A paper published in Applied and Environmental Microbiology identified 23 fungal strains isolated from NASA spacecraft assembly cleanrooms capable of surviving ultraviolet radiation exposure.
NASA’s Curiosity Mars rover. Credit:
NASA/JPL-Caltech/MSSS
Using experimental simulation facilities, the researchers conducted a comprehensive assessment of microbial survivability and morphology on the most resilient spacecraft-associated microorganisms.
Simulating Mars and sterilization protocols
The researchers used a Mars Simulation Chamber, a gas-tight chamber replication Martian UV-visible radiation, atmospheric composition (95.8% CO₂, 2.1% Ar, 1.9% N₂ and 0.2% O₂) and pressure (6 mbar) to test the fungal strains. They also exposed the strain to temperatures of 125 and 150 degrees Celsius to test the efficacy of common spacecraft sterilization protocols.
The scientists also exposed the fungal strains to Californium-252 to provide a chronic low-dose neutron field of 2.16 mGy per day, mimicking the galactic cosmic rays encountered during deep-space travel. During initial screening, 23 fungal strains survived 3,000 Jm-2 of UVC exposure. All 23 UVC-resistant strains belonged to filamentous ascomycete fungi.
One strain, Aspergillus calidoustus, demonstrated remarkable survival under simulated Martian conditions, withstanding up to 1,440 minutes of Martian solar irradiation, Mars atmospheric pressure and composition and the presence of Martian regolith. Lethality only occurred under combined irradiation and cooling to negative 60 degrees Celsius, the mean Mars surface temperature. A. calidoustus survived long-duration neutron radiation exposure and dry-heat microbial reduction technique.
A. calidoustus exhibited high resilience to neutron radiation, with a 35% reduction in viable conidia after one month of exposure and a 57% reduction in viable conidia after six months of exposure. The strain also survived 120 minutes at 125 degrees Celsius with only a 3-log loss, proving more heat-resistant than the standard bacterial bioindicator, Bacillus pumilus.
The CO2-rich simulated Martian atmosphere actually appeared to have a cryoprotective effect on A. calidoustus during cooling. However, this was negated by the presence of Martian regolith, likely due to increased thermal conductivity.
Scanning Electron Microscopy (SEM) revealed deep pitting and scarring on the conidium surface following exposure to the Martian atmosphere and irradiation.
Missions might have already brought fungi to Mars
This was the first study to perform and end-to-end evaluation of eukaryotic microbial survival across conditions that occur during preparation for, travel to and robotic exploration of Mars. The capacity for fungal conidia to survive multiple space-relevant conditions suggests their potential as forward contaminants, capable of being transported to and persisting on Mars. The results also raise the possibility that previous missions may have already brought microbes from Earth to Mars.
The research also has potential implications for biocontamination within the food, pharmaceutical and medical sectors.
