The effect of light/dark cycles on the growth of the red microalga Porphyridium so. was studied in a tubular loop bioreactor with light/dark cycles of different frequencies and in two 35-L reactors : a bubble column reactor and an air-lift reactor. Photon flux densities were in the range of 50 to 300 mu E m(-2) s(-1), and flow rates were 1 to 10 L min(-1). Under conditions of low illumination and high flow rates, similar results were obtained for the bubble column and air-lift reactors. However, higher productivities-in terms of biomass and polysaccharide-were recorded in the air-lift reactor under high light intensity and low gas flow rates. The interactions of both photosynthesis and photoinhibition with the fluid dynamics in the bioreactors was taken as the main element that allowed us to interpret the differences in performance of the bubble column and the air-lift reactor. It is suggested that the cyclic distribution of dark periods in the air-lift reactor facilitates better recovery from the photoinhibition damage suffered by the cells.