In this study phytoplankton viruses were investigated from a point of view of their genotypic richness, ecology and role in controlling two microalgae species: Emiliania huxleyi and Phaeocystis pouchetii. Host specificity determined for Emiliania huxleyi-virus (EhV) isolates revealed a highly variable host range suggesting a relation between virus specificity and genetic or phenotypic variations within E. huxleyi strains and EhVs. Subsequently the dynamics and genetic richness of Emiliania huxleyi and EhVs were monitored in mesocosm experiments and during the progression of a natural bloom in the sea. The results confirmed the role of virus infection in regulating the intraspecific succession of E. huxleyi in the ocean. Furthermore, they revealed significant differences in genotypic composition and dynamics among blooms. The mesocosm setup appeared to be a very robust experimental system, which allowed reproducibility. The most important factor determining the development of the blooms in the enclosures was the experimental manipulation (i.e. nutrient addition), whereas the effect of filling of the enclosures, delay in nutrient addition and position in the raft were of minor importance. Further laboratory experiments revealed differences in the genomic content of different EhVs. EhV isolates from the English Channel carry a putative phosphate permease gene (ehv117) while the only available EhV from a Norwegian fjord has replaced ehv117 with a putative endonuclease, suggesting different propagation strategies among closely related EhVs. Culture studies using one of the English Channel isolates and E. huxleyi CCMP 1516 showed that the lack of phosphate (P) reduced the growth rate of the host and inhibited the production of viral particles. Furthermore, P availability was shown to have an effect on the level of ehv117 expression. In addition, other mesocosm studies revealed that specific viruses (PpVs) play a significant role in the termination of induced Phaeocystis pouchetii blooms. However, the role of PpVs may be significant only for the flagellated stage of P. pouchetii. Phenotypic characteristics of PpVs isolated during these studies indicate that they are probably members of the Phycodnaviridae family.

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