Photosynthetic and Antioxidant Activities in Extremophile Microalgae Dunaliella Salina, Cylindrotheca Closterium and Phormidium Versicolor According to NaCl Concentration and Irradiance

Abstract

Dunaliella salina (Chlorophyceae), Phormidium versicolor (Cyanophyceae) and Cylindrotheca closterium (Bacillariophyceae) were isolated from three ponds in the solar saltern of Sfax (Tunisia) having an average salinity of 350, 100 and 90 respectively. Growth, pigment contents, photosynthetic and antioxidant enzyme activities were measured under controlled conditions: three light levels (300, 500 and 1000 µmol photons m-2 s-1) and three NaCl concentrations (40, 80 and 140 g L-1). The highest salinity reduced the growth of D. salina and P. versicolor, and strongly inhibited that of C. closterium. These results are in accordance with the species distibution in the salt marshes. Irradiance rise only induced a significant increase of net photosynthesis in C. closterium probably due to the efficient nonphotochemical quenching and antioxidative enzyme activities. According to □PSII values, the photosynthetic apparatus of P. versicolor was stimulated by increasing salinity whereas that of D. salina and C. closterium was decreased by irradiance rise. The production of carotenoids in D. salina and P. versicolor was stimulated when salinity and irradiance increased whereas it decreased in the diatom. Antioxidant activity of carotenoids could compensate the low antioxidant enzyme activity measured in D. salina.