Improving the production rate of high-value nutraceutical compounds, such as astaxanthin and polyunsaturated fatty acids (PUFAs), is important for the commercialization of Haematococcus pluvialis biorefineries. Here, the effects of a phytohormone, strigolactone analog rac-GR24, on cell growth and astaxanthin and fatty acid biosynthesis in H. pluvialis were investigated. Four concentrations (2, 4, 6, and 8 µM) of rac-GR24 were initially added during 30 days of photoautotrophic cultivation. The addition of rac-GR24 improved cell number density and chlorophyll concentration in H. pluvialis cultures compared to the control; the optimal concentration was 8 µM. Despite a slightly reduced astaxanthin content of 30-d-old cyst cells, the astaxanthin production (26.1 ± 1.7 mg/L) improved by 21% compared to the rac-GR24-free control (21.6 ± 1.5 mg/L), owing to improved biomass production. Notably, at the highest dosage of 8 µM rac-GR24, the total fatty acid content of the treated H. pluvialis cells (899.8 pg/cell) was higher than that of the untreated cells (762.5 pg/cell), resulting in a significant increase in the total fatty acid production (361.6 ± 48.0 mg/L; 61% improvement over the control). The ratio of PUFAs, such as linoleic (C18:2) and linolenic (C18:3) acids, among total fatty acids was high (41.5–44.6% w/w) regardless of the rac-GR24 dose.

Efficient induction of astaxanthin (AXT) biosynthesis remains a considerable challenge for the industrialization of the biorefinement of the microalga Haematococcus lacustris. In this study, we evaluated the technical feasibility of photosynthetic electrotreatment to enhance AXT accumulation in H. lacustris. The AXT content of H. lacustris electrotreated at an optimal current intensity (10 mA for 4 h) was 21.8% to 34.9% higher than that of the untreated control group, depending on the physiological state of the initial palmella cells. The contents of other carotenoids (i.e., canthaxanthin, zeaxanthin, and β-carotene) were also increased by this electrotreatment. However, when H. lacustris cells were exposed to more intense electric treatments, particularly at 20 and 30 mA, cell viability significantly decreased to 84.2% and 65.6%, respectively, with a concurrent reduction in the contents of both AXT and the three other carotenoids compared to those of the control group. The cumulative effect of electric stimulation is likely related to two opposing functions of reactive oxygen species, which facilitate AXT biosynthesis as signaling molecules while also causing cellular damage as oxidizing radicals. Collectively, our findings indicate that when adequately controlled, electric stimulation can be an effective and eco-friendly strategy for inducing targeted carotenoid pigments in photosynthetic microalgae.

The slow growth and complex life cycle of Haematococcus lacustris pose significant challenges for cost-effective astaxanthin production. This study explores the use of microfluidic collision treatment to stimulate the germination of dormant seed cysts, thereby improving photosynthetic cell growth and astaxanthin productivity in H. lacustris cultivated in well plate and flask cultures. The flow rate (1.0–3.0 mL/min) and the number of T-junction loops (3–30) were optimized in the microfluidic device. Under optimal conditions (a flow rate of 2.0 mL/min with 10 loops), the total cell number density in well plate cultures increased by 44.5% compared to untreated controls, reaching 28.9 ± 2.0 × 10⁴ cells/mL after 72 h. In flask cultures, treated cysts showed a 21% increase in astaxanthin productivity after 30 d, reaching 0.95 mg/L/d, due to higher biomass concentrations, while the astaxanthin content per cell remained constant. However, excessive physical collision stress at higher flow rates and loop numbers resulted in reduced cell viability and cell damage. These findings suggest that carefully controlled cyst mechanostimulation can be an effective and environmentally friendly strategy for Haematococcus biorefining, enabling the production of multiple bioactive products.

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.