This paper evaluates the effect of the irradiance, pH and temperature on the photosynthetic activity (PA) of Scenedesmus obtusiusculus under N-replete and N-deplete conditions through oxygen measurements. The highest PA values were 160 mg(O2) g(b) (1) h (1) at 620 mu mol m (2) s (1), 35 degrees C and pH of 8 under N-replete conditions and 3.3 mg(O2) g(b) (1) h (1) at 100 mu mol m (2) s (1), 28.5 degrees C and pH of 5.5 for N-deplete conditions. Those operation conditions were tested in a flat-panel photobioreactor. The biomass productivity was 0.97 g(b) L (1) d (1) under N-replete conditions with a photosynthetic efficiency (PE) of 4.4% yielding 0.85 g(b) mol(photon)(1). Similar biomass productivity was obtained under N-deplete condition; and the lipid productivity was 0.34 g(L) L (1) d (1) with a PE of 7.8% yielding 0.39 g(L) mol(photon)(1). The apparent activation and deactivation energies were 16.1 and 30 kcal mol (1), and 11.9 and 15.3 kcal mol (1), for N-replete and N-deplete conditions, respectively. (C) 2015 Elsevier Ltd. All rights reserved.