Physiological and transcriptomic analyses reveal the effects of phytohormones on growth and astaxanthin accumulation in Haematococcus pluvialis

Haematococcus pluvialis is recognized as the richest natural source of astaxanthin, a high-value ketocarotenoid with potent antioxidant properties. Commercial production typically relies on a two-stage cultivation strategy, comprising a green vegetative phase for biomass accumulation and a red inductive phase for astaxanthin synthesis. However, a major bottleneck in this process is the balance between cell growth and pigment accumulation, as the stress conditions required for astaxanthin synthesis often inhibit biomass productivity. Phytohormones are critical signaling molecules that regulate growth and metabolism in photosynthetic organisms. Exploring their potential to decouple these conflicting physiological processes is crucial for optimizing industrial astaxanthin production. In this study, physiological and transcriptomic analyses were performed to evaluate the effects of five phytohormones (IAA, ABA, MeJA, ZT, and IPR) on H. pluvialis during its two-stage cultivation. Physiological data indicated distinct hormonal requirements for each stage: 0.01 mg L⁻¹ IAA significantly enhanced biomass in the green stage (0.37 g L⁻¹ vs. 0.29 g L⁻¹ in control), whereas 0.1 mg L⁻¹ MeJA maximized astaxanthin content in the red stage (29.22 mg g⁻¹ vs. 23.09 mg g⁻¹ in control). Transcriptomic profiling revealed that IAA treatment was associated with higher biomass and accompanied by increased expression of genes involved in primary carbon metabolism (PPDK), cell cycle progression (CDKB1-1), and nutrient uptake (ZIP9). Conversely, MeJA treatment was associated with a transcriptomic shift consistent with enhanced secondary metabolism, including upregulation of ketolase genes (BKT1 and BKT3) while downregulating competing carotenoid pathway genes (CRTZ and LCYE) and photosynthesis-related transcripts. Based on transcriptomic analysis, this study proposes hypotheses explaining how phytohormones regulate the balance between growth and secondary metabolism in H. pluvialis across different life stages. The findings suggest that the targeted application of IAA to boost biomass and MeJA to induce pigmentation offers a practical hormone guided strategy to improve two-stage astaxanthin production and provides molecular targets for further optimization.