Summary Cyanobacteria play a key role in marine photosynthesis, which contributes to the global carbon cycle and to the world oxygen supply. Genes encoding the photosystem‐II (PSII) reaction centre are found in many cyanophage genomes, and it was suggested that the horizontal transfer of these genes might be involved in increasing phage fitness. Recently, evidence for the existence of phages carrying Photosystem‐I (PSI) genes...