The harmful effect of direct air sparging on Phaeodactylum tricornutum microalgal cultures was investigated in bubble columns and airlift photobioreactors with various superficial air velocities and two types of spargers, which generated different sizes of bubbles. Small bubbles bursting at the surface of the culture were apparently the main cause of cell damage in batch cultures in laboratory-scale bubble columns. Other mechanisms of cell damage also were a contributing factor to the observed cell loss in outdoor pilot-scale bubble columns. Supplementation of the microalgal culture medium with carboxymethyl cellulose at concentrations of 0.02% and greater is shown to protect the algal cells against aeration-induced hydrodynamic stress.