Thermal responses of Baltic Sea Ostreococcus vary in their levels of intra-and interspecific variability

O_LITrait variation within and between phytoplankton species can shape community dynamics and ecosystem processes. A better understanding of this variability will improve the prediction of future ocean scenarios. We investigated thermal response variation within and among three Ostreococcus species from the Baltic Sea area and eastern North Sea. C_LIO_LIThirty-one strains were exposed to a range of temperatures. Cell counts, mean size, and chlorophyll a (Chla) content were measured by flow cytometry. Maximum growth rates were estimated using the modified Gompertz model, thermal performance curves were fitted and parameters of interest extracted. Generalized additive models assessed size and Chla responses across temperature. C_LIO_LIThermal performance traits varied substantially within and among species. There were trends for higher Topt and peak rate in Ostreococcus tauri, thermal breadth and Ea varied mainly within species. For size and Chla Ostreococcus mediterraneus showed high intraspecific variability and delayed responses at colder temperatures. Ostreococcus tauri exhibited more linear responses, the hybrid species was intermediate. C_LIO_LIHigh intra- and interspecific variation in Ostreococcus indicates evolutionary potential to buffer temperature changes and provide ecosystem resilience. Temperature induced shifts in community composition could impact nutrient cycling and energy transport, highlighting the need to account for such variability in ecosystem models. C_LI