## 3 Results and Discussion
The amount of biomass obtained in a culture medium with a neutral pH, regardless of the type of lighting and the time of lighting culture, was generally significantly different from the other values. [PERSON] et al. [34] investigated the effect of pH on the growth of _Dunialella bardawil_ and _Chlorella ellipsoidea_ and found that although these microalgae could grow over a wide pH range from 4 to 9/10, the greatest dry matter gain for _D. bardawil_ was at pH 7.5, and for _C. ellipsoidal_, at alkaline pH (9-10). In a study by [PERSON] et al. [35], the highest increase in _Chlorella sorkinian_ biomass was obtained at pH 6. These results indicate the need to conduct research on the optimization of biomass production conditions, which change significantly depending on the type of microalgae. According to [PERSON] et al. [36], it is very important to keep the pH constant during cultivation. Methods with control of optimal pH value increased the growth of microalgae by over 56%. Chlorophyll content at the beginning of the experiment in all objects was 46 \(\pm\) 6.17 mg\(\cdot\)m\({}^{-3}\) (Figure 5). In the photobioreactors illuminated by SON-T Agro light, the highest values were determined on the fifth day of the experiment and a significant decrease in the content of chlorophyll and in microalgae cells in subsequent dates. Similar relationships were presented by [PERSON] et al. [37] who, while examining the growth of _C. vulgaris_ in tubular photobioreactors, found that in the initial phase of growth, the content of chlorophyll a increased, and then, after the 5 th day of the experiment, the amount of assimilation pigment decreased almost to zero. Such a significant decrease in chlorophyll  
Figure 4: The average amount of microalgae biomass in SON-T Agro light (**A**) and LED light (**B**) during different light:dark cycle. Figure 3: Biomass productivity under optimal culture conditions in SON-T Agro light (**A**) and LED light (**B**). content, according to these authors, was related to the reduction of nutrients in the culture medium. In the presented research, a similar situation was observed in the conditions of cultivating with LED light. A significant increase in the content of chlorophyll was observed already with the second measuring time. The highest content of assimilated pigment was determined in the photobioreactor at 18/6 h cycle and pH 7. Similar results were obtained by [PERSON] et al. [38] while breeding _Chlamydomonas reinhardtii_ and _Chlorella_ sp. in a gas-tight photobioreactor, and [PERSON] et al. [39] conducted a photoautotrophic culture of _C. vulgaris_. According to [PERSON] et al. [37], determining the content of chlorophyll a is not a useful measure of biomass concentration due to the lack of correlation between the content of chlorophyll and the optical density of the culture. The content of chlorophyll a may also be misleading due to the differences in the size of the analyzed cells [40]. Figure 5: Dynamics of chlorophyll a content in microalgal in SON-T Agro light (**A**) and LED light (**B**) during different light:dark cycle. The highest chlorophyll productivity was on day 5 (Figure 6). Content of microalgal chlorophyll in optimal condition was 24.21 \(\pm\) 8.89 and 26.34 \(\pm\) 2.01 mg\(\cdot\)m\({}^{-3}\cdot\)d\({}^{-1}\), under HPS and LED lightening, respectively. The mean content of chlorophyll a with SON-T Agro illumination ranged from 70 + 10.11 to 135\(\pm\) 6.17 mg\(\cdot\)L\({}^{-1}\) (Figure 7). The highest content of assimilated pigment for this type of light was determined in a photobioreactor illuminated for 18 h and at pH 7. In the study by [PERSON] et al. [33], two strains of _Dunaliella bardavil_ and _Chlorella ellipsoidea_ at different pH variants showed that the neutral pH of the culture medium favored the accumulation of chlorophyll a and other assimilation pigments. The pH of the culture medium affects the entire process of microalgae cultivation. Unfavorable pH values influence the efficiency of nutrient absorption and metabolite production [41].