Relatively large (0.19 m column diameter, 2 m tall, 0.06 m(3) working volume) outdoor bubble column and airlift bioreactors (a split-cylinder and a draft-tube airlift device) were compared for monoseptic fed-batch culture of the microalga Phaeodactylum tricornutum. The three photobioreactors produced similar biomass versus time profiles and final biomass concentration (similar to4 kg m(-3)). The maximum specific growth rate observed within a daily illuminated period in the exponential growth phase, had a value of 0.08 h(-1) on the third day of culture. Because of night-time losses of biomass, the specific growth rate averaged over the 4-days of exponential phase was 0.021 h(-1) for the three reactors. The biomass in the vertical column reactors did not experience photoinhibition under conditions (photosynthetically active daily averaged irradiance value of 1150 +/- 52 muE m(-2) s(-1)) that are known to cause photoinhibition in conventional thin-tube horizontal loop reactors. Because of good gas-liquid mass transfer, the dissolved oxygen concentration in the reactors at peak photosynthesis remained < 120% of air saturation; thus, oxygen inhibition of photosynthesis and photo-oxidation of the biomass did not occur. Carbohydrate accumulation (up to similar to13% w/w) by the biomass was favored during light-limited linear growth. A declining light intensity caused a more than five-fold increase in cellular carotenoids but the chlorophylls increased only by about 2.5-fold during the course of the culture. In the stationary phase, up to 2% of the biomass was chlorophylls and carotenoids constituted up to 0.5% of the biomass dry weight.