## 6 Methods of Microalgae Cultivation Monitoring
### Monitoring of O\({}_{2}\) Concentration in the Culture Medium and Gaseous Metabolites  
The analysis of the O\({}_{2}\) content in the culture medium of PBRs is a decisive factor for the optimisation of biotechnological processes. Oxygen plays an important role in the metabolism of these organisms. In PBRs, oxygen is present in both the liquid and the gas phase. As a product of photosynthesis, it exerts an inhibitory effect on the growth rate of the culture when the oxygen concentration exceeds 200-500% of the saturation values, i.e., above 15-25 mg O\({}_{2}\)/L [160]. As a rule, oxygen is continuously removed from the culture medium by aerating the culture with CO\({}_{2}\)-enriched air or other gas mixtures. This process simultaneously integrates three interdependent functions: removal of O\({}_{2}\), supply of CO\({}_{2}\), and pH control in the culture system. The measurement of dissolved oxygen concentration is considered a reliable and sensitive indicator of the physiological state of microalgae in terms of biomass growth rate and productivity of target metabolites [117]. The most commonly used methods for measuring oxygen in the culture medium are oxygen electrodes, such as the Clark polarographic electrode and galvanic electrodes [191]. These work on the principle of oxygen reduction at the cathode and generate an electric current that is proportional to the oxygen concentration in the sample. These methods offer high sensitivity and precision but require regular calibration and can be affected by temperature fluctuations and the presence of other gases. An alternative to electrodes are optical sensors that utilise oxygen-induced luminescence quenching [192]. This method uses luminescent dyes whose light emission is quenched in the presence of oxygen. The change in luminescence intensity is proportional to the oxygen concentration in the medium. Optical methods are less invasive and do not consume oxygen during the measurement, which is a considerable advantage. The polarographic analyser (also known as an anodic oxygen analyser) is based on the electrochemical reduction of oxygen at the electrode surface [193]. The oxygen dissolved in the liquid (e.g., water) is reduced at the cathode and generates an electric current that is proportional to the oxygen concentration in the sample. Changes in the electrode voltage or current allow the oxygen to be quantified. In polarography, the voltage fluctuations due to the interaction of the oxygen with the electrolyte are measured, and the current fluctuations are used to determine the oxygen content. Polarographic oxygen analysers such as the YSI ProDSS (YSI, Yellow Spring, OH, USA), the Orion 9830/9835 (Boston, MA, USA), and the AquaSensors O\({}_{2}\) Sensor (Thermo Scientific, Beverly, MA, USA) are widely used in applications that require precise measurement of dissolved oxygen, especially in microalgae cultivation, aquaculture, the food industry, and environmental monitoring. They are valued for their accuracy, ease of use, and ability to continuously monitor water quality. Classical chemical methods, such as Winkler titration, involve the chemical binding of oxygen and subsequent titration of the reaction products [194]. Although these methods are less practical and more time-consuming than instrumental methods, they are still used in some laboratory studies. Sensor and analyser measurements are subsequently used to regulate the air flow or other gas supply to the culture. The oxygen content in the exhaust gas can also be measured using a paramagnetic gas analyser [195]. The paramagnetic analyser makes use of the unique magnetic properties of oxygen. Oxygen is a paramagnetic gas, i.e., its molecules have unpaired electrons that react to magnetic fields. Paramagnetic oxygen (O\({}_{2}\)) interacts with an external magnetic field, which attracts the oxygen molecules. The analyser uses a magnet and a detector to measure changes in the attraction or repulsion of oxygen molecules.