Real time light intensity based carbon dioxide feeding for high cell-density microalgae cultivation and biodiesel production in a bubble column photobioreactor under outdoor natural sunlight

•A novel real time sunlight intensity based CO2 feeding strategy was developed.•High biomass of 6.8 and 9.0 g L−1 were achieved in two different outdoor conditions.•The strategy is more effective than known pH-control and biomass based CO2 feeding.•The biodiesel of 3.3 g L−1 and productivity of 120 g L−1 day−1 was achieved.•Specific light availability affects the lipid production and productivity. Outdoor high cell-density microalgae cultivation is highly challenging due to unavailability of appropriate CO2 feeding strategy under diurnal sunlight intensities. Hence, a novel real time light based CO2 feeding strategy was firstly developed under diurnal simulated sunlight (LED) to test on Chlorella sp. in a 10 L scale bubble column photobioreactor. The strategy yielded a biomass titer of 5.12 g L−1 under simulated sunlight, far higher than existing biomass-density and pH-control based CO2 feeding strategies. In outdoor culturing, the proposed feeding strategy yielded high biomass titers of 6.8 and 9.0 g L−1 in growth-phase of two-stage and single-stage lipid induction studies respectively with same biomass productivity of 0.8 g L−1 day−1. Subsequently, two-stage lipid induction strategy of 6.8 g L−1 titer yielded biodiesel productivity of 120 g L−1 day−1, whereas single-stage strategy of 9.0 g L−1 titer was unable to induce lipid. Moreover, specific light availability affects the lipid production.