Long-term stress induced by nitrate deficiency, sodium chloride, and high light on photosystem II activity and carotenogenesis of green alga Scenedesmus sp

The green alga Scenedesmus sp. was exposed to a long-term physiological stress of 40 days induced by the combination of nitrate deficiency, different concentrations of sodium chloride, and high light irradiance. Under these conditions, the change of photosystem II (PSII) photochemical reactions of photosynthesis and cellular growth were investigated along with the synthesis of secondary carotenoids, including astaxanthin and canthaxanthin. From 10 to 40 days, the biosynthesis of these secondary carotenoids was highly induced after the deterioration of PSII complexes when chlorophyll synthesis and cell division were inhibited. In this long term study, the highest induction of carotenoid synthesis happened after the deterioration of PSII reaction centers. There was no direct relationship between the induction of secondary carotenoids and the decline of PSII photochemical efficiency, although it may contribute to carotenogenesis. Under this stressful condition, viability of cells was noted by the small change of unbleached cell density compared with control after 40 days. Therefore, Scenedesmus sp. was tolerant to the long-term chronic stress condition caused by the production of secondary carotenoids, such as astaxanthin and cantaxanthin, participating in antioxidative mechanisms to protect algal cellular system.