Isolation and Characterization of the First Freshwater Cyanophage Infecting Pseudanabaena

Cyanobacteria are the major primary producers in both freshwater and marine environments. However, the majority of freshwater cyanophages remain unknown due to the limited number of cyanophage isolates. In this study, we present a novel lytic freshwater cyanophage, PA-SR01, which was isolated from the Singapore Serangoon Reservoir. To our knowledge, this is the first isolate of a cyanophage that has been found to infect the cyanobacterium Pseudanabaena PA-SR01 has a narrow host range, a short latent period, and is chloroform sensitive. Distinct from the majority of cyanophage isolates, PA-SR01 has a tailless morphology. It is a double-stranded DNA virus with a 137,012-bp genome. Functional annotation for the predicted open reading frames (ORFs) of the PA-SR01 genome identified genes with putative functions related to DNA metabolism, structural proteins, lysis, host-derived metabolic genes, and DNA packaging. Out of 166 predicted ORFs, only 17 ORFs have homology with genes with known function. Phylogenetic analysis of the major capsid protein and terminase large subunit further suggests that phage PA-SR01 is evolutionary distinct from known cyanophages. Metagenomics sequence recruitment onto the PA-SR01 genome indicates that PA-SR01 represents a new evolutionary lineage of phage which shares considerable genetic similarities with phage sequences in aquatic environments and could play key ecological roles.IMPORTANCE This study presents the isolation of the very first freshwater cyanophage, PA-SR01, that infects Pseudanabaena, and fills an important knowledge gap on freshwater cyanophages as well as cyanophages infecting Pseudanabaena.