Functional features of Scenedesmus obliquus: wild type 276–6 strain (WT) and its two mutants reported as photosystem I-deficient (mutant 56.80) and photosystem II-deficient (mutant 57.80) were characterized. Algae were cultured aseptically under continuous light or in darkness on mineral bold basal medium (BBM), yeast extract-enriched BBM and yeast extract to evaluate the physiology of algal cells under photoautotrophic, mixotrophic and heterotrophic conditions. Growth, superoxide dismutase activity and photosynthetic parameters, including polyphasic fluorescence rise during the first seconds of chlorophyll a illumination (OJIP), were analyzed to find relationships between the photosynthetic/respiratory activity of the cells, occurrence of oxidative stress and trophic conditions applied to PSs-deficient algae. Despite the highest superoxide dismutase activity, indicating the presence of oxidative stress, mixotrophic conditions appeared to be optimal for S. obliquus WT and mutant strains kept in non-aerated cultures. OJIP analysis indicated that in mutant 56.80 part of photosystem (PS) I was functional and in mutant 57.80 residual PS II activity was found.