Dinoflagellates have proven extremely difficult to culture because they are inhibited by low-level shear forces. Specific growth rate of the toxic dinoflagellate Protoceratium, reticulatum was greatly decreased compared with static control culture by intermittent exposure to a turbulent hydrodynamic environment with a bulk average shear rate that was as low as 0.3 s(-1). Hydrodynamic forces appeared to induce the production of reactive oxygen species (ROS) within the cells and this caused peroxidation of cellular lipids and ultimately cell damage. Exposure to damaging levels of shear rate correlated with the elevated level of lipoperoxides in the cells, but ROS levels measured directly by flow cytometry did not correlate with shear induced cell damage. This was apparently because the measured level of ROS could not distinguish between the ROS that are normally generated by photosynthesis and the additional ROS produced as a consequence of hydrodynamic shearforces. Continuously subjecting the cells to a bulk average shear rate value of about 0.3 s(-1) for 24-h caused an elevation in the levels of chlorophyll a, peridinin and dinoxanthin, as the cells apparently attempted to counter the damaging effects of shear fields by producing pigments that are potential antioxidants. In static culture, limitation of carbon dioxide produced a small but measureable increase in ROS. The addition of ascorbic acid (0.1 mM) to the culture medium resulted in a significant protective effect on lipid peroxidation, allowing cells to grow under damaging levels of shear rates. This confirmed the use of antioxidant additives as an efficient strategy to counter the damaging effects of turbulence in photobioreactors where shear sensitive dinoflagellates are cultivated. (C) 2009 American Institute of Chemical Engineers Biotechnol. Prog., 25: 792-800, 2009