Individual and combined effects of three phytohormones on lipid productivity and biodiesel quality in Nannochloropsis gaditana under nitrogen starvation

Abstract

To address the biomass-lipid trade-offs in microalgal biodiesel production, this study investigated biodiesel production using Nannochloropsis gaditana ffrc-1 by optimizing fulvic acid (FA), jasmonic acid (JA) and salicylic acid (SA) under nitrogen starvation, along with mechanistic insights into lipid biosynthesis. Using 20 mg/L FA under nitrogen starvation, the lipid content, biomass productivity and lipid productivity peaked at 30.50%, 0.48 g/L/day and 0.1450 g/L/day. With 20 mg/L JA under nitrogen starvation, the lipid content, biomass productivity and lipid productivity peaked at 31.50%, 0.74 g/L/day and 0.2335 g/L/day. Under nitrogen starvation with 2 mg/L SA, the lipid content, biomass productivity and lipid productivity peaked at 30.20%, 0.84 g/L/day and 0.2540 g/L/day. The combination of 22 mg/L FA, 17 mg/L JA and 2.4 mg/L SA by response surface methodology increased lipid productivity to 0.24 g/L/day under nitrogen starvation. The mRNA expression of lipid synthesis enzymes (acetyl-CoA carboxylase beta and diacylglycerol acyltransferase) was consistent with the trend of lipid accumulation. Moreover, the fatty acid profile and key biodiesel properties of N. gaditana ffrc-1 under nitrogen starvation with the supplementation of these phytohormones complied with international standards with improved cold flow performance. This indicated its promise as a sustainable biodiesel feedstock for cold-weather operations. In summary, this study proves that coordinated FA, JA, and SA application under nitrogen starvation effectively breaks the growth-lipid trade-off in N. gaditana, delivering both high lipid productivity and superior biodiesel cold-flow properties.