Sensitivity of Photosystem II to Photoinhibition in Chlamydomonas reinhardtii Under Conditions of Decreasing CO2 Depends on a Luminal Carbonic Anhydrase.

Some mutants of the green alga Chlamydomonas reinhardtii, including the cia3 mutant deficient in the carbonic anhydrase CAH3 in the thylakoid lumen, strongly require high CO2 supplementation for survival, which is usually explained by disruption at some step(s) of the carbon-concentrating mechanism. This study aimed to determine whether CAH3 contributes to photosystem II (PSII) functional stability under low CO2 conditions. Two wild-type C. reinhardtii strains and the cia3 mutant were compared for PSII photoinhibition sensitivity, D1 protein stability, pigment content, and PSII photoprotective responses before and after short-term acclimation to low CO2 growth conditions. Compared with the two wild-type strains, the cia3 mutant exhibited greater PSII sensitivity to increased light intensity after acclimation to low CO2. In addition, this was accompanied by more pronounced degradation of the D1 protein of PSII, indicating that the absence of CAH3 also affects the structural stability of PSII. At the same time, no differences in pigment content were observed between the strains, and a low contribution of PSII photoprotective mechanisms was detected, indicating that the growth light conditions used were non-stressful for the algal photosynthetic apparatus. The observed data indicate that CAH3 contributes to maintaining the functional and structural stability of PSII under low CO2, even under non-stressful light growth conditions.