Impact of surplus P with N and S modulation for enhanced microalgal lipid productivity in closed reactor system

Microalgae growth and lipid production via two-stage cultivation is challenging on a large scale due to the high energy demand for transferring biomass from the first to the second stage. To address this, the present study implements a continuous two-stage (CTS) cultivation strategy using Chlorella sp. BRE5 to enhance lipid productivity without compromising biomass growth. Shake flask experiments were initially conducted to evaluate the effects of varying phosphorus (P) doses (100 %, 250 %, 500 %, 750 %, and 1000 %) under four sets of nitrogen (N) and sulphur (S) supplementation conditions during the second stage of CTS cultivation: (i) N0-S0, (ii) N0-S100, (iii) N100-S0, and (iv) N100-S100. Results from these experiments guided a comparative photobioreactor (PBR) study, which revealed that the highest biomass productivity of 300.6 mg L^-1 d^-1 was achieved under PBR (N0-S100-P250) conditions, i.e., N deficiency with S (100 %) re-supplementation and surplus P (250 %) on the 8th day of the second stage of CTS. Under these conditions, lipid productivity was 2.74 times higher than that of the PBR (Control). The fatty acid profile composed of C16/C18, showing more than 88 % of the total fatty acids, and the estimated biodiesel properties meet international standards (EN-14214 and ASTM D-6751). These findings highlight that luxury P supply, when applied in the second stage of CTS cultivation, promoted lipid productivity in Chlorella sp. BRE5 offers a promising approach for enhanced biodiesel production.