Novel freshwater cyanomyophages highlight the picocyanobacteria-cyanophage genomic and proteomic diversification across the salinity divide
Oxygenic photosynthesis performed by picocyanobacteria is a fundamental biological process contributing to approximately 25% of the ocean net primary production. Here, viruses (cyanophages) infecting these globally distributed phototrophs are responsible for releasing ca. 20% of globally fixed carbon into the ecosystem. Marine picocyanobacteria/cyanophages have been studied for decades, whilst the dearth of data of freshwater representatives has only recently been addressed. We show that freshwater cyanophage genomes are distinct from their marine counterparts with isoelectric point acid shift and prevalence for negatively charged amino acids. Additioanlly, freshwater cyanophages show a distinct pattern of genes inolved with metabolism and lack key genes common in marine counterparts. Coverage in metagenomic datasets unequivocaly discards the presence of marine phages in freshwater lakes and freshwaters on oceanic regimes. Therefore, salinity dictates a phage diversification where transitions between marine and freshwater systems would require extensive changes in the proteomes. Yet despite this, phylogenetic analsysis shows freshwater cyanophages have emerged more recently and multiple times from their marine counterparts. This is at odds with the phylogenetic relationships seen in their cognate hosts, where marine picoyanobacteria have evolved once from fhreshwater ancestors. This has important implications for horizontal gene transfer across the salinity divide