Microbial activity in Martian analog soils after ionizing radiation: implications for the preservation of subsurface life on Mars

Abstract

At present, the surface of Mars is affected by a set of factors that can prevent the survival of Earth-like life. However, the modern concept of the evolution of the planet assumes the existence more favorable for life climate in the past. If in the past on Mars had formed a biosphere, similar to the one that originated in the early Earth, it is supposed that it is preserved till now in anabiotic state in the bowels of the planet, like microbial communities inhabiting the ancient permafrost of Arctic and Antarctic. In the conditions of modern Martian regolith, this relic life seems to be deprived of the possibility of damage reparation (or these processes occur on a geological time scale), and ionizing radiation should be considered the main factor inhibiting such anabiotic life. In the present study, we studied soil samples, selected in two different extreme habitats of the Earth: ancient permafrost from the Dry Valleys of Antarctica and Xerosol soil from the mountain desert in Morocco, gamma-irradiated with 40 kGy dose at low pressure (1 Torr) and low temperature (-50 °C). Microbial communities inhabiting these samples showed in situ high resistance to the applied effects, retained high number of viable cells, metabolic activity, and high biodiversity. Based on the results, it is assumed that the putative biosphere could be preserved in the dormant state for at least 500 thousand years and 8 million years in the surface layer of Mars regolith and at 5 m depth, respectively, at the current level of ionizing radiation intensity.

Keywords: Mars; astrobiology; gamma radiation; microbial communities; radioresistance.

Figure 1.. Effect of gamma radiation (40 kGy), low temperature (−50 °C), and low pressure (1 Torr) on the bacterial CFU count: (a) S1 sample; (b) A-6/99-6 sample. The number of CFU/g on PYG medium—white columns; the number of CFU/g on ½R2A medium—gray columns; the total number of prokaryotes (cells/g)—black columns. The error bars is within the confidence intervals at p < 0.05. Statistically different pairs of values are marked with asterisks.

Figure 2.. Effect of gamma radiation (40 kGy), low temperature (−50 °C), and low pressure (1 Torr) on the number of metabolically active cells of Bacteria and Archaea: (a) sample S1, (b) sample A-6/99-6; number of cells/g: archaea—white columns, bacteria—black columns. The error bars is within the confidence intervals at p < 0.05. Statistically different pairs of values are marked with asterisks.

Figure 3.. Substrates consumption by microbial communities: (a) sample S1, (b) sample A-6/99-6, before and after exposure to gamma radiation (40 kGy), low pressure (1 Torr) and low temperature (−50 °C). Control samples—white columns; irradiated samples—black columns. P: pentoses; H: hexoses; O: oligoses; Alc: alcohols; Am: amino acids; SoCA: salts of carboxylic acids; PM: polymers; AAN: amides, amines, nucleosides. Statistically different pairs of values are marked with asterisks.

Figure 4.. Cluster analysis (by Ward's method) of substrate consumption spectra by microbial communities of control and irradiated samples. Linkage distance: 1-Pearson r, n = 47.