Page 69 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Syntrophobacterales were relatively more abundant in DL, with an average 3.0% abundance compared to 1.3% in AS sample 2. Metagenomic binning resulted in one nearly complete Syntrophobacterales MAG containing 18.1% of total mapped DL reads and no reads from AS. The partial 16S and 23S rRNA genes were retrieved (1,564 and 2,016 nt) with the highest identity for the 16S rRNA gene to Syntrophaceae-related clones (96%) and Syntrophus aciditrophicus strain SB (93%), which was also the best hit for the 23S rRNA gene (Jackson et al. 1999). The 16S rRNA gene was most identical to sequences from a methanogenic hexadecane-degrading culture enriched in Syntrophaceae (Cheng et al. 2013). For additional 30S and 50S ribosomal subunit and functional gene analyses, see Tables S12 and S13 in the supplemental material. Some Syntrophobacterales species can reduce sulfate, but most syntrophically oxidize fatty acids and alcohols to acetate, H2, and CO2 (Plugge et al. 2011). Their metabolism could thus fuel methanogenic archaea. Co-occurrence of Syntrophobacterales and both acetoclastic and hydrogenotrophic methanogens is common in soils and lake sediments (Chauhan, Ogram and Reddy 2004; Conrad et al. 2014). A study by Lueders et al. (2004) showed that Syntrophobacterales syntrophically oxidized propionate in cooccurrence with Methanobacterium and Methanosarcina species. These findings indicate that Syntrophobacterales potentially have key roles in organic compound conversion to methanogenic substrates in the organic-rich sediments surrounding the metal sheet piles.
The actinobacterial order OPB41 was enriched in the deposit layers and had an average relative abundance of 4.7%. These organisms co-occur with Methanobacteriaceae in coal beds (Kirk et al. 2015; Robbins et al. 2016b). In coal-bearing strata of the Cherokee Basin, USA, the actinobacterial order OPB41 cooccurred with Clostridia, Deltaproteobacteria, and an archaeal community that was dominated (>80% relative abundance) by methanogens (Methanococcales, Methanobacteriales, Methanomicrobiales, and Methanosarcinales) (Kirk et al. 2015). Here, we also found cooccurrence of OPB41 with both hydrogenotrophic and acetoclastic methanogens.
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