An approach based on energy gain was utilized to optimize algal cultivation in bubble columns. Net energy gain was estimated considering the energy input for mixing and providing carbon dioxide, and the energy that can be generated from the lipids extracted from the algal biomass. Energy input for sparging was minimized based on the gas-to-culture volume ratio and energy output from lipid production was maximized based on nitrate and CO2 levels. Sparging at a gas-to-culture volume ratio of 0.18 min(-1) with CO2-enrichment of 0.5% and initial nitrate concentration of 1 mM was optimal for improving net energy gain with Nannochloropsis sauna. Sparging with CO2-enriched air of 0.5% along with nitrogen starvation resulted in 50% more lipid productivity than sparging with ambient air. (C) 2012 Elsevier Ltd. All rights reserved.