## 3 Results and Discussion  
### Evaluation of B. aggregatus BM5/15 Cell Morphology  
Cell morphology is an important diagnostic feature of microalgal cultures, especially carotenogenic species, because it reflects metabolic status and division rate of cells [24; 39; 40]. It is also useful to detect culture contamination. At the vegetative growth stage, _B. aggregatus_ BM5/15 cell suspension was green. The culture was presented predominantly by small spherical palmelloid cells (so-called vegetative cells) 5-10 um diameter (Figure 1a-c). Rarely, large cells up to 25 um diameter were observed. It was in accordance with morphological features of _B. aggregatus_ BM5/15 after its isolation [53]. In the first 1-4 days of cultivation under vegetative growth conditions cells formed large clusters (Figure 1a). Later, the clusters became smaller, and individual cells were predominant. At 5-6 days of cultivation auto- (Figure 1b) and zoosporanga (Figure 1c) appeared in the culture. This period was also characterized by a massive release of motile zoospores (Figure 1d). Zoospores were small (2.5-3.5 um length) with two flagella on the apex (Figure 1e). To the best of our knowledge, there are no published photographs of _Bracteacoccus_ zoospores, but the cells in the _B. aggregatus_ BM5/15 culture (Figure 1e) were very similar to drawn illustrations of motile stages of this microalga from [PERSON] et al. [71]. High number of zoospores and dividing cells is a sign of actively growing cultures of green carotenogenic algae [60; 72; 73; 74]. Thus, their mass appearance in _B. aggregatus_ BM5/15 cultures might reflect the high potential of their vegetative growth. At the same time, non-motile vegetative cells are more resistant to adverse conditions [73]. Therefore, older cultures with low number of zoospores (10-12 days old) were better for induction of carotenoid accumulation. Carotenoid synthesis induction by distilled water led to massive cell death and total lysis of the culture at 2 nd day of cultivation (data not shown). This method was previously successfully used for induction of astaxanthin synthesis in _H. lacustris_ bubbled by air [57]. However, it caused cell lysis in _H. lacustris_ cultures bubbled with air-gas mixture enriched with 5% (by volume) CO\({}_{2}\)[60]. Thus, the transferring of _B. aggregatus_ BM5/15 cells to the BG-11\({}_{0}\)[60] medium was applied for induction of carotenoid accumulation in further experiments even in the case of bubbling with air. No cell division was observed under the inductive conditions, only spherical non-motile cells were in the culture. The cells became yellow (Figure 1f). Visually, carotenoid accumulation in the _B. aggregatus_ BM5/15 cultures cultivated in bubble-column photobioreactors was observed as changing the color of cell suspensions from green (Figure 1g) to orange (Figure 1h). Initially, _B. aggregatus_ BM5/15 was isolated as a bacteria-containing unialgal culture (i.e., containing only one strain of oxygenic phototroph, but not free of bacteria) [53]. Single bacteria attached to the surface of algal cells or freely dispersed in the medium at the vegetative and inductive stages of the cultivation were also observed for the strain cultivated in photobioreactors. Often, in laboratory cultures [28; 29; 75] and natural habitats [75; 76] carotenogenic algae exist in a form of complex bacterial communities. Particularly, the carotenogenic chlorophyte _H. lucstris_ forms microbial consortia with _Alcaligenaceae_, _Flavobacteriumaceae_, _Sphingomonadaceae_, _Commandaceae_, and _Caulobacteriaceae_[28; 75; 76]. Many of them are related to plant growth promoting bacteria, which produce vitamins and other important compounds [77; 78]. In some studies, a positive  
Figure 1: Studies of _Bracteococcus aggregatus_ BM5/15 (IPPAS C-2045) culture by light microscopy and its visual observation in bubble-column photobioreactors.