The effect of hydrodynamic forces generated by air bubbles on cell growth of continuous culture of Synechocystis PCC 6803 was studied in a flat-panel photobioreactor. Keeping all relevant parameters constant enables the optimization of individual parameters, for which a continuous cultivation approach has significant advantages. Continuous culture of Synechocystis PCC 6803 was cultivated under different gas velocities from 0.022 m s(-1) up to 0.128 m s(-1). Based on direct determination of effective growth rate at constant cell densities, cell damage due to shear stress induced by the increasing gas velocity at the sparger was directly observed. A significant decrease of effective growth rate was observed at gas velocity of 0.085 m s(-1) generated at the gas flow rate of 200 ml min(-1), indicating cell damage by shear stress. Optimization of gas volume and the development of an effective aeration system corresponding to a given reactor setup is important to realize a reliable cell growth.