In modern energy, various technologies for absorbing carbon dioxide from the atmosphere are being considered, including photosynthetic microalgae. An important task is to obtain maximum productivity at high concentrations of CO₂ in gas–air mixtures. In this regard, the aim of the investigation is to study the effect of light intensity on the biomass growth and biochemical composition of five different microalgae strains: Arthrospira platensis, Chlorella ellipsoidea, Chlorella vulgaris, Gloeotila pulchra, and Elliptochloris subsphaerica. To assess the viability of microalgae cells, the method of cytochemical staining with methylene blue, which enables identifying dead cells during microscopy, was used. The microalgae were cultivated at 6% CO₂ and five different intensities: 80, 120, 160, 200, and 245 μmol quanta·m⁻²·s⁻¹. The maximum growth rate among all strains was obtained for C. vulgaris (0.78 g·L⁻¹·d⁻¹) at an illumination intensity of 245 µmol quanta·m⁻²·s⁻¹. For E. subsphaerica and A. platensis, similar results (approximately 0.59 and 0.25 g·L⁻¹·d⁻¹ for each strain) were obtained at an illumination intensity of 160 and 245 µmol quanta·m⁻²·s⁻¹. A decrease in protein content with an increase in illumination was noted for C. vulgaris (from 61.0 to 46.6%) and A. platensis (from 43.8 to 33.6%), and a slight increase in lipid content was shown by A. platensis (from 17.8 to 21.4%). The possibility of increasing microalgae biomass productivity by increasing illumination has been demonstrated. This result can also be considered as showing potential for enhanced lipid microalgae production for biodiesel applications.

The influence of elevated CO₂ concentrations on the growth and viability of various microalgae strains was studied. Arthrospira platensis, Chlorella ellipsoidea, Chlorella vulgaris, Gloeotila pulchra, and Elliptochloris subsphaerica were tested. The cultivation of microalgae was carried out at constant CO₂ concentrations (0.04, 3, 6, or 9%—sequentially from lower to higher concentrations), under constant (24 h·day⁻¹) illumination with an intensity of 74.3 µmol quanta·m⁻²·s⁻¹, and a constant temperature of 23.5 ± 0.5 °C. The optical density of the microalgae biomass, pH, and the chemical composition of the culture medium were measured. Microscopy (including the cytochemical microscopic method) was conducted to monitor the state of the microalgae. The highest biomass growth rate (0.37 g·L⁻¹·day⁻¹), among all experiments, was achieved for Chlorella vulgaris at CO₂ = 3% and for Chlorella ellipsoidea at CO₂ = 6 and 9%. The lowest growth rate (0.12 g·L⁻¹·day⁻¹) was achieved for Arthrospira platensis at CO₂ = 3 and 9%. The microscopy results showed the absence or a minimum number of dead cells of the strains under selected conditions. The ability to maintain the viability of cultures up to significant concentrations of CO₂ = 9% was due to adaptation (gradual increase in CO₂ concentrations in the experiments).

Direct study of CO₂ capture efficiency during microalgae Arthrospira platensis cultivation at high CO₂ concentrations was carried out. Microalgae were grown in a 90 L photobioreactor on Zarrouk’s medium prepared with distilled water. Three 15-day experiments were carried out with different initial CO₂ concentrations: 1, 5, and 9 vol.%. During the experiments, both the change in the optical density of the microalgae suspension and the direct change in the CO₂ concentration in the chamber were measured. The maximum decrease in CO₂ concentration due to the growth of microalgae was 0.10 vol.% (CO₂)/day in the experiment with an initial CO₂ concentration of 5 vol.%. Growth rate of biomass density was 79.4, 76.3, and 48.4 (mg/L)/day at 1, 5, and 9 vol.% CO₂ concentrations, respectively. During the experiment with initial CO₂ concentrations of 1 and 5 vol.%., pH of the culture medium was increased, but pH was decreased from 9.2 to 8.8 at 9 vol.%. In general, good viability (high quality of biomass and high rate of its growth) of Arthrospira platensis was established at 1 and 5 vol. (CO₂)%, while massive death of Arthrospira platensis cells was observed in the experiment with 9 vol. (CO₂)%. Biochemical analysis of the resulting biomass revealed a decrease in the content of lipids and proteins with an increase in CO₂ concentration.

In this work, the path from the cultivation of Arthrospira platensis at an increased concentration of CO₂ to the production of bio-oil by hydrothermal liquefaction (HTL) of the grown biomass is realized. The cultivation was carried out in a 90 L photobioreactor at an initial CO₂ concentration of 8 vol.% for 15 days. During the cultivation stage, the optical density for microalgae suspension, pH and chemical composition of nutrient medium were monitored. The grown biomass was separated from the nutrient medium with a 100 µm mesh and then subjected to HTL at 330 °C for 1 h. The biomass growth rate was 82 ± 4.1 mg × L⁻¹day⁻¹ and the pH was in the range from 9.08 ± 0.22 to 8.9 ± 0.24. Biochemical and CHNS analyses were applied for the obtained biomass. The contents of carbohydrates, proteins and lipids in the grown biomass were 38.7 ± 0.4 wt.%, 37.4 ± 0.5 wt.% and 3.8 ± 0.4 wt.%, respectively. Bio-oil yield after the HTL procedure was 13.8 wt.%. The bio-oil composition and properties were determined by GH-MS, TLC-PID and ICP-MS techniques. ICP-MS revealed the contents of 51 metals in bio-oil.