Changes in Lipid Metabolism during Adaptation of the Dunaliella salina Photosynthetic Apparatus to High CO2 Concentration

The effects of CO₂ on the content and composition of lipid fatty acids (FA) and on the photosynthetic characteristics of unicellular halophilic green alga Dunaliella salina (known to be susceptible to CO₂ stress) were investigated. It was shown that even one-day-long increase in the CO₂ concentration (from 2 to 10%) provoked an increase in the total amount of FA on the dry weight basis by 30%. After 7-day-long growth at 10% CO₂, this value was 2.7-fold higher than that at 2% CO₂. The difference in the FA content and composition indicated the activation of FA synthesis de novo and inhibition of their elongation and desaturation, as well as the increase in the relative content of saturated FA at 10% CO₂. It was demonstrated that, after one-day-long CO₂ stress, the MGDG/DGDG ratio increased fourfold without change in the sum of their FA, which indicates the increase in the proportion of lipids predisposed to micellar (hexagonal phase) but not lamellar structure formation. Under short-term CO₂ stress, the ratio of 3/6 FA increased and the content of E-16:113 FA in phosphatidylglycerols increased sharply. The drop in protein content especially in the photosystem I (PSI) preparations, as well as diminishing the ratio of F ₇₀₀-to-F ₆₈₆ nm fluorescence (F ₇₀₀/F ₆₈₆) under short-term CO₂ stress argued for the significant damage to PSI. The reversibility of these changes at more prolonged treatment (7 and 10 days) demonstrated that D. salina cells could restore the functional activity of PSI. The lower level of F ₇₀₀/F ₆₈₆, chlorophyll a (Chla)/Chlb, and 3/6 FA ratio in line with the higher level of E-16:113 in the cells growing for a long time at the high CO₂ concentration is characteristic for the new structural and functional state of the photosynthetic apparatus providing for the effective photosynthesis of D. salina under these conditions.