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 (Sate et al., Eur. J. Biochem., 230 (1995) 987-993). 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 degrees C, while similar for the parent and hf-9 at 39 degrees C. The cells grown at 24 degrees C revealed that both activities of CO2-dependent oxygen evolution and photosystem II were higher in the parent than in hf-9 in the range between 7 and 40 degrees C. In contrast, hf-9 surpassed the parent in both activities at 45 degrees C. Optimal temperatures for both activities were at around 35 degrees C and 40 degrees C in the parent and hf-9, respectively. Incubation of the cells at 41 and 45 degrees 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.