Modeling of Gas Exchange in the BIOS‑3 Closed Ecosystem

The creation of biotechnological life support systems (BLSS) is impossible without the development of their mathematical models, which in turn require verification based on available experimental data. This study is devoted to the development of a mathematical model of gas exchange in a closed ecosystem of BIOS‑3 and its verification. The key feature of gas exchange in the BIOS‑3 experiments was the high (1–2 %) concentration of carbon dioxide in the household compartment of the system, which was in a quasi-stationary state during each of the longest experiments (in 1973 and 1978). That could mean that the system had mechanisms for self-stabilization. However, the most obvious mechanism of autostabilization due to plant photosynthesis does not work at these concentrations because photosynthesis reaches carbon dioxide saturation, which occurs at a concentration of 0.15 % for wheat and vegetables. Two possible explanations for the abnormal concentration of carbon dioxide in the BIOS-3 system have been proposed: spatial heterogeneity of the gas composition and the inhibition of human respiration at concentrations of carbon dioxide close to 1.5 %, which is described in the literature. A computer model of BIOS-3 has been created that contains both of the proposed mechanisms. The model describes the interactions between the biological components of the BLSS: plants and humans. The equations and coefficients of the model have been determined based on experimental data from the BIOS‑3 facility. Simulations of isolation experiments have been carried out, which have shown the possibility of reproducing quasi‑stationary high concentrations within the framework of both proposed mechanisms. The results obtained are valuable for the development of future BLSS systems. In particular, the study demonstrates the importance of controlling the gas exchange between the compartments of the system.