The affinity for HCO3− and carbonic anhydrase activity (CA; EC 4.2.1.1) were determined in the red macroalga Bostrychia scorpioides. The affinity for HCO3− was assessed by using three independent methods: the effect of a high pH on photosynthetic rates, photosynthetic conductance for CO2 estimated from the initial slope of photosynthesis rate versus inorganic carbon concentration curves, and the CO2 compensation point. The results obtained from the three methods were consistent in demonstrating that B. scorpioides has a very low affinity for HCO3−, although a certain capacity for its use must not be ruled out. High activities of both external and internal CA were found. Internal CA activity was 10-fold higher than external. External and internal CA were necessary for photosynthesis at pH values higher than 7·0 because O2 evolution was inhibited after adding acetazolamide and ethoxyzolamide (inhibitors of CA). The effect of changing O2 concentration on photosynthesis was also examined. Bostrychia scorpioides exhibited a high sensitivity to O2, demonstrating a C3-like photosynthetic gas exchange physiology. Therefore, the high level of CA activity in B. scorpioides was not enough to support a high affinity for HCO3− and to prevent photorespiration.