From Field Metagenomes to Mutant Genomes: Coevolution of Cyanophages and Synechococcus in Estuarine Ecosystems

Picocyanobacteria, represented by Prochlorococcus and Synechococcus, are major photosynthetic organisms in aquatic ecosystems, and their viruses (cyanophages) significantly impact cyanobacterial ecology and evolution. Here, we combined metagenomics of Synechococcus communities along four representative estuaries in China and whole-genome analyses of laboratory-evolved Synechococcus mutants to link viral diversity to host adaptation and evolution. We assembled 83 cyanophage genomes (mainly cyanomyoviruses), with expanded auxiliary metabolic genes encoding glycosyltransferases and radical S-adenosyl methionine proteins involved in amino acid and lipopolysaccharide metabolism. Metagenome-assembled cyanobacterial genomes revealed mutations predominantly in membrane-associated functions linked to phage infection. In parallel, we identified genetic pathways conferring phage resistance in 18 evolved Synechococcus mutant strains that are resistant to phage infection. Notably, mutations in carbohydrate (rfbA) and photosynthetic energy transfer (cpeT) of Synechococcus mutants recurred in both cultured isolates and recovered metagenomes. These results indicate that cyanophages in estuaries leverage broader metabolic toolkits, while Synechococcus repeatedly evolves resistance. Together, these findings outline a reciprocal adaptive landscape that helps explain the persistence and turnover of picocyanobacterial populations in estuarine environments.