Genome Sequence of Ostreococcus tauri Virus OtV-2 Throws Light on the Role of Picoeukaryote Niche Separation in the Ocean

<jats:title>ABSTRACT</jats:title> <jats:p> <jats:named-content content-type="genus-species">Ostreococcus tauri</jats:named-content> , a unicellular marine green alga, is the smallest known free-living eukaryote and is ubiquitous in the surface oceans. The ecological success of this organism has been attributed to distinct low- and high-light-adapted ecotypes existing in different niches at a range of depths in the ocean. Viruses have already been characterized that infect the high-light-adapted strains. <jats:named-content content-type="genus-species">Ostreococcus tauri</jats:named-content> virus (OtV) isolate OtV-2 is a large double-stranded DNA algal virus that infects a low-light-adapted strain of <jats:named-content content-type="genus-species">O. tauri</jats:named-content> and was assigned to the algal virus family <jats:named-content content-type="genus-species">Phycodnaviridae</jats:named-content> , genus <jats:named-content content-type="genus-species">Prasinovirus</jats:named-content> . Our working hypothesis for this study was that different viruses infecting high- versus low-light-adapted <jats:named-content content-type="genus-species">O. tauri</jats:named-content> strains would provide clues to propagation strategies that would give them selective advantages within their particular light niche. Sequence analysis of the 184,409-bp linear OtV-2 genome revealed a range of core functional genes exclusive to this low-light genotype and included a variety of unexpected genes, such as those encoding an RNA polymerase sigma factor, at least four DNA methyltransferases, a cytochrome <jats:italic> b <jats:sub>5</jats:sub> </jats:italic> , and a high-affinity phosphate transporter. It is clear that OtV-2 has acquired a range of potentially functional genes from its host, other eukaryotes, and even bacteria over evolutionary time. Such piecemeal accretion of genes is a trademark of large double-stranded DNA viruses that has allowed them to adapt their propagation strategies to keep up with host niche separation in the sunlit layers of the oceanic environment. </jats:p>