Diversity of Viruses Infecting the Green Microalga Ostreococcus lucimarinus

Abstract

The functional diversity of eukaryotic viruses infecting a single host strain from seawater samples originating from distant marine locations is unknown. To estimate this diversity, we used lysis plaque assays to detect viruses that infect the widespread species Ostreococcus lucimarinus, which is found in coastal and mesotrophic systems, and O. tauri, which was isolated from coastal and lagoon sites from the northwest Mediterranean Sea. Detection of viral lytic activities against O. tauri was not observed using seawater from most sites, except those close to the area where the host strain was isolated. In contrast, the more cosmopolitan O. lucimarinus species recovered viruses from locations in the Atlantic and Pacific Oceans and the Mediterranean Sea. Six new O. lucimarinus viruses (OlVs) then were characterized and their genomes sequenced. Two subgroups of OlVs were distinguished based on their genetic distances and on the inversion of a central 32-kb-long DNA fragment, but overall their genomes displayed a high level of synteny. The two groups did not correspond to proximity of isolation sites, and the phylogenetic distance between these subgroups was higher than the distances observed among viruses infecting O. tauri. Our study demonstrates that viruses originating from very distant sites are able to infect the same algal host strain and can be more diverse than those infecting different species of the same genus. Finally, distinctive features and evolutionary distances between these different viral subgroups does not appear to be linked to biogeography of the viral isolates. IMPORTANCEMarine eukaryotic phytoplankton virus diversity has yet to be addressed, and more specifically, it is unclear whether diversity is connected to geographical distance and whether differential infection and lysis patterns exist among such viruses that infect the same host strain. Here, we assessed the genetic distance of geographically segregated viruses that infect the ubiquitous green microalga Ostreococcus. This study provides the first glimpse into the diversity of predicted gene functions in Ostreococcus viruses originating from distant sites and provides new insights into potential host distributions and restrictions in the world oceans.

Host-derived viral transporter protein for nitrogen uptake in infected marine phytoplankton

“…Phycodnaviridae infect a broad range of eukaryotic algae (38). To date, 12 genome sequences of viruses infecting the prasinophyte alga Ostreococcus-a widely distributed marine Mamiellophyceae (39,40) and the smallest known free-living eukaryotic cell-are available (41)(42)(43)(44)(45)(46). These include viruses that infect one of three species: O. tauri, Ostreococcus lucimarinus, and Ostreococcus mediterraneus.…”

“…2 (90), edited with the trimAl v1.4 "strict" algorithm (91), and manually inspected and corrected. For the phylogeny based on the 22 core proteins shared among green algal viruses (46), the OtV6 predicted proteome was parsed using BLASTP and homologous sequences from other Ostreococcus spp. viruses were used as search queries.…”

Marine Prasinoviruses and Their Tiny Plankton Hosts: A Review

“…Sequenced Micromonas genomes are more divergent than the Ostreococcus genomes [28] and Ostreococcus tauri has fewer than 8000 genes. The genetic distances between different clades within a genus of these prasinophytes revealed two Ostreococcus strains belonging to two clades actually only showed a divergence of less than 0.5% but approximately 25% divergence in amino-acid identity in their orthologous protein coding genes [34]. …”

“…TEM analysis confirmed viruses are localised to a region of the cytoplasm and do not associate with the nucleus or other organelles [30]. Ostreococcus and their viruses are globally distributed in coastal and open-ocean euphotic environments [34,36,43]. Viruses that infect O. tauri have been reported to be prevalent in coastal sites [32,33,36], whilst OlVs have been detected in more widespread marine locations, including oligotrophic sites in the Atlantic and Pacific Oceans [34,36,71,72], although this geographical distribution is not related to genetic distance, based on analysis of the polB gene [36].…”

“…Genome sequences are now available for the host prasinophyte picophytoplankton species Ostreococcus tauri [26,27], Ostreococcus lucimarinus [27], Micromonas pusilla [28] and Bathycoccus prasinos [29], as well as 12 whole prasinovirus genome sequences [30,31,32,33,34] (Table 1). Analysis of the NCLDVs present in microbial metagenomes collected during the TARA Oceans survey indicated that prasinoviruses were the most abundant NCLDVs, outnumbering the next most abundant members of the Megavirales, Mimivirus [35].…”