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 Micromonas commoda, doubling the number of published genomes of Micromonas 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 205-212 kb, and phylogenetic analysis shows that all four are members of the genus Prasinovirus. Three of the viruses form a clade that is adjacent to previously sequenced Micromonas viruses, while the fourth virus is relatively divergent from previously sequenced prasinoviruses. We identified 61 putative genes not previously found in prasinoviruses, including a phosphate transporter from a family novel to viruses, 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., Micromonas, Ostreococcus, or Bathycoccus), 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.