Cobaviruses – a new globally distributed phage group infecting Rhodobacteraceae in marine ecosystems

Bacteriophages are widely considered to influence bacterial communities, however most phages are still unknown or not studied well enough to understand their ecological roles. We have isolated two phages infecting Lentibacter sp. SH36, affiliated with the marine Roseobactergroup, and retrieved similar phage genomes from publicly available metagenomicsdatabases. Phylogenetic analysis placed the new phages within the Cobavirus group, in the here newly proposed genus Siovirus and subfamily Riovirinae of the Podoviridae. Gene composition and presence of direct terminal repeats in cultivated cobaviruses point toward a genome replication and packaging strategy similar to the T7 phage. Investigation of the genomes suggests that viral lysis of the cell proceeds via the canonical holin-endolysin pathway. Cobaviral hosts include members of the genera Lentibacter, Sulfitobacterand Celeribacter of the Roseobactergroup within the family Rhodobacteraceae( Alphaproteobacteria). Screening more than 5,000 marine metagenomes, we found cobaviruses worldwide from temperate to tropical waters, in the euphotic zone, mainly in bays and estuaries, but also in the open ocean. The presence of cobaviruses in protist metagenomesas well as the phylogenetic neighborhood of cobaviruses in glutaredoxinand ribonucleotide reductasetrees suggest that cobaviruses could infect bacteria associated with phototrophicor grazing protists. With this study, we expand the understanding of the phylogeny, classification, genomic organization, biogeographyand ecology of this phage groupinfecting marine Rhodobacteraceae.