## 1 Introduction
In turn, the CO\({}_{2}\) concentration in the gas-air environment during the microalgae cultivation affects not only the growth of the culture but also the biochemical composition of the biomass, which makes it possible to obtain a biomass richer in lipids or carbohydrates for further processing of the biomass into biodiesel or bio-alcohol, respectively. Therefore, some factors affecting the content of carbohydrates in the biomass of _Arthrospina platensis_ were studied in [14]. Cultivation was carried out in a Zarrouk-modified medium with a reduced content of nitrogen-containing salts and with the use of fly ashes and a constant CO\({}_{2}\) concentration of 2%. At the same time, a carbon dioxide-air mixture was added to the bubbling reactor every 40 min in the ratio of 1:9. Such an experiment made it possible to increase the content of carbohydrates in the final biomass up to 50%, so it can later be used for more efficient production of bioethanol. In [15], an experiment was carried out for _Arthrospina platensis_ cultivation in an atmosphere containing 15% CO\({}_{2}\). It was shown that adding CO\({}_{2}\) leads to an increase in the total sugar concentration for the further producing of biomass into biogas. The experiment was also carried out to study the efficiency of _Arthrospina platensis_ biomass growth in atmospheres containing 2, 5, 10 and 15 vol.% of carbon dioxide [16]. The experiment showed that a CO\({}_{2}\) concentration increase had a positive effect on the growth of microorganisms in comparison with the control sample that was grown under atmospheric conditions. However, the maximum growth rate was shown by microalgae grown in CO\({}_{2}\) at 10%; this means that further increases in carbon dioxide concentration could have a harmful effect on the cultivation efficiency. It was also seen in this experiment that when increasing the concentration of carbon dioxide, the biochemical composition of the biomass changed in favor of a significant increase in carbohydrate content. According to [17], _Arthrospina platensis_ is tolerant to a CO\({}_{2}\) concentration of 12% in the atmosphere, which also confirms the possibility of its use as a CO\({}_{2}\)-capture agent. In this way, we can say that the use of _Arthospira platensis_ is very effective for capturing atmospheric CO\({}_{2}\) and that this process allows the further processing of the biomass into biofuel. A lot of research has also been carried out to study the effectiveness of CO\({}_{2}\) fixation and the further processing of the obtained biomass into biofuel for other types of microalgae. The influence of CO\({}_{2}\) concentration in the atmosphere on lipid content in biomass for further processing into biodiesel has been studied [2; 18; 19]. The growth rate of different strains of microalgae according to CO\({}_{2}\) concentration in the atmosphere and in flue gas was compared in [2]. It was shown that the growth of an organism can be inhibited due to the presence of such strong oxidizing agents as sulfur- and nitrogen-containing oxides. It was found that a CO\({}_{2}\) concentration increase in the gas-air environment has a different effect on different types of microalgae: depending on microalgae strain, the CO\({}_{2}\) concentration increase in the gas-air environment leads to an increase or decrease of the biomass growth rate. Another reported investigated _Desmarestia viridis_ growth in conditions of increased CO\({}_{2}\) concentration and nitrogen deficiency, which led to a 21% increase in the lipid part of the biomass [18]. The strain _Microactinium_ sp. was also cultivated using different CO\({}_{2}\) concentrations, light intensities and cultivation times [19]. As with the previous samples, the microalgae showed a greater productivity with the increased CO\({}_{2}\) concentration in the atmosphere and was also characterized by a high lipid concentration.