A removable and cosmopolitan dsRNA Toti-like virus causes asymptomatic but productive infection in a model diatom strain

In this study, we report the discovery of an RNA molecule in the DNA extracts of the Phaeodactylum tricornutum strain CCMP2561 (Pt1), which possesses a well-annotated genome and has been used as a diatom model system since 1956. We confirmed this molecule to be a double-stranded linear RNA molecule and, through sequencing, demonstrated it to be a virus in the Totiviridae family that is prevalent in the ocean. We also detected this virus in Phaeodactylum tricornutum strain Pt3, which was collected in 1930s from a similar geographic location to Pt1, suggesting its prevalence within the region. By employing various inhibitors of the viral RNA-dependent RNA polymerase, we successfully generated a virus-free line isogenic to Pt1, establishing a model system to investigate the impact of RNA viruses on diatom physiology. The virus-free lines did not display obvious growth advantages or defects, indicating a tendency of the virus towards latent infection. Furthermore, we generated a robust antibody against the coat protein of this virus. By performing immunoprecipitation coupled with mass spectrometry, we found that translation-related proteins are enriched. Our results suggest that potential viral impacts in molecular research should be considered when Pt1 and Pt3 are used for studying translation-related processes. Additionally, our study unveiled a temperate mode of interaction between viruses and marine algal hosts that differs from the commonly-reported virulent, lytic infections.