Unsteady state experiments in microalgal cultivation are important to evaluate the influence of the irradiance on the dynamic photosynthetic response and to develop dynamic models for the design of photobioreactors. In this paper, the short-term effect of incident light fluctuations on the oxygen production by the microalga Scenedesmus obtusiusculus cultivated in a 20 L air-lift photobioreactor was performed at different operation times under nitrogen-replete or nitrogen-starved conditions. It was possible to reach steady states in the oxygen production indicating short-term photosynthetic acclimation and the highest values during the incremental light step-changes were between 103 and 207 mg(o2) g(b)(-1) h(-1) for light intensities between 141 and 505 mu mol m(-2) s(-1) at 0.13 g(b) L-1. The photosynthetic response was not symmetric in the increase/decrease light step-changes due to temperature variations caused by the illumination system. Moreover, in nitrogen-starved conditions the dynamic photosynthetic response was slower than in nitrogen-replete levels declining to 70% at 0.5 g(b) L-1 at the maximum light intensity. Furthermore, a mathematical model was developed to estimate the dynamic oxygen response and the biomass productivity. The simulations predicted the highest O-2 concentrations at 35 degrees C and irradiances above 600 mu mol m(-2) s(-1) and the highest biomass productivity was 0.78 g(b) L-1 d(-1). (C) 2014 Elsevier B.V. All rights reserved.