Abstract EANA2025-90

While assessing the potential for life on present-day Mars, it is important to examine a range of environmental factors, with the presence of water being especially crucial. While surface features suggest that Mars once had extensive bodies of liquid water, the planet’s current environment makes it difficult for water in a liquid state to remain stable on the surface. Nonetheless, liquid water could still exist, although only temporarily, in the form of liquid brines possibly found on the Martian surface or in shallow subsurface areas.

This study investigated the effects of three Martian-relevant salts – sodium perchlorate, sodium chlorate, and sodium chloride – on the motility and survivability of Escherichia coli. These three salts have been previously detected at different locations across Mars and, thus, could point toward: (1) the existence of liquid water on or just below the planet’s surface and (2) the planet’s potential for habitability. E. coli is a well-known model organism for understanding microbial responses to various environmental stressors. We investigated bacterial survivability under varying concentrations of each salt at different timepoints, by standard colony plating and, subsequent, CFU/mL estimation. Regarding motility, we performed a semi-automated motility analysis in two-dimensions, based on motion history image creation, in which we extracted a single population-based metric (i.e., the percentage of motile cells) and multiple individual-based metrics (e.g., the mean average speed of cells). In addition, a combined analysis incorporating both survivability and motility data was performed, by estimating the percentage of motile cells among viable organisms post-salt-exposure.

Our results showed that both survivability and motility were generally affected by salt concentration and exposure time, with the extent of these effects differing depending on the salt type. Particularly, sodium perchlorate was the most toxic salt to the cells, followed by sodium chlorate, with sodium chloride being the least harmful sodium compound. Notably, we also observed a short-lived increase in overall motility despite rapid declines in cellular survivability at certain salt concentrations, particularly under sodium chlorate and sodium perchlorate stresses. These observations suggest a stress response mechanism related to motility.

Given that motility can enhance an organism's ability to navigate harsh and variable environments while searching for more suitable conditions for growth (i.e., taxis or directional movement), it holds promise as a key biosignature for the search of life on Mars. Nevertheless, to improve the overall chances of detecting life during future space exploration missions to our neighbouring red planet, further research is still needed to fully assess the potential of motility as a reliable biosignature as well as to more precisely identify locations on Mars that could likely contain liquid water.

Keywords: motility; survivability; biosignature; Mars; sodium perchlorate; sodium chlorate; and sodium chloride.