## 3 Results and Discussion
### Carotenoid Profile in the B. aggregatus BM5/15 Biomass at the Induction Stage
In chemical terms, carotenoids are representatives of terpenoids, molecules of biological origin, of which the C5 hydrocarbon isoprene is a structural unit. They are related to tetraterpenes (C40-isoprenoids), i.e., they consist of eight isoprene building blocks [67]. In cells of higher plants and most of algae they take their origin in plastids, where their precursor, isopentenyl pyrophosphate (or 'activated isoprene'), is synthesized via 1-deoxy-\(D\)-xylulose-5-phosphate pathway [2; 94; 32]. However, there are some exceptions. Particularly, euglenophytes synthesize carotenoids via mevalonate pathway and cyanobacteria realize all stages of their synthesis in cytosol [32]. C-5 precursors are condensed into the C-40 terpenoid phytoene and then desaturated to the non-cyclic lycopene. After desaturation, \(\alpha\) and/or \(\beta\)-ionone are formed on the ends of carotenoid molecules [2; 94; 32]. Carotenoids are divided into oxygen-free carotenes and oxygen-containing xanthophylls [94]. Astaxanthin is produced from \(\beta\)-carotene by addition of two keto- and two hydroxyl groups with the help of corresponding enzymes (Figure 3b). Some authors proposed a particular chain of reactions in different carotenogenic microalgae. According to [PERSON] et al. [95], in _H. lacustris_, ketogroups are added initially, thus astaxanthin is formed through echinone, canthaxanthin and adonirubin, whereas in _Ch. zofingiensis_ hydroxyl groups are formed first leading to formation of the adonirubin intermediate. In other works, all types of reactions are principally possible in one microcalga [43; 44; 72]. Based on results of our work the following metabolic nexus of astaxanthin biosynthesis was possible in _B. agregatus_ BM5/15 (Figure 3b). Flux of \(\beta\)-carotene precursors was being implemented through both two pathways: with either primary ketolation or hydroxylation of ionone rings. In the first case, the pigment was formed through echinone, canthaxanthin and adonirubin. In the second case, zeaxanthin and adonirubin were astaxanthin synthesis intermediates. Principally, the formation of adonixanthin from echinone and adonirubin from cryptoxanthin were also possible, but corresponding intermediates, 3-hydroxychinenone and 3'-hydroxychinenone, were not detected in _B. agregatus_ BM5/15. Pigment composition has been evaluated for some carotenogenic microalgae after carotenoid synthesis induction. In _H. lacustris_, astaxanthin is a single predominant carotenoid, only a minor fraction of other pigments is presented [57; 72; 97]. In other species the fraction and diversity of intermediates are higher [39; 40; 42; 43; 44; 45]. The strains defined as _Neochloris wimmeri_ CCAP-213/4, _Coolastrella oocysfiformis_ SAG-277/1, and _Protosiphon botryoides_ SAG731/1a accumulate significant amounts of astaxanthin monoesters and minor fraction of other ketocarotenoids [97]. The strain _Scenedesmus vacuolatus_ SAG-211/15 accumulates predominantly canthaxanthin and lutein [97]. The microcalga _Bracteacoccus minor_ accumulates a mixture of ketocarotenoids: astaxanthin, canthaxanthin, echinone, adonixanthin and adonirubin [24; 70]. Astaxanthin disasters are its predominant form [24; 70]. High astaxanthin yield in _H. lacustris_ may be explained by effective channeling of substrates due to high degree of interaction between enzymes of xanthophyll synthesis [98]. At the same time, less intense channeling and enzymes activity make it possible to obtain valuable intermediates, e.g., \(\beta\)-carotene. Most likely, the last situation was possible in _B. agregatus_ BM5/15. Thus, the microcalga may be considered as a source of both carotenoids, astaxanthin and \(\beta\)-carotene.