Quantum Requirements of Photosynthetic Oxygen Evolution and 77 K Fluorescence Emission Spectra in Unicellular Green Algae Grown Under Low‐ and High‐CO₂‐Conditions

Quantum requirements of photosynthetic oxygen evolution at 682 nm and fluorescence spectra at liquid nitrogen temperature (77 K), were investigated in Dunaliella tertiolecta, Chlamydomonas reinhardtii C‐9, Chlorella vulgaris 11g, Chlorella vulgaris C3, and Chlorella pyrenoidosa 8b grown under low‐ and high‐CO 2 conditions. Dunaliella, Chlamydomonas and C. vulgaris 11g show higher quantum requirements and a higher ratio of F 710–740 /F 680–695 fluorescence when grown under low‐CO 2 conditions, indicating a change in excitation energy distribution towards PS I. In C. pyrenoidosa the quantum requirement for low‐CO 2 grown cells is higher than in high‐CO 2 grown cells, but there was practically no change in the fluorescence ratio. In C. vulgaris C3, the quantum requirements of low‐ and high‐CO 2 grown cells are the same, but the fluorescence ratio is higher in high‐CO 2 grown cells than in low‐CO 2 grown cells. These results indicate that most of the low‐CO 2 grown cells require more PS I light than high‐CO 2 grown cells. It is possible that this energy is used for cyclic electron flow. In C. vulgaris C 3, a mechanism may exist for excitation energy distribution which leads to the same quantum requirements under low‐ and high‐CO 2 conditions.