Effects of atmospheric pressure, and of the partial pressures of CO2 and N2, on the growth rates of Anabaena sp. PCC 7938: Assessment and implications for cyanobacterium cultivation on Mars

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.

Resource-efficiency of cyanobacterium production on Mars: Assessment and paths forward

“…Further uncertainty comes from the provision of gaseous nutrients from the Martian atmosphere. While we here assume that pressurized, nitrogen-enriched gas will be a byproduct of carbon dioxide purification envisioned for, e.g., propellant or polymer production) (32), additional processing steps may be required (14). On the other hand, dinitrogen would not be the only source of nitrogen: nitrates have been detected in Martian meteorites (33,34) and within Gale crater materials (35).…”

“…Other gases from the Martian atmosphere (predominantly, argon), introduced together with dinitrogen as removing them from the dinitrogen source would cause unnecessary costs, vary similarly over time and peak at 20.98 mbar. The total pressure, on which depend the required level of robustnessand therefore the massof the cultivation hardware, as well as energy needs for pressurization (13,14), peaks at 121.11 mbar (183 mbar with full hydrostatic pressure;…”

“…The half velocity constants for dissolved inorganic carbon (𝐾 𝐷𝐼𝐶 , 𝑚𝑜𝑙 𝐷𝐼𝐶 𝑚 −3 ) and dissolved nitrogen (𝐾 𝑁2𝐿 , 𝑚𝑜𝑙 𝑁2𝐿 𝑚 −3 ), as well as the associated maximum growth rate (µ 𝑚𝑎𝑥𝐺𝐴𝑆 , 𝑠 −1 ), were determined previously (14). The growth rates dictated by the concentrations of dissolved inorganic carbon (µ 𝐷𝐼𝐶 , 𝑠 −1 ) or dissolved nitrogen (µ 𝑁2𝐿 , 𝑠 −1 )…”

“…Its partial pressure is calculated with standard methodology (51-53), as previously described (14). The total gas pressure inside the reactor is approximated to the sum of the partial pressures (Dalton's law):…”

“…One strategy proposed to feed these organisms lies in using selected species of diazotrophic, rock-weathering cyanobacteria as primary producers (12). An expanding body of work suggests that these could be fed with local resources: water mined from the ground and atmosphere; carbon and nitrogen sourced from the atmosphere as CO2 and N2 (13,14); and other nutrients leached from the local regolith (the Martian soil) (15)(16)(17). The cultivated cyanobacteria could then produce various consumables directly, such as O2, dietary proteins or biomaterials (18,19), but also be used for feeding heterotrophic microorganisms (13,20,21), plants (22), or other secondary producers.…”