Effects of Desiccation and Freezing on Microbial Ionizing Radiation Survivability: Considerations for Mars Sample...

Area of Science:

• Astrobiology and Planetary Protection
• Microbial Radiation Resistance
• Space Exploration

Background:

• National space agencies are planning Mars sample return missions, necessitating robust planetary protection measures.
• Understanding microbial survival under Martian surface conditions is crucial to prevent extraterrestrial contamination.
• Ionizing radiation on Mars poses a significant challenge to microbial life over geological timescales.

Purpose of the Study:

• To investigate the impact of desiccation and freezing on the radiation survival of key microorganisms.
• To assess the potential for microbial contamination during Mars sample return missions.
• To determine the long-term survivability of microbes on the Martian surface and subsurface.

Main Methods:

• Tested the ionizing radiation survival of six model microorganisms (Deinococcus radiodurans, Escherichia coli, Saccharomyces cerevisiae, and three Bacillus species) under various conditions.
• Exposed microbial cells and endospores to desiccation and freezing, both separately and in combination, before irradiation.
• Measured survival rates after exposure to ionizing radiation, correlating with genomic characteristics and antioxidant levels.

Main Results:

• Desiccation and freezing synergistically enhanced radiation survival in polyploid vegetative cells, notably extending Deinococcus radiodurans survival to 140 kGy.
• Monogenomic and digenomic Bacillus cells and endospores showed limited radiation survival (12 kGy) without synergistic protection.
• Polyploid organisms with high Mn antioxidant levels and multiple genome copies exhibit extreme radioresistance.

Conclusions:

• Polyploid microorganisms, particularly Deinococcus radiodurans, can survive extreme radiation levels on Mars for millions of years when desiccated and frozen.
• Findings suggest permanent forward contamination of Mars is likely and backward contamination is a possibility.
• The study underscores the importance of stringent planetary protection protocols for Mars sample return missions.