## 3 Results and Discussion  
### Algal Culture Growth Rates and Biomass Yield under Various Growth Conditions  
In this work, photoautotrophic cultivation of _Scenedesmaceae_ alga, strain EZ-B1, was carried out in modified BBM. Parameters such as temperature, pH, aeration, and mixing rates were maintained at the same level in all experiments by using an automatic photo-bioreactor control system. To assess the effect of light conditions on the growth of the algal culture, two modes were selected. During the first mode, with a 16 h photoperiod, a wide range of daily PPFD was tested, from a minimum of 37.44 mol m\({}^{-2}\) day\({}^{-1}\) to a maximum of 74.88 mol m\({}^{-2}\) day\({}^{-1}\). During the second mode of the study, a 24 h photoperiod with 56.16 mol photons m\({}^{-2}\) day\({}^{-1}\) and 112.32 mol photons m\({}^{-2}\) day\({}^{-1}\) was evaluated. At this stage, in addition to the different PPFD values, the influence of various CO\({}_{2}\) concentrations ranging from 1.0% to 4.0% (aeration rate: 0.7 L min\({}^{-1}\)) was further assessed. The experimental design is presented in Table 1.  
The growth dynamics of the algal culture _Scenedesmaceae_ strain EZ-B1 are presented based on the average optical density values measured at a wavelength of 750 nm (Figure 1A,B). As the photosynthetic photon flux density increased (a 16 h photoperiod; 2% CO\({}_{2}\)), the exponential growth duration of the microalga decreased from approximately 256 h at 37.44 mol m\({}^{-2}\) day\({}^{-1}\) (PL1) to approximately 208 h at 74.88 mol m\({}^{-2}\) day\({}^{-1}\) (PL5). In the range of 37.44-56.16 mol photons m\({}^{-2}\) day\({}^{-1}\), the final values of the OD\({}_{750}\) did not differ, while at 65.55 mol photons m\({}^{-2}\) day\({}^{-1}\) and 74.88 mol photons m\({}^{-2}\) day\({}^{-1}\), higher OD\({}_{750}\) values were reached. The maximum OD\({}_{750}\) value in experiments with periodic light was 8.80 \(\pm\) 0.15 obtained at 74.88 mol photons m\({}^{-2}\) day\({}^{-1}\) (PL5) at 232 h of cultivation (Figure 1A). During the dark period, the optical density of the algal culture did not change substantially (not shown in the figure). Similar growth patterns of the strain EZ-B1 were observed with a 24 h photoperiod, but higher PPFD values resulted in higher biomass yields (when the CL1 and CL3 treatments were compared). Thus, the exponential phase at 112.32 mol photons m\({}^{-2}\) day\({}^{-1}\) ended after about 184 h, whereas the growth of the culture at 56.16 mol photons m\({}^{-2}\) day\({}^{-1}\) was delayed. Furthermore, different CO\({}_{2}\) concentrations were tested at the maximum PPFD (when the CL2, CL3, and CL4 treatments were compared). The final OD\({}_{750}\) values were comparable over the CO\({}_{2}\) concentration range studied, but 2% CO\({}_{2}\) was found to be optimal in terms of biomass production (Figure 1B). It is worth noting that in the experiment with a high daily PPFD, no photoinhibition effect was observed, which, in addition to the resistance of the strain to higher light intensity, is also associated with the presence of a shadow effect on microalgae when cultivated in a photobioreactor, which was also observed in another study [12]. The optimal growth of microalgae at high PPFD values was also indicated by the specific growth rate (Figure 2). The minimum and maximum values of the specific growth rate with a 16 h photoperiod were 0.47 \(\pm\) 0.01 day\({}^{-1}\) (at 37.44 mol photons m\({}^{-2}\) day\({}^{-1}\)) and 0.61 \(\pm\) 0.01 day\({}^{-1}\) (at 74.88 mol photons m\({}^{-2}\) day\({}^{-1}\)). With a 24 h photoperiod, the specific growth rates obtained at 112.32 mol photons m\({}^{-2}\) day\({}^{-1}\) and 2% CO\({}_{2}\) were higher than the values obtained at 56.16 mol photons m\({}^{-2}\) day\({}^{-1}\) at the same CO\({}_{2}\) level and amounted to 0.72 \(\pm\) 0.01 day\({}^{-1}\). A decrease or an increase in CO\({}_{2}\) concentrations to 1% and 4%, respectively, resulted in a decrease in the specific growth rates.