Susceptibility of colonies and colonial cells of Phaeocystis pouchetii (Haptophyta) to viral infection

Viruses play a significant role in the microbial food web, as controlling agents in community composition and succession, and in termination of blooms. The flagellated stage of the polymorphic Phaeocystispouchetii (Hariot) Lagerheim was previously shown to be readily infected by the species-specific PpV-virus. In the present study, we investigated if colonial cells of P. pouchetii were susceptible 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 and added viral concentrate, and monitoring viral abundance during 2 different growth seasons in large outdoor mesocosm experiments. In the incubation experiments, colonial cells of F pouchetii at various growth stages, embedded in and detached from the mucus, were inoculated with different concentrations of PpV-AJ96. Viral lysis of colonial P. pouchetii cells was not observed, regardless of virus concentrations and stage of P. pouchetii colony growth. However, flagellated cells of P. pouchetii were readily infected with the same virus strain. In the mesocosm sampling studies, the development of populations of PpV-like particles along with a bloom of P. pouchetii was followed in 2 separate growth seasons. These studies showed a dynamic PpV-like abundance over time that was closely linked to the host population. PpV-like viruses were present and readily detected in both mesocosm experiments in which P. pouchetii appeared. The results from these experiments suggest that colonial cells of P. poucheffi were not infected by the virus strain PpV-AJ96 and that the colonial stage of P. pouchetii provides protection against viral infection.