A giant virus awakens virophage-like PLVs in the green alga Tetraselmis

Giant double-stranded DNA viruses infecting microalgae often interact with polinton-like viruses (PLVs), yet their dynamics within natural hosts remain poorly understood. Here, we report the isolation of a new Tetraselmis striata giant virus (Allomimiviridae) from Mediterranean coastal waters, which we designate Oceanusvirus lionense (TetV-2). From the same sample, we co-isolated a PLV, Tsa-S2b, that replicates concomitantly with TetV-2. Remarkably, TetV-2 infection also reactivates at least two additional PLVs integrated into the host genome. We show that these PLVs exert a virophage-like effect, reducing the fitness of the giant virus. Comparative genomics reveals that these virophagic PLVs are related to the autonomous lytic PLV TsV-N1, sharing structural but not replication-associated genes. This newly described pathosystem provides a unique model to investigate the evolution of PLVs and the parasitic mechanisms underlying virophage–giant virus interactions.