The Hypopiezotolerant Bacterium, , Failed to Grow in Mars Analog Soils under Simulated Martian Conditions at 7 hPa.

The search for life on Mars is predicated on the idea that Earth and Mars life (if present) should be both carbon- and water-based with similar forms of evolution. However, the astrobiology community can currently only investigate plausible Martian microbial ecosystems by using Terran life-forms as proxies. In order to examine how life might persist on Mars, we used a hypopiezotolerant bacterium (def., able to grow at 7-10 hPa)--in growth assays with four Mars analog soils conducted under a subset of simulated Martian conditions including 7 hPa, 0 °C, and a CO-enriched anoxic atmosphere (called conditions). The four Mars analog soils included an dust analog, the Mars analog, a lander-site simulant, and a high- analog. cells were able to grow at 30 °C in a liquid minimal basal medium (MBM) supplemented with 10- or 20-mM sucrose, Spizizen salts, and micronutrients. When the four analog soils were doped with both MBM and cells of , and subsequently incubated at 30 °C for 72 h, cell densities increased between 2-logs ( analog) and 4-logs ( and analogs); the analog led to complete inactivation of within 24 h. In contrast, when the experiment was repeated, but incubated under conditions, cells were either killed immediately by the analog, or decreased by > 5 logs over 28 d by the , , and analogs. The failure of to grow in the analog soils under conditions was attributed to the synergistic interactions among six factors (i.e., low pressure, low temperature, anoxic atmosphere (i.e., the conditions), low-pH in the soil, dissolved salts in all analogs, and oligotrophic conditions) that increased the biocidal or inhibitory conditions within the analog soils. Results suggest that even if a hypopiezotolerant Terran microbe is displaced from a spacecraft surface on Mars, and lands in a hydrated and nutrient-rich niche, growth in the Martian regolith is not automatically assured.