Fluorescence in situ hybridization (FISH) in combination with polynucleotide probes revealed that the two major groups of planktonic Archaea (Crenarchaeota and Euryarchaeota) exhibit a different distribution pattern in the water column of the Pacific subtropical gyre and in the Antarctic Circumpolar Current system. While Euryarchaeota were found to be more dominant in nearsurface waters, Crenarchaeota were relatively more abundant in the mesopelagic and bathypelagic waters. We determined the abundance of archaea in the mesopelagic and bathypelagic North Atlantic along a south-north transect of more than 4,000 km. Using an improved catalyzed reporter deposition-FISH (CARD-FISH) method and specific oligonucleotide probes, we found that archaea were consistently more abundant than bacteria below a 100-m depth. Combining microautoradiography with CARD-FISH revealed a high fraction of metabolically active cells in the deep ocean. Even at a 3,000-m depth, about 16% of the bacteria were taking up leucine. The percentage of Euryarchaeota and Crenarchaeaota taking up leucine did not follow a specific trend, with depths ranging from 6 to 35% and 3 to 18%, respectively. The fraction of Crenarchaeota taking up inorganic carbon increased with depth, while Euryarchaeota taking up inorganic carbon decreased from 200 m to 3,000 m in depth. The ability of archaea to take up inorganic carbon was used as a proxy to estimate archaeal cell production and to compare this archaeal production with total prokaryotic production measured via leucine incorporation. We estimate that archaeal production in the mesopelagic and bathypelagic North Atlantic contributes between 13 to 27% to the total prokaryotic production in the oxygen minimum layer and 41 to 84% in the Labrador Sea Water, declining to 10 to 20% in the North Atlantic Deep Water. Thus, planktonic archaea are actively growing in the dark ocean although at lower growth rates than bacteria and might play a significant role in the oceanic carbon cycle.

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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.

During a recent research cruise to investigate the nature and continuity of the Mozambique Current, we observed that the flow in the Mozambique Channel is dominated by a train of large anti‐cyclonic eddies (diameters >300 km) that reach to the channel bottom and propagate southward. At a frequency of 4 per year they cause a net poleward transport of about 15 Sv (1 Sv = 106 m3/s). In the deep sea, a Mozambique Undercurrent flows equatorward along the continental slope. Using a lowered acoustic Doppler current profiler maximum observed velocities are about 0.2 m/s around 2400 m with another current core around 1000 m. It carries about 5 Sv of intermediate (AAIW) and deep waters (NADW) of Atlantic origin into the Channel. Subsequently, the equatorward flowing AAIW is largely entrained by the eddies and, while mixing with intermediate water from the North Indian Ocean in the eddy core, returned to the Agulhas Retroflection region.

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.