## 1 Introduction  
Carotenoids are widely used by humans. Due to the high number of healthy properties, they are key components of drugs, cosmetics, biologically active additives and other health food products [1; 2; 3; 4; 5; 6; 7; 8]. They are also considered as lipophilic food colorants [3; 9]. Carotenoids are powerful natural antioxidants. Particularly, antioxidant activity of astaxanthin is 100 times higher than of vitamin E [6]. Many carotenoids, such as \(\beta\)-carotene, are precursors of vitamin A. Therefore, they are an essential component of human diet [2]. Astaxanthin is not a pro-vitamin A. At the same time, it exhibits anti-inflammatory, antitumor, UV-protective and many other health benefit effects [4; 6; 10; 11; 12; 13]. Astaxanthin determines the colorful appearance of some animals. Such red colorations of shells of crayfish, lobsters, crabs and shrimps, meat of salmonids, as well as a rose plumage of flamingo are due to the presence of this carotenoid in their diet [14; 15; 16; 17]. It makes astaxanthin an essential component of feed, especially in aquaculture. For the last 10 years astaxanthin and \(\beta\)-carotene have retained leading positions on the world market of carotenoids. According to the Grand View Research council ([[https://www.grandviewresearch.com/](https://www.grandviewresearch.com/)]([https://www.grandviewresearch.com/](https://www.grandviewresearch.com/)), accessed on 10 April 2021), in 2017, the total carotenoid world market was estimated at USD 2 billion. A gradual increase by 2027 is projected. In industry, most carotenoids are obtained by chemical synthesis. Costs of the synthetic pigments are lower than natural ones [1; 8], but their application has some limitations. Particularly, they cannot be used for medicines, cosmetics and food. Therefore, the task of obtaining carotenoids with the help of metabolic pathways of living things is still necessary. On an industrial scale, microalgae are cultivated either extensively or intensively. In the first case, the cultures are grown in open ponds occupying large areas [1; 18; 19; 20; 21; 22; 23; 24; 25]. In outdoor ponds the culture is often illuminated by sunlight. Such systems are characterized by poorly controlled conditions; the culture there is highly vulnerable to contamination. At the same time, operation costs of culturing in open ponds are relatively low. In the second case, cultivation takes place in closed fermenters, photobioreactors, where microalgae can be illuminated by artificial light [8; 26; 27]. A photobioreactor is an enclosed, illuminated culture vessel designed for controlled biomass production [27]. There, it is possible to provide sterility or stable composition of satellite bacterial communities of an alga. Particularly, _H. lacustris_ cultures in photobioreactor are characterized by specific robust bacterial composition different from that of an outside laboratory [28]. Using photobioreactors allows to maintain relatively stable culturing conditions, and therefore culture productivity in photobioreactors is more predictable. The most common strategy for production of natural carotenoids from microalgae is two-phase (or two-stage) cultivation [20; 21; 29; 30; 31]. At the first, \"vegetative\", stage biomass accumulation occurs. At the second, \"inductive\", stage carotenoids accumulation is promoted under stressful conditions. Another approach, one-stage cultivation, is based on reconciliation of vegetative growth and carotenoid accumulation [23; 30]. Some microalgae accumulate high amounts of carotenoids. They are considered as an industrial source of natural pigments [1; 32; 33]. Two species of green microalgae are cultivated on the industrial scale for production of carotenoids: _Dunaliella salina_ (formerly _D. bardawil_) (Chlamydomonadales) [19; 20; 21; 31; 34] and _Haematococcus lacustris_ (formerly _H. pluvialis_) (Chlamydomonadales) [14; 35; 36; 37; 38]. Industrial strains of _D. salina_ accumulate up to 14% \(\beta\)-carotene of cell dry mass (DM) [1; 20; 21]. _H. lacustris_ is a source of astaxanthin.