Characteristics of Vacuolar Inclusions in Coelastrella rubescens Namsu R1 Green Microalgae Cells in Low- and High-Intensity Light

Abstract

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.