| Summary: | <i>Dunaliella salina</i> (Chlorophyceae), <i>Phormidium versicolor</i> (Cyanophyceae), and <i>Cylindrotheca closterium</i> (Bacillariophyceae) were isolated from three ponds in the solar saltern of Sfax (Tunisia). Growth, pigment contents, and photosynthetic and antioxidant enzyme activities were measured under controlled conditions of three light levels (300, 500, and 1000 µmol photons m<sup>−2</sup> s<sup>−1</sup>) and three NaCl concentrations (40, 80, and 140 g L<sup>−1</sup>). The highest salinity reduced the growth of <i>D. salina</i> and <i>P. versicolor</i> NCC466 and strongly inhibited that of <i>C. closterium</i>. According to ΦPSII values, the photosynthetic apparatus of <i>P. versicolor</i> was stimulated by increasing salinity, whereas that of <i>D. salina</i> and <i>C. closterium</i> was decreased by irradiance rise. The production of carotenoids in <i>D. salina</i> and <i>P. versicolor</i> was stimulated when salinity and irradiance increased, whereas it decreased in the diatom. Catalase (CAT), Superoxide dismutase (SOD), and Ascorbate peroxidase (APX) activities were only detected when the three species were cultivated under E1000. The antioxidant activity of carotenoids could compensate for the low antioxidant enzyme activity measured in <i>D. salina</i>. Salinity and irradiation levels interact with the physiology of three species that have mechanisms of more or less effective stress resistance, hence different resistance to environmental stresses according to the species. Under these stress-controlled conditions, <i>P. versicolor</i> and <i>C. closterium</i> strains could provide promising sources of extremolyte for several purposes.
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