| Summary: | Laboratory experiments on the physiological response of picophytoplankton to light, temperature and nutrient limitations were conducted. The impact of climate change in a RCP8.5 model scenario simulation was investigated. An acclimated and a dynamic photosynthesis response model reproduce the physiological response to light. Long-term damage (accumulated over days) through photoinhibition is underestimated by the dynamic model. The maximum rate of photosynthesis is significantly lower for picoprokaryotes (0.81 − 1.44 d-1) than for picoeukaryotes (1.93 − 4.93 d-1). Also, their affinity for light is higher (7.15 − 12.42 g C m2 (mol photons g Chl)-1 compared to 3.42 − 9.81 g C m2 (mol photons g Chl)-1). Optimum growth rates differ significantly between the groups (0.47 ± 0.17 d-1 for picoprokaryotes and 1.05 ± 0.47 d-1 for picoeukaryotes). The temperature tolerance range is higher for picoeukaryotes (2.8°C − 32.4°C compared to 13.7°C − 27°C). The maximum picophytoplankton community growth has a Q10 value of 2.3. For picoprokaryotes the Q10 value is even higher (4.9). The cell composition in both groups deviates significantly from the Redfield ratio under nutrient saturated conditions with a lower phosphorus demand in picoprokaryotes. Under nutrient limitation nitrogen: carbon is reduced by 15 − 42%, and phosphorus: carbon by 37 − 65%. Chlorophyll a: carbon is significantly lower under both nitrogen (-50 − -82% ) and phosphorus (-62 − -91 %) limitations. The half-saturation constants are in the range between 0.01 ± 0.02 and 0.19 ± 0.23 μmol NH4+ L-1 for individual picoeukaryotes. These findings agree with theoretical assumptions related to size with an advantage in subtropical oligotrophic light limited environments and highlight the requirement of data on picoeukaryotes. Climate change leads to enhanced stratification of the water column, reduced availability of nutrients and an increased contribution of picophytoplankton to total phytoplankton biomass weakening the biological pump.
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