Use of plant growth regulators and their combined application to enhance lipid productivity in the filamentous microalga Tribonema sp.

Oleaginous Tribonema sp. has shown great potential in biodiesel production. However, effective strategies are still needed to increase overall lipid productivity for cost-effectiveness. Although the effects of plant growth regulators (PGRs) on enhancing microalgal growth and lipid content have been widely demonstrated, the synergistic effects of their combinations on improving lipid productivity in Tribonema sp. remain unclear. In this study, we investigated the comparative effects of five PGRs (e.g., fulvic acid, melatonin, gibberellin, γ-aminobutyric acid [GABA], and indoleacetic-3-acid [IAA]) at different concentrations (0.1–10 mg/L) on the growth, nitrogen uptake, and lipid content of Tribonema sp. The results showed that 10 mg/L GABA and 0.1 mg/L IAA were optimal conditions, contributing to an approximately 33.00 % increase in lipid productivity compared with the control (without PGRs). Based on these findings, the combination of optimized concentrations of GABA and IAA, especially in equal proportions, resulted in a more desirable fatty acid profile and produced the highest biomass, lipid content, and lipid productivity, with increases of 23.55 %, 25.62 %, and 54.95 %, respectively, over the control. This combination also significantly increased nitrogen removal efficiency in the medium, which was 46.24 % higher that of the control. These results were attributed to the significant upregulation of genes associated with nitrogen metabolism mediated by the synergy of IAA and GABA, promoting nitrate uptake by algal cells in the medium. Genes involved in photosynthesis and lipid synthesis were also upregulated, facilitating simultaneous biomass and lipid accumulation to maximize lipid productivity. Overall, this study is the first to report the combined application of GABA and IAA as an effective strategy to enhance lipid productivity in Tribonema sp., providing insights into the underlying mechanisms. These findings offer a promising solution for economically improving microalgal lipid production.