Evolution in temperature-dependent phytoplankton traits revealed from a sediment archive: do reaction norms tell the whole story?

The high evolutionary potential of phytoplankton species allows them to rapidly adapt to global warming. Adaptations may occur in temperature-dependent traits, such as growth rate, cell size and life cycle processes. Using resurrection experiments with resting stages from living sediment archives, it is possible to investigate whether adaptation occurred. For this study, we revived resting cysts of the spring bloom dinoflagellate Apocalathium malmogiense from recent and 100-year-old sediment layers from the Gulf of Finland, and compared temperature-dependent traits of recent and historic strains along a temperature gradient. We detected no changes in growth rates and cell sizes but a significant difference between recent and historic strains regarding resting cyst formation. The encystment rate of recent strains was significantly lower compared with historic strains which we interpret as an indication of adaptation to higher and more rapidly increasing spring temperatures. Low encystment rates may allow for bloom formation even if the threshold temperature inducing a loss of actively growing cells through resting cyst formation is exceeded. Our findings reveal that phenotypic responses of phytoplankton to changing temperature conditions may include hidden traits such as life cycle processes and their regulation mechanisms. This study emphasizes the potential of living sediment archives to investigate plankton responses and adaptation to global warming.