Influence of heterotrophs on phage infection of marine picocyanobacteria

Picocyanobacteria Prochlorococcus and Synechococcus coexist with lytic phages and heterotrophic bacteria in the oceans. These phages are a significant cause of mortality, while heterotrophic bacteria can increase Prochlorococcus fitness by reducing oxidative stress and cross-feeding during extended darkness. Studies of Prochlorococcus-phage interactions are often done with xenic cultures, as it has been historically difficult to obtain and maintain heterotroph-free cultures. Here, we examine how heterotrophic bacteria affect phage infection dynamics in Prochlorococcus and Synechococcus by comparing infections in cultures with and without heterotrophs. We found that Prochlorococcus populations resumed growth following infection only when heterotrophs were present, independent of phage-host or heterotroph-host ratios. In Synechococcus phage-host pairings, outcomes varied, suggesting that heterotrophic effects depend on the phage-host interaction. When hosts recovered from infection, heterotrophs appeared to facilitate this by mitigating oxidative stress and possibly supplying organic carbon sources that support post-infection growth. Recovered Prochlorococcus and Synechococcus populations were resistant to reinfection when transferred to fresh media. However, comparisons of host sequences before and after infection argue against genetic change as the mechanism of tolerance. Instead, we infer that hosts undergo non-genetic adaptations during recovery in the presence of heterotrophs, likely driven by heterotroph-derived organic compounds that reshape host metabolism and confer protection against future lysis. During this process, heterotrophic community composition shifted dynamically across sequential transfers, yet host recovery remained consistent, suggesting that either community function or host adaptations maintained support for growth.

This study uncovers a previously unrecognized role of heterotrophic bacteria in shaping phage infection outcomes in marine cyanobacteria. Our findings demonstrate that the presence of heterotrophs can enable Prochlorococcus to recover from phage infection, irrespective of phage:host or heterotroph:host ratios. In contrast, Synechococcus exhibited variable outcomes, suggesting that the impact of heterotrophs is dependent on the heterotroph-host pairing. Recovery after phage infection appears to be driven by heterotroph-facilitated mitigation of oxidative stress and provision of organic carbon, which may alter host metabolism and contribute to protection against future phage infection. These results highlight the importance of the microbial community, particularly heterotrophs, in shaping cyanobacteria-phage dynamics and highlight the need to reframe host-phage interactions within a broader ecological framework.