Seasonality and strain specificity drive rapid co-evolution in a Ostreococcus-virus system from the Western Baltic Sea

Marine viruses are a major driver of phytoplankton mortality and thereby influence biogeochemical cycling of carbon and other nutrients. Phytoplankton-targeting viruses are important components of ecosystem dynamics, but broad-scale experimental investigations of host-virus interactions remain scarce. Here, we investigated in detail a picophytoplankton (size 1 µm) host’s responses to infections by species-specific viruses from distinct geographical regions and different sampling seasons. Specifically, we used <em>Ostreococcus tauri </em>and<em> O. mediterraneus</em> and their viruses (size ca. 100 nm). <em>Ostreococcus</em> sp. are globally distributed and, like other picoplankton species, play an important role in coastal ecosystems at certain times of the year. Further,<em> Ostreococcus</em> sp. are model organisms, and the <em>Ostreococcus</em>-virus system is well-known in marine biology. However, only few studies have researched its evolutionary biology and the implications thereof for ecosystem dynamics. The <em>Ostreococcus</em> strains used here stem from different regions of the Southwestern Baltic Sea that vary in salinity and temperature and were obtained during several cruises spanning different sampling seasons. Using an experimental cross-infection set-up, we explicitly confirm species and strain specificity in <em>Ostreococcus</em> sp. from the Baltic Sea. Moreover, we found the timing of virus-host co-existence, was driver of infection patterns as well. In combination, these findings prove that host-virus co-evolution can be rapid in natural systems.