Stressor interactions modify ecological and evolutionary responses of phytoplankton to warming

How phytoplankton respond and whether they will be able to adapt to rising temperatures, including temperature extremes, is a major question for determining the fate of aquatic ecosystems under global change. Thermal performance curves (TPCs) characterize phytoplankton growth at different temperatures and are used to assess and predict the responses of species or strains to changing temperatures. We showed experimentally that TPCs are sensitive to nutrient concentrations, with the maximum growth rates and the optimum temperatures (Topt) declining under nitrogen (N) or phosphorus limitation. This sensitivity may potentially make nutrient-limited phytoplankton more vulnerable to high temperatures. Moreover, in our long-term thermal evolution experiments, we found that while N-replete strains of diatoms can readily adapt to high temperatures and become more tolerant of high temperature extremes, N limitation prevented thermal adaptation. We developed an eco-evolutionary model that suggests that the increased nitrogen requirements in high-temperature adapted strains could not be met under N limitation and, consequently, it may have prevented the selection of high-temperature tolerant genotypes and evolutionary rescue. Our results demonstrate that nutrient limitation effects on TPCs and on thermal adaptation should be taken into account when estimating the ecological and evolutionary responses of phytoplankton to warming.