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 Euryand Crenarchaeota, assessed by catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH), was significantly higher in the western branch (17% and 24% of 49,69-diamidino-2phenylindole (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. The mesoand bathypelagic realm of the ocean makes up more than 70% of the global ocean’s volume. It is commonly accepted that microbial biomass and activity are extremely low in the dark ocean and depend on the 30% of the organic matter, on average, exported from the euphotic layer into the mesopelagic realm (Nagata et al. 2000). Direct measurements indicate one to two orders of magnitude decrease in bacterial abundance and production from the euphotic layer to the bathypelagic waters (Patching and Eardly 1997; Nagata et al. 2000; Hansell and Ducklow 2003). The distribution of planktonic prokaryotes in a given oceanic habitat appears to be determined by local environmental conditions and not by restricted dispersal (Pedrós-Alió 1993; Finlay 2002). With the exceptions of the Mediterranean and the Sulu Sea, the global dark ocean exhibits a temperature range of 0–6uC and a salinity range between 34.6 and 37.8. The lack of geographic barriers for dispersion and the homogeneity of environmental conditions in the deep ocean may suggest a cosmopolitan distribution of most of the microbial inhabitants. Over the past few years, several studies on the distribution of the main prokaryotic groups have led to the conclusion that bacterial abundance decreases with depth, Crenarchaeota 1 To whom correspondence should be addressed. Present address: Departamento Ecoloxı́a e Bioloxı́a Animal, Universidade de Vigo, Vigo, Spain ([email protected]). Acknowledgments We thank the captain and crew of the R/V Pelagia for their help during work at sea, Karel Baker for inorganic nutrient analyses, and Philippe Catala for flow cytometry work. This research was supported by a Marie Curie Fellowship of the European Community (HPMF-CT-2002-01738) to E.T., by a grant of the Earth and Life Science Division of the Dutch Science Foundation (NWO-ALW; project 811.33.004) to G.J.H., and by the 5th Framework Program of the Commission of the European Union (BASICS project). Limnol. Oceanogr., 51(5), 2006, 2131–2144 E 2006, by the American Society of Limnology and Oceanography, Inc.