## 3 Results and Discussion  
### Growth and Morphology of H. pluvialis with rac-GR24  
Figure 2 shows the changes in the total cell number density, optical shape, and relative morphological distribution of _H. pluvialis_ during 30-d photosynthetic cultivation. To enhance cell growth and astaxanthin and fatty acid production in _H. pluvialis_, strigolactone analog _rac_-GR24 was initially added at 2.0-8.0 \(\upmu\)M. Overall, the cell number density of the algal cultures tended to increase almost proportionally with increasing _rac_-GR24 dosage in the tested range. For the highest _rac_-GR24 dosage (8.0 \(\upmu\)M), a considerably higher cell number density trend (\(\sim\)429.4 \(\times\) 10\({}^{3}\) cells/mL) was observed during the photosynthetic culture, which was approximately 25% higher than that observed in the _rac_-GR24-free control (\(\sim\)342.5 \(\times\) 10\({}^{3}\) cells/mL). Regardless of _rac_-GR24 addition, the cell number densities of all _H. pluvialis_ cultures increased almost linearly with time for the initial 12 days. However, as the nitrogen source was depleted, the growth rate of _H. pluvialis_ decreased, and all algal cultures soon reached stationary growth phases with astaxanthin induction [2]. It should be noted that the difference in cell number density observed at _rac_-GR24 concentrations of 2-8 \(\upmu\)M appeared to be much smaller than between them and zero (control). This implies that, although not investigated in this study, there may be a threshold value below 2 \(\upmu\)M. This could be an interesting topic for further investigation. The optical appearance of _H. pluvialis_ cells during the photosynthetic life cycle (Figure 2b) can be divided into five different cell types, such as red cyst cells, germinating cells, biflagellate cells, green palmella cells, and brownish-green/brown palmella cells, similar to previous observations [1,12]. When inoculated into fresh medium, red cyst cells started to germinate rapidly. The germinating cells formed sporangia and proliferated internal daughter cells. The cell membrane then broke to open, releasing motile biflagellate cells. With cell aging, the motile biflagellate cells lose their flagella and transform into vegetative non-motile green palmella. The palmella cells reproduced asexually with 2, 4, 8, 16, or 32 daughter cells. When exposed to adverse conditions, such as nutrient limitation and excessive light irradiation, astaxanthin biosynthesis is induced in the palmella cells as an adaptive defense event [1,21]. Astaxanthin accumulation was observed in the center and spread to the periphery, gradually changing the entire cell color from green to brownish-green and brown in turn. Eventually, they were transformed into a dormant cyst with a rigid cell wall structure that was rich in astaxanthin and exhibited a bright red color. Overall, the morphological distribution patterns of _H. pluvialis_ cell types during 30 d cultivation were similar regardless of _rac_-GR24 concentration (Figure 2c). However, it should be noted that the _rac_-GR24-treated groups (8.6 \(\pm\) 2.0%) showed slightly higher germination rates during the first two days than the control (7.9 \(\pm\) 0.8%). This might have contributed to the improvement in the cell number density of the former (Figure 2a). Interestingly, after 30 d cultivation, the cell size of _H. pluvialis_ showed a tendency to slightly increase as the _rac_-GR24 concentration increased from 0 to 8 uM (Figure 3). Strigolactone  
Figure 2: Time-course profiles of changes in the cell number density (**a**), optical shape (**b**), and relative morphological distribution of cell type (**c**) of _Haematococcus pluvialis_ during 30 d photosynthetic cultivation under different _rac_-GR24 concentrations (2–8 uM). _H. pluvialis_ cells were classified into 5 distinct types (red cyst, germinating cell, biflagellate cell, green palmella, and brownish-green/brown palmella) and counted using an improved Neubauer counting chamber under an optical microscope.