Page 71 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

archaea (Denonfoux et al. 2013) (Table S5). There were no hits to McrA sequences of anaerobic methanotrophic (ANME) archaea.
The methanogenic community of deposit layers was dominated by a Methanobacteriales MAG related to a Methanobacterium sp.
Due to its high abundance in the deposit layers (8.7% of mapped reads) and completeness of the MAG (98.9%), the first Methanobacteriales MAG was investigated more closely. Phylogenetic analysis based on a partial 16S rRNA gene sequence (925 nt) showed the highest identity to Methanobacterium oryzae strain FPi (NR_028171.1, 97%) isolated from a rice paddy field soil (Joulian et al. 1998, 2000). The 23S rRNA gene (2,970 nt) was most identical to the hydrogenotrophic methanogen Methanobacterium palustre strain F (KF882001.1, 97%), which was also detected in the 16S rRNA gene de novo assembly approach. M. palustre was isolated from a peat bog (Zellner et al. 1988). Both M. oryzae and M. palustre were isolated from organic carbon-rich freshwater environments. For further identification a total of 55 30S and 50S ribosomal proteins were identified. Forty-nine proteins gave best hits to multispecies Methanobacterium sequences (89% average aa identity); at the species level, the average identity was 84%, with most hits to Methanobacterium congolense (35% of proteins, 83% aa identity), Methanobacterium sp. strain SMA-27 (16% of proteins, 84% aa identity), and Methanobacterium formicicum (11% of proteins, 85% aa identity) (Table S8). These findings and the high strain heterogeneity observed by CheckM indicates the presence of multiple novel Methanobacteriales strains in DL. For detailed functional gene analyses, see Table S9.
Elemental iron is a potential substrate for acetogenic and methanogenic communities
Methanogens were relatively more abundant and more diverse at the iron sheet piles than in the surrounding and top sediment. The hydrogen partial pressure does not seem to be the driving factor of methanogen community composition since the more-abundant Methanobacterium spp. have lower affinity for H2 than the less-abundant Methanomicrobiales (Sakai et al. 2009). Since sulfate is not relevant at the study site, significant competition with sulfate-reducing hydrogen oxidizers is unlikely (Lupton and Zeikus 1984). This suggests that there might be a central role for elemental iron in structuring the microbial community in CPLs.
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