| Summary: | Viral abundance was assessed in different water masses of the NW Atlantic, and the development of viral abundance, lytic viral infection and lysogeny was followed for the first ca. 5000 km (corresponding to ca. 50 yr in the oceanic conveyor belt) of the western branch of the North Atlantic Deep Water (NADW). Viral abundance was significantly higher in the 100 m layer than in the NADW (2400-2700 m depth) and the Denmark Strait Overflow Water (2400-3600 m depth). The virus-to-prokaryote ratio (VPR) increased with depth, ranging from 32-43 for different water masses of the bathypelagic ocean, thus corroborating the enigma of high viral abundance in the dark ocean. The O2-minimum layer (250-600 m) also showed high viral abundance and VPRs. Viral abundance, a viral subgroup and VPRs decreased in a non-linear form with distance from the NADW origin. Viral production (range: 0.2-2.4 × 107 viruses l-1) and the fraction of lytically infected cells (range: 1-22%) decreased with increasing distance from the formation site of the NADW. Conservative estimations of virus-mediated mortality of prokaryotes in the NADW averaged 20 ± 12%. The fraction of the prokaryotic community with lysogens (i.e. harboring a functional viral DNA) in the NADW averaged 21 ± 14%. Hence, we conclude that (1) viral abundance and subgroups differ between water masses, (2) virus-mediated mortality of prokaryotes as well as lysogeny are significant in the dark ocean and (3) the lysogenic life strategy became more important than the lytic life style during the early formation of the NADW.
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