## 1 Introduction  
The imbalance of the food rations of the population in megalopolises in the absence of the required amount of essential food micronutrients is a global problem of modern civilization. The deficiency of vitamin A (retinol) was found to be about 10-30%, and that of vitamin E (tocopherol) was 15-35%. This is a major problem in all groups of the population, regardless of age and/or profession. The deterioration of the ecological status of urban areas, the industrialization of the food industry (including the deep processing of the raw materials, long-term storage, refining, and the use of dry substitutes for natural products, etc.) leads to a high necessity for the additional enrichment of the diet with biologically valuable components which are necessary for the prevention of altimentary-dependent diseases [1]. One of the possible solutions of this problem is to expand the range of functional food products and food additives obtained from the alternative types of plant materials. These plants contain numerous biologically active substances, including natural pigments (carotenoids), \(\alpha\)- and \(\beta\)-carotene, fucoxanthin, astaxanthin, lutein, violaxanthin, and zeaxanthin [2]. The system of conjugated double bonds in carotenoid molecules, as well as structural similarity with some fat-soluble vitamins, determines their high biological value for plants and humans, including their role as a provitamin, their detoxifying activity, the strengthening of the immune system, the chemoprevention of cancer, and the prevention of cardiovascular diseases, cataracts, and other degenerative disorders [3]. Currently, unconventional raw materials, including microalgae, are considered to be significant sources of carotenoids [4]. Single-celled green algae \(D\). _salina_ are known for their ability to accumulate about 0.5 to 10% \(\beta\)-carotene per unit of dry matter [5]. \(\beta\)-carotene in \(D\). _salina_ has a high biological activity due to its isomeric composition, which cannot be achieved by chemical synthesis [6]. The microalgae _Chlorella_ are considered to be the most valuable, because of their high content of essential amino acids, carbohydrates, vitamins, and lipids, as well as lipid-like compounds, including carotenoids and other valuable plastid pigments [7; 8; 9; 10]. Algae contain xanthophylls of a unique chemical structure. This is characteristic of the individual taxa, as algae's oxycarotenoids are derivatives of not only \(\beta\)- but also \(\alpha\)- and \(\gamma\)-carotenes. Thus, algae of the _Chlorophyta_ group contain special pigments: phycocyanin, siphonoxanthin, astaxanthin, loroxanthin, zeinoxanthin, and neoxanthin, as well as allene and acetylenic carotenoids such as diatoxanthin and diadinoxanthin, and many others. Depending on the cultivation conditions, the nutrient composition of the _Chlorella_ cell can vary significantly. Taking into account the specificity of the _Chlorella_ pigments' application and the variety of directed cultivation techniques, it is possible to stimulate the biosynthesis of secondary carotenoids in order to isolate and apply them further for nutraceuticals and para-pharmaceuticals. The effect of the culture medium's composition and bubbling on the quantitative accumulation of pigments, including carotenoids, in the microalgae _Chlorella vulgaris_ was studied [11]. Due to their high adaptive activity, unicellular algae are able to synthesize a large number of secondary carotenoids under various stress conditions, e.g., the osmotic shock of nitrogen and/or phosphorus deficiency, a high intensity of absorbed light, non-optimal temperatures, and the presence of reactive oxygen species in the culture medium or their synthesis. The connection between the presence of reactive oxygen species in the culture medium and the biosynthesis of astaxanthin by haematococci was proven [12]. Therefore, it is possible to stimulate the biosynthesis of secondary carotenoids with a simple variation in the cultivation conditions.