## 6 Methods of Microalgae Cultivation Monitoring
### Culture Temperature Monitoring  
The optimum cultivation temperature for microalgae is species- or strain-specific. In general, the optimum range for most strains is between 20 \({}^{\circ}\)C and 30 \({}^{\circ}\)C. Certain species of cyanobacteria can grow effectively at higher temperatures, up to 35 \({}^{\circ}\)C. There are also strains, including genetically modified strains, for which temperatures in the range of 10 \({}^{\circ}\)C to 15 \({}^{\circ}\)C are sufficient to achieve satisfactory cultivation results [167]. Deviations from the optimum temperature significantly reduce the overall productivity of the biomassand inhibit the synthesis of the desired metabolites by the microalgae. Temperatures exceeding the optimal range should be avoided, as they cause a significant decrease in the quantum efficiency of photosynthesis. In larger cultivation systems, shading, immersion of the PBR in water tanks, or spraying with water are commonly used for temperature control [168]. However, shading reduces productivity. In open ponds and water channels, the temperature is rarely controlled. Low temperatures inhibit the rate of biochemical reactions, which has a direct impact on the technological outcome of the cultivation process. Temperature is usually measured using a standard industrial Pt-100 sensor (Eppendorf AG, Hamburg, Germany), which is often integrated into control and automation systems in more advanced PBR systems [169]. When the temperature exceeds a threshold, the sensor signal triggers automation functions to stabilise the temperature, including various heating or cooling mechanisms. In lab-scale bioreactors or smaller units for cultivating microalgae, multiparameter sensors that combine temperature measurement with monitoring of other environmental parameters are often used [170]. Sensors for temperature, pH, and suspended solids concentration are often installed and functionally integrated with transducers with RS-485 communication interfaces [171]. This solution enables precise monitoring of the cultivation conditions in real time and allows the operator to react quickly. Advanced thermostatic temperature measurement systems are less commonly used. Examples include Peltier modules, which transfer heat between two sides of the device in response to an electrical current flow. These modules allow precise temperature measurement and control in the range of 4 \({}^{\circ}\)C to 95 \({}^{\circ}\)C [172]. Such systems are primarily used in UV/VIS spectrophotometric analyses where temperature control is critical to measurement accuracy. Although they are more commonly used for laboratory analyses, their application in microalgae cultivation can improve environmental control. External thermostatting modules are sometimes used for temperature regulation in PBRs, and such solutions are increasingly being adapted for microalgae cultivation monitoring. Thermostatting, i.e., the precise control and stabilisation of temperature, is a key factor in microalgae cultivation. Temperature affects the growth rate, photosynthetic efficiency, and biochemical composition of the microalgae biomass [173]. Therefore, the application of appropriate methods for temperature monitoring and regulation is essential for the optimisation of cultivation processes. In open systems, such as open ponds, continuous flow bioreactors, and other open tanks, temperature control is difficult due to direct exposure to ambient conditions. In such cases, the culture temperature depends on the local climatic conditions, which can lead to undesirable fluctuations [109]. In closed cultivation systems, such as various PBR designs, electric heaters are often used, especially in small units. More commonly, temperature-controlled heating and cooling jackets with circulating fluid are used [174]. This approach allows the precise maintenance of optimal thermal conditions inside the PBR regardless of the external conditions. Heat exchangers, both internal and external, are used in bioreactors for various purposes.