Short‐term effect of the superficial gas velocity and light–dark cycles on the photosynthetic activity of the microalga <scp> <i>Scenedesmus obtusiusculus</i> AT </scp> ‐ <scp>UAM</scp>

Abstract BACKGROUND Dynamic operation strategies enable the analysis of short‐term physiological adaptations of microalgal cultures in photobioreactors. In this study, dynamic experiments were conducted to evaluate the O 2 production and CO 2 consumption rates of Scenedesmus obtusiusculus AT‐UAM at superficial gas velocities of 0.28, 0.51, and 0.75 m·min −1 in a 20‐L bubble‐column photobioreactor under continuous illumination. Furthermore, photosynthetic activity and biomass productivity were assessed under LED‐induced light–dark cycles of 5:5 min, 1:1 min, 30:30 s, 6:6 s and 1:1 s in a 2‐L flat‐panel photobioreactor. RESULTS The highest CO 2 consumed and O 2 production rates were 287 mg O2 g b −1 h −1 and 658 mg CO2 g b −1 h −1 , respectively, at a superficial gas velocity of 0.75 m min −1 and a biomass concentration of 0.4 g b L −1 . The increase in O 2 production rate was due to the combined effects of CO 2 and nutrient mass transfer, as well as light–dark cycles induced by changes in mixing conditions. Therefore, it was important to assess the independent contribution of light–dark cycles on photosynthetic activity. The highest biomass productivity was 1.1 g b L −1 d −1 with continuous illumination and decreased to 0.53 and 0.05 g b L −1 d −1 for light–dark cycles of 5:5 min and 1:1 s, respectively. CONCLUSIONS These results showed that short light–dark cycles do not lead to higher photosynthetic activity, because photosystems do not absorb sufficient light, and the microalga does not store enough energy to sustain growth at the same level as under continuous illumination. Therefore, the mixing‐induced light–dark cycles are not the main contribution to increment activity photosynthetic in large‐scale photobioreactors. © 2025 Society of Chemical Industry (SCI).