Differentiating leucine incorporation of <i>Archaea</i> and <i>Bacteria</i> throughout the water column of the eastern Atlantic using metabolic inhibitors

The abundance (based on catalyzed reporter deposition-fluorescence in situ hybrid ization, CARD-FISH) and leucine incorporation rates of Archaea and Bacteria were determined throughout the water column in the eastern Atlantic. Bacteria dominated throughout the water column, although their contribution to total prokaryotic abundance in the bathypelagic layer (1000 to 4000 m depth) was lower than in the surface and mesopelagic layers (0 to 1000 m depth). While marine Crenarchaeota Group I (MCG I) contributed 28 +/- 12% to the total prokaryotic abundance, with a generally higher contribution in the bathypelagic layer than in the surface and mesopelagic layers, marine Euryarchaeota Group II contributed < 5% throughout the water column. Using microautoradiography in combination with CARD-FISH, we tested the specificity of erythromycin and diphtheria toxin and found them to selectively inhibit bacterial and archaeal activity, respectively. These inhibitors were thus used to determine the contribution of Bacteria and Archaea to total leucine incorporation: Bacteria contributed 69 +/- 15%, and this value decreased with depth; Archaea contributed 32 +/- 16% over the entire water column, with no significant difference between surface and mesopelagic waters and the bathypelagic realm. The mean cell-specific leucine incorporation rate of MCG I (5.3 +/- 3.0 x 10(-20) mol cell(-1) d(-1)) was 3.5-fold lower than that of Bacteria (18.6 +/- 18.2 x 10(-20) mol cell(-1) d(-1)) in the surface and mesopelagic layer. In the bathypelagic layer, cell-specific leucine incorporation rates of Crenarchaeota were similar to those of Bacteria (2.3 x 10(-20) mol cell(-1) d(-1) for MCG I, 2.9 x 10(-20) mol cell(-1) d(-1) for Bacteria). In the surface and mesopelagic waters of the subtropical eastern North Atlantic, MCG I exhibited a lower heterotrophic activity on a per-cell level than Bacteria. In the bathypelagic zone, cell-specific heterotrophic activities of Bacteria and MCGI were similar