Carbon acquisition of bloom‐forming marine phytoplankton

Carbon acquisition in relation to CO 2 supply was investigated in three marine bloom‐forming microalgae, the diatom Skeletonema costatum , the flagellate Phaeocystis globosa , and the coccolithophorid Emiliania huxleyi . In vivo activities of extracellular (eCA) and intracellular (iCA) carbonic anhydrase activity, photosynthetic O 2 evolution, CO 2 and HCO 3 − uptake rates were measured by membrane inlet mass spectrometry in cells acclimated to pCO 2 levels of 36, 180, 360, and 1,800 ppmv. Large differences were obtained between species both with regard to the efficiency and regulation of carbon acquisition. While eCA activity increased with decreasing CO 2 concentration in S. costatum and P. globosa , consistently low values were obtained for E. huxleyi . No clear trends with pCO 2 were observed in iCA activity for any of the species tested. Half saturation concentrations ( K 1/2 ) for photosynthetic O 2 evolution, which were highest for E. huxleyi and lowest for S. costatum , generally decreased with decreasing CO 2 concentration. In contrast, K 1/2 values for P. globosa remained unaffected by pCO 2 of the incubation. CO 2 and HCO 3 − were taken up simultaneously by all species. The relative contribution of HCO 3 − to total carbon uptake generally increased with decreasing CO 2 , yet strongly differed between species. Whereas K 1/2 for CO 2 and HCO 3 − uptake was lowest at the lowest pCO 2 for S. costatum and E. huxleyi , it did not change as a function of pCO 2 in P. globosa. The observed taxon‐specific differences in CO 2 sensitivity, if representative for the natural environment, suggest that changes in CO 2 availability may influence phytoplankton species succession and distribution. By modifying the relative contribution of different functional groups, e.g., diatomaceous versus calcareous phytoplankton, to the overall primary production this could potentially affect marine biogeochemical cycling and air—sea gas exchange.