Life-Style and Genome Structure of Marine <i>Pseudoalteromonas</i> Siphovirus B8b Isolated from the Northwestern Mediterranean Sea

<div><p>Marine viruses (phages) alter bacterial diversity and evolution with impacts on marine biogeochemical cycles, and yet few well-developed model systems limit opportunities for hypothesis testing. Here we isolate phage B8b from the Mediterranean Sea using <i>Pseudoalteromonas</i> sp. QC-44 as a host and characterize it using myriad techniques. Morphologically, phage B8b was classified as a member of the <i>Siphoviridae</i> family. One-step growth analyses showed that this siphovirus had a latent period of 70 min and released 172 new viral particles per cell. Host range analysis against 89 bacterial host strains revealed that phage B8b infected 3 <i>Pseudoalteromonas</i> strains (52 tested, >99.9% 16S rRNA gene nucleotide identity) and 1 non-<i>Pseudoaltermonas</i> strain belonging to <i>Alteromonas</i> sp. (37 strains from 6 genera tested), which helps bound the phylogenetic distance possible in a phage-mediated horizontal gene transfer event. The <i>Pseudoalteromonas</i> phage B8b genome size was 42.7 kb, with clear structural and replication modules where the former were delineated leveraging identification of 16 structural genes by virion structural proteomics, only 4 of which had any similarity to known structural proteins. In nature, this phage was common in coastal marine environments in both photic and aphotic layers (found in 26.5% of available viral metagenomes), but not abundant in any sample (average per sample abundance was 0.65% of the reads). Together these data improve our understanding of siphoviruses in nature, and provide foundational information for a new ‘rare virosphere’ phage–host model system.</p></div