Dependence of cyanobacterium growth and Mars-specific photobioreactor mass on total pressure, pN2 and pCO2

Abstract

In situ resource utilization systems based on cyanobacteria could support the sustainability of crewed missions to Mars. However, their resource-efficiency will depend on the extent to which gases from the Martian atmosphere must be processed to support cyanobacterial growth. The main purpose of the present work is to help assess this extent. We therefore start with investigating the impact of changes in atmospheric conditions on the photoautotrophic, diazotrophic growth of the cyanobacterium Anabaena sp. PCC 7938. We show that lowering atmospheric pressure from 1 bar down to 80 hPa, without changing the partial pressures of metabolizable gases, does not reduce growth rates. We also provide equations, analogous to Monod's, that describe the dependence of growth rates on the partial pressures of CO₂ and N₂. We then outline the relationships between atmospheric pressure and composition, the minimal mass of a photobioreactor's outer walls (which is dependent on the inner-outer pressure difference), and growth rates. Relying on these relationships, we demonstrate that the structural mass of a photobioreactor can be decreased - without affecting cyanobacterial productivity - by reducing the inner gas pressure. We argue, however, that this reduction would be small next to the equivalent system mass of the cultivation system. A greater impact on resource-efficiency could come from the selection of atmospheric conditions which minimize gas processing requirements while adequately supporting cyanobacterial growth. The data and equations we provide can help identify these conditions.

Fig. 1. System integrated to Atmos for sampling cultures without altering the atmospheric conditions in the vessels.

Left: Close-up of the sample lifting system, composed of a Teflon sample container (a) placed inside the vessel and of an outer piece (b) connected to it by two pairs of neodymium magnets. Right: View of one vessel. Liquid samples can be brought to within a few centimetres of the lid by using the sample lifting system, then collected with a syringe and a needle inserted through a sampling port (c) integrated into the lid.

Fig. 2. Impact of the partial pressures of CO₂ and N₂, and of the total pressure, on the growth rates of Anabaena sp.

An equilibrium is assumed between the atmosphere and the culture medium. Left: Specific growth rate (SGR) as a function of pN₂ under 10 hPa of pCO₂ (top), or as a function of pCO₂ under 800 hPa of pN₂ (bottom). Dots correspond to experimental data (six biological replicates per condition); lines represent equations, analogous to Monod’s, whose parameter values are indicated. Top right: SGR as a function of total pressure, in an Ar atmosphere (carbon and nitrogen sources were provided in the culture medium). Horizontal lines show average values for six biological replicates (dots); differences are not significant (two-way ANOVA, p > 0.05). Bottom right: Heatmap showing SGR as a function of pCO₂ and pN₂.

Fig. 3. Minimum pCO₂, pN₂, partial pressure of water vapour (pH₂O) and total gas pressure required to support the photoautotrophic, diazotrophic growth of Anabaena sp. at given specific growth rates (SGR).

An equilibrium is assumed between the atmosphere and the culture medium. Two scenarios are considered for the total pressure: one where N₂ is provided as Martian atmospheric gases purified from CO₂ (total pressure), the other where N₂ has been further purified by removing Ar and trace gases (total pressure [N₂ purified]).

Fig. 4. Impact of pressure requirements on the minimal mass of a photobioreactor’s outer walls.

The assumed photobioreactor is a bubble column with a liquid phase twice as high as wide (except for the bottom-right panel) and of one cubic metre, a gas phase increasing the total height by one tenth, and a maximal gas superficial velocity of 0.08 m s⁻¹. The ambient (outer) pressure is assumed to be 6 hPa. Top left: Minimal total pressure (including gas and hydrostatic pressures), pN₂ and pCO₂ required, as a function of biomass concentration (C_x), to maintain a specific growth rate of 0.2 day⁻¹. Bottom left: Minimal total pressure, pN₂ and pCO₂ required, as a function of C_x, to maintain a biomass productivity of 100 g m^-3 day⁻¹ (assuming a molar mass of 32.965 g mol_x⁻¹ for Anabaena sp., as previously assessed). Top right: Minimal mass of a photobioreactor’s outer walls required as a function of total pressure, for different materials. Bottom right: Minimal mass of a PMMA photobioreactor’s outer walls required to withstand an inner pressure of 100 hPa, as a function of reactor height (under a constant volume of 1 m³).