Future warming stimulates growth and photosynthesis in an Arctic microalga more strongly than changes in light intensity or pCO₂

We assessed the responses of solitary cells of Arctic Phaeocystis pouchetii grown under a matrix of temperature (2 degrees C vs. 6 degrees C), light intensity (55 vs. 160 mu mol photons m(-2) s(-1)) and pCO(2) (400 vs. 1000 mu atm CO2, i.e., 40.5 vs. 101.3 Pa). Next to acclimation parameters (growth rates, particulate and dissolved organic C and N, Chlorophyll a content), we measured physiological processes in vivo (electron transport rates and net photosynthesis) using fast-repetition rate fluorometry and membrane-inlet mass spectrometry. Within the applied driver ranges, elevated temperature had the most pronounced impacts, significantly increasing growth, elemental quotas and photosynthetic performance. Light stimulations manifested more prominently under 6 degrees C, underlining temperature's role as a "master-variable". pCO(2) was the least effective driver, exerting mostly insignificant effects. The obtained data were used for a simplistic upscaling simulation to investigate potential changes in P. pouchetii's bloom dynamics in the Fram Strait with increasing temperatures over the 21(st) century. Although solitary cells might not be fully representative of colonial cells commonly observed in the field, our results suggest that global warming accelerates bloom dynamics, with earlier onsets of blooms and higher peak biomasses. Such a temperature-induced acceleration in the phenology of Phaeocystis and likely other Arctic phytoplankton might cause temporal mismatches, e.g., with the development of grazers, and therefore substantially affect the biogeochemistry and ecology of the Arctic.