The effect of three nitrogen (N) sources in the nutrient medium – sodium nitrate (NaNO3), urea (CO(NH2)2), and ammonium chloride (NH4Cl) – on the morphological and physiological characteristics of the green microalga Chromochloris (Chlorella) zofingiensis, a potential commercial producer of lipids and a ketocarotenoid astaxanthin, was studied. The alga was batch-cultivated in glass conical flasks from starting cell density (n) around 2.3·106 per mL and dry weight (DW) content of 0.06 g·L−1 in all variants at 120 μmol·m−2·s−1 PAR, +20…+21 °C, and air bubbling at a rate of 0.3 L·min−1·L−1. The concentration of nitrogen sources (as elemental N) in the modified BBM nutrient medium was 8.83 mmol·L−1, the cultivation duration was 17 days. The dynamics of n and cell volumes, DW content, chlorophylls a and b (Chla and Chlb), total carotenoids (Car), and lipids (Lip) in the cultures, concentration of N sources in the nutrient medium, and its pH were recorded. It was shown that the growth rate, size distribution of the cell populations, and the biomass chemical composition depended significantly on the nitrogen source in the nutrient medium. Using NH4Cl as N source caused on the second day growth inhibition, cell swelling, aggregation, and discoloration; by the seventh day, it caused culture crash. C. zofingiensis cells took up NaNO3 and CO(NH2)2 from the medium at a similar rate (0.626 and 0.631 mmol N·L−1·day−1, respectively), but the growth of the culture fed with CO(NH2)2 lagged; its cell volume and Chla, Chlb, and total Car contents declined profoundly. The average dry matter productivity (PDW) in the culture grown on CO(NH2)2 [(0.086 ± 0.004) g·L−1·day−1] was 32.6 % lower than in the culture grown on NaNO3 [(0.114 ± 0.005) g·L−1·day−1]. At the same time, lipid productivity (PLip) of the urea-fed culture was comparable with that of the nitrate-fed culture (PLip of 28 and 26 mg·L−1·day−1, respectively). The lipid DW percentage of the former exceeded significantly that of the nitrate-fed culture (31.6 % vs 23.1 %, respectively). From the standpoint of profitability, the lag in biomass accumulation recorded in the urea-fed culture on PDW is not critical since it is compensated by lowering the cost of nitrogen source for the nutrient medium (approximately by 230 %) and a higher biomass lipid content. C. zofingiensis grown in media with urea as the only N source deserves further investigation.

A strain of aeroterrestrial green microalgae Coelastrella rubescens IBSS-156, isolated from an epilithic lichen, has been previously shown to efficiently produce green biomass and accumulate significant amounts of secondary carotenoids. In this study, using a two-stage batch culture, we analyzed time-course changes in variable chlorophyll a (Chl a) fluorescence parameters. Additionally, regression models were developed to correlate autofluorescence signals with spectrophotometric measurements of Chl a and total carotenoid content. Maximum quantum efficiency of photosystemII (F_v/F_m) remained high throughout the vegetative stage. At the end of this stage, under nutrient-limited conditions, the relative electron transport rate (rETR) declined to half its peak value during exponential growth. Stress induced a strong response in the algal photosynthetic apparatus during the early red stage. Within the first three days, F_v/F_m and rETR remained extremely low, but both increased sharply by day 5. During secondary carotenoid accumulation, fluorescence parameters remained at 70–80% of the vegetative-stage maximum, followed by a sharp decline toward the end of the red stage. Therefore, changes in variable fluorescence parameters can serve as markers of C. rubescens cellular physiology during biotechnological cultivation, denoting the completion of specific stages. Flow cytometry and pigment assay regression enabled real-time monitoring of C. rubescens biomass and carotenoids.

On the example of green microalgae of the family Scenedesmaceae Coelastrella rubescens, Coelastrella sp., Coelastrella aeroterrestrica, Pseudospongiococcum protocols and Desmodesmus sp. different fixation methods were tested for scanning electron microscopy in order to obtain images of taxonomically relevant cell surface structures (ribs, poles, spines, warts, etc.). Depending on the physiological state of the cells (young, actively dividing or at the secondary carotenogenesis stage), 0.5–2.5% glutaraldehyde in 0.15 M PBS, as well as in 5 and 6.67 mM phosphate buffer, were used for fixation. The pH range of the fixing reagent was 6,8–7,4. Dehydration was performed in graduated ethanol (20–100%). It was concluded that the SEM method is highly informative for the identification of Scenedesmaceae with a structured cell wall.