Millennia-Long Evolution of Temperature and Salinity Dependence of a Baltic Sea Diatom Revealed by Resurrection Experiments

Many species have demonstrated persistence through past climate phases, but the recent accelerated anthropogenic climate change severely impacts species composition, altering aquatic biodiversity and phytoplankton communities. Uncovering how species have responded to natural climate variability in the past is of great value for understanding adaptive dynamics and predicting future adaptations. Here, we investigated the Holocene adaptation dynamics of the cosmopolitan diatom species Skeletonema marinoi by reviving dormant phytoplankton cells that have accumulated in the sediment of the Baltic Sea. The Baltic Sea is strongly affected by current climate change and has undergone fundamental environmental changes throughout its Holocene history, including glacial rebound, alternating warmer and cooler periods, and changes in salinity and nutrient availability. Using resurrected temporal cohorts from up to 6800-year-old sediment horizons, we studied past adaptation dynamics by performing growth experiments and morphological measurements under different temperature and salinity conditions. Our results demonstrate that S. marinoi temporal cohorts exhibit differences in their morphological trait values and environmental optima, partially reflecting past ambient environments. Moreover, divergences from expected adaptation patterns demonstrate the complexity of evolution in natural ecosystems. Based on our findings, we expect S. marinoi to cope well with projected environmental changes for the Baltic Sea. These findings highlight the resilience of phytoplankton and emphasize their capacity for phenotypic adaptation to changing conditions. Furthermore, this research underscores the importance of understanding past adaptation processes in predicting phytoplankton responses to future climate change.