The role of extracellular carbonic anhydrase activity in inorganic carbon utilization of Phaeocystis globosa (Prymnesiophyceae): A comparison with other marine algae using the isotopic disequilibrium technique

The utilization of inorganic carbon species by the marine microalga Phaeocystis globasa (Prymnesiophyceae) and several other algal species from different taxa, was investigated by determining the time course of C-14 incorporation in isotopic disequilibrium experiments. From these kinetic data, conclusions can be drawn about the carbon species, CO2 or HCO3-, that is being utilized. By comparing the uptake kinetics in the absence and presence of acetazolamide (AZ) or dextran-bound sulfonamide, inhibitors of external carbonic anhydrase CCA), it was determined that P. globosa, Dunaliella tertiolecta, and some strains of Emiliania huxleyi do use HCO3- by extracellular, CA-catalyzed conversion to CO2, which then diffuses across the membrane. Nannochloropsis, Thalassiosira pseudonanna, and often Synechococcus use HCO3- without extracellular conversion. Thalasiosira punctigera, some strains of E. huxleyi, and Rhodomonas sp. use exclusively free CO2. The presence of extracellular CA activity in Phaeocystis is not constitutive but is induced under low inorganic-carbon conditions. Thus, marine microalgae show variability in carbon acquisition strategy for one single species, depending on external conditions, and in carbon acquisition strategy between species. Determining AZ-induced changes in carbon uptake kinetics provides a sensitive test for the presence of extracellular CA activity. With the potentiometric method, no CA activity could be measured, whereas with the isotopic disequilibrium technique, significant CA activity could be detected.