Genome of <i>Phaeocystis globosa</i> virus PgV-16T highlights the common ancestry of the largest known DNA viruses infecting eukaryotes

<jats:p> Large dsDNA viruses are involved in the population control of many globally distributed species of eukaryotic phytoplankton and have a prominent role in bloom termination. The genus <jats:italic>Phaeocystis</jats:italic> ( <jats:italic>Haptophyta</jats:italic> , <jats:italic>Prymnesiophyceae</jats:italic> ) includes several high-biomass-forming phytoplankton species, such as <jats:italic>Phaeocystis globosa</jats:italic> , the blooms of which occur mostly in the coastal zone of the North Atlantic and the North Sea. Here, we report the 459,984-bp-long genome sequence of <jats:italic>P. globosa</jats:italic> virus strain PgV-16T, encoding 434 proteins and eight tRNAs and, thus, the largest fully sequenced genome to date among viruses infecting algae. Surprisingly, PgV-16T exhibits no phylogenetic affinity with other viruses infecting microalgae (e.g., phycodnaviruses), including those infecting <jats:italic>Emiliania huxleyi</jats:italic> , another ubiquitous bloom-forming haptophyte. Rather, PgV-16T belongs to an emerging clade (the Megaviridae) clustering the viruses endowed with the largest known genomes, including Megavirus, Mimivirus (both infecting acanthamoeba), and a virus infecting the marine microflagellate grazer <jats:italic>Cafeteria roenbergensis</jats:italic> . Seventy-five percent of the best matches of PgV-16T–predicted proteins correspond to two viruses [Organic Lake phycodnavirus (OLPV)1 and OLPV2] from a hypersaline lake in Antarctica (Organic Lake), the hosts of which are unknown. As for OLPVs and other Megaviridae, the PgV-16T sequence data revealed the presence of a virophage-like genome. However, no virophage particle was detected in infected <jats:italic>P. globosa</jats:italic> cultures. The presence of many genes found only in Megaviridae in its genome and the presence of an associated virophage strongly suggest that PgV-16T shares a common ancestry with the largest known dsDNA viruses, the host range of which already encompasses the earliest diverging branches of domain Eukarya. </jats:p>