## 3 Results and Discussion
### Carbon and Nitrogen Nutrition  
To determine the efficiency of the assimilation of carbon dioxide supplied to the photobioreactor (CL2-CL4 treatments), the CO\({}_{2}\) output was measured (Figure 6). Since it was previously established that the concentration of CO\({}_{2}\), as well as the assimilation of ammonium by microalgae with the simultaneous release of hydrogen ions (protons), destabilizes the pH level and leads to its decrease [71], the pH level was maintained throughout all treatments in the automatic mode by supplying 2 M NaOH and kept at 7.0 \(\pm\) 0.02. Figures 1 and 6 illustrate that the period of intensive growth of the culture was accompanied by a decrease in the carbon dioxide output and, consequently, by effective carbon assimilation. With an increase in cell concentration and complete utilization of ammonium from the growth medium, an increase in the CO\({}_{2}\) concentration in the output was observed, which can be explained by the simultaneous assimilation of CO\({}_{2}\) and its active release by cells. In our recent work [38], cultivation of _Chorella sorokiniana_ in a growth medium in the presence of ammonium ions stimulated cells to more efficiently uptake carbon dioxide than during cultivation in the presence of nitrate ions or in effluent from biogas reactors. Effluent from biogas reactors is a by-product of the anaerobic digestion process that must be disposed of to prevent harmful effects on the environment [72;73]. In  
Figure 5: Protein content of the biomass of _Scenedesmaceae_ alga, strain EZ-B1, under periodic light (PL) and continuous light (CL). Means that do not share a letter are significantly different (ANOVA, Tukey method, \(\alpha\) = 0.05). turn, an additional CO\({}_{2}\) supply affects the removal of nutrients from the growth medium, as shown by [PERSON] et al. [27], where nitrate removal was increased from 5.0 to 87.2% when the CO\({}_{2}\) concentration was increased from 0.0 to 3.0%. It is worth noting that the additional supply of carbon dioxide during the cultivation of microalgae not only optimizes the growth of photoautotrophic microorganisms but also has an advantage in the environmental aspect. In all experiments, the initial total ammonium nitrogen concentration was about 165 mg L\({}^{-1}\) (Figure 7). The dynamics of ammonium nitrogen removal from the nutrient medium were influenced by both the PPFD level and the concentration of supplied CO\({}_{2}\). At 37.44-56.16 mol photons m\({}^{-2}\) day\({}^{-1}\) (PL1-PL3), almost all nitrogen was removed during 112 h of cultivation. With a further increase in PPFD level (PL4 and PL5), complete removal of nitrogen was observed after 88 h of cultivation. Cultivation under continuous light (112.32 mol m\({}^{-2}\) day\({}^{-1}\)) with a supply of 1 and 2% CO\({}_{2}\) (CL2 and CL3) shifted the point of almost complete nitrogen removal to 64 h. In the case of phosphate utilization by algal cells, 90-98% of phosphates were removed from the culture media by the end of cultivation. Nitrogen can be supplied in various forms to support the growth of green algae, but different species prefer specific forms and concentrations of nitrogen [38; 39; 74; 75]. One previous study investigated the effect of different forms of nitrogen (NaNO\({}_{3}\), CO(NH\({}_{2}\))\({}_{2}\), and NH\({}_{4}\)Cl) in different concentrations on the growth of _Chlorella_ sp. GN1 in flat photobioreactors [74]. The authors demonstrated that _Chlorella_ sp. GN1 grew significantly better on media containing CO(NH\({}_{2}\))\({}_{2}\) and NaNO\({}_{3}\), while algal cell growth was inhibited by increasing the NH\({}_{4}\)Cl concentration. In the following work [75], CO(NH\({}_{2}\))\({}_{2}\), NaNO\({}_{3}\), KNO\({}_{3}\), NH\({}_{4}\)NO\({}_{3}\), NH\({}_{4}\)Cl, NH\({}_{4}\)CH\({}_{3}\)CO\({}_{2}\), (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\), and NH\({}_{4}\)HCO\({}_{3}\) were tested for the growth of _Chlorella_ sp. HS2.