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Fluorescence characteristics of the diatom Cylindrotheca closterium previously adapted to light intensities of 17, 200, and 800 μE∙m−2∙s−1 were investigated. Possibility of using fluorescence parameters for express score of both the algae functional state and the identification of a range of optimal light intensities for their growth was shown. The variable fluorescence coefficient (Fv/Fm) allows to evaluate in express mode the algae functional state in intensive cultivation conditions. It was shown that the maximum of Fv/Fm was of 0.65–0.7 for algae grown at light intensities of 17 and 200 μE∙m−2∙s−1; it decreased to 0.48–0.57 for algae adapted to light intensities of 800 μE∙m−2∙s−1. Light response curves of the electron transport rate, photochemical and non-photochemical quenching of chlorophyll fluorescence, and the Fv'/Fm' coefficient values were obtained. These parameters indicate the degree of algae resistance to the light factor level. It was shown that saturating light intensity of about 200 μE∙m−2∙s−1 is optimal for the growth of C. closterium. The high values of yield of fluorescence per chlorophyll unit under extreme light intensity (800 μE∙m−2∙s−1) may indicate the degree of inactivation of part of photosystem II reaction centers.
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