Microalga Euglena gracilis is able to photosynthesis and can also absorb different organic substrates from the environment both under the light conditions and in the darkness. In comparison with autotrophy, mixotrophic cultivation of E. gracilis with ethanol as carbon source has a significant influence on the metabolic characteristics of the microalgal cells. For most microalgae ethanol is toxic, but due to the unique biochemical organization, E. gracilis cells are able to effectively utilize ethanol with accumulation of the storage polysaccharide paramylon. Ethanol as a source of easily digestible carbon retards the light-dependent formation of the photosynthetic apparatus of the microalgal cells. Analysis of photosynthetic pigments at different light intensities showed that E. gracilis microalga is a shade-tolerant organism with a special way to adapt to shade. Light intensity also changes the rate of organic substrates uptake, which can affect the growth rate of a culture and the content of pigments in the cells. Effect of three variants of light intensities on the cultures of E. gracilis, which were grown mixotrophically in the presence of ethanol and ethanol and glutamate in nutrition medium, was investigated in this work. Autotrophic culture was the control variant. The density of the cultures and the content of photosynthetic pigments (chlorophylls and carotenoids) in the cells were fixed in every 3 days during 20 days of the investigation. At the stationary growth phase the maximal cell concentration was registered in the culture with ethanol and sodium glutamate under illumination 100 mmol/(m2 · s). Decreased chlorophylls content in the cells of mixotrophic cultures in comparison with autotrophic cultures at the beginning of exponential growth phase can be explained by catabolic repression of photosynthetic pigments synthesis. It was shown that chlorophylls and carotenoids content, chlorophylls a/b ratio increased with duration of mixotrophic cultures growth.
Keywords: Euglena gracilis, microalga, mixotrophy, ethanol, growth rate, light intensity, chlorophylls, carotenoids
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