## 3 Results
### CO\({}_{2}\) Absorption Efficiency  
The data from the CO\({}_{2}\) concentration changes in the atmospheric gas chamber during the cultivation of _Arthrospira platensis_ at a CO\({}_{2}\) volume of 8% are shown in Figure 3. The ratio of the gas-air environment volume to the working volume of the microalgae suspension was 120 (12 m\({}^{3}\)/0.1 m\({}^{3}\)). The value of CO\({}_{2}\) changed from 8 vol.% at the beginning of the experiment to 6.47 vol.% at the end of the experiment. The average rate of CO\({}_{2}\) change over the whole experiment was 0.109 vol.%/day; the average rate over the first half of the experiment was 0.107 vol.%/day, whereas in the second half of the experiment it was 0.11 vol.%/day. In our previous work with this strain [27; 34], the average rates varied from 0.14 vol.%/day to 0.12 vol.%/day throughout the experiment. The experimental conditions and nutrient medium were identical to the experimental conditions and nutrient medium in the current work, the only exception was the initial concentration of carbon dioxide, which was 6 vol.% [27] or 5 vol.% [34]. At the same time, the spread between the average speeds of the first and second halves of the experiment was significant (additionally, in the 2 nd half of the experiments the average speeds were lower than those in the first half), which was caused by the adaptation of _Arthrospira platensis_ to the increased concentrations of carbon dioxide in the gas-air environment [27; 34].  
\begin{table}
\begin{tabular}{c c} \hline
**Experiment Day** & **Biomass Growth Rate, mg \(\times\) L\({}^{-1}\)day\({}^{-1}\)** \\ \hline
0 & \\
5 & 55 \(\pm\) 3.9 \\
10 & 80 \(\pm\) 7.6 \\
15 & 82 \(\pm\) 4.1 \\ \hline \end{tabular}
\end{table}
Table 1: Growth rate of biomass during the entire cultivation stage of _Arthrospira platensis_.  
Figure 2: Biomass density and pH changes during the 15-day cultivation of _Arthrospira platensis_.  
When considering the average rate of CO\({}_{2}\) absorption, we also need to take into account the losses of CO\({}_{2}\) from the chamber to the atmosphere. The gas losses in the chamber decrease in relative terms depending on the initial volume percentages of the studied gas medium in the chamber. Additionally, losses can be due to the methods of the experiment, the method of supplying gas to the PBR, the duration of maintaining the gas-air environment in the PBR and so on. With an initial CO\({}_{2}\) volume of 8 vol.%, the losses were 0.67% or 0.04 vol.%/day, which was also obtained directly. Therefore, the CO\({}_{2}\) absorption takes on average 0.11 \(-\) 0.04 = 0.07 vol.%/day in the atmospheric gas chamber during the process of photosynthesis.  
In the current work, we managed \"to include\" the biomass presence into the new conditions from the very beginning of the experiment, which shows the excellent adaptive ability of _Arthospira platensis_ to new extreme conditions in the shortest possible time. The biomass growth rate turned out to be linear, with the average rate of biomass growth in the first half of the experiment (the first 7 days of the experiment) being the same as the rate of biomass growth in the second half of the experiment (the last 7 days of the experiment).