## 3 Results and Discussion  
Various methods are used to assess the efficiency of biomass production. The most popular of them are the determination of dry matter [23], measurement of optical density, determination of chlorophyll content using a spectrophotometer, and a method based on counting cells using a counting chamber [24]. In the presented work, differences between the optimal process conditions determined with the use of individual methods were noted. Based on the statistical analysis of the test results, it was found that the photoperiod used, and the pH of the culture medium had a significant effect on the amount of biomass and the content of chlorophyll in a microalgae cells of the _C. vulgaris_ species. However, the factors analyzed in the study did not have a significant effect on the number of microalgae cells. The amount of biomass in individual photobioreactors at the beginning of the experiment was on average 417 \(\pm\) 14.43 mg:l\({}^{-1}\). Under LED lighting conditions, the time of the culture entering the stationary growth phase occurred slightly earlier than under SON-T Agro lighting (Figure 2). Depending on other factors, it was generally between the 7 th and 9 th day of breeding. Similar dependencies in their research were presented by [PERSON] et al. [25]), who assessed the optimal conditions for the production of microalgae _Chorella_ sp., _Dunaliella salina_, and _Dunaliella_ sp. biomass and found that the biomass content increased by the 8 th measurement day. Similar results were obtained by [PERSON] et al. [26] in a study on the production of _Chorella minutissimu_ biomass using aquaculture wastewater. After 10 days, the breeding entered the stationary phase. The inhibition of the growth and development of microalgae between 8 and 10 days may be related to the depletion of nutrients in the culture medium [27]. In optimal culture conditions, higher biomass productivity was obtained on day 4--27.08 \(\pm\) 7.80 mg:L\({}^{-1}\).d\({}^{-1}\) under SON-T Agro lighting and 25.00 \(\pm\) 5.10 mg:L\({}^{-1}\).d\({}^{-1}\) under LED lighting (Figure 3). Similar results for _C. vulgaris_ biomass productivity (0.04 \(\pm\) 0.03 g:L\({}^{-1}\).d\({}^{-1}\)) were obtained by [PERSON] et al. [28]. The average amount of biomass in the culture with SON-T Agro illumination ranged from 506 \(\pm\) 6.74 to 546 \(\pm\) 7.88 mg:L\({}^{-1}\) (Figure 4). The concentration of biomass in the culture depends on the initial inoculum density, but the amount of biomass obtained in the presented studies on the F/2 substrate was within the range of the results obtained by other authors. [PERSON] et al. [29] in the modified BBM medium obtained 0.59 g:L\({}^{-1}\) of biomass. Similar results were presented by [PERSON] et al. [30], who cultivated microalgae biomass in diluted nitrogen-rich waste and obtained a maximum of 510 mg:L\({}^{-1}\). The highest amount was determined in a photobioreactor illuminated for 18 h and at pH 7. Similar results were obtained by [PERSON] et al. [31], who investigated the effect of the irradiation cycle on the growth of biomass of _Nannochloropsis_ sp. In addition, the highest values were recorded in 18/6 h cycle. In studies conducted by [PERSON] et al. [32], the highest increase in the biomass of microalgae _Chlorella vulgaris_ was recorded in the 16/8 h (light/dark) photoperiod. Different results are presented by [PERSON] et al. [33], who found in their research that the optimal culture conditions for the microalgae C. _vulgaris_ occurred in the 12/12 h photoperiod (light/dark), while the lowest biomass increases with continuous lighting of the culture. Figure 2: Dynamics of the growth of microalgae biomass in SON-T Agro light (**A**) and light-emitting diode (LED) light (**B**) during different light:dark cycle. Under LED light conditions, the average amount of biomass ranged from 489 \(\pm\) 5.00 to 543 \(\pm\) 1.92 mg\(\cdot\)L\({}^{-1}\). As in the case of sodium lighting, the highest values were recorded in the 18/6 h cycle and pH 7.