Further, although N. maritimus most likely uses the same reaction sequences as described for Metallosphaera sedula, not all

reactions are catalyzed by identical enzymes [52]. It is still not clear whether ammonia oxidizing archaea are dependent on autotrophy or not. A mixotrophic lifestyle has been indicated for Nitrosopumilus and other (mainly this website marine) group I.1a Thaumarchaeota, while heterotrophic growth has been observed for Thaumarchaeota of group I.1b (most common in soils) [52–55]. Since 4-hydroxybutyryl-CoA dehydratase/vinylacetyl-CoA-Delta-isomerase, a characteristic key gene of the 3HP/4HB cycle [56], has been detected by the KEGG Automatic Annotation Server (KAAS) [57, 58] among metagenomic reads assigned to N. maritimus from the Troll metagenomes in a separate study [59] it is likely

that Nitrosopumilus in the Troll area KPT-8602 chemical structure has the genetic potential for autotrophy. Conclusions Most taxa were present in all metagenomes see more and differences in community structure and metabolic potential between them were mainly due to abundance variation. Despite detection of a few reads assigned to key enzymes for methane oxidation in Tpm1-2, our analyses revealed no general increase in the potential for methane oxidation in the surface sediments of Troll pockmarks compared to the Oslofjord. The analyses are thereby supporting geological analyses indicating no, or very low, methane seepage at the present time. Despite high concentrations of hydrocarbons in the Troll area, compared to the Oslofjord, significantly increased Tryptophan synthase potential for hydrocarbon degradation could only be detected in two of the Troll metagenomes. Overrepresentation of subsystem and key enzymes supported an increased potential for aromatic hydrocarbon degradation in these samples. The proposed extended use of aromatic hydrocarbons as a carbon source could

be a result of the lower alkane concentrations measured in these samples compared to the other Troll samples. Given the placement of the sampling sites, less bioavailability of nutrients essential for hydrocarbon degradation is a likely factor limiting the hydrocarbonoclastic subcommunities at the other sites. The most evident difference between the two sampling areas was an overabundance of predominantly autotrophic nitrifiers, especially Nitrosopumilus, in the Troll metagenomes compared to the Oslofjord. Given the great depth of the hydrocarbon-containing sediments in the Troll area, substantial sequential anaerobic degradation and oxidation of hydrocarbons is likely to occur. Migration of degradation products, including CO2, up through the sediments could provide an additional source of carbon for the nitrifiers thriving in the area. This subcommunity could therefore play an important role turning CO2, partially originating from hydrocarbon degradation, back into organic carbon in these dark oligotrophic sediments.