CARBONIC-ANHYDRASE ACTIVITY AND INORGANIC CARBON FLUXES IN LOW-CI ANDHIGH-CI CELLS OF CHLAMYDOMONAS-REINHARDTII AND SCENEDESMUS-OBLIQUUS

Carbonic anhydrase (CA) activity associated with high- and low-dissolv ed inorganic carbon (C(i)) grown cells was examined in whole cells by measuring O-18 exchange from doubly labeled CO2 ((COO))-C-13-O-18-O-18 . Both algal species showed the presence of extracellular (periplasmic ) as well as intracellular CA activity, which were both greatly increa sed in low-C(i) cells. The periplasmic CA activity was at least 40-fol d higher in low-compared to high-C(i) cells in both C reinhardtii and S. obliquus, while low-C(i) cells of S. obliquus showed the highest ac tivity of internal CA. The CA inhibitor ethoxyzolamide showed a strong inhibition of the C(i) uptake process in both C. reinhardtii and S. o bliquus as in cyanobacteria, which may indicate that the nature of the primary uptake process is similar in both green algae and cyanobacter ia. By using a mass spectrometric disequilibrium technique it was poss ible to separate the C(i) fluxes of net HCO3--uptake and net CO2-uptak e during steady-state photosynthesis in high- and low-C(i) grown cells of Chlamydomonas reinhardtii (WT, 2137+) and Scenedesmus obliquus (WT , D3). It was found that both high- and low-C(i) cells of the two alga e can utilize both CO2 and HCO3- for photosynthesis, although low-C(i) cells have a higher affinity for the uptake of both C(i) species. Ind uction at low-C(i) causes an increase in the affinity of both species for HCO3- and CO2; changes in net CO2-uptake were, however, significan tly greater.