Monitoring of the Photosynthetic Activity of the Microalgae Chlorella under Nitrogen Depletion Conditions

—The paper presents the results of monitoring of the photosynthetic activity of the green microalga Chlorella. A thermophilic strain of Chlorella with a high rate of cell division was cultivated in a photobioreactor. Three medium variants with different mineral composition were used. Chlorophyll fluorescence induction curves and microalgal suspension absorption spectra were recorded every hour during 40-hour culture growth for each medium. The method of spectral multiexponential approximation we have developed was used to analyze 120 induction curves obtained after monitoring. This method revealed the dynamics of gradual changes in the number of phases of induction curves, their amplitudes and characteristic times during culture growth. The relationship between the parameters of induction curves and electron transfer processes in the reaction center of photosystem II allowed for interpretation of the changes in the shape of the induction curves in terms of the processes in the reaction center of photosystem II. The spectral multiexponential approximation method was shown to be highly sensitive to physiological state of microalgae and suitable for analyzing a huge amount of experimental data. The physiological processes accompanying the depletion of nitrogen in the environment are discussed.