Activities of Photosystems I and II in Chlamydomonas segnis Adapted and Adapting to Air and Air‐enriched with Carbon Dioxide*

Activities of photosystems I and II were compared at a saturating irradiance in air‐ and 5% CO 2 ‐adapted and adapting Chlamydomonas segnis at the active phase of photosynthesis during the cell cycle. PSII activity was 200% greater in air‐ than in 5% CO 2 ‐adapted cells, while PSI activity was similar in both types of cells and matched the level of PSII activity in air‐adapted cells. As a result, air‐ and 5% CO 2 ‐adapted cells were characterized by low and high PSI/PSII ratios, respectively. In air‐adapted cells, the greater PSII activity (rate of O 2 evolved) exceeded that of photosynthetic (Ps) O 2 evolution, resulting in a Ps/PSII ratio below unity. This was associated with higher levels of catalase activity, lower l ‐ascorbate content, and higher dehydro‐ l ‐ascorbate content than in 5% CO 2 ‐adapted cells. During adaptation to air or 5% CO 2 for 6 h in light, PSI rather than PSII was sensitive to changes in the concentration of CO 2 , and the adapting cells acquired the characteristics of air‐ and 5% CO 2 ‐adapted cells as indicated by PSI/PSII, Ps/PSII, catalase activity, l ‐ascorbate and dehydro‐ l ‐ascorbate contents. The results are discussed in the light of changes in the molecular organization of the thylakoid membranes and enhanced non‐cyclic electron transport coupled with O 2 ‐uptake (Mehler reaction) for the generation of the ATP required for CO 2 /HCO − 3 ‐transport in air‐adapted and adapting cells.