## 6 Methods of Microalgae Cultivation Monitoring
### Monitoring of pH Changes in the Culture Medium and CO\({}_{2}\) Content  
The pH value has a direct effect on the growth rate of the biomass by characterising the environmental conditions specific to the microalgae species and strains. This parameter also influences the availability and utilisation efficiency of the nutrients present in the culture medium. The acceptable pH range for most microalgae species is between 7 and 9, with a typical optimum at 8.2-8.7. Some species are adapted to more acidic or alkaline environments [176]. In intensive photoautotrophic microalgae cultivation carried out at light intensities above the compensation point--where the photosynthetic rate equals the respiration rate--the pH of the culture tends to increase. This occurs because during active microalgae growth there is an efficient uptake of CO\({}_{2}\) and HCO\({}^{3-}\) from the culture medium and an assimilation of nitrates. Conversely, CO\({}_{2}\) introduced into the culture medium from outside dissolves easily and forms carbonic acid (H\({}_{2}\)CO\({}_{3}\)), which dissociates into bicarbonate (HCO\({}^{3-}\)) and hydrogen ions (H\({}^{+}\)). An increase in the CO\({}_{2}\) concentration leads to a higher H\({}^{+}\) ion concentration and lowers the pH value (acidification). Conversely, a decrease in the CO\({}_{2}\) concentration reduces the H\({}^{+}\) concentration and increases the pH value [176; 177]. Therefore, in practice, the monitoring and stabilisation of the pH value in microalgae culture systems is often combined with the control of the CO\({}_{2}\) concentration in the culture medium. Controlled CO\({}_{2}\) dosing is often used to prevent excessive pH increase, maintain acid-base balance, and create optimal conditions for microalgae growth [178]. pH is measured with electrodes or optical pH probes and is usually controlled by injecting CO\({}_{2}\) into the culture or inlet gas stream. The most commonly used method for pH measurement uses glass electrodes, which are characterised by high precision and fast response time [179]. In biotechnology, including microalgae cultivation, electrodes with pressurised liquid electrolytes and integrated temperature sensors are used to minimise measurement error and ensure stable readings [180]. An alternative to conventional electrodes are optical sensors that utilise the fluorescence properties of pH-sensitive materials [181]. Although they are less widely used, they offer advantages such as lower susceptibility to interference and no need for calibration. pH control with mineral acids can be advantageous in some cases [110]. More advanced pH control algorithms have been investigated to improve the efficiency of CO\({}_{2}\) utilisation [182]. Improved pH control through control algorithms reduces the pH gradients within the culture, thereby increasing the average photosynthetic rate and biomass productivity. Methods for measuring CO\({}_{2}\) concentration in the culture medium are based on electrochemical sensors that perform direct CO\({}_{2}\) measurements. These sensors recognise changes in the electrical potential in response to the presence of CO\({}_{2}\) and thus make it possible to determine the concentration. Optical sensors, on the other hand, utilise the absorption of infrared radiation by CO\({}_{2}\) molecules. Changes in absorption correspond to the CO\({}_{2}\) concentration, which enables precise measurement. Effective regulation of the CO\({}_{2}\) concentration in the culture medium is also possible based on the relationship between pH and carbonate hardness [183]. Monitoring the pH provides information about the CO\({}_{2}\) content, while the carbonate hardness determines the buffering capacity of the medium to stabilise the pH in response to changes in CO\({}_{2}\) concentration. Proper regulation of these parameters is critical to ensure optimal growth conditions for microalgae.