ABSTRACT:

We examined the full-depth distributions of viruses at 38 stations along a cruise track of ~17000 km in the Southern Ocean and the central Pacific. In the layer between 200 and 2000 m, viral abundance decreased sharply with depth, and prokaryote abundance accounted for a large fraction (60%) of the variability in viral abundance. In contrast, in the deeper layer (>2000 m), the depth-dependent decrease in viral abundance was less evident, and the relationship between viral and prokaryote abundances was weak. Viral abundances were high in the Antarctic bottom water and tended to decrease along the paths of the Circumpolar Deep Water and the Pacific Deep Water. Outstanding peaks of virus-to-prokaryote abundance ratios (mean ± SE: 46 ± 1.2) were found in the bottom waters (depth >3500 m) of the subtropical regions, where sinking particle fluxes are known to be generally low. These results are interpreted as an indication that large-scale viral distribution patterns are generally, if not solely, influenced by transport mediated by the meridional overturning circulation, underscoring the necessity of future studies to clarify prevailing mode and mechanisms for viral production and decay in deep oceans, which appear to be largely distinct from those in upper oceans.