Diving into the hidden viral world of marine protists.

As the most abundant biological entities in the ocean, viruses of microbes play important roles in regulating host population dynamics and influencing biogeochemical cycles. Metagenomic surveys have revealed an astounding reservoir of viral genetic diversity in single-celled marine eukaryotes known as protists, but the vast majority of these viruses have not been directly observed, and information about their protist hosts remains fragmentary. The 2023 discovery of mirusviruses provides a striking example, whereby metagenomic surveys of samples collected by the Tara Oceans expedition led to the discovery of a new phylum of viruses, the Mirusviricota, with remarkable chimeric genomes encoding structural proteins from herpesviruses and enzymes from giant eukaryotic viruses. However, because mirusviruses were detected indirectly by metagenomics, their host range remained unclear, and their biological properties unexplored. Here, we provide new insights into research approaches to identify bona fide protist hosts for marine viruses and characterize virus-host interactions. A greater understanding of these viruses and their natural hosts will unlock opportunities to understand the roles that they play in regulating biogeochemical processes in marine habitats.