Abstract
Aqueous solutions on Mars are theorized to contain very different ion compositions than those on Earth. To determine the effect of such solutions on typical environmental micro-organisms, which could be released from robotic spacecraft or human exploration activity, we investigated the resistance of Sphingomonas desiccabilis to brines that simulate the composition of martian aqueous environments. S. desiccabilis is a desiccation-resistant, biofilm-forming microbe found in desert crusts. The viability of cells in both planktonic and biofilm forms was measured after exposure to simulated martian brines. Planktonic cells showed a loss of viability over the course of several hours in almost all of the seven brines tested. Biofilms conferred greater resistance to all the brines, including those with low water activity and pH, but even cells in biofilms showed a complete loss of viability in <6 h in the harsher brines and in <2 days in the less harsh brines. One brine, however, allowed the microbes to maintain viability over several days, despite having a water activity and pH lower and ionic strength higher than brines that reduced viability over the same timescales, suggesting important ion-specific effects. These data show that biofilm-forming cells have a greater capacity to resist martian aqueous extremes, but that evaporative or deliquescent brines are likely to be destructive to many organisms over relatively short timescales, with implications for the habitability of Mars and for micro-organisms dispersed by robotic or human explorers.
Highlights
The search for habitable environments in the universe centers on the availability of liquid water, and aqueous environments are known to exist on a number of planetary bodies, including Mars, Europa, Enceladus, and Titan (Fortes, 2000; Kivelson et al, 2000; Mellon and Phillips, 2001; Squyres et al, 2004; Postberg et al, 2011)
To determine the effect of such solutions on typical environmental micro-organisms, which could be released from robotic spacecraft or human exploration activity, we investigated the resistance of Sphingomonas desiccabilis to brines that simulate the composition of martian aqueous environments
We sought to address the hypothesis that as on Earth, specific adaptations of microbial growth would influence the extent to which organisms could persist in extreme extraterrestrial aqueous environments
Summary
The search for habitable environments in the universe centers on the availability of liquid water, and aqueous environments are known to exist on a number of planetary bodies, including Mars, Europa, Enceladus, and Titan (Fortes, 2000; Kivelson et al, 2000; Mellon and Phillips, 2001; Squyres et al, 2004; Postberg et al, 2011). Biofilms are important in understanding microbial survival and growth in extremes because they offer resistance against a number of extreme conditions, including bactericidal chemicals (Luppens et al, 2002), rapid changes in temperature and pH (Koerdt et al, 2010), and ultraviolet radiation (Niemira and Solomon, 2005) These physical stresses are all associated with martian surface conditions. The definition of ‘‘special regions’’ (Rettberg et al, 2016) and whether terrestrial organisms accidently introduced into extraterrestrial brines by spacecraft could persist in these regions require an understanding of their potential habitability with respect to terrestrial organisms These implications are relevant for Mars and for the icy moons in the outer solar system, which are thought to have high levels of dissolved salts in their liquid water interiors (Hand and Carlson, 2015). Understanding how different microbes are able to survive in these environments is important for understanding their habitability
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