## 6 Methods of Microalgae Cultivation Monitoring
### Photosynthetic Activity  
Numerous devices are available on the market for analysing the photosynthetic activity of microalgae using various measurement techniques such as chlorophyll fluorescence, gas exchange, and electron transport measurement in photosystems. A popular instrument is the bbe Moldaenke Chlorophylll Fluorometer, which allows direct measurement of chlorophyll fluorescence and assessment of the photosynthetic performance of microalgae by analysing the emitted light signal [210]. This fluorometer works by exciting the chlorophylll at specific wavelengths and recording the resulting fluorescence emission. This enables the identification of different microalgae groups based on their characteristic fluorescence signatures. The device also enables the assessment of photosystem II (PSII) efficiency and parameters such as Fv/Fm (variable fluorescence relative to maximum fluorescence), which are important indicators of the photosynthetic state of microalgae and their response to changing environmental conditions. By using multiple excitation wavelengths, the bbe Moldaenke Chlorophylll Fluorometer can distinguish the major microalgae groups, including cyanobacteria, green algae, diatoms, and cryptomonads. This instrument is widely used in ecotoxicological studies, surface water monitoring, and controlled microalgae cultures, where rapid and non-destructive analysis of photosynthetic activity is essential for optimising growth conditions and assessing the health of the culture. It can be operated online, enabling continuous monitoring of cultures in real time, which is especially useful in industrial PBRs and in applications related to bioremediation and the production of biofuels from microalgae.  
Similar opportunities are offered by pulse amplitude modulated (PAM) fluorometers such as the Walz WATER-PAM and the MINI-PAM-II, which measure the variability of chlorophyll fluorescence in response to light of different intensities, enabling the assessment of PSII efficiency and microalgae response to environmental stress [211]. Another solution is the Turner Designs Trilogy Fluorometer, which allows the measurement of chlorophyll fluorescence at different wavelengths and can be used to analyse the biomass and photosynthetic activity of phytoplankton [212]. More compact and portable devices include the AquaPen-C 100 from Photon Systems Instruments (PSI) (Drasov, Czech Republic), which enables rapid measurements of photosynthetic parameters under both field and laboratory conditions [213]. Similarly, the Handy PEA from Hansatech Instruments records chlorophyll fluorescence kinetics and allows detailed analysis of photosynthetic dynamics [214]. For more advanced applications, the LI-COR LI-6800 Portable Photosynthesis System (Lincoln, NB, USA) combines the measurement of gas exchange with chlorophyll fluorescence analysis, enabling the simultaneous monitoring of CO\({}_{2}\) uptake, oxygen production, and photosynthetic efficiency of microalgae [215]. The choice of a suitable device depends on the specific research objectives, the required measurement accuracy and the experimental conditions.