Effect of oxygen concentration on the growth of Nannochloropsis sp. at low light intensity

In large-scale microalgal production in tubular photobioreactors, the build-up of O2 along the tubes is one of the major bottlenecks to obtain high productivities. Oxygen inhibits the growth, since it competes with carbon dioxide for the Rubisco enzyme involved in the CO2 fixation to generate biomass. The effect of oxygen on growth of Nannochloropsis sp. was experimentally determined in a fully controlled flat-panel photobioreactor operated in turbidostat mode using an incident photon flux density of 100 µmol photons m-2 s-1 and with only the oxygen concentration as variable parameter. The dissolved oxygen concentration was varied from 20 to 250% air saturation. Results showed that there was no clear effect of oxygen concentration on specific growth rate (mean of 0.48¿±¿0.40 day-1) upon increasing the oxygen concentration from 20% to 75% air saturation. Upon further increasing the oxygen concentration, however, a linear decrease in specific growth rate was observed, ranging from 0.48¿±¿0.40 day-1 at a dissolved oxygen concentration of 75% air saturation to 0.18¿±¿0.01 day-1 at 250% air saturation. In vitro data on isolated Rubisco were used to predict the quantum yield at different oxygen concentrations in the medium. The predicted decrease in quantum yield matches well with the observed decrease that was measured in vivo. These results indicate that the effect of oxygen on growth of Nannochloropsis sp. at low light intensity is only due to competitive inhibition of the Rubisco enzyme. At these sub-saturating light conditions, the presence of high concentrations of oxygen in the medium induced slightly higher carotenoid content, but the increased levels of this protective antioxidant did not diminish the growth-inhibiting effects of oxygen on the Rubisco