In view of an efficient industrial autotrophic cultivation of microalgal biomass, the role of light into photobioreactor is still to be fully ascertained. The influence of different light intensities was investigated using both continuous illumination and alternation of light and dark cycles with different frequencies, which mimic illumination variations in a photobioreactor due to mixing. In order to minimize the effect of cell self- shading all experiments were carried out in thin layer flat plate reactor (which can be operated both in batch and continuous mode), where microalgae were cultivated under a non-limiting CO2 supply, with CO2- enriched air bubbled through the culture. In this apparatus, we measured growth rate, lipid content and photosynthetic performances of Scenedesmus obliquus. The species showed a maximum growth rate at about 150 µmol of photons m-2 s-1. Above this value the growth was inhibited, while the algae was found still able to exploit light, even if at lower efficiency, and showed a decreased pigment content. The resistance of the species to high irradiances was confirmed also in continuous experiments, where microalgae were found able to adapt to high illumination, leading to an increased steady state concentration and higher productivity at 350 µmol m-2 s-1 (about 3 g L-1 d-1). In addition, by changing the residence time, a maximum in productivity was found. The lipid content, constitutively high, was not affected by the variation light intensity, making this species suitable to industrial application.
Furthermore, the alternation of light and dark cycles with different frequencies was studied both in batch and continuous experiments. Cultures exposed to pulsed light shows a drastically reduced growth compared to continuous light.