Virus dynamics in a coccolithophore-dominated bloom in the North Sea

We used analytical flow cytometry (AFC) to determine virus concentrations through vertical profiles in a coccolithophore-dominated bloom in the northern North Sea during June 1999. We present the first high-intensity sampling data of viruses from a lagrangian survey to gain a unique insight into the temporal and spatial dynamics of viruses in an open-water sight. Virus abundances ranged from 2.6 x 10(5) to 5.4 x 10(6) ml(-1), which is within the range expected for open-water environments. The highest concentrations were invariably observed in surface waters. During the lagrangian experiment there was a net decrease in virus numbers, suggesting that they were actively infecting hosts. Large viruses could be easily discriminated from small viruses since there was at least an order of magnitude difference in their AFC side-scatter values. Large viruses, assumed to infect DMS-producing algae, did not appear to influence DMS/DMSP production. It is likely that microzooplankton out-competed viruses for coccolithophore prey/hosts. Lower small virus to bacteria ratios (VBR) were observed in a subsurface layer compared to the more productive surface layers. The subsurface layer was dominated by a species of alpha-proteobacteria related to the genus Roseobacter, and the low VBR may indicate that viruses were infecting Roseobacter in this layer. Application of AFC is an excellent technique for high-definition sampling of virus communities, although it is recognised that we are working at the limit of detection for many small viruses using currently available nucleic acid stains. (C) 2002 Elsevier Science Ltd. All rights reserved.