## 1 Introduction  
Microalgae have long been utilized as industrial microorganisms to produce high-value nutraceutical biocompounds, such as astaxanthin and polyunsaturated fatty acids (PUFAs) [1]. Recently, astaxanthin (3,3-dihydroxy-\(\beta\)-carotene-4,4-dione, Figure 1a), a secondary ketoconetenoid pigment, has received considerable attention in various bioindustries, including food supplements, cosmetics, and aquaculture [2]. The antioxidant activity of astaxanthin is significantly higher than that of \(\beta\)-carotene and vitamin E. This pigment has various therapeutic effects, such as immunomododulation, anti-aging activity, anti-cancer potential, and cardiovascular protection [3].  
_Haematococcus pluvialis_, a freshwater unicellular green microalga, is considered one of the best sources of astaxanthin (\(\sim\)5% of dry weight) [1, 4]. Algal astaxanthin is usually synthesized as a self-defense mechanism to protect the organism from oxidative stress. In _H. pluvialis_, astaxanthin biosynthesis is induced during the transformation of vegetative cells into dormant cysts under unfavorable culture conditions [5]. Astaxanthin biomolecules accumulate inside neutral lipid bodies forming ester bonds with long-chain fatty acids, enabled by closely interconnected metabolic networks of lipogenesis and carotenogenesis [6]. To improve the astaxanthin productivity of photosynthetic _H. pluvialis_ cultures, various stress-inducing strategies, such as nitrogen starvation [7], strong light irradiance [8], high salinity [9], high temperature [10], mechanical stress [11], electrical treatment [12],and aminoclay nanoparticle addition [2] have been applied. However, these approaches are time-consuming, require high energy input and large amounts of chemicals; additionally, they may substantially inhibit cell growth. Therefore, it is necessary to develop an efficient astaxanthin production method applying environmentally friendly large-scale biorefinement of _H. pluvialis_.  
Strigolactone, a carotenoid-derived phytohormone, functions as an effective chemical messenger that regulates cellular activity in plants in response to nutrient availability and environmental changes [13]. Strigolactone also plays an important role in shoot branching, leaf senescence, and symbiosis with microorganisms [14]. Recently, [PERSON] et al. [13] have reported that the addition of strigolactone analog _rac_-GR24 (0.001-1 uM) (see Figure 1b for the chemical structure) enhances the photosynthetic growth rate of _Chlorella vulgaris_ FACHB-8 by increasing chlorophyll \(a\) content compared to the control. Interestingly, _rac_-GR24 (0.2-5 uM) also promotes lipid accumulation in _Monoraphidium_ sp. QLY-1 when CaCl\({}_{2}\) supplementation [14] or nitrogen deficiency [15] strategies are combined. These results suggest that, although cell growth and lipid biosynthesis are generally inversely related, _rac_-GR24 may help improve overall lipid productivity in other algal species if applied carefully.  
Notably, astaxanthin and lipid biosynthetic pathways in _H. pluvialis_ are closely linked [1,4]. In addition, strigolactone is expected to enhance the germination rate of dormant cysts of _H. pluvialis_, similar to that observed in plant seeds [16]. In this study, we aimed to investigate the technical feasibility of using _rac_-GR24 to improve astaxanthin and fatty acid production in photosynthetic _H. pluvialis_ cultures.