## 3 Results and Discussion
### Parameters of B. aggregatus BM5/15 Cultured in Photobioreactor  
Chlorophyll (Figure 2a) and carotenoid (Figure 2b) contents were gradually increasing in the _B. aggregatus_ BM5/15 under the vegetative growth conditions. After 14 days of cultivation, the content of total chlorophyll and carotenoid increased from 2.5 \(\pm\) 0.0 to 47.5 \(\pm\) 0.2 mg\(\cdot\)L\({}^{-1}\) and from 0.4 \(\pm\) 0.0 to 8.1 \(\pm\) 0.1 mg\(\cdot\)L\({}^{-1}\), respectively. DM increased at the vegetative growth stage from 0.2 \(\pm\) 0.0 to 1.4 \(\pm\) 0.2 mg\(\cdot\)mL\({}^{-1}\) (Figure 2c). In previous studies on _B. bullatus_ growth [81], maximal DM referred to the same period of time was as high as 2.4 mg\(\cdot\)mL\({}^{-1}\), which was larger than in the current work. Thus, optimization of the vegetative growth conditions (e.g., increasing initial culture density, changing of culturing medium and light intensity, CO\({}_{2}\) enrichment) should be the matter of further research. Calculated values of the growth curve parameters by DM accumulation were as follows: \(\mu\) = 0.33 day\({}^{-1}\), \(DM_{msx}\) = 1.3 mg\(\cdot\)mL\({}^{-1}\), \(L\) = 7 days (Table 1). The values of \(\mu\) and \(DM_{max}\) were comparable with the data on the kinetics of carotenogenic algae autotrophic growth. Particularly, _H. lacustris_ cultures are characterized by following maximal specific growth rates: \(\mu\) = 0.32 day\({}^{-1}\)[82], \(\mu\) = 0.31 day\({}^{-1}\)[83], \(\mu\) = 0.94 day\({}^{-1}\) (by cell number) [72], \(\mu\) = 0.2 day\({}^{-1}\) (by cell number) [84]. For _H. alpinus_ it is 0.31 day\({}^{-1}\)[63]. _C. rubescens_ is characterized by \(\mu\) = 0.35 day\({}^{-1}\) (by cell number) [39]. For _Ch. zofingiensis_ it is estimated as 0.77 day\({}^{-1}\)[44], as 0.5 day\({}^{-1}\) (by cell number) [85], as 0.96 day\({}^{-1}\)[22], or as 0.72 day\({}^{-1}\) (by cell number) [22]. In lutein-producing _Senedesmus_ sp. CCALA 1074, cultivated in a thin-layer photobioreactor, \(\mu\) is of 0.74 day\({}^{-1}\)[49]. Thus, the value of \(\mu\) of _B. aggregatus_ BM5/15 culture is comparable with that reported for other main carotenogenic microalgae. At the same time, the lag phase was too long. Therefore, increasing biomass yield would be achieved by increasing the initial culture density and performing multiple reseeding of the culture before growth in photobioreactors in order to obtain the population of actively dividing cells. Total carotenoid content in the algal biomass did not change significantly under the conditions of vegetative growth and was in the range of 0.2-1.0% of DM (Figure 2d). \begin{table}
\begin{tabular}{c c} \hline
**Culture Parameter** & **Value** \\ \hline Maximum specific growth rate (\(\mu\)), day\({}^{-1}\) & 0.33 \\ \hline Maximum biomass at the vegetative stage (\(DM_{max}\)), mg\(\cdot\)L\({}^{-1}\) & 1.2–1.6 \\ \hline Maximal carotenoid content at the inductive stage, mg\(\cdot\)L\({}^{-1}\) & 24.5–27.1 \\ \hline Maximal carotenoid content at the inductive stage, \% of dry biomass & 2.2–2.4 \\ \hline Average productivity, mg\(\cdot\)L\({}^{-1}\cdot\)day\({}^{-1}\) & 100–200 \\ \hline Carotenoid yield, mg\(\cdot\)L\({}^{-1}\cdot\)day\({}^{-1}\) & 3.6–3.8 \\ \hline \end{tabular}
\end{table} 
Table 1: Main parameters of the _Bractaococcus aggregatus_ BM5/15 (IPPAS C-2045) growth and carotenoid accumulation in the bubble-column photobioreactors. Under the inductive conditions, chlorophyll content was gradually increasing during the first four days of cultivation up to 52.1 \(\pm\) 2.8 mg:L\({}^{-1}\). Then, it decreased sharply (Figure 2a). Increasing chlorophyll content at the induction stage of cultivation is not a typical feature of carotenoid algae. In _H. lacustris_, it decreased monotonously after transferring to new conditions [38; 60; 86; 87]. The same is true about _Ch. zofingiensis_ cultures [41; 45]. Chlorophylls are directly involved in photosynthesis. Decrease of their content under inductive conditions is accompanied by downregulation of photosynthetic activity [60; 87; 88; 89; 90].