Optimization of continuous TAG production by Nannochloropsis gaditana in solar-nitrogen-limited culture

Abstract

Nitrogen limitation and changing solar conditions are both known to affect triacylglycerol (TAG) production in microalgae. This study investigates the optimization of TAG production with a continuous nitrogen-limited culture of Nannochloropsis gaditana in simulated day-night cycles (DNc). The effect of DNc was first investigated in nitrogen-deprived condition (i.e., batch culture), emphasizing a significant change in mechanical resistance of the strain during the night. The concept of released TAG, which shows how much of the TAG produced is actually recovered in the downstream stages, that is, after cell disruption, was shown here of interest. For a maximum released TAG, the optimum harvesting time was suggested as being 4 h into the night period, which minimizes the losses due to a too great cell mechanical resistance. The protocol for continuous nitrogen-limited culture was then optimized, and a continuous nitrogen addition was compared to a pulsed-addition. For the latter, nitrogen was supplied in a single pulse at the beginning of the light periods, while the bulk medium was supplied separately at a slow but constant dilution rate of $0.005h^{-1}$ . The pulse dose was calculated after the study of nitrogen consumption and TAG production/consumption during the DNc. The estimated released TAG for the pulsed-addition of 1.4 × 10^-3 kg/m² d found significantly higher than the one achieved in batch culture (0.3 × 10^-3 kg/m² d) but lower than for continuous nitrogen addition which obtained the highest released TAG of $3\times 10^{-3}\text{kg}∕m^{2}d$ .

Keywords: biodiesel; cells mechanical resistance; day-night cycles; lipids; nitrogen limitation.