Endospore integrity and viability of Bacillus subtilis on simulated Martian regolith in rotational UVC radiation exposures

Current understanding of Martian regolith has advanced due to various rover explorations. Due to this, there are now several variants of Martian regolith that are chemically known and commercially available. These simulants are vital for panspermia models that suggest a transfer of simple life throughout the solar system via ejecta containing life from Mars when its surface was more favourable to host life. Bacteria that produce endospores are suitable candidates for these models as they have adequate protection from the harsh conditions of space, particularly against solar radiation. To assess this, the simple endospore former, Bacillus subtilis, was assessed for growth on several Martian regolith that represented different locations and epochs of Mars. This consisted of a conventional Martian regolith, a sulphur rich regolith, and a simulant of the Jezero crater which was thought to have once been flooded with water. We found that the sulphur rich regolith inhibited endospore formation, while the other two variants favoured endospore production. Interestingly, we also identified that pulsing of UVC in a simulation of endospores on rotational ejecta show that endospores break down faster with lower rotational frequency despite receiving the same UVC dose. Moreover, and most strikingly is that viable endospores after surviving UVC dosage displayed elevated expression of the DNA damage SOS response gene, RecA. Importantly, this study suggests that astrobiological approaches that utilise endospore viability as a benchmark for survival require reassessment as genomic integrity may be compromised.