From earth to space: how bacterial consortia and green compost improve lettuce growth on lunar and martian simulants

The possibility of establishing human colonies on the Moon or Mars depends on the ability to regenerate environmental resources and produce food on-site. In this context, plant-based systems, such as Bioregenerative Life Support Systems, offer a promising solution for regenerating environmental resources, primarily air and water, and producing fresh food, partly recycling crew’s organic waste. This study investigates the role of selected consortium of plant growth-promoting bacteria (PGPB) Azotobacter chroococcum 76A, Priestia megaterium EL5, Methylobacterium populi VP2, and Kosakonia pseudosacchari TL13 in enhancing plant growth on Martian and Lunar regolith simulants, focusing on the effect on nutrient bioavailability in a system with zero external input aside from green compost derived from plant residues (pH 8.25 and a low C/N ratio), using lettuce as a model plant. The compost amendment improved lettuce biomass, particularly on the Lunar simulant (moving from 0.1 to 1.3 g plant−1), and nutrient bioavailability in all the amended substrates (+ 23% on average). PGPB treatment enhanced leaf biomass by 35% and its mineral nutritional content (+ 26% on average), indicating that the microbial consortium could exert positive impact on plants grown in nutrient-limited substrates. Detailed analysis of the effect of microbial inoculation revealed an increase in functional genes linked with nitrogen fixation and phosphorus solubilization in non-amended inoculated substrates. High throughput sequencing revealed an increment of bacterial diversity in amended substrates, and a significant shift in microbial communities due to inoculation in non-amended substrates. Moreover, the positive correlation between nutrient bioavailability in the rhizosphere and the presence of PGPB in Martian simulant substrates suggests that PGPB inoculation can enhance nutrient uptake efficiency under Martian soil conditions. Our study indicates that the application of microbial consortium along with green compost is a promising strategy to improve regolith simulants fertility and thus, plant development, passing nutrient-deficient conditions and offering applications for future long-term space missions.