## 5 Monitored and Controlled Indicators Critical to Microalgae Cultivation
### Oxygen Concentration  
High concentrations of dissolved oxygen, especially in combination with intense lighting, can lead to photoinhibition, which significantly reduces photosynthetic efficiency and decreases the productivity of the microalgal biomass [147]. An increase in oxygen levels to 29 mg/L can lead to a significant decrease in photosynthetic activity--studies have shown that photosynthetic efficiency can decrease by up to 98% under such conditions [148]. These changes can trigger oxidative stress, which leads to cell damage in microalgae and has a negative effect on their growth.  
In addition, increased oxygen levels favour the development of parasitic organisms such as _Chytidium_ spp. fungi, which can infect microalgae and further impair cultivation performance [149]. In such cases, an effective strategy is to control the oxygen concentration in the culture system, which can be achieved by periodically reducing the oxygen content [150]. These measures can limit the growth of aerobic parasites and improve the overall efficiency of microalgae biomass production in PBRs. Regular monitoring and control of oxygen levels in combination with optimised cultivation conditions can help to minimise the risks associated with photoinhibition and infection. Table 4 shows a number of important environmental parameters that influence the growth efficiency of microalgae biomass and the production of economically valuable end products.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Value** & **Microalgae Species** & **References** \\ \hline \multirow{5}{*}{Nitrogen (N)} & 75–300 mg/L (NO\({}_{3}\)\({}^{-}\), NH\({}_{4}\)\({}^{+}\), mocznik) & _Chlorella vulgaris_ & [109; 151] \\ \cline{2-4}  & 50–500 mg/L (NO\({}_{3}\)\({}^{-}\)) & _Nannochloropsis_ sp. & [152] \\ \cline{2-4}  & 100–250 mg/L (NH\({}_{4}\)\({}^{+}\), NO\({}_{3}\)\({}^{-}\)) & _Scenedesmus obliquus_ & [153] \\ \cline{2-4}  & 100–400 mg/L (NO\({}_{3}\)\({}^{-}\), mocznik) & _Spirulina platensis_ & [116] \\ \hline \multirow{5}{*}{Phosphorus (P)} & 5–50 mg/L (PO\({}_{4}\)\({}^{3-}\)) & _Chlorella vulgaris_ & [151] \\ \cline{2-4}  & 1–20 mg/L (PO\({}_{4}\)\({}^{3-}\)) & _Nannochloropsis_ sp. & [152] \\ \cline{2-4}  & 2–30 mg/L (PO\({}_{4}\)\({}^{3-}\)) & _Scenedesmus obliquus_ & [153] \\ \cline{2-4}  & 5–25 mg/L (PO\({}_{4}\)\({}^{3-}\)) & _Spirulina platensis_ & [116] \\ \hline \multirow{2}{*}{Microelements} & Fe: 0.1–5 mg/L, Zn: 0.01–0.5 mg/L, Mn, & \multirow{2}{*}{Most species} & [112; 151] \\ \cline{2-4}  & Cu, Mo, B & & \\ \hline \multirow{5}{*}{Light} & 50–300 \(\mu\)mol m\({}^{-2}\) s\({}^{-1}\) & _Chlorella vulgaris_, & [109; 151] \\ \cline{2-4}  & 100–500 \(\mu\)mol m\({}^{-2}\) s\({}^{-1}\) & _Dunaliella salina_, & \\ \cline{2-4}  & _Nannochloropsis_ sp. & [151; 152] \\ \cline{2-4}  & 30–150 \(\mu\)mol m\({}^{-2}\) s\({}^{-1}\) & _Spirulina platensis_ & [116] \\ \hline \multirow{5}{*}{pH} & 6.5–8.5 & _Chlorella vulgaris_ & [109; 151] \\ \cline{2-4}  & 7.5–8.5 & _Nannochloropsis_ sp. & [152] \\ \cline{2-4}  & 6.5–9.0 & _Scenedesmus obliquus_ & [153] \\ \cline{1-1} \cline{2-4}  & 9.0–10.5 & _Spirulina platensis_ & [116] \\ \hline \hline \end{tabular}
\end{table}
Table 4: Range of applied technological and environmental parameters for biomass production of selected microalgae species.