Cultivation of an indigenous Chlorella sorokiniana with phytohormones for biomass and lipid production under N-limitation

Nitrogen starvation promotes lipid accumulation in microalgal cells, but leads to low biomass production which directly influences the lipid yield. Growth stimulators may activate simultaneous increase in biomass and metabolite productivity under nutrition stress conditions. In this work, the dual effect of phytohormone (indole-3-acetic acid (IAA), diethyl aminoethyl hexonate (DA-6)) supplementation and nitrogen limitation conditions on the biomass and lipid production of an indigenous microalga Chlorella sorokiniana DPK-5 was thoroughly investigated by monitoring the fatty acid compositions, oxidative stress alleviation and anti-oxidative defense system, including gene expression level. The optimized concertation of IAA (10− 5 M) or DA-6 (10− 9 M) were applied under a series of N limitation in range 100% to 12.5% to promote the both biomass and lipid production of DPK-5. Interestingly, very minimal dose of IAA and DA-6 under 50% N concentration produced the highest biomass productivity of 201 mg/L/d (22% increase) and 236 mg/L/d (43% increase) from the control (166 mg/L/d), leading to outstanding lipid productivity of 69 mg/L/d (49% increase) and 86 mg/L/d (84% increase) from the control (32 mg/L/d). In addition, abundance of saturated fatty acid (SFA) and monounsaturated fatty acid (MUFA) as a measurement of biodiesel quality were also enhanced. Furthermore, it was proven that treatment with IAA and DA-6 induced the activity of antioxidants enzymes, which protect cells from damages caused by abiotic stresses. In addition, the intracellular levels of RuBisCO and ACCase were considerably up-regulated by IAA and DA-6 under N limitation. These findings indicate that hormone addition-based autotrophic-N limitation is a promising cultivation strategy to improve the lipid production potential of microalgae without substantial loss in biomass for generation of high quality biodiesel.