Active viral infection during blooms of a dinoflagellate indicates dinoflagellate-viral co-adaptation

Viruses are generally believed to cause cell mortality and terminate algal blooms. However, how the dinoflagellate-virus interaction shapes the dynamics of host dinoflagellate blooms remains poorly understood. Here, we profile viral composition and metabolic landscape in two blooms of the widely distributed dinoflagellate Prorocentrum shikokuense. Our data show that P. shikokuense was infected dominantly by Mimiviridae and Phycodnaviridae viruses in both blooms. Strikingly, these viruses were transcriptionally very active during the bloom that extended for weeks, suggesting a paralleled development of both the bloom and chronic infection. Furthermore, our data indicate that these viruses maintained a stable infection process by generating polymorphic variants to sustain the exploitation of host intracellular machinery, suggesting co-adaptation between the viruses and the bloom-causative dinoflagellate. This novel insight will be valuable for fully understanding and modeling the role of viruses in regulating blooms of dinoflagellates and other algae. IMPORTANCE This study represents the first that investigates in situ virus infection in dinoflagellate blooms. Our findings reveal highly similar viral assemblages that infected the bloom species Prorocentrum shikokuense and a co-adapted metabolic relationship between the host and the viruses in the blooms, which varied between the prolonged and the short-lived blooms of the same dinoflagellate species. These findings fill the gap in knowledge regarding the identity and behavior of viruses in a dinoflagellate bloom and shed light on what appears to be the complex mode of infection. The novel insight will be potentially valuable for fully understanding and modeling the role of viruses in regulating blooms of dinoflagellates and other algae.