Large-scale distribution patterns of virioplankton in the upper ocean

We used flow cytometry to investigate large-scale distribution patterns of viruses and their subclusters in the upper 200 m water column of the central Pacific Ocean and the Pacific sector of the Southern Ocean. The abundances of 3 groups of photoautotrophic picoplankton (APP), including Synechococcus, picoeukaryotes, and Prochlorococcus, accounted for a large fraction of viral abundance variability (r(2) = 0.570, n = 354, p < 0.001). The relative contribution of the viral subcluster with high fluorescence intensity (HV) increased with increasing abundance of APP. These data are consistent with the notion that viruses infecting APP are abundant, and the HV subcluster might contain large numbers of these viruses especially in subtropical and tropical oceans. Relationships between viral abundance and biotic variables differed among regions, indicating that controls of virus-host systems are largely distinct among different oceanic regions. We found a conspicuous peak of viral abundance in the mid-latitude region of the North Pacific. This 'viral hot spot' coincided with a dissolved oxygen anomaly (the excess above saturation) indicated by negative values (-15 to -59 mu mol kg(-1)) of apparent oxygen utilization. Our results support the notion that large-scale distribution patterns of viruses and their subclusters in the upper oceans are affected by a combined effect of host distributions and physical processes.