The effect of low temperature and nitrogen starvation on the morphological and physiological characteristics of two strains of green microalgae of the genus Lobosphaera sp.

The effect of nitrogen starvation and, for the first time, low temperature, as well as their simultaneouseffect, on the physiology and ultrastructure of cells of microalgae of the genus Lobosphaera (Chlorophyta, Trebouxiophyceae) was studied. Nitrogen deficiency in both strains led to a decrease in the content ofchlorophyll by three times and an increase in the proportion of carotenoids by two times. A decrease in thecontent of both chlorophyll and carotenoids was observed at +10°C. The simultaneous effect of two factorsresulted in a threefold decrease in the chlorophyll content in NAMSU 924/2 and a sixfold decrease inNAMSU (CALU) 1497; the proportion of carotenoids in both strains decreased by 1.5–2 times. Data onultrastructural changes in cells of microalgae of the genus Lobosphaera under the influence of stress factorshave been obtained. A similar nature of the response in both strains to stress conditions was noted. Nitrogendeficiency led to the accumulation of numerous lipid droplets in the cytoplasm of cells along the cell wall. Long-term incubation on a nitrogen-free medium led to the filling of the entire volume of cells with lipiddroplets, disassembly of the membrane system of chloroplasts, that reduction in sizeand being locatedbetween densely lying lipid droplets. At low temperatures, the number of thylakoids decreased, while theinterthylakoid space and the size of chloroplasts increased. With simultaneous exposure to nitrogen starvation and low temperature, numerous lipid droplets accumulated, the number of thylakoids decreased, the interthylakoid space and the size of the chloroplast increased, which was noted under separate exposure to stress factors. The pyrenoid in both strains did not undergo significant changes in all cases.