This study evaluated growth and lipid productivity of Nannochloropsis sauna in bubble columns sparged with ambient air. Experiments were conducted in batch mode at gas-to-culture volume ratios of 0.08-0.28 min(-1), under constant light input. The net energy from the process, considering the energy equivalence of the lipid produced and the energy input for sparging, is proposed as the basis to optimize the sparging rate. Based on this energetic measure, the optimal gas-to-culture volume was found to be 0.18 min(-1). Under this optimal condition, the specific growth rate was 0.224 d(-1); the gravimetric lipid content was 59.7%; and the carbon dioxide utilization efficiency was 57%. Considering the temporal profiles of biomass growth and pH, it is concluded that sparging with ambient air could limit biomass growth due to limited carbon availability, supporting the case for sparging with carbon dioxide-enriched air. (C) 2012 Elsevier Ltd. All rights reserved.