Resistance in marine cyanobacteria differs against specialist and generalist cyanophages

Marine unicellular cyanobacteria coexist in the oceans with lytic phages that infect and kill them. Yet, the cyanobacteria persist, fulfilling their role as important primary producers, due to population diversity that results in different sensitivity and resistance profiles to co-occurring cyanophages. Here, we report a surprising dichotomy in modes of resistance against specialist versus generalist phages: Resistance is primarily extracellular against specialist phages but intracellular against generalist phages. Known intracellular resistance mechanisms are absent from most marine Synechococcus and Prochlorococcus strains, suggesting that currently unknown defense mechanisms exist in marine cyanobacteria. Furthermore, phage DNA entry and replication, coupled with survival of the cyanobacterial cell, provide a means for horizontal transfer of genetic material into, and evolution of, marine cyanobacteria.