## 3 Results and Discussion
Additionally, pH conditions can cause calcium precipitation in some microalgae cultures [42]. [PERSON] [43] found that the optimal pH for photosynthesis is generally lower than the requirements for growth and showed that pH values ranging from 6 to 9 stimulate carorogenesis for _Muriellopsis_ sp. Under LED lighting, the mean amount of chlorophyll ranged from 62 \(\pm\) 6.72 to 136 \(\pm\) 14.13 mg\(\cdot\)L\({}^{-1}\). Contrary to sodium lighting, the highest values were obtained at the photoperiod 12/12 and pH 6. No significant differences were found between the content of chlorophyll in microalgae cells at the highest values of chlorophyll a. The study of [PERSON] et al. [44] on the effect of different wavelengths of LED light on the cultivation of _C. vulgaris_ confirmed the effectiveness of this type of lighting in the production of biomass from algae; the use of red light has a positive effect on the content of chlorophyll a. [PERSON] et al. [45] confirmed the dependence of red light on the increase in chlorophyll content and in _C. vulgaris_ cells and the decrease in biomass content. The initial number of microalgae cells in the culture was 9.3 \(\times\) 10\({}^{5}\) (Figure 8). A significant increase in the number was recorded between the first and second measurement dates for SON-T Agro and LED light-illuminated photobioreactors. After this measurement date, the culture entered the stationary growth phase. With LED illumination at the end of the experiment in the two photoperiods, the culture had already reached the death phase. Many authors have shown a similar relationship. [PERSON] et al. [46], examining the effect of glucose on the growth of _C. vulgaris_, showed that on the fifth day of the experiment, cell growth entered the stationary phase (20 \(\times\) 10\({}^{6}\) of microalgal cells). The studies of [PERSON] et al. [47] on the effect of the addition of nitrate and glucose to the culture medium on the growth of _C. vulgaris_ and _Nammochloropsis__collata_ cells showed, as in the presented work, a large increase in the initial phase of the experiment. The number of C. vulgaris cells ranged from 9.4 \(\times\) 10\({}^{9}\) to 12 \(\times\) 10\({}^{9}\). This was due to the addition of nutrients to the medium, which stimulates the growth and development of microalgae [48]. On the other hand, studies by [PERSON] et al. [49], concerning the effect of nutrients on the growth of  
Figure 6: Chlorophyll productivity of _C. vulgaris_ in SON-T Agro light (**A**) and LED light (**B**). _C. vulgaris_ biomass, showed the growth of algae cells only up to the 8 th day of measurement, after which the culture entered the death phase. The mean values of the number of microalgae cells indicate that the time of lighting culture, and the pH had no significant effect on the obtained values (Figure 9). Under SON-T Agro lighting, the highest values were recorded at the 18/6 h photoperiod (19.3 \(\times\) 10\({}^{5}\)-20.8 \(\times\) 10\({}^{5}\)), and under LED lighting, the highest average number of microalgae cells was recorded at 12/12 h (18.9 \(\times\) 10\({}^{5}\)-20.8 \(\times\) 10\({}^{5}\)), however, as shown in Figure 5, they did not differ significantly from other lighting conditions. There was also no significant influence of medium pH on the analyzed parameters. On the other hand, the research by [PERSON] et al. [50] were carried out under constant lighting conditions and results showed significant differences in the number of _C. vulgaris_ cells depending on cultivation conditions (batch growth or semi-continuous culture) and types of culture media. The analysis of the number of cells used to quantify the biomass does not define the stage of development of the culture and the size of the cells [51].