Growth of natural microorganisms in Martian regolith simulant at reduced water activities

Water activity (aw) measures the free water available for microbial growth. While liquid water is unstable on Mars, there is significant water exchange between the atmosphere and the upper regolith (surface layer of fine mineral grains). Daytime aw on Mars is typically low, but nighttime aw can reach saturation (aw=1). For Martian habitability, it is assumed that Earth-like life requires aw > 0.5. However, the effect of aw on cell replication in Martian regolith remains untested. This study examines microbial growth in a Martian regolith simulant (MMS-2) at different aw levels provided by atmospheric vapour. Growth was monitored by measuring DNA mass. In MMS-2 and Bacillus subtilis-spiked MMS-2 at aw=1, DNA mass peaked after 7 and 3 days, respectively. At lower aw levels (0.75 and 0.65), DNA mass took 20 and 30 days to peak, with reduced growth at 0.34 aw. These findings suggest atmospheric water can be adsorbed into the regolith, enabling microbial growth in Martian-like conditions at lower aw than previously thought.