Changing and fluctuating (biotic and abiotic) environments have been the driving force for adaptive evolution over the past billion years, leading from the first simple single cell to complex multicellular organisms. Due to global warming, ocean temperatures are rising on a time scale that might be too fast for evolutionary processes, and we currently know little about phytoplankton plasticity, adaption or evolution. In natural environments, adaptive evolution is typically the consequence of multiple interacting selection pressures which makes it difficult to investigate. However, adaptive evolution can be studied in controlled laboratory experiments, by focusing on specific traits (e.g. fitness, nutrient uptake, nutrient storage) or selective agents or combinations of selective agents (temperature, pH, oxygen, salt, nutrients, light, competition, etc). In order to identify cellular process that occur during and aid long-term temperature acclimation, we are conducting experimental evolution experiments with two model diatoms (Thalassiosira pseudonana and Fragilariopsis cylindrus). This will provide first insights into their evolutionary potential under temperature stress.