ABSTRACT: Viruses play a significant role in the microbial food web, as controlling agents in communitycomposition and succession, and in termination of blooms. The flagellated stage of the polymorphicPhaeocystis pouchetii (Hariot) Lagerheim was previously shown to be readily infected by thespecies-specific PpV-virus. In the present study, we investigated if colonial cells of P. pouchetii weresusceptible to virus infection and if the growth stage of the host population affected viral infectivity,based on 2 types of observations: incubation experiments with natural P. pouchetii colonies andadded viral concentrate, and monitoring viral abundance during 2 different growth seasons in largeoutdoor mesocosm experiments. In the incubation experiments, colonial cells of P. pouchetii at variousgrowth stages, embedded in and detached from the mucus, were inoculated with different concentrationsof PpV-AJ96. Viral lysis of colonial P. pouchetii cells was not observed, regardless of virusconcentrations and stage of P. pouchetii colony growth. However, flagellated cells of P. pouchetiiwere readily infected with the same virus strain. In the mesocosm sampling studies, the developmentof populations of PpV-like particles along with a bloom of P. pouchetii was followed in 2 separategrowth seasons. These studies showed a dynamic PpV-like abundance over time that was closelylinked to the host population. PpV-like viruses were present and readily detected in both mesocosmexperiments in which P. pouchetii appeared. The results from these experiments suggest that colonialcells of P. pouchetii were not infected by the virus strain PpV-AJ96 and that the colonial stage ofP. pouchetii provides protection against viral infection.
