VIRUS-LIKE PARTICLES IN A SUMMER BLOOM OF EMILIANIA-HUXLEYI IN THE NORTH-SEA

The importance of viruses in controlling a bloom of the coccolithophor id Emiliania huxleyi in the North Sea was investigated during summer 1 993. Viral infection of E. huxleyi was highest in the decaying phase o f the bloom. Up to 50% of E. huxleyi cells were visibly infected. For E, huxleyi, 2 types of virus-like particles are reported. They differe d in size and were occassionally found within the same cell. The infec tion level of the large virus-like particles was never higher than 25% . Viral lysis of E, huxleyi within the nitrogen-limited decaying phase of the bloom seemed an important source of organic carbon utilized by bacteria. In addition to infection in E. huxleyi, we also found sever e viral infection in Chrysochromulina sp. Our results show that in nat ural ecosystems viruses can be a significant source of phytoplankton m ortality, influencing phytoplankton (bloom) dynamics and the microbial food web.