Depth-dependent and seasonal variability in archaeal community structure in the subarctic and subtropical western North Pacific

Archaea represent an important component of marine picoplankton. Marine archaeal communities are generally dominated by Marine Group I (MGI) and Marine Group II (MGII), which are affiliated to the phyla Thaumarchaeota and Euryarchaeota. We used pyrosequencing-based analysis of archaeal 16S rRNA genes to investigate archaeal community composition throughout the water column (0-5000 m) at the subarctic (station K2) and subtropical (station S1) stations in western North Pacific. Sampling was conducted on four occasions between 2010 and 2011 to examine seasonal variability in community composition. Four phylogenetic groups (MGI-alpha, MGI-gamma, MGII-alpha, and MGII-beta) belonging to Thaumarchaeota and Euryarchaeota were found throughout the water column, but the relative abundance of each group varied depending on depth. In the surface water, archaeal communities displayed marked seasonal variability at both stations, whereas in deeper waters (300-5000 m), temporal variability in community composition was less evident. Sequences similar to those of the uncultured Marine Benthic Group A, to date found only in marine sediments, were detected throughout the water column at station S1.