The capacity for HCO (3) (-) use by Porphyra leucosticta Thur. in Le Jolis grown at different concentrations of inorganic carbon (C(i)) was investigated. The use of HCO (3) (-) at alkaline pH by P. leucosticta was demonstrated by comparing the O(2) evolution rates measured with the O(2) evolution rates theoretically supported by the CO(2) spontaneously formed from HCO (3) (-) . Both external and internal carbonic anhydrase (CA; EC 4.2.1.1) were implied in HCO (3) (-) use during photosynthesis because O(2) evolution rates and the increasing pH during photosynthesis were inhibited in the presence of azetazolamide and ethoxyzolamide (inhibitors for external and total CA respectively). Both external and internal CA were regulated by the C(i) level at which the algae were grown. A high C(i) level produced a reduction in total CA activity and a low C(i) level produced an increase in total CA activity. In contrast, external CA was increased at low C(i) although it was not affected at high C(i). Parallel to the reduction in total CA activity at high C(i) is a reduction in the affinity for C(i), as estimated from photosynthesis versus C(i) curves, was found. However, there was no evident relationship between external CA activity and the capacity for HCO (3) (-) use because the presence of external CA became redundant when P. leucosticta was cultivated at high C(i). Our results suggest that the system for HCO (3) (-) use in P. leucosticta is composed of different elements that can be activated or inactivated separately. Two complementary hypotheses are postulated: (i) internal CA is an absolute requirement for a functioning C(i)-accumulation mechanism; (ii) there is a CO(2) transporter that works in association with external CA.