Flow cytometric analyses of viral infection in two marine phytoplankton species, Micromonas pusilla (Prasinophyceae) and Phaeocystis pouchetii (Prymnesiophyceae)

Cell characteristics of two axenic marine phytoplankton species, Micromonas pusilla (Butscher) Manton et Parke and Phaeocystis pouchetii (Hariot) Lagerheim, were followed during viral infection using flow cytometry, Distinct differences between noninfected and infected cultures were detected in the forward scatter intensities for both algal species. Changes in side scatter signals on viral infection were found only for P, pouchetii. Chlorophyll red fluorescence intensity per cell decreased gradually over time in the infected cultures, DNA analyses were performed using the nucleic acid-specific fluorescent dye SYBR Green I. Shortly after infection the fraction of algal cells with more than one genome equivalent increased for both species because of the replication of viral DNA in the infected cells. Over time, a population of algal cells with low red autofluorescence and low DNA fluorescence developed, likely representing algal cells just prior to viral lysis, The present study provides insight into basic virus-algal host cell interactions. It shows that flow cytometry can be a useful tool to discriminate between virus infected and noninfected phytoplankton cells.