Enhancement of Co-Production of Astaxanthin and Total Fatty Acids in Haematococcus lacustris by Combined Treatment with Exogenous Indole-3-Acetic Acid and Abscisic Acid

Phytohormones play pivotal roles in regulating metabolic processes in microalgae. This study investigates the individual and combined effects of indole-3-acetic acid (IAA; 0.1–100 µM) and abscisic acid (ABA; 1–100 µM) on cell morphology, growth, astaxanthin biosynthesis, and fatty acid production in Haematococcus lacustris during 30 d photoautotrophic cultivation. IAA significantly enhanced cell density but did not alter intracellular astaxanthin levels, whereas ABA increased astaxanthin accumulation while reducing cell density. The optimal response was observed with co-application of 1 µM IAA and 50 µM ABA at the onset of cultivation, improving cell density (255.0 × 103 cells/mL), astaxanthin content (22.5 mg/g cell), and astaxanthin production (44.7 mg/L), corresponding to increases of 21%, 45%, and 122%, respectively, compared to untreated controls. Fatty acid content (mg/g cell) and volumetric production (mg/L) were increased by 24% and 90%, respectively. These results demonstrate that strategic, early-stage co-application of IAA and ABA can effectively enhance astaxanthin and lipid coproduction in H. lacustris, offering a promising approach for improving the efficiency of microalgal biorefineries.