A Polinton-like Virus of C. parva Inhibits the Population Growth of a Newly Isolated Relative of Tethysvirus ontarioense

The previous discovery of genomes of Polinton-like viruses (PLVs) associated with viruses of <i>Chrysochromulina parva</i> stimulated this research to determine the biological nature of these putative viral hyperparasites. Purification of <i>C. parva</i> viruses to enable co-infection experiments led to the discovery of a previously unknown virus, CpV-BQ3, which, based on sequence information and electron microscopy, is a species of <i>Tethysvirus</i>, a genus within the <i>Megaviricetes</i>. Purification and TEM imaging of CpV-PLV Moe revealed naked icosahedral particles morphologically similar to other cultivated virophages and PLVs. Mixed-infection experiments with the putative Polinton-like virus CpV-PLV Moe demonstrated that CpV-BQ3 supports its replication, whereas the putative Phycodnavirus CpV-BQ1 does not. Further, experimental infections with differing proportions of the Moe and its helper virus CpV-BQ3 revealed a dose-effect whereby high levels of Moe had a greater negative impact on BQ3 replication compared to lower levels. Conversely, high levels of Moe relative to BQ3 provided greater protection for <i>C. parva,</i> allowing enhanced cell survival, whereas low doses of Moe did not prevent cell lysis. Overall, the results of this study demonstrated the intimate relationship of CpV-PLV Moe with the newly discovered virus, CpV-BQ3, and <i>C. parva</i>, and illustrate the complex ecology of algal viruses.