Isolation of prasinoviruses of the green unicellular algae Ostreococcus spp. on a worldwide geographical scale

Abstract

Ostreococcus spp. are extremely small unicellular eukaryotic green algae found worldwide in marine environments, and they are susceptible to attacks by a diverse group of large DNA viruses. Several biologically distinct species of Ostreococcus are known and differ in the ecological niches that they occupy: while O. tauri (representing clade C strains) is found in marine lagoons and coastal seas, strains belonging to clade A, exemplified by O. lucimarinus, are present in different oceans. We used laboratory cultures of clonal isolates of these two species to assay for the presence of viruses in seawater samples from diverse locations. In keeping with the distributions of their host strains, we found a decline in the abundance of O. tauri viruses from a lagoon in southwest France relative to the Mediterranean Sea, whereas in the ocean, no O. tauri viruses were detected. In contrast, viruses infecting O. lucimarinus were detected from distantly separated oceans. DNA sequencing, phylogenetic analyses using a conserved viral marker gene, and a Mantel test revealed no relationship between geographic and phylogenetic distances in viruses infecting O. lucimarinus.

FIG. 1.

Phylogenetic tree of prasinoviruses with environmental sequences from the partial DNA polymerase gene (for a final length of 195 amino acids [aa]) reconstructed by Bayesian inference (BI) and maximum likelihood (ML). Numbers are posterior probabilities (BI) and bootstrap proportions (ML) reflecting clade support (ML; values below 50 are indicated by dashes). Geographic origins of sequences are given on the right. GenBank accession numbers are as follows: MiV130, FJ267495; MiV93, FJ267493; OtV63, FJ267501; OtV64, FJ267502; BpV132, FJ267517; and OtV5, EU304328.