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

The activity and subcellular distribution of carbonic anhydrase in a c occolithophorid alga, CCMP 299, was examined. The enzyme could not be detected in crude cell homogenates but was present at high specific ac tivity (27.5 unit . mg-1 protein) in chloroplasts (density, 1.14 g . c m-3) isolated in a sucrose gradient. The carbonic anhydrase activity w as 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 inhi bition previously observed for chloroplastic (as compared to periplasm ic) carbonic anhydrase. In this organism, carbonic anhydrase was local ized in the chloroplast stroma. These findings are discussed in terms of the relationship among dissolved inorganic carbon interconversions, photosynthesis, and calcification.