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CARBONIC ANHYDRASE IN THE CHLOROPLAST OF A COCCOLITHOPHORID (PRYMNESIOPHYCEAE) 1

The activity and subcellular distribution of carbonic anhydrase in a coccolithophorid alga, CCMP 299, was examined. The enzyme could not be detected in crude cell homogenates but was present at high specific activity (27.5 unit·mg−1 protein) in chloroplasts (density, 1.14 g·cm−3) isolated in a sucrose gradient. The carbonic anhydrase activity was sensitive to known inhibitors. Inhibition at 50% (I50) was obtained with concentrations of 4.60 mM and 2.65 mM for acetazolamide and NaN3, respectively. These levels are more consistent with patterns of inhibition previously observed for chloroplastic (as compared to periplasmic) carbonic anhydrase. In this organism, carbonic anhydrase was localized in the chloroplast stroma. These findings are discussed in terms of the relationship among dissolved inorganic carbon interconversions, photosynthesis, and calcification.