Carbonic anhydrase activity and inorganic carbon fluxes in low‐ and high‐C1 cells of Chlamydomonas reinhardtü and Scenedesmus obliquus

Abstract

Carbonic anhydrase (CA) activity associated with high‐ and low‐dissolved inorganic carbon (C1) grown cells was examined in whole cells by measuring 18O exchange from doubly labeled CO2 (13C18O18O). Both algal species showed the presence of extracellular (periplasmic) as well as intracellular CA activity, which were both greatly increased in low‐C1 cells. The periplasmic CA activity was at least 40‐fold higher in lowcompared to high‐C1 cells in both C. reinhardtii and S. obliquus. while low‐C1 cells of S. obliquus showed the highest activity of internal CA. The CA inhibitor ethoxyzolamide showed a strong inhibition of the C1 uptake process in both C. reinhardtii and S. obliquus as in cyanobacteria. which may indicate that the nature of the primary uptake process is similar in both green algae and cyanobacteria. By using a mass spectrometnc disequilibrium technique it was possible to separate the C1 fluxes of net HCO−3‐uptake and net CO2‐uptake during steady‐state photosynthesis in high‐ and Sow‐C1 grown cells of Chlamydomonas reinhardtii (WT. 2137+) and Scenedesmus obliquus (WT. D3). It was found that both high‐ and low‐C1 cells of the two algae can utilize both CO2 and HCO−3 for photosynthesis, although low‐C1 cells have a higher affinity for the uptake of both C1 species. Induction at low‐C1 causes an increase in the affinity of both species for HCO−3 and CO2; changes in net CO2‐uptake were, however, significantly greater.