We measured prokaryotic production and respiration in the major water masses of the North Atlantic down to a depth of ,4,000 m by following the progression of the two branches of North Atlantic Deep Water (NADW) in the oceanic conveyor belt. Prokaryotic abundance decreased exponentially with depth from 3 to 0.4 3 10 5 cells mL 21 in the eastern basin and from 3.6 to 0.3 3 10 5 cells mL 21 in the western basin. Prokaryotic production measured via 3 H-leucine incorporation showed a similar pattern to that of prokaryotic abundance and decreased with depth from 9.2 to 1.1 mmol C m 23 d 21 in the eastern and from 20.6 to 1.2 mmol C m 23 d 21 in the western basin. Prokaryotic respiration, measured via oxygen consumption, ranged from about 300 to 60 mmol C m 23 d 21 from ,100 m depth to the NADW. Prokaryotic growth efficiencies of ,2% in the deep waters (depth range 1,200-4,000 m) indicate that the prokaryotic carbon demand exceeds dissolved organic matter input and surface primary production by 2 orders of magnitude. Cell-specific prokaryotic production was rather constant throughout the water column, ranging from 15 to 32 3 10 23 fmol C cell 21 d 21 in the eastern and from 35 to 58 3 10 23 fmol C cell 21 d 21 in the western basin. Along with increasing cell-specific respiration towards the deep water masses and the relatively short turnover time of the prokaryotic community in the dark ocean (34-54 d), prokaryotic activity in the meso-and bathypelagic North Atlantic might be higher than previously assumed.

We determined the contribution of the three major prokaryotic groups (Bacteria, Crenarchaeota, and Euryarchaeota) on the uptake of D-and L-aspartic acid (Asp) in the major water masses of the North Atlantic (from 100-to 4,000-m depth) with the use of microautoradiography combined with catalyzed reporter deposition fluorescence in situ hybridization (MICRO-CARD-FISH). The percentage of prokaryotic cells that assimilated D-and L-Asp ranged from Ͻ5% to 25%. In the meso-and bathypelagic waters of the North Atlantic, Archaea are more abundant (42% Ϯ 2% of 4Ј,6Ј-diamino-2-phenylindole [DAPI]-stained cells) than Bacteria (30% Ϯ 1% of DAPI-stained cells), and more archaeal than bacterial cells are actively incorporating D-Asp (62% Ϯ 2% vs. 38% Ϯ 2% of total D-Asp active cells). In contrast, Bacteria and Archaea almost equally contribute to L-Asp use in the deep waters of the North Atlantic (47% Ϯ 2% vs. 53% Ϯ 2% of total L-Asp active cells). The increase in the D-Asp : L-Asp uptake ratio in the prokaryotic community with depth appears to be driven by the efficient uptake of D-Asp by, especially, the Crenarchaeota in the deep waters. Because Archaea, and particularly Crenarchaeota, commonly dominate the prokaryotic communities in the ocean's interior, we suggest that they represent a previously unrecognized sink of D-amino acids in the deep ocean.The formation of the North Atlantic Deep Water (NADW) is the major driving force of the oceanic conveyor belt system that, in turn, influences the global climate (Broecker 1997). The turnover time of this oceanic conveyor belt system is about 2,000 yr, whereas that of the dissolved organic carbon (DOC) in the oceanic deep water is about 6,000-8,000 yr (Williams 2000). Hansell and Carlson (1998) showed that the deep water DOC concentrations decline from the deep North Atlantic (ϳ45 mol L Ϫ1 ) to the opposite end of the conveyor belt circulation, the deep Pacific (ϳ37 mol L Ϫ1 ), indicating net removal of DOC. Despite recent advances in the phylogenetic characterization of deep-water prokaryotic communities, little is known about the metabolically active fraction of the prokaryotic community that drives the biogeochemical cycles in the 1 To whom correspondence should be addressed. Present address: Departamento de Ecología y Biología Animal, Universidad de Vigo, 36200, Vigo, Spain (teira@uvigo.es). AcknowledgmentsWe thank the captain and crew of the R/V Pelagia for their help during work at sea.

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 49,69-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.The meso-and 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 (Pedró s-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 ...

[1] Amongst the most energetic motions in the deep ocean are internal waves supported by stable vertical stratification in density. Previously, these waves were considered freely propagating as described by a smooth continuum internal wave band (IWB) frequency spectrum. We studied details of the IWB using yearlong current observations from the Bay of Biscay. Instead of observing continuum IWB spectra near a continental slope, we show that (at least) the first half-decade of the IWB is dominated by motions at localized frequencies determined by strong non-linear interactions between waves at the fundamental semidiurnal tidal and atmospherically induced inertial frequencies.

There is growing evidence that the interocean exchange south of Africa is an important link in the global overturning circulation of the ocean, the so‐called ocean conveyer belt. At this location, warm and salty Indian Ocean waters enter the South Atlantic and are pulled by currents that eventually reach the North Atlantic, where water cools and sinks. A major contributor to the exchange is the frequent shedding of ring eddies from the termination of the Agulhas Current south of the tip of Africa.This shedding is controlled by developments far upstream in the Indian Ocean, and variations in this ‘Agulhas Leakage’ can lead to changes in the rate and stability of the Atlantic overturning, with possible associated global climate variations [Weijer et al., 1999]. Regional climate variations in the tropical and subtropical Indian Ocean are known to affect the whole system of the Agulhas Current, including the interocean exchanges. This article reports on some of the seminal results of ongoing multinational, multidisciplinary projects that explore these issues.