A giant virus awakens polinton-like virophages in the green alga Tetraselmis, revealing an inducible antiviral defense system

Giant double-stranded DNA viruses profoundly influence algal population dynamics, yet their interactions with co-infecting mobile elements remain poorly understood. Here, we describe a natural tripartite infection system in the marine unicellular alga Tetraselmis striata, where a newly isolated giant virus, Oceanusvirus lionense (TetV-2), triggers the productive replication of multiple polinton-like viruses (PLVs). One PLV (Tsv-S2b) was co-isolated from the same seawater sample, while two others (Tsv-S2a and Tsv-S3b) derive from endogenous elements integrated in the algal genome. These PLVs depend strictly on TetV-2 for propagation and exert a virophage-like effect, reducing TetV-2 yields in a dose-dependent manner. Comparative genomics shows that virophagic PLVs retain a conserved structural module but harbor divergent replication and integration genes, supporting modular evolution of the PLV lineage. The reactivation of integrated PLVs mirrors the behavior of endogenous virophages in Cafeteria burkhardae, suggesting that specific Oceanusvirus-PLV compatibilities govern reactivation. This work provides direct evidence that integrated PLVs in green algae can transition to a virophage lifestyle upon infection by a giant virus, highlighting an inducible antiviral defense mechanism within photosynthetic protists. Author SummaryMarine microalgae are frequently infected by giant viruses, but how these infections are regulated is poorly understood. In the green alga Tetraselmis striata, we discovered that a giant virus can "wake up" small DNA viruses known as polinton-like viruses (PLVs) that are normally integrated and silent within the algal genome. Once activated, these PLVs multiply using the giant viruss replication machinery and, in turn, reduce its production. This helper-dependent relationship mirrors the behavior of virophages previously known only in non-photosynthetic protists. Our results reveal that microalgae possess an inducible antiviral system based on the reactivation of endogenous viral elements, suggesting that such interactions may be widespread in the oceans and play an important role in controlling algal virus epidemics.