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Abstract:
The Martian surface and shallow subsurface lacks stable liquid water, yet hygroscopic salts in the
regolith may enable the transient formation of liquid brines. This study investigated the combined
impact of water scarcity, UV exposure, and regolith depth on microbial survival under Marslike
environmental conditions. Both vegetative cells of Debaryomyces hansenii and Planococcus
halocryophilus, alongside with spores of Aspergillus niger, were exposed to an experimental chamber
simulating Martian environmental conditions (constant temperatures of about − 11 °C, low pressure
of approximately 6 mbar, a CO2
atmosphere, and 2 h of daily UV irradiation). We evaluated colonyforming
units (CFU) and water content at three different regolith depths before and after exposure
periods of 3 and 7 days, respectively. Each organism was tested under three conditions: one
without the addition of salts to the regolith, one containing sodium chlorate, and one with sodium
perchlorate. Our results reveal that the residual water content after the exposure experiments
increased with regolith depth, along with the organism survival rates in chlorate-containing and
salt-free samples. The survival rates of the three organisms in perchlorate-containing regolith were
consistently lower for all organisms and depths compared to chlorate, with the most significant
difference being observed at a depth of 10–12 cm, which corresponds to the depth with the highest
residual water content. The postulated reason for this is an increase in the salt concentration at this
depth due to the freezing of water, showing that for these organisms, perchlorate brines are more
toxic than chlorate brines under the experimental conditions. This underscores the significance of
chlorate salts when considering the habitability of Martian environments.
