## 4 Discussion  
### Oxygen Saturation and pH  
During light and dark cycles, oxygen concentrations in a photobioreactor shows a diurnal pattern [48; 49; 50]. Since our experiments were carried out under continuous irradiation, the diurnal pattern of oxygen content changes was not observed. Lower than average initial oxygen levels in the hybrid reactor could be explained by the much more complex geometry, which requires a longer time for the even gas distribution in the reactor than in the case of the tank reactor. The higher oxygen levels in the 25%-N cultures in both types of reactors are probably the result of the lower salt content: the lack of nitrate made it possible to dissolve more oxygen within these cultures. The significantly higher oxygen levels in the tank reactor in control cultures and the 50%-N treated cultures on certain days than in the hybrid reactor in the same cultures is not easy to explain. Cell density differences or geometry differences do not explain why the oxygen contents of these cultures differed significantly from each other and the others not. Although photosynthetic activity calculations were not done, the moderate increase of oxygen levels can be explained by the photosynthetic activity of the cultures in the tank reactor. In the hybrid reactor, increased oxygen levels could be the result of the insufficient aeration of the tubular part of the reactor, as it was observed by [PERSON] et al. [51] in a similar device. In summary, the results suggest that the gas supply of the tank reactor is more even in the applied volume than in the hybrid reactor with a complex geometry. The literature also reported more uneven gas distribution in tubular reactors [52] and a more balanced gas supply in column reactors [53; 54]. At the same time, it also has to be taken into account that scalability of tubular reactors is considered much higher than that of column reactors [4].  
The changes in pH are also strongly related to reactor geometry, proper mixing, and the physiological processes of algal cells. Jaworski's Medium contains H\({}_{2}\)PO\({}_{4}\)\({}^{-}\) and HPO\({}_{4}\)\({}^{2-}\) ions, which can work as a buffer system. However, these ions are also a phosphorous source for the algae, so their uptake can decrease the buffer capacity of the medium. As shown in the Results section, intensive phosphate uptake was observed in all cultures in the tank reactor, which could explain the increase of pH in these cultures. Another reason for the increasing pH is the photosynthetic carbon consumption [48; 49]. Furthermore, pH increases could also be caused by the consumption of the nitrogen source, nitrate ions, because of the parallel excretion of OH\({}^{-}\) ions [55]. During dark and light cycles, pH shows changes \"adjusted\" to the dark and light periods because of the alternation of the ratio of photosynthesis and respiration [48; 49]. Since our cultures were kept under continuous irradiation, the resulting photosynthesis / respiration ratio did not allow for a decrease in pH. Although microalgal cultures could be considered carbon limited at the measured pH values due to the dissolution properties of CO\({}_{2}\)[48; 49], growth characteristics did not show this limitation in the tank reactor (also suggesting that light limitation due to increasing culture density did not occur significantly during the experiment). The pH remained more stable and at lower values in the hybrid reactor, probably because of the lower metabolic activity due to lower cell density.