## 1 Introduction  
Carotenoids are widely occurring pigments in nature, mainly found in higher plants, algae, cyanobacteria, fungi, and some non-photosynthetic bacteria [1]. Carotenoids belong to terpenoids whose structure consists of eight isoprenoids with a basic skeleton of C\({}_{40}\)H\({}_{56}\) and are fat-soluble pigments. Carotenoids are mainly divided into carotenes and lutein, and the structural composition of carotenes consists of only two atoms, carbon and hydrogen, whereas lutein consists of three atoms: carbon, hydrogen and oxygen (oxygen functional groups). \(\alpha\)-, \(\beta\)- and \(\gamma\)-Carotene and lycopene belong to carotenoids, whereas lutein, zeaxanthin and astaxanthin, which is a market hotspot, all belong to oxygenated carotenoids [2]. Among these carotenoids, astaxanthin is currently one of the most commercially valuable pigments of the carotenoid market due to its intense orange-red color and much higher antioxidant activity than that of other carotenoids and their derivatives. Astaxanthin is a ketocartenoid whose structure consists of four isoprenoids with hydrophilic hydroxyl-bearing ones at both ends of the \(\beta\)-viologen ring; the molecular formula is C\({}_{40}\)H\({}_{52}\)O\({}_{4}\)Previous studies have shown that astaxanthin has a higher antioxidant activity than \(\beta\)-carotenoid and \(\alpha\)-tocopherol do [3], while astaxanthin has also been shown clinically to have anticancer, anti-Alzheimer, vision preservation, and skin aging prevention functions. Studies on Astaxanthin absorption in humans have shown that astaxanthin as a dietary supplement also provides good effects to human health via neutralizing singlet oxygen, scavenging free radicals, and inhibiting lipid peroxidation, among other effects [4]. Carotenoids mainly constitute capture antenna complexes in higher plants and play an important role in photosynthesis. Animals (including fish, crustaceans and mammals) cannot synthesize carotenoids by themselves. For example, the human eye contains lutein and zeaxanthin, constituting the macula of the human retina that functions to protect vision [5]. Humans therefore need carotenoid supplementation from food. The workhorse 'products' of the current carotenoid market are \(\beta\)-Carotene and astaxanthin ([[https://www.fortunebusinessights.com/industry-reports/carotenoids-market-100180](https://www.fortunebusinessights.com/industry-reports/carotenoids-market-100180)]([https://www.fortunebusinessights.com/industry-reports/carotenoids-market-100180](https://www.fortunebusinessights.com/industry-reports/carotenoids-market-100180)), accessed on October 2020), which are mainly applied in the feed, health products, and cosmetic industries. The growing demand for production has created great opportunities for the market, and the continuous development of technologies for the microbial production of carotenoids in recent years has brought new opportunities to the expanding market. _Haematococcus pluvialis_ and _Pluffia rhodozyma_ are among the first two microorganisms to be applied to produce astaxanthin, with _Dunaliella_ is mainly used for the production of \(\beta\)-carotenes [6; 7; 8]. Microalgae, as one of the most potential biological factories at present, can produce large amounts of high-value metabolites. Through the heterotrophic cultivation of microalgae, the microalgal growth environment can be controlled in a closed environment, and a large amount of biomass is harvested after continuous culture at a high concentration [9]. The heterotrophic safety, controllability, short harvest time, and high yield of microalgae are currently well-established for commercial production. The use of microalgae for the heterotrophic production of carotenoids is one of the current hotspots due to the development of microalgal heterotrophic culture techniques [10]. For example, _Haematococcus pluvialis_ can obtain biomass containing 1.5-3% astaxanthin via heterotrophic cultivation in acetate [11].