Phytoplankton are responsible for a major part of global primary production due to the immensity of the marine realm and are heavily implicated in global biosphere equilibriums by driving elemental chemistry in surface oceans, exporting massive amounts of C to sediments and influencing ocean-atmosphere gas exchange. Climate change will alter the marine environment within the next 100 years. Increasing atmospheric CO2 has already caused higher aquatic pCO2 levels and lower pH (ocean acidification) and rising temperature will impact ocean stratification, and hence light and nutrient conditions. Phytoplankton will be affected by these Earth system transformations in many ways, altering the complex balance of biogeochemical cycles and climate feedback mechanisms. Prediction of how phytoplankton may respond at the cellular and ecosystem levels is a key challenge in global change research. The proposed project will investigate physiological reactions of 3 important phytoplankton groups (diatoms, coccolithophores, cyanobacteria) to environmental factors which will be affected by global change (pCO2/pH, light, nutrients). Using an innovative combination of cutting-edge mass-spectrometric and fluorometric techniques, a suite of in vivo assays will be applied in lab and field experiments to develop a process-based understanding of cellular responses. Specific biogeochemical issues will be addressed since diatoms are the main drivers of vertical organic C fluxes, coccolithophores regulate ocean alkalinity through calcification, and N2-fixing cyanobacteria control availability of reactive N. These are relevant in different marine zones, from Southern Ocean to equatorial oligotrophic waters. Data will significantly improve understanding of key processes in phytoplankton and will be exploited in multidisciplinary contexts ranging from molecular to ecological processes and, through cellular and ecosystem models, to predictions of marine biosphere responses to future global change