Growth and biochemical characterization of microalgal biomass produced in bubble column and airlift photobioreactors: studies in fed-batch culture

Relatively large (0.19 m column diameter, 2 m tall, 0.06 m3 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 (∼4 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 μE 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 ∼13% 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.