Influence of mixing and shear stress on Chlorella vulgaris, Scenedesmus obliquus, and Chlamydomonas reinhardtii

Photosynthetic activity (PA) and growth of different microalgae species (Chlorella vulgaris, Scenedesmus obliquus, and Chlamydomonas reinhardtii) depends in addition to other factors on mixing (tip speed) and shear stress (friction velocity) and was studied in a stirring tank (microcosm). In order to detect cause–effect relationships for an increase in photosynthetic activity, experiments were conducted under different pH values (6.0–8.5) and CO₂ concentrations (0.038 and 4 % (v/v)). The PA was determined as the effective quantum yield by pulse amplitude modulation during a stepwise increase of the tip speed from 0 to 589 cm s^?1 (friction velocity: 0–6.05 cm s^?1) in short-term experiments. The increase caused a distinctive pattern of PA of each species. Compared to 0 cm s^?1, C. vulgaris and S. obliquus showed a 4.0 and 4.8 % higher PA at the optimum tip speed of 126 cm s^?1 (friction velocity of 2.09 cm s^?1) and a 48 and 71 % higher growth, respectively. At 203 cm s^?1, the PA dropped to the value of the unstirred control, while at 589 cm s^?1, the PA decreased of up to 7 and 8 %. In contrast, C. reinhardtii showed 7 % stronger growth at 126 cm s^?1, while the PA decreased about 15 % at an increase of tip speed to 589 cm s^?1. For all investigated microalgae, the pattern of PA and higher growth was not only explained by the main contributing factors like light supply, nutrient supply, and overcoming diffusion gradients. The results indicate that hydrodynamic forces have a stimulating effect on the physiological processes within the cells.