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

<p>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 &micro;m) host&rsquo;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.&nbsp;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.</p>