## 3 Results and Discussion
### Astaxanthin and Fatty Acid Production  
Figure 5 shows the cellular content and volumetric production of astaxanthin and total fatty acids in 30-d-grown _H. pluvialis_ cells at different doses of _rac_-GR24 (2.0-8.0 \(\upmu\)M). Contrary to the results of the cell growth promotion (Figure 2a), the astaxanthin contents of the _rac_-GR24-treated cells (60.1-65.0 pg/cell) were slightly lower than that of the control (73.3 pg/cell). This indicates that _rac_-GR24 does not promote astaxanthin biosynthesis in _H. pluvialis_ cells under the present photoautotrophic culture conditions. Cell growth rate and astaxanthin accumulation in _H. pluvialis_ generally show opposite trends under photosynthetic conditions [2; 28]. However, it should be noted that at the highest dosage of 8 \(\upmu\)M _rac_-GR24, the volumetric astaxanthin production (26.1 \(\pm\) 1.7 mg/L) considerably improved by 21% compared to the control (21.6 \(\pm\) 1.5 mg/L) due to the improvement of cell number density. Oxidative stress, such as nutrient depletion, electric stress, and strong light irradiation, induces the generation of toxic ROS, which severely damages the cell membrane, DNA, and proteins of algal cells. In terms of cellular defense mechanisms, ROS also serves as an important signal that triggers the metabolic synthesis of various antioxidant biomolecules, such as astaxanthin [29]. This suggests that _rac_-GR24 may modulate the cellular antioxidant system to eliminate excess ROS while simultaneously interfering with the astaxanthin-induced event. However, [PERSON] et al. [30] reported an enhancement of astaxanthin production in _H. pluvialis_ 192.80 by combining an organic carbon substrate acetate with another phytohormone, salicylic acid, under photoheterotrophic conditions. [PERSON] et al. [31] improved astaxanthin content by 2.36-fold in _H. pluvialis_ LUGU using 10 \(\upmu\)M melatonin (\(N\)-acetyl-5-methoxyryptamine) under photoautotrophic conditions. The effects of phytohormonal types and nutritional modes on _H. pluvialis_ may be subject to further investigation. Figure 4: Time course profile of chlorophyll \(a\) and \(b\) concentration during the 30-d photosynthetic cultivation of _H. pluvialis_ under different _rac_-GR24 concentrations (2.0–8.0 \(\upmu\)M). Interestingly, the effect of _rac_-GR24 on the accumulation of total fatty acids in _H. pluvialis_ cells was different from that of astaxanthin (Figure 5). At the lowest dosage of 2 uM _rac_-GR24, the fatty acid content of _H. pluvialis_ was lower as 628.2 pg/cell compared to the untreated control (762.5 pg/cell), similar to that of astaxanthin. However, at concentrations above 2 uM, with increasing _rac_-GR24 dose, the algal fatty acid content showed an increasing trend. Notably, the highest dose of 8 uM _rac_-GR24 resulted in a considerable increase in the total fatty acid content (899.8 \(\pm\) 119.5 pg/cell) than the control (762.5 \(\pm\) 108.5 pg/cell). Furthermore, due to the enhanced cell number density (Figure 2), the volumetric total fatty acid production increased almost proportionally with increasing the _rac_-GR24 dose in the range of 2-8 uM, obtaining a 1.6-fold improvement (361.6 \(\pm\) 48.0 mg/L) at 8 uM compared to the untreated control (224.5 \(\pm\) 31.9 mg/L). This suggests that _rac_-GR24 can enhance lipid production in the photosynthetic _H. pluvialis_ culture. Enhancement of algal lipid biosynthesis using several phytochemicals has been reported. [PERSON] et al. [14] observed that 1 uM _rac_-GR24 enhanced the lipid productivity of \(M\). sp. QLY-1 by 54.6% with CaCl\({}_{2}\) supplementation compared to the untreated control. Endogenous NO and Ca\({}^{2+}\) levels were triggered by adding _rac_-GR24. Chemical genetic data demonstrated that Ca\({}^{2+}\) accumulation induced by _rac_-GR24 stimulates lipid accumulation by regulating the transcription levels of lipid biosynthesis-related genes and NO signaling. They also reported a 1.3-fold improvement in lipid productivity with the same _rac_-GR24 dosage under nitrogen-deficient conditions [15].