EXTRACELLULAR AND INTRACELLULAR CARBONIC-ANHYDRASE IN RELATION TO CULTURE AGE IN A HIGH-CALCIFYING STRAIN OF EMILIANIA-HUXLEYI LOHMANN

The relationships between extra- and intra-cellular carbonic anhydrase , calcification rate, utilization of dissolved inorganic carbon (DIC) and culture age were investigated in a high-calcifying strain of Emili ania huxleyi. The detection of carbonic anhydrase was dependent on cul ture age; neither the extra-cellular activity of intact cells nor acti vity in crude homogenates was detected until the stationary phase. By the stationary phase DIC in the medium was totally depleted and the ca lcification rate had decreased by 60%. Extra- and intra-cellular carbo nic anhydrase was stimulated by Na+ but not Cl ions. The isolation of intact organelles by isopycnic gradient centrifugation from exponentia l and stationary phase cultures of high calcifying cells showed high c arbonic anhydrase activity in the chloroplast fraction but carbonic an hydrase was not detected in low-calcifying cells even after gradient c entrifugation. Although 50 mu M ethoxyzolamide was an effective inhibi tor of carbonic anhydrase activity in vitro, photosynthetic (CO2)-C-14 fixation was only inhibited 30%, calcification rate 10% and the inter nal inorganic carbon pool unaffected in intact cells. It is proposed t hat chloroplast carbonic anhydrase maintains the steady state flux of CO2 from the chloroplast envelope to Rubisco, much of the CO2 arising by release from HCO3- in the cytosol, required to maintain cytoplasmic pH near neutrality in high-calcifying cells.