High-temperature thermoluminescence of chlorophyll as a method to study lipid peroxidation in planktonic algae

Chlorophyll-containing cells of algae, photosynthetic bacteria, and plants often exhibit strong red luminescence when heated to temperatures above 70 °C. This high-temperature thermoluminescence (HTL) originates mostly from thermally induced degradation of lipid cycloperoxides present in thylakoid membranes. The degrading cycloperoxides can transiently form carbonyl species in excited triplet state, whereas chlorophyll molecules readily accept energy from the triplet carbonyls and emit it as luminescence. The HTL emission was measured in phytoplankton collected from different waters and in laboratory cultures of algae and cyanobacteria. The intensity of HTL generally increased at mid-day in the phytoplankton sampled in oligotrophic waters (Lake Baikal, Kandalakshskaya Bay of the White Sea) but showed little changes throughout the day in waters with high concentration of mineral nutrients (Lake Geneva, Marseille Gulf of the Mediterranean Sea, Kotorska Gulf of the Adriatic Sea). When the culture of Chlorella pyrenoidosa was grown under the nitrogenor phosphorus-limited conditions at moderate light intensity, an increase in the HTL was observed. However, the HTL emission remained low when C. pyrenoidosa or other species of algae were grown in nutrient-rich media even if the algae were exposed to a strong light. Consequently, nutrient availability may be of importance for cell resistance to the light-induced peroxidation of membrane lipids. The HTL is an easy and convenient method to access lipid peroxidation in natural phytoplankton assemblages.