Warming disrupts infection patterns and primarily accelerates Ostreococcus-virus infection dynamics in the Baltic Sea.

Warming affects phytoplankton host-virus dynamics, but only a few model systems in a handful of ecosystems have been experimentally explored. Viruses influence biogeochemical cycling, phytoplankton population dynamics, and drive coevolution and adaptation. Assessing how host-virus interactions are affected by changing environmental conditions is crucial for understanding and predicting shifts in phytoplankton community structure and function. We first characterized general infection network dynamics through a comprehensive cross-infection experiment involving 31 Ostreococcus strains and 83 viruses, predominantly from the Baltic Sea. We found a modular network structure, characterized by generalist and specialist infections, coevolutionary history and phylogeny. To assess the impact of warming on infection dynamics, we then performed cross-infections with a representative subset of 12 selected host strains and 17 virus lysates per host each at 20°C, 22°C, and 24°C, temperatures generally below the optimal growth temperature for most hosts. We measured spot-plaque growth rates by tracking the timing and growth dynamics of spot-plaque formation through daily photos. Increasing temperature significantly affected susceptibility, spot-plaque formation, and growth, generally increasing rates but with variable responses among host-virus pairs, especially among generalist viruses. Such differences, often decoupled from host growth, indicate that warming adds more complexity to infection dynamics and further increases unpredictability of infection patterns. These results suggest that a shift in infection dynamics could potentially alter the role of the viral shunt in future oceans, reshape infection networks, and therefore affect biogeochemical cycling and food webs.