## 6 Methods of Microalgae Cultivation Monitoring
### Dynamics of Growth, Concentration, and Biomass Composition  
There are numerous analytical methods for assessing these parameters, which provide precise information on the growth rate of microalgae and their biochemical composition. Monitoring these parameters is not only important in scientific research but also in industry, particularly in biotechnology, biofuel production, and water treatment. One of the basic methods for monitoring the growth dynamics of microalgae biomass is the measurement of optical density using spectrophotometers. In this technique, the absorption of light by the microalgae sample is measured at a specific wavelength, typically 680 nm, which corresponds to the characteristic absorption of chlorophyll. Changes in absorbance over time allow an estimation of the growth rate of the microalgae, as higher biomass leads to higher absorbance. A commonly used instrument for this method is the Hach DR 6000 UV-Vis spectrophotometer (Dusseldorf, Germany), which allows fast and precise absorbance measurements in different spectral ranges. Another widely used method is dry weight measurement, where the microalgae biomass sample is dried in an oven to a constant weight. Although this method is more time-consuming, it is considered one of the most accurate methods for determining total biomass. It can be combined with chemical analyses to determine the content of specific components such as carbohydrates, proteins, lipids, and others. High-precision analytical balances such as the Mettler Toledo XPR6 (Greifensee, Switzerland) are used for this purpose [20]. Optical turbidimetric sensors that measure the turbidity of the culture medium are also used to monitor the biomass concentration in real time. Biomass growth correlates with increasing turbidity, as microalgae particles scatter the light and thus cause higher turbidity values. Devices such as the OTT Optic 410 (Kempten, Germany) or ABB Turbidity Analyser (Zurich, Switzerland) are used to continuously monitor turbidity in the culture medium [205]. Fluorometric methods for quantifying biomass based on chlorophyll fluorescence measurement are another technique for evaluating the growth dynamics of microalgae. Microalgae absorb light at specific wavelengths and emit fluorescence proportional to their biomass content. Fluorometric analysers such as the Trilogy Fluorometer from Turner Designs allow accurate monitoring of chlorophyll fluorescence and provide a rapid and non-destructive method for assessing the status of microalgae culture [206]. For more detailed analyses of the biochemical composition of the microalgae biomass, techniques such as high-performance liquid chromatography (HPLC) and mass spectrometry (MS) are used. These methods enable the precise determination of various biomass components, including lipids, proteins, carbohydrates, and pigments such as chlorophylls, carotenoids, and other bioactive compounds. Instruments such as the Agilent 1200 HPLC system enable precise separation and analysis of biochemical components, while mass spectrometers such as the Thermo Fisher LTQ Orbitrap XL (Cleveland, OH, USA) allow detailed identification and quantification of individual compounds [207]. Elemental analysers for carbon/hydrogen/nitrogen (C/H/N), such as the Elementar Vario EL Cube (Hanau, Germany), are used to determine the organic carbon content in the microalgae biomass. These devices allow accurate quantification of carbon in the samples, which is essential for the evaluation of photosynthetic efficiency and biomass production [208]. Regarding the composition of microelements in biomass, techniques such as inductively coupled plasma optical emission spectroscopy (ICP-OES) and atomic absorption spectroscopy (AAS) are used. These methods enable precise quantification of micronutrients such as copper, zinc, manganese, molybdenum, and iron.