[48]) might discriminate against short reads, and that lowering of the threshold

would result in decreased EGS [49]. A decreased EGS would in turn result in a reduction of the estimated fraction of the community carrying the marker genes mcrA, pmoA and dsrAB. Differences in copy number for organisms carrying the gene might also affect the expected number of hits. Aerobic methane oxidation Due to limited oxygen penetration, active aerobic methane oxidation is probably limited to a thin surface layer. The maximum oxygen penetration at the nearby Brian seep sediments was measured to a depth of 1.4 cm [24]. Due to high tar content, oxygen penetration in the sediments of the Tonya seep is expected Kinase Inhibitor Library screening to be more restricted than at the Brian seep. Methane monooxygenase (EC: 1.14.13.25) was Z-IETD-FMK mouse only detected in the 0-4 cm metagenome after plotting of KO

and EC numbers onto KEGG pathway maps. Overrepresentation of aerobic methanotrophic genera and pmoA (based on library comparison) in the 0-4 cm metagenome compared to the 10-15 cm metagenome further support aerobic oxidation of methane in the 0-4 cm sediment sample (see Figures 4 and 6). Both taxonomic binning of reads and marker gene classification point to type I methanotrophs of Methylococcaceae as the most important aerobic methane oxidizers in our samples. While Methylococcus was the aerobic methanotrophic genus with most reads assigned (see Figure 4), most of the detected pmoA reads were assigned to unclassified Methylococcaceae (see Figure 6). This indicates that uncultured type I methanotrophs might play an important role in aerobic methane oxidation at the Tonya Seep. Also in microbial mats and sediments of the nearby Shane and Brian seeps aerobic type I methanotrophs have been identified, while no type II methanotrophs

were detected at either of these sites [21, 22]. This is consistent with type I methanotrophs dominating over type II methanotrophs in most marine settings ([50]and refs therein). Anaerobic methane oxidation Genes for AOM were detected in both metagenomes (see Figure 5). The taxonomic binning of reads points to AMNE-1 as the predominant anaerobic oxidizer of methane old in the Tonya seep sediment, especially in the 10-15 cm sediment sample. It is however, important to notice that ANME-1, due to the genome sequencing efforts [12], is the most sequenced ANME-clade, and therefore overrepresented in the database. This could skew our relative abundance results. However, the presence and dominance of ANME-1 was further supported by the mcrA reads in our metagenomes (see Figure 6). This gene is identified in all ANME-clades, still all reads matching mcrA in the 10-15 cm metagenome were assigned to ANME-1. Taken together, these results provide strong evidence of ANME-1 being the most important clade for anaerobic methane oxidation in the Tonya seep sediments. In contrast, only ANME-2 was detected at the nearby Brian Seep [24].