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 Phaeocy stis globasa (Prymnesiophyceae) and several other algal species from differ ent taxa, was investigated by determining the time course of C-14 incorpora tion in isotopic disequilibrium experiments. From these kinetic data, concl usions can be drawn about the carbon species, CO2 or HCO3-, that is being u tilized. By comparing the uptake kinetics in the absence and presence of ac etazolamide (AZ) or dextran-bound sulfonamide, inhibitors of external carbo nic anhydrase CCA), it was determined that P. globosa, Dunaliella tertiolec ta, and some strains of Emiliania huxleyi do use HCO3- by extracellular, CA -catalyzed conversion to CO2, which then diffuses across the membrane. Nann ochloropsis, Thalassiosira pseudonanna, and often Synechococcus use HCO3- w ithout extracellular conversion. Thalasiosira punctigera, some strains of E . huxleyi, and Rhodomonas sp. use exclusively free CO2. The presence of ext racellular CA activity in Phaeocystis is not constitutive but is induced un der low inorganic-carbon conditions. Thus, marine microalgae show variabili ty in carbon acquisition strategy for one single species, depending on exte rnal conditions, and in carbon acquisition strategy between species. Determ ining AZ-induced changes in carbon uptake kinetics provides a sensitive tes t for the presence of extracellular CA activity. With the potentiometric me thod, no CA activity could be measured, whereas with the isotopic disequili brium technique, significant CA activity could be detected.