Effect of altered nitrogen partial pressure on Chlorellaceae for spaceflight applications

Algal-based photobioreactors offer a promising bioregenerative technology that can perform the functions of air revitalization, water recycling, and food production in a human spacecraft life support system. In order to assess their performance in this context, this study exposed a non-axenic green algal culture (Chlorellacea) to an atmospheric pressure of 8.2psia [56.5 kPa] with an oxygen concentration of 34%, which are currently proposed cabin atmosphere conditions for future space exploration missions. Growth and metabolism of green algae were compared to a control experiment kept at 14.7psia [101.3 kPa] with oxygen and carbon dioxide partial pressure maintained at constant levels of 3.1 and 0.147psia [21.3 and 1.0 kPa], respectively, which represent concentrations of 21 and 1% at standard sea level pressure. To conduct the evaluation, a flow-through photobioreactor was developed that can precisely control atmospheric compositions such as partial pressures of oxygen and carbon dioxide and set the total pressure solely by altering the nitrogen partial pressure. Algal performance was measured by daily cell counts and optical density measurements. Additionally, continuous measurements of dissolved oxygen and pH in the liquid phase, as well as evolved oxygen and absorbed carbon dioxide in the gaseous phase, were collected. A metagenomic analysis was conducted on the algal culture to assure the composition of the algal culture remained consistent during the experimental phase, as well as to characterize the nature of coexisting contaminants in the non-axenic culture. The performance of the algal culture was compared to related, previously published literature values. The hypothesized increase in performance of the algal culture under reduced pressures was not verified, rather it was shown that the nitrogen partial pressure neither benefits nor inhibits algal growth under the tested conditions.