A possible role for carbonic anhydrase in the lumen of chloroplast thylakoids in green algae

Abstract

In order to understand the function of the lumen carbonic anhydrase (CA) which is bound to PSII at the lumenal side of the thylakoids in chloroplasts of eukaryotic algae, thylakoids were isolated from chloroplasts of Tetraedron minimum, Chlamydomonas noctigama, the cell wall-less mutant Chlamydomonas reinhardtii CW15, and a C. reinhardtii CW15/CIA3 mutant which lacks the lumen CA. The isolated thylakoids produced O₂ on illumination and exhibited electron flow between PSII and PSI, indicating that the thylakoids were intact and the photosynthetic apparatus were functional. We could not detect any uptake of HCO₃–^,nor efflux of CO₂, from the thylakoids upon illumination, making it improbable that the CA present in the lumen of the thylakoids would play a role in furnishing CO₂ for Rubisco. We were able to determine ATP production upon illumination in isolated thylakoids. Under high inorganic carbon (Ci; 5 mM), all species showed significant amounts of ATP being produced. Under low Ci (200 M), we could not detect ATP formation from C. reinhardtii CW15/CIA3 upon illumination. This mutant was not able to survive more then 4 h of low Ci in culture. We therefore suggest that the lumen CA is not involved in the CO₂ concentrating mechanism, but might play a role in the formation of a proton gradient across the thylakoid membranes.