Salt Tolerance and UV Protection of Bacillus subtilis and Enterococcus faecalis under Simulated Martian Conditions.

ABSTRACT

Ultraviolet (UV) irradiation on the surface of Mars is an important factor that affects the survivability of microorganisms on Mars. The possibility of martian brines made from Fe2(SO4)3, MnSO4, and MgSO4 salts providing a habitable niche on Mars via attenuation of UV radiation was investigated on the bacteria Bacillus subtilis and Enterococcus faecalis. Results demonstrate that it is possible for brines containing Fe2(SO4)3 on Mars to provide protection from harmful UV irradiation, even at concentrations as low as 0.5%. Brines made from MnSO4 and MgSO4 did not provide significant UV protection, and most spores/cells died over the course of short-term experiments. However, Fe2(SO4)3 brines are strongly acidic and thus were lethal to E. faecalis, when cells were exposed for 7 days. In contrast, B. subtilis, a spore-forming bacterium resistant to pH extremes, was unaffected by the acidic conditions of the brines and did not experience any significant lethal effects in Fe2(SO4)3. Any extant microbial life in martian Fe2(SO4)3 brines (if present) would need to be capable of surviving acidic environments, if these brines are to be considered a possible habitable niche. The results from this work are important to the search for life on planets with atmospheres that do not significantly attenuate UV radiation (i.e., like Mars) and to planetary protection, since it is possible that terrestrial bacteria in the genus Bacillus are likely to survive in Fe-sulfate brines on Mars.