Genomic diversity and global distribution of four new prasinoviruses from the tropical north Pacific

Viruses that infect phytoplankton are an integral part of marine ecosystems, but the vast majority of viral diversity remains uncultivated. Here, we introduce four near-complete genomic assemblies of viruses that infect the widespread marine picoeukaryote <i>Micromonas commoda</i>, doubling the number of reported genomes of <i>Micromonas</i> dsDNA viruses. All host and virus isolates were obtained from tropical waters of the North Pacific, a first for viruses infecting green algae in the order Mamiellales. Genome length of the new isolates ranges from 205 to 212 kb, and phylogenetic analysis shows that all four are members of the genus <i>Prasinovirus</i>. Three of the viruses form a clade that is adjacent to previously sequenced <i>Micromonas</i> viruses, while the fourth virus is relatively divergent from previously sequenced prasinoviruses. We identified 61 putative genes not previously found in prasinovirus isolates, including a phosphate transporter and a potential apoptosis inhibitor novel to marine viruses. Forty-eight genes in the new viruses are also found in host genome(s) and may have been acquired through horizontal gene transfer. By analyzing the coding sequences of all published prasinoviruses, we found that ~25% of prasinovirus gene content is significantly correlated with host genus identity (i.e., <i>Micromonas</i>, <i>Ostreococcus</i>, or <i>Bathycoccus</i>), and the functions of these genes suggest that much of the viral life cycle is differentially adapted to the three host genera. Mapping of metagenomic reads from global survey data indicates that one of the new isolates, McV-SA1, is relatively common in multiple ocean basins.IMPORTANCEThe genomes analyzed here represent the first viruses from the tropical North Pacific that infect the abundant phytoplankton order Mamiellales. Comparing isolates from the same location demonstrates high genomic diversity among viruses that co-occur and presumably compete for hosts. Comparing all published prasinovirus genomes highlights gene functions that are likely associated with adaptation to different host genera. Metagenomic data indicate these viruses are globally distributed, and one of the novel isolates may be among the most abundant marine viruses.