Synergistic effect of selenium and gibberellic acid for enhanced biomass, lipid and improved biodiesel quality from Tetradesmus obliquus through response surface methodology

[Display omitted] •Selenium15 µg/L supplementation for T. obliquus increased the lipid content.•The addition of 50 µM gibberellic acid increased T. obliquus biomass productivity.•Selenium and gibberellic acid synergistically raised lipid and biomass of T. obliquus.•Selenium and gibberellic acid enhanced ACC, FAD gene expression in T. obliquus.•Selenium and gibberellic acid extended T. obliquus biodiesel stability to 14.96 hrs. Biodiesel production from microalgae presents an innovative solution for renewable energy. This study investigates biodiesel production using Tetradesmus obliquus ON506010.1 by optimizing substrates, selenium and gibberellic acid. Using 15 µg/L selenium, lipid content and biomass productivity reached 35.45 %±0.92 and 0.178 g/L/day ± 0.051. With 50 µM gibberellic acid, biomass productivity and lipid content peaked at 0.785 ± 0.101 g/L/day and 38.95 %±0.35, surpassing the control. Fatty acid composition, biodiesel properties, and mRNA expression of lipid synthesis enzymes (acetyl CoA carboxylase (ACC) and fatty acid desaturase (FAD)) correlated. Combining 10 µg/L selenium with 75 µM gibberellic acid with response surface methodology (RSM) increased lipid content (42.80 % ±0.11) and biomass productivity (0.964 g/L/day ± 0.128). ACC and FAD upregulation validated this enhancement, with a 4.4-fold increase in FAD expression. Fatty acid composition and most biodiesel properties met international standards demonstrating Tetradesmus obliquus ON506010.1′s potential for sustainable biodiesel production with better cold flow property and oxidative stability.