Role of carbonic anhydrase in photosynthesis and inorganic-carbon assimilation in the red alga Gracilaria tenuistipitata

The mechanism of inorganic-carbon (C_i) accumulation in the red seaweed Gracilaria tenuistipitata Zhang et Xia has been investigated. Extracellular and intracellular carbonic-anhydrase (CA) activities have been detected. Photosynthetic O₂ evolution in thalli and protoplasts of G. tenuistipitata were higher at pH 6.5 than at pH 8.6, where HCO ₃ ^– is the predominant form of C_i. Dextran-bound sulfonamide (DBS), a specific inhibitor of extracellular CA, reduced photosynthetic O₂ evolution at pH 8.6 and did not have any effect at pH 6.5. After inhibition with DBS, O₂ evolution was similar to the rate that could be supported by CO₂ from spontaneous dehydration of HCO ₃ ^– . The rate of photosynthetic alkalization of the surrounding medium by the algal thallus was dependent on the concentration of C_i and inhibited by DBS. We suggest that the general form of C_i that enters through the plasma membrane of G. tenuistipitata is CO₂. Bicarbonate is utilized mainly by an indirect mechanism after dehydration to CO₂, and this mechanism involves extracellular CA.Abbreviations C_i inorganic carbon (CO₂ + HCO ₃ ^– ) - CA carbonic anhydrase - DIC dissolved inorganic carbon (total) - DBS dextran-bound sulfonamide - EZ ethoxyzolamide - NSW natural seawater - PPFD photosynthetic photon flux density - REA relative enzyme activity - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase This research was supported by the Deutsche Forschungsgemeinschaft (Bonn) as a programme of the Sonderforschungsbereich 251 der Universität Würzburg and by the Fonds der Chemischen Industrie (Frankfurt). Joint work in Würzburg was possible thanks to travel grants from the Chancellor of the University of Würzburg, Professor R. Günther, from the Australian National University under the auspices of its Overseas Studies Programme, and from the New Zealand — Federal Republic of Germany Scientific and Technological Exchange Programme, which are gratefully acknowledged. We thank Dr. A. Meyer and Ms. E. Kilian for untiringly conducting part of the experimental work, Ms. G. Theumer and Ms. D. Faltenbacher-Werner for their valuable assistance, and Mr. H. Walz (Walz Company, Effeltrich, FRG) for his skilled help with the calibration of our gas-exchange system for measurements with helox. The Department of Conservation, New Zealand, is thanked for permission to collect lichens.