Contribution of lowered unsaturation levels of chloroplast lipids to high temperature tolerance of photosynthesis in Chlamydomonas reinhardtii

A mutant of Chlamydomonas reinhardtii designated as hf-9 is impaired in fatty acid desaturation of chloroplasts, and showed lowered unsaturation levels of chloroplast lipids, as compared with the parent. The effects of temperature on photosynthesis were compared between hf-9 and the parent for investigation whether or not unsaturation levels of chloroplast lipids are correlated with the thermal properties of photosynthesis. Growth rates determined by turbidity were higher in the parent than in hf-9 at both 10 and 24°C, while similar for the parent and hf-9 at 39°C. The cells grown at 24°C revealed that both activities of CO₂-dependent oxygen evolution acid photosystem II were higher in the parent than in hf-9 in the range between 7 and 40°C. In contrast, hf-9 surpassed the parent in both activities at 45°C. Optimal temperatures for both activities were at around 35°C and 40°C in the parent and hf-9, respectively. Incubation of the cells at 41 and 45°C demonstrated that the activity of photosystem II in hf-9 was more tolerant to the high temperatures than that in the parent. These results suggest that lowered unsaturation levels of chloroplast lipids contributed to high temperature tolerance of photosystem II, and eventually to that of photosynthesis.