The microalga Coelastrella rubescens dwells in habitats with excessive solar irradiation; consequently, it must accumulate diverse compounds to protect itself. We characterized the array of photoprotective compounds in C. rubescens. Toward this goal, we exposed the cells to high fluxes of visible light and UV-A and analyzed the ability of hydrophilic and hydrophobic extracts from the cells to absorb radiation. Potential light-screening compounds were profiled by thin layer chromatography and UPLC-MS. Coelastrella accumulated diverse carotenoids that absorbed visible light in the blue–green part of the spectrum and mycosporine-like amino acids (MAA) that absorbed the UV-A. It is the first report on the occurrence of MAA in Coelastrella. Two new MAA, named coelastrin A and coelastrin B, were identified. Transmission electron microscopy revealed the development of hydrophobic subcompartments under the high light and UV-A exposition. We also evaluate and discuss sporopollenin-like compounds in the cell wall and autophagy-like processes as the possible reason for the decrease in sunlight absorption by cells, in addition to inducible sunscreen accumulation. The results suggested that C. rubescens NAMSU R1 accumulates a broad range of valuable photoprotective compounds in response to UV-A and visible light irradiation, which indicates this strain as a potential producer for biotechnology.

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Coelastrella rubescens Kaufnerová & Eliás (Chlorophyceae) is a green, single-celled algae that lives in the terrestrial-air environment. Under stress conditions, its cells go into a state characterized by low photosynthetic activity and high content of reserve lipids and secondary carotenoids. For the first time, a comparative morphological, ultrastructural, and elemental analysis of vacuolar inclusions in the C. rubescens NAMSU R1 strain when cultivated on a mineral medium under conditions of low and high (causing stress) light intensity. Microalgae cells stained with the fluorescent dye DAPI showed signs of the presence of polyphosphates. Polarization microscopy in cells of C. rubescens has identified structures capable of refracting polarized light, which is typical of crystals. Cell analysis of C. rubescens with the transmission electron microscopy (TEM) method revealed the presence of various vacuoles with heterogeneous contents (autophagic bodies, crystalloids, and rounded globules of inhomogeneous electron density). With the exception of autophagic bodies noted in cells only in bright light, these inclusions were characteristic of microalgae cells, regardless of the intensity of illumination. The elemental composition of vacuolar inclusions was characterized by TEM in combination with energy-dispersive X-ray spectroscopy: the predominant content of nitrogen, phosphorus, or both elements simultaneously was established in them. The potential physiological role of C. rubescens vacuolar inclusions is discussed.

An Erratum to this paper has been published: https://doi.org/10.1134/S1021443722330020