Distribution and activity of Bacteria and Archaea in the deep water masses of the North Atlantic

We determined the distribution and activity of the major prokaryotic groups (Bacteria, Cren-, and Euryarchaeota) inhabiting the deep water masses of the North Atlantic by following the path of the North Atlantic Deep Water (NADW) from its formation in the Greenland-Iceland-Norwegian (GIN) Sea along its two major branches covering approximately the first 50 yr of the NADW in the oceanic conveyor belt system. The relative abundance of Eury- and Crenarchaeota, assessed by catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH), was significantly higher in the western branch (17% and 24% of 4′,6′-diamidino-2-phenylindole (DAPI)-stained cells, respectively) than in the eastern (9% and 17%, respectively) branch of the NADW. In contrast, the relative abundance of Bacteria (30% of DAPI-stained cells) did not differ between the western and the eastern basin. Prokaryotic production and turnover rates, however, were higher in the western than the eastern basin. Generally, the contribution of Euryarchaeota to total picoplankton was correlated positively with oxygen concentrations (p < 0.001) and negatively with salinity (p < 0.001) and temperature (p < 0.001). The contribution of Crenarchaeota to total picoplankton correlated positively with oxygen (p < 0.05) and negatively with salinity (p < 0.001). There was a positive correlation between the crenarchaeotal contribution to picoplankton and nitrite concentration (p < 0.001), especially in the oxygen minimum layer, suggesting their potential involvement in the marine nitrogen cycle as nitrifiers. The observed variability in archaeal abundance in relation to bulk prokaryotic activity supports the emerging notion that Archaea are a highly dynamic and metabolically active component of the deep ocean prokaryotic community.