## 6 Methods of Microalgae Cultivation Monitoring
### Monitoring of Nutrient Content in the Culture Medium  
The determination of nutrient concentrations in microalgae culture media is a key element of quality control of the culture environment, as it directly affects the efficiency of microalgae growth and biomass production. Nutrient analysis methods include a variety of techniques, including spectrophotometry, chromatography, flame photometry, and electrochemical methods. Each method is used depending on the type of substance being analysed [197]. Nitrogen compounds such as ammonium nitrogen (NH\({}_{4}\)\({}^{+}\)), nitrate nitrogen (NO\({}_{3}\)\({}^{-}\)), and nitrite nitrogen (NO\({}_{2}\)\({}^{-}\)) are of crucial importance in the cultivation of microalgae, as these microorganisms intensively utilise nitrogen in their metabolic processes [198]. Ammonium nitrogen can be determined by a colourimetric method, in which ammonia reacts with reagents such as phenol, or by an electrochemical method using ion-selective electrodes. Nitrate and nitrite nitrogen are usually measured spectrophotometrically using the reaction with the Griess reagent, which produces a characteristic coloured compound. Ion-selective electrodes are also used for these types of nitrogen. To determine the total nitrogen, the Kjeldahl method is used, in which the sample is digested with sulphuric acid and the ammonium nitrogen is then quantified, or the persulphate method, in which the nitrogen in the sample is oxidised to nitrate and then measured spectrophotometrically. Phosphorus is another important nutrient in microalgae culture media, which is usually determined as orthophosphate (PO4\({}^{3-}\)). The standard method for phosphorus determination is the molybdenum blue method, in which orthophosphates react with ammonium molybdate under acidic conditions to form a blue-coloured complex. The colour intensity is measured spectrophotometrically, normally at 880 nm. Other spectrophotometric reagents can be used to increase selectivity in the presence of interfering substances. Total phosphorus can also be measured spectrophotometrically after the sample has been mineralised with sulphuric acid [199]. Iron, an essential micronutrient, must also be monitored in microalgae cultures. Iron in the forms Fe2\({}^{+}\) and Fe3\({}^{+}\) is usually determined spectrophotometrically, where Fe2\({}^{+}\) reacts with 1,10-phenanthroline to form a red complex. The absorbance of this complex is measured at 510 nm. Alternatively, a colourimetric method using 2,2\({}^{\prime}\)-bipyridyl reagent, which forms a characteristic colour with iron, is also used [200]. The chemical oxygen demand (COD) is a measure of the oxygen demand of water due to the oxidation of organic and inorganic compounds. The manganese method is most commonly used for COD determination, in which organic compounds are oxidised with potassium permanganate and the reagent consumption is measured spectrophotometrically. This is a standard method for analysing COD in water. Alternative methods include rate-based approaches that monitor the changes in oxygen concentration during the oxidation reaction [201]. Biochemical oxygen demand over 5 days (BOD5) quantifies the oxygen consumption by microorganisms during a 5-day incubation at 20 \({}^{\circ}\)C. The standard method involves incubating the sample for 5 days and measuring the oxygen consumption before and after incubation. This method is time-consuming but accurate. A faster approach is the respirometric method, which allows real-time monitoring of oxygen consumption [202]. Potassium, an important macronutrient in microalgae cultivation, is often determined using flame photometry. The sample is exposed to a flame, which excites potassium atoms that emit light at a characteristic wavelength. The emission intensity is proportional to the potassium concentration in the sample [203]. Another method is inductively coupled plasma optical emission spectroscopy (ICP-OES), which enables the simultaneous determination of potassium and other micronutrients.