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.

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.