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Spectral properties of cell suspensions, individual cells, and extracts of the unicellular green alga Parietochloris incisa (Reisigl) Shin Watan. grown under low light were studied. Long-term nitrogen (N) deprivation resulted in a decrease of chloroplast volume, appearance of numerous large cytoplasmic oil bodies, and the deposition of triacylglycerols with a high proportion of arachidonic acid. Chlorophylls a and b underwent a synchronous decline, whereas carotenoids (Car) showed a relative increase. Simultaneously, significant qualitative changes in the spectral properties of P. incisa individual cells, cell extracts, and cell suspensions were observed. To a large extent, the spectral changes observed in cell suspension could be attributed to a decrease in overall pigment content, leading to a gradual weakening of the socalled package effect and accumulation of additional amounts of Car over chl, most probably, in oil bodies. Several optical characteristics of cell suspensions could serve as sensitive indicators of N-deficiency in P. incisa. Furthermore, the absorption ratios, A 476 ⁄ A 676 and A 650 ⁄ A 676, showed close correlations with the Car-to-chl ratio and relative arachidonic acid (AA) content, respectively. The latter makes it possible to suggest that the increase in AA percentage in P. incisa proceeds in parallel with a decrease in cell chl content, accounting for the weakening of the package effect. N-replenishment resulted in complete recovery of cell optical properties. The possible significance of the changes in cell ultrastructure, pigments, lipids, and optical properties is discussed with special reference to the ability of algae to adapt to and survive under conditions of long-term nutrient deficiency.

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Vacuole is a multifunctional compartment central to a large number of functions (storage, catabolism, maintenance of the cell homeostasis) in oxygenic phototrophs including microalgae. Still, microalgal cell vacuole is much less studied than that of higher plants although knowledge of the vacuolar structure and function is essential for understanding physiology of nutrition and stress tolerance of microalgae. Here, we combined the advanced analytical and conventional transmission electron microscopy methods to obtain semi-quantitative, spatially resolved at the subcellular level information on elemental composition of the cell vacuoles in several free-living and symbiotic chlorophytes. We obtained a detailed record of the changes in cell and vacuolar ultrastructure in response to environmental stimuli under diverse conditions. We suggested that the vacuolar inclusions could be divided into responsible for storage of phosphorus (mainly in form of polyphosphate) and those accommodating non-protein nitrogen (presumably polyamine) reserves, respectively.The ultrastructural findings, together with the data on elemental composition of different cell compartments, allowed us to speculate on the role of the vacuolar membrane in the biosynthesis and sequestration of polyphosphate. We also describe the ultrastructural evidence of possible involvement of the tonoplast in the membrane lipid turnover and exchange of energy and metabolites between chloroplasts and mitochondria. These processes might play a significant role in acclimation in different stresses including nitrogen starvation and extremely high level of CO and might also be of importance for microalgal biotechnology. Advantages and limitations of application of analytical electron microscopy to biosamples such as microalgal cells are discussed.

Nitrogen (N) is an essential macronutrient for microalgae, influencing their productivity, composition, and growth dynamics. Despite the dramatic consequences of N starvation, many free-living and endosymbiotic microalgae thrive in N-poor and N-fluctuating environments, giving rise to questions about the existence and nature of their long-term N reserves. Our understanding of these processes requires a unequivocal identification of the N reserves in microalgal cells as well as their turnover kinetics and subcellular localization. Herein, we identified crystalline guanine as the enigmatic large-capacity and rapid-turnover N reserve of microalgae. The identification was unambiguously supported by confocal Raman, fluorescence, and analytical transmission electron microscopies as well as stable isotope labeling. We discovered that the storing capacity for crystalline guanine by the marine dinoflagellateAmphidiniumcarteraewas sufficient to support N requirements for several new generations. We determined that N reserves were rapidly accumulated from guanine available in the environment as well as biosynthesized from various N-containing nutrients. Storage of exogenic N in the form of crystalline guanine was found broadly distributed across taxonomically distant groups of microalgae from diverse habitats, from freshwater and marine free-living forms to endosymbiotic microalgae of reef-building corals (Acropora millepora,Euphyllia paraancora). We propose that crystalline guanine is the elusive N depot that mitigates the negative consequences of episodic N shortage. Guanine (C5H5N5O) may act similarly to cyanophycin (C10H19N5O5) granules in cyanobacteria. Considering the phytoplankton nitrogen pool size and dynamics, guanine is proposed to be an important storage form participating in the global N cycle.

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.

We report on common and strain-specific responses to nitrogen (N) starvation recorded in four closely related symbiotic Desmodesmus strains from taxonomically very distant animals (hydroids, a sponge and a polychaete) dwelling in the White Sea. A number of common for the studied strains and free-living microalgae as well as some specific patterns of acclimation to the N starvation were documented. The common responses included a slowdown of cell division, a reduction of photosynthetic apparatus and a vast expansion of storage subcompartments of the cell. Although these responses were qualitatively similar to those known in free-living chlorophytes, in the studied strains they occurred in a strain-specific manner. The specific N-starvation responses comprised formation of chloroplast envelope membrane twirls, thinning of the appressed thylakoid membranes and a loss of the luminal depositions and channeling of the fixed carbon to cell wall polysaccharide layer. Desmodesmus sp. from a hydroid featured a unique, among the studied strains, capability of 'emergency' degradation of Rubisco, apparently to salvage the N contained in this protein. The obtained results are discussed in view of the remarkable physiological plasticity of the symbiotic Desmodesmus spp. and their survival under the harsh conditions of the subarctic sea habitat.