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.

Carotenogenic microalgae are unicellular photosynthetic organisms with the ability to accumulate carotenoids. Carotenoid accumulation is a protective reaction against environmental stress factors, such as bright light, extreme temperatures, etc. It makes the survival of these microorganisms under harsh environmental conditions possible. The diversity of carotenogenic microalgae has been described in detail for Central Europe and North America, as well as for tropical and subtropical latitudes with relatively favorable environmes. However, data about these microorganisms in polar and subpolar latitudes is scarce and restricted to few reports. We isolated several strains of carotenogenic microalgae from the coastal zone of the White Sea, where they were abundant. The obtained microalgae related to four species of Chlorophytes: Haematococcus lacustris, H. rubicundus, Coelastrella aeroterrestrica, and Bracteacoccus aggregatus. The last three species have been reported for polar latitudes for the first time. Most likely, carotenogenic algae in the White Sea coast are abundant due to their high physiological and metabolic plasticity, which is essential for surviving under adverse conditions of the northern regions. Pigment composition of the strains is provided. Their predominant carotenoids were astaxanthin and β-carotene. Further, the obtained strains may be considered as potential producers of natural pigments for biotechnology.

Haematococcus lacustris is a natural source of a valuable ketocarotenoid astaxanthin. Under autotrophic growth conditions, it exists in the form of a community with bacteria. The close coexistence of these microorganisms raises two questions: how broad their diversity is and how they interact with the microalga. Despite the importance these issues, little is known about microorganisms existing in Haematococcus cultures. For the first time, we characterize the dynamic of the H. lacustris microbiome of the microbiome of Haematococcus (a changeover of the bacterial associated species as function of the time) cultivated autotrophically in a photobioreactor based on 16S rRNA metabarcoding data. We found that Proteobacteria and Bacteroidetes are predominant phyla in the community. The Caulobacter bacterium became abundant during astaxanthin accumulation. These data were supported by microscopy. We discuss possible roles and interactions of the community members. These findings are of potential significance for biotechnology. They provide an insight into possible bacterial contamination in algal biomass and reveal the presence of bacteria essential for the algal growth.