and Brune 2007; Negandhi, Laurion and Lovejoy 2016; Wang et al. 2016). However, little is known about their role in the environment (Brune 2018). In our study, two MAGs were identified as Elusimicrobiales (Table 1). The MAG identifications were supported by 16S/23S and rpoB analyses that showed highest identity to Elusimicrobia-related sequences (data not shown). Both MAGs showed highest abundances in the unamended sediments (9.0-9.9%) but covered less than 2.5% of the nutrient-amended communities. Interestingly, both MAGs contained the nrfA gene encoding nitrite reductase (Supplementary Table S8). Furthermore, the MAGs contained acetate and butyrate kinases, which indicates a fermentative lifestyle (Supplementary Table S7). Interestingly, our observations highlight that increased nutrient availability reduces their relative abundance. Further experimental evidence is needed to confirm their role in the environment.
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