Marine phytoplankton are responsible for ~50% of global primary production and encompass a wide range of microorganisms characterized by being phototrophs. Within this group are the marine picocyanobacteria encompassing the phylogenetically closely related genera Synechococcus and Prochlorococcus. Major biotic causes of mortality of these organisms include viral lysis as well as grazing by eukaryotic protists. However, picocyanobacterial populations show no extinction, suggesting high rates of production counteract this mortality, or that specific phage-resistance and prey-selectivity mechanisms exist.
Here, we characterised the phenotype and genotype of several phage-resistant Synechococcus mutants. Phenotypic analysis revealed several cyanophage-resistant mutants that were adsorption mutants, a similar phenotype to that reported for Prochlorococcus mutants (Avrani et al., 2011 Nature). However, we also isolated several non-adsorption mutants indicative of novel mechanisms involving inhibition of phage replication or preventing release of viral particles from the host cell. Furthermore, whole genome sequencing of Synechococcus sp. WH7803 adsorption and non-adsorption mutants identified a plethora of SNPs in each mutant and with SNPs occurring at various frequencies. We correlate these observations with the oligoploid nature of this strain and suggest that ploidy may facilitate novel virus resistance mechanisms in this organism.