## 1 Introduction  
Microalgae are highly efficient photosynthetic unicellular organisms that efficiently generate chemical energy from solar energy [1]. The mass cultivation of microalgae is highly valued for the bioproduction of carotenoids (e.g., \(\beta\)-carotene, astaxanthin, lutein, phycoerythrin, and phycocyanin), polysaccharides, enzymes, pigments, hydrocarbons, antibiotics, sterols (e.g., sitosterol, stigmasterol, 24-ethylcholesterol, campesterol, and 24-propyline-cholesterol), proteins (e.g., microcolin A, cyanovirin-N, and superoxide dismutase), polyunsaturated fatty acids (e.g., \(\gamma\)-Inelastic acid, docoshexaenoic acid, sitosterol, eicosquentaenoic acid, and stigmastrol), vitamins (e.g., A, C, and E), and biomass and sugars for biofuel production. Microalgal biomass is also used as a component of food supplements, animal fodder, and fertiliser [2; 3; 4]. Microalgal cultivation for various purposes has been reported in a number of countries, including the USA, China, Japan, Taiwan, India, Australia, Denmark, Canada, the Netherlands, France, Chile, and Israel [4; 5; 6; 7; 8; 9]. The commercial mass cultivation of microalgae began with the culture of _Chorella_ in the early 1960s in Japan. In the early 1970s, _Arthospira_ cultivation and harvesting systems were developed in Mexico. By 1980, 46 companies were involved in the production of microalgae (particularly _Chorella_) in Asia, where more than 1000 kg of microalgae was manufactured per month. In 1986, _Dunaliella salinu_ became the third major microalga to be industrially cultivated, with the introduction of mass production systems in Australia. Later, in the 1990s, Japan became the foremost Asian country to authorise the consumption of _Haematococcus-derived_ astaxanthin as a common food or food supplement. Following this, the use of astaxanthin as a food additive started in Europe in 1995 and in North America in the late 1990s. The market for astaxanthin as a food supplement grew and became available in Malaysia, Indonesia, and Thailand during 2004-2006, in Singapore in 2012, and in the Philippines in 2018 [10]. Several plants were established in the USA, India, and Australia to produce astaxanthin from _Haematococcus pluvialis_[11]. Commercially, astaxanthin is produced from _H. pluvialis_ by the following companies: Cyanotech Corporation (USA), AstaReal Inc. (USA), Fuji Chemical Industry (Japan), Beijing Ginko Group (China), Algatechnologies Ltd. (China), Parry's Pharmaceuticals (India), etc. [12]. The mass cultivation of microalgae has also focused on biofuel production [13]. The xanthophyll astaxanthin (3,3\({}^{\prime}\)-dihydroxy-\(\beta\)-carotene-4,4\({}^{\prime}\)-dione) is a 40-carbon tetraterpene with linked isoprene units; it is composed of two terminal \(\beta\) rings and a linear polyene chain. The characteristic pink and red colours of astaxanthin are determined by its 11 conjugated double-bond systems. Astaxanthin was initially derived in 1938 from lobsters [14]. Chemically synthesised astaxanthin is abundantly available commercially with low production costs but is utilised mainly in the poultry and aquaculture industries as a pigmentation source and has not been authorised for human consumption. These products have 20 times lower antioxidant activity than natural astaxanthin. Therefore, there is high demand for the natural form of astaxanthin, which has not been met due to the arduous procedures required for its manufacture [15; 16; 17]. The astaxanthin market was valued at USD 1.0 billion globally in 2019 and is projected to increase at a compound annual growth rate of 16.2% to USD 3.4 billion by 2027 [18]. Natural astaxanthin has very powerful antioxidative properties, which have applications in the nutraceutical, cosmetic, and pharmaceutical industries [17; 19]. The antioxidant capacity of natural astaxanthin is 500 and 38 times greater than that of vitamin E and \(\beta\)-carotene, respectively [20].