## 1 Introduction  
Astaxanthin (3,3\({}^{\prime}\)-dihydroxy-\(\beta\)-carotene-4,4\({}^{\prime}\)-dione) is a type of red-orange carotenoid with strong biological antioxidant capacity [1] and important applications in human health and in the nutraceuticalital, cosmetics, food, and feed industries [2]. During the process of astaxanthin synthesis, isopentenyl diphosphate (IPP) reacts with dimethylallyl diphosphate (DMAPP) to form geranyl pyrophosphate (GPP), which joins another molecule of IPP to form farnesyl pyrophosphate (FPP). FPP joins IPP to create geranylgeranyl pyrophosphate (GGPP). Two molecules of GGPP are connected to generate phytoene, which undergoes multiple dehydrogenation reactions to produce lycopene. Then, lycopene undergoes two reactions to produce \(\beta\)-carotene and ultimately to generate astaxanthin [3]. Compared with other species of microalgae, _Haematococcus pluvialis_ has shown the highest accumulation of astaxanthin--up to 4% of dry cell weight (DCW) [4]. In the production of astaxanthin, _H. pluvialis_ goes through two developmental stages, including the dividing stage (morphologically green, producing macrozooids or zoospores and microzooids) and the astaxanthin accumulating stage (morphologically red, producing palmella and hematocysts or aplanospores). In the astaxanthin accumulating stage, the green dividing cells of _H. pluvialis_ are transformed into palmella, which further develop into red aplanospores, while cells of red aplanospores continue to produce astaxanthin [5]. As a type of secondary metabolite, astaxanthin is mainly accumulated once the cell divisions in the microalgae largely terminate [6]. When the cells of _H. pluvialis_ stop dividing, both the chloroplast size and the chlorophyll content are reduced. Astaxanthin is generally deposited in cytoplasmic lipid droplets (CLD) of _H. pluvialis_[7]. The accumulation of astaxanthin could be caused by various types of environmental stresses, including high light intensities, low nitrogen, salt stress, and high temperature [8]. Due to its less heat dissipation with long life-expectancy and small size, the light-emitting diode (LED) has become an important factor in the microalgal industry [9]. Blue, green, and red lights are important for the growth and development of microalgae [10]. For example, blue light increases the cell size of algae, lipid accumulation, and growing period in _Chlamydomonas reinhardtii_[11; 12], while green light of high intensities could significantly promote photosynthesis [13], and red light increases growth rates in _Dunaliella salina_ and _Spirulina_[14]; meanwhile, a red LED at a relatively low intensity enhances cell growth in _H. pluvialis_[15]. Studies have shown that different light qualities increase the production of astaxanthin, biomass, and fatty acid content in _H. pluvialis_[16; 17], while high-intensity blue light increases the accumulation of astaxanthin in _H. pluvialis_[18]. Carotenoids in microalgae primarily function as the accessory pigments in light-harvesting antenna by absorbing blue light that is inadequately absorbed by chlorophyll, thus improving light absorption. Furthermore, astaxanthin has been shown to help prevent damage to the photosynthesis system in algae [7]. Studies have shown that addition of chemical regulators enhances the yield of microalgal biomass and biological products [19]. As a type of important plant hormone and signal molecule, salicylic acid (SA) shows critical functions in plant growth, immunity, and development, which have been extensively investigated [20]. Specifically, SA is involved in the molecular regulation of metabolic response to both abiotic stresses, i.e., heat, cold, drought, heavy metal, salt stresses [21; 22; 23], and biotic stresses, e.g., a plant's defense against pathogens [24]. Our previous studies on SA inductions at the transcriptional level identified a group of plant-specific transcription factor families (e.g., MYB, AP2/ERF, WRKY, and HSF) in _H. pluvialis_ in response to the treatment with SA [25; 26].