EFFECT OF DISSOLVED INORGANIC CARBON CONCENTRATION ON THE PHOTOSYNTHESIS OF THE SYMBIOTIC SEA-ANEMONE AIPTASIA-PULCHELLA CARLGREN - ROLE OF CARBONIC-ANHYDRASE

Photosynthetic rate of the anemone Aiptasia pulchella Carlgren, containing symbiotic dinoflagellates (zooxanthellae), was measured as a function of dissolved inorganic carbon (DIC) concentration in seawater. Photosynthetic rate increased as [DIC] increased at concentrations below 2.2 mM (ambient) but saturated at higher concentrations, a pattern similar to Michaelis-Menten enzyme kinetics and carrier transport. Further, photosynthetic rate at 5 mM DIC was on average 26% higher than at ambient DIC, suggesting that DIC is limiting to photosynthesis in ambient seawater. Photosynthetic rate vs [DIC] was also measured in the presence of 10(-4) M acetazolamide a non-competetive inhibitor of carbonic anhydrase (CA, EC 4.2.1.1.), an enzyme that catalyzes the interconversion of CO2 and HCO3- At all concentrations of DIC except 0.0 mM, the presence of the inhibitor reduced photosynthesis by over 50%. CA was localized in A. pulchella using fractionation by centrifugation and colloidal gold immunocytochemistry. In the fractionation studies, CA activity was detected in the soluble animal fraction, indicating that animal CA is not intrinsically bound to any animal membranes. However, CA, localized by immunocytochemistry, appeared in the animal endodermal cells, on or near the vacuolar membranes surrounding the zooxanthellae. No CA was detected in the ectoderm. With its function in carbon delivery and its location on the membranes surrounding the zooxanthellae, CA could be an example of a specialized cnidarian host adaptation with benefits to both partners of the mutualism.