Microbial Life on Mars: The Response of Halophilic Archaea to Simulated Martian Conditions

One of the oldest and most exciting questions in science is: are we alone in the universe? During the last four billion years of Earth’s history, countless organisms have inhabited almost every environmental niche on the planet, from the deepest sea to driest deserts. However, so far no extraterrestrial life has been found. Studying the propensity for life on our neighboring planet, Mars, helps us understanding its potential for past and present life, and guides future missions. Liquid water is a prerequisite for life as we know it and recently, evidence of transient night time liquid brines on the surface of present day Mars have been theorized. These brines may be hyper-saline with high ionic strengths and varying pH-values. Halobacterium salinarum is an extremophilic (salt-loving) halophilic archaeon whose natural habitat includes hyper-saline brines, desiccating conditions and exposure to high fluences of solar UV radiation. Herein, we report the response of Hbt. salinarum following exposure to simulated Martian conditions, with regard to survival and DNA integrity. The simulated conditions include the synthetic Martian Brine Analogues (MBAs), diurnal-nocturnal temperature cycling, prolonged desiccation and Mars-like solar UV (200-400 nm) radiation. We also addressed the prolific space hardware contaminant, Bacillus subtilis whose endospores show substantial resistance against space conditions. The ambition was to investigate potential interplanetary forward contamination by Hbt. salinarum, should it have bacterial spores available as nutrients in the Martian brines. Halophiles are some of our best candidates for studying unicellular life on Mars and other bodies where liquid water is also stabilized by high salt concentrations. Moreover, Hbt. salinarum was able to survive over one month in the Martian brines, albeit with growth limited to one particularly hospitable brine. It displayed survival in the brines at relevant temperatures and with diurnal-nocturnal cycling but only when first desiccated to remove prevent water crystal formation. The radiation resistance was highly dependent on the choice of brine in which Hbt. salinarum was confined and desiccated. Even in the hospitable brines, the halophile lost over 90% of its viable population following irradiation equal to one Martian day, in our experimental setup. The inter-brine difference in DNA fragmentation following irradiation confirmed the difference in survival. Hbt. salinarum was subsequently unable to digest B. subtilis endospores for nutrient exploit and responded no differently than when nutrient-deprived. Surprisingly, the addition of otherwise available nutrients in the brines caused a hurried decrease in survival, with the exception of the hospitable brine. Despite its extremophilic and polyextremotolerant character, Hbt. salinarum is unlikely to survive, not to mention thrive, in a combination of all tested stressors