Temperature-induced changes in photosynthetic activities and thylakoid membrane properties of Euglena gracilis

Euglena gracilis Klebs strain Z Pringsheim was grown at 26°C, and the effects of temperature on photosynthesis and thylakoid parameters were determined. Arrhenius plots of oxygen evolution accompanying CO₂ or p-benzoquinone reduction by intact cells showed a discontinuity at approx. 6-7°C. Plots of oxygen evolution accompanying ferricyanide or benzoquinone reduction by isolated thylakoids showed a discontinuity at approximately 5.5-7°C. A high sensitivity differential scanning calorimetric study of isolated thylakoids showed that a very sharp and reversible transition occurred at 6.2°C. We conclude that a temperature-induced change in the structure of thylakoid lipids resulted in the discontinuity observed in the Arrhenius plots. Calorimetric experiments also demonstrated that several transitions occurred at temperatures higher than 20°C with one at approx. 63°C being the most prominent. As in the case of thylakoids of most higher plants, all of the higher temperature transitions were irreversible.