## 1 Introduction  
Carotenoids, a kind of natural isoprenoid pigments, are distinguished for various biological functions and commercial applications as colorants, antioxidants, pharmaceuticals and matraceutical agents [1; 2; 3; 4]. However, the rising commercial demand for carotenoids has not been satisfied due to the high cost of natural extraction and mixed stereoisomers of chemical synthesis [5]. Microbially heterologous biosynthesis has exhibited great potentials for efficient carotenoid production [6; 7; 8; 9]. As a typical carotenoid and precursor of vitamin A, \(\beta\)-carotene is also in great demand. The availability of novel key genes and the elucidation of a synthetic pathway facilitate production improvements of \(\beta\)-carotene. The biosynthesis of \(\beta\)-carotene is derived from the condensation of a universal five-carbon (C5) isoprene unit. The basic C5-isoprene building block is isopentenyl pyrophosphate (IPP; C5), which is synthesized through the mevalonate (MVA) pathway in eukaryotes or the methylerythintol (MEP) pathway in plastids and prokaryotes [10; 11; 12; 13]. The IPP is reversibly converted into its isomer, dimethylallyl pyrophosphate (DMAPP), which is catalyzed by the isoptenenyl pyrophosphate isomerase (IDI). Subsequently, the IPP condensed with DMAPP results in geranyl pyrophosphate (GPP; C10). The continuous addition of the IPP unit generates farnesyl pyrophosphate (FPP; C15) and geranylgeranyl pyrophosphate (GGPP; C20) catalyzed by the geranylgeranyl pyrophosphate synthase (GGPPS/CrtE) [14; 15; 16; 17]. The GGPP is further recruited as the direct precursor for the synthesis of carotenoids (tetraterpene, 40C), including phytoene, lycopene and \(\beta\)-carotene, in turn, by using the corresponding carotenogenic enzymes [18; 19; 20]. As for the de novo \(\beta\)-carotene biosynthesis pathway, we divided it into the precursor module and the product module bounded with GGPP. Due to the important role of isoprenoid precursors (IPP, DMAPP, GPP, FPP and GGPP) for carotenoid synthesis, genes in the precursor module have attracted increasing attentions [21; 22; 23]. Among them, IDI catalyzes the reversible conversion of IPP into DMAPP, guiding the carbon flow to the synthesis of GPP, FPP and GGPP. GGPPS is responsible for GGPP synthesis, channeling the FPP flow towards carotenoids (40C) instead of sesquiterpenes (15C) and triterpenes (30C). Thereby, IDI and GGPPS define the availability of IPP, DMAPP and GGPP, functioning as the key regulatory nodes by directing metabolic flux to carotenoid biosynthesis. The two key precursor synthases have been derived from different species [24; 25; 26; 27; 28; 29], and it has proved to be an effective strategy to strengthen DMAPP and GGPP supply through overexpression of _IDI_ or/and _GGPPS_, diverting the metabolite flow towards carotenoid biosynthesis [8; 29; 30; 31]. As heterotrophic fungus-like protists, thraustochytrids, including _Aurantiochytrium_, _Schizochytrium_ and other genera, accumulated high levels of docosahexaenoicenoic acid (DHA) [32; 33; 34; 35]. Therefore, for its beneficial effects on human health, the microorganic source of DHA was developed commercially in food and pharmaceutical manufacturing [36; 37; 38]. In thraustochytrids, _Aurantiochytrium_ and _Schizochytrium_ were remarkable and attracted increasing attention due to their superior properties for DHA production [39; 40; 41]. Interestingly, thraustochytrids were also found to accumulate certain types of carotenoids [42; 43; 44; 45]. High concentrations of butanol, methanol and ethanol were observed to remarkably strengthen the accumulation of carotenoids, especially astaxanthin [46; 47; 48]. These physiological phenomena indicated that a whole biological pathway for the de novo synthesis of carotenoids existed in _Aurantiochytrium_ and _Schizochytrium_. Based on the availability of genomic and transcriptome data, researches of gene mining for carotenoid synthesis have been performed. The results were not satisfactory as no impressive carotenoid yields were found.