Influence of a low atmospheric pressure on the growth and lipid composition of Chlorella vulgaris

Within crewed missions to Mars, microalgae could be used for the production of essential consumables. Their cultivation there could be less resource-intensive if the pressure inside bioreactors was reduced. However, whether this would decrease their growth is unclear; and how the composition of their lipidome would change (for instance, to compensate for the tendency of low pressure to increase membrane fluidity) is unknown. Here we demonstrate that the growth of Chlorella vulgaris is unaffected by a reduction in total pressure from 1000 hPa (ca. Earth at sea level) to 100 hPa if the partial pressure of carbon dioxide is constant. We then show that the microalga can grow vigorously, reaching above 1.5 g L−1 within 4 days, under cultivations conditions analogous to these foreseen on Mars: a leachate of a Martian regolith simulant mixed with synthetic urine, under an atmosphere of 100 hPa of carbon dioxide. Conversely, a decrease in pressure altered the lipidome. Some changes may have helped preserve membrane viscosity: as an example, the MGDG-to-DGDG ratio increased. However, other alterations do not clearly point in that direction: the overall ratio of unsaturated-to-saturated fatty acids, as well as the average degree of unsaturation in thylakoid membrane lipids, for instance, only decreased with pressure when carbon dioxide (rather than carbonate) was the carbon source. Regardless, these alterations are small when compared to those induced by a change in the inorganic carbon source. Our results overall suggest that relying on a low pressure would benefit the resource-efficiency of microalgal cultivation on Mars.