Intraspecific variability exceeds interspecific variability in thermal responses of Baltic Sea Ostreococcus (Mamiellophyceae) traits

Abstract Trait variation within and between phytoplankton species as well as within regions, can shape community dynamics and ecosystem processes. A better understanding of this variability will improve the prediction of future ocean scenarios and the possible resilience of different phytoplankton groups. We investigated thermal response variation within and among three Ostreococcus species—a prominent picophytoplankton genus—using 31 strains from the Baltic Sea area and eastern North Sea. Across a temperature gradient ranging from 18°C up to 32°C, we quantified growth, cell size, and a proxy for chlorophyll a (Chl a ) and characterized their thermal responses using modeling approaches. We further assessed how these traits vary with temperature and quantified interspecific, intraspecific and intraregional variability. Thermal performance traits showed high variability within species and regions and moderate variability between species. Ostreococcus tauri showed significantly higher thermal optimum and a trend for higher peak rates than Ostreococcus mediterraneus . Thermal safety margin and the sensitivity of growth to temperature varied substantially within all species. For size and Chl a , the species showed distinct temperature responses: Ostreococcus mediterraneus exhibited highest intraspecific variability and delayed responses at colder temperatures, Ostreococcus tauri responded linearly, with the Ostreococcus mediterraneus variant showing intermediate patterns. High intraspecific variation in Ostreococcus indicates substantial evolutionary potential, while interspecific differences may contribute to ecosystem stability under changing temperatures. This is especially relevant with predicted temperature-induced community shifts favoring smaller phytoplankton groups, highlighting the need to incorporate trait variability in ecosystem models.