# Propagation of Inoculum for _Haematococcus pluvialis_
## 4 Discussion  
While conducting the research and development activities related to scaling-up the process of cultivating microalgae, it is necessary to properly evaluate and extrapolate the efficiency of the designed system. Over the course of conducting developmental activities, it is essential to use storage tanks of a similar type that work similarly to the photobioreactors intended for the full-scale cultivation [8]. All the photobioreactors used in our system were of the airlift type, as well as the photobioreactors used in the last stage of preparation of inoculate and the final photobioreactors in the semi-technical scale. They had a similar system for regulating pH and mixing. All experiments must be conducted in the same operating conditions in which the commercial installation will be operating; Large volume, commercial photobioreactors should not be designed. Using smaller storage tanks or modular constructions allows maintaining the necessary flexibility and introducing improvements using trial and error. In the case of large volume containers, every problem which arises (e.g., pollution) influences the entire culture and entire production batch [8]. For that reason, the described inoculate preparation comprises of smaller units. The units are entirely separate and work independently of each other. Even in cases when sporadic contamination or irregularities occurred over the course of conducting the cultivation, they were always related only to a single photobioreactor. That allowed to maintain continuity and reliability of the work of the entire system. Switching between automated and manual modes of cultivating microalgae is a very good solution. It is especially useful to have the separate parameters control opportunity in individual modules (photobioreactors), which enables to carry out the cultivation process using various parameters e.g., light intensity, agitation rate [15]. The main environmental factors ensuring that the process of photosynthesis in microalgae proceeds satisfactorily is light (sunlight or artificial light), temperature, and pH [16]. Figure 9: Changes in the temperature and pH value over the course of the eight-day microalgae cultivation processes. The best temperature range for the proper functioning of the _Haematococcus pluvialis_ microalgae cultivation is the 20-27 \({}^{\circ}\)C temperature range [17]. Exceeding this range by 2-4 \({}^{\circ}\)C may have a significant influence on the course of enzymatic reactions in the microalgae cells (especially photosynthesis) and may lead to loss of the entire cultivation [18]. It was not planned for the station to use a temperature regulation system. However, the system used for monitoring temperature allowed for controlling it, which is very important for maintaining the highest possible productivity [19]. The results of measurements collected over the course of experiments confirmed that the work of the station in an air-conditioned room at a constant temperature of 22 \({}^{\circ}\)C prevents the upper-temperature threshold from being exceeded, as proposed by other authors, e.g., in [20; 21]. The systematic increase of temperature over the course of the experiments (Figures 6 and 9) was caused by the heat released by the LED lighting installed inside each storage unit. The temperature did not exceed the acceptable upper threshold for the _H. pluvialis_ microalgae. As a result, there was no need to use the temperature control system. Among all of the environmental parameters, pH has the most influence on the rate of cell growth and synthesis of chlorophyll of the _Haematococcus pluvialis_ microalgae [22]. It also determines the solubility and diffusion of carbon dioxide in the media, which in turn has an influence on the productivity of photosynthetic carbon bonds and has a direct or indirect impact on metabolism in microorganisms [23]. In the presented inoculate propagation station both, the mixed cultures as well as the control of pH were implemented by means of introducing gases into the photobioreactor.