# Propagation of Inoculum for _Haematococcus pluvialis_
## 4 Discussion
Compressed gases were used to agitate the content of the photobioreactors. Air and carbon dioxide were administered alternately to ensure continuity of agitation. The flow rate of dosed air and carbon dioxide was maintained at a constant of 0.5 dm\({}^{3}\) gas per minute. Due to that, the mixing of the medium remained steady, regardless of the introduced type of gas (air, carbon dioxide). The use of the air-lift system allowed for efficient mixing of gases with the culture medium in the internal tube (riser). Additionally, the microalgal agglomerate was broken-up by turbulent flow. After reaching the top of the photobioreactor, the medium would spill over the edge of the riser to the external tube (downcomer) and flow in a laminar manner. The process of agitating the suspension of microalgae is a crucial aspect of cultivation as it prevents sedimentation of cells and has a positive influence on the distribution of carbon dioxide, oxygen, and nutrients contained in the culture media. Additionally, the mixing process provides required access of microalgae cultures to light and facilitates the exchange of heat [24]. Providing carbon dioxide ensures simultaneously access to the source of carbon and adjustment of the pH value of the culture medium. This method works exceptionally well in vertical photobioreactors with aeration [19]. The monitoring of pH over the course of cultivation enabled to control the amount of dosed carbon dioxide to maintain a neutral pH. The undisturbed photosynthesis process and the growth of microalgae take place when pH is maintained at the level of 7.0. Indirectly, this parameter provides information about the photosynthetic activity of the microalgae cultivation [15]. The on-off pH control algorithm was satisfactorily used over the course of the experiments. The control had a pH set point of 7.0 and 0.2 hysteresis. It is consistent with the research results [25] in which the highest value of chlorophyll was achieved at pH 7.0. Additionally, in [PERSON] et al. [23], the maximum increase in cell count and maximum concentration of chlorophyll a and b was noted at pH 7.0. A gradual increase in the pH of the medium, between 5.78 and 6.92, was observed during the experiments (Figure 7). This tendency is typical for microalgae cultivations, which limits the enzymatic activity of microalgae and, in consequence, inhibits their growth [26]. Over the course of the experiments, the volume of carbon dioxide was not measured. The control system, based on the on-off control algorithm with hysteresis, made the decisions related to dosing the carbon dioxide when the pH of the medium would rise above pH 7.0 (Figure 9). It was indicative of, among other things, of running out of carbon dioxide introduced into the photobioreactor in the previous control cycle. With this, the dosing of carbon dioxide took place as a response to the demand of microalgae and indicated indirectly by the change in pH value. A similar pH control system was used in research, where the microalgae were used to capture and recycle CO\({}_{2}\)[27]. While analyzing the operating frequency of the control algorithm, it can be seen that as time went by the dosing of carbon dioxide happened more frequently. It serves as an indication of the increasing demand for carbon, sourced from the dosed carbon dioxide. This increase was a result of the natural course of microalgae growth and biomass growth, which accompanying it [27]. Light is one of the most vital factors in the cultivation of microalgae because it provides the energy required in the process of photosynthesis [28]. Energy captured by microalgae is used to bind carbon dioxide, release oxygen as a byproduct, and produce biomass. Optimizing the access to light is crucial for obtaining high efficiency of the cultivating system as well as for reducing the cost of its functioning [28]. In the presented system electroluminescent LED diodes were used to illuminate the photobioreactors. The diodes replaced fluorescent lights, which were used in 5 dm\({}^{3}\) laboratory bottles solution.