Physicochemical constraints on the habitability of extreme, Mars relevant brines

Critically evaluating the habitability of extraterrestrial environments requires an understanding of the factors that restrict the biosphere. Life is dependent on the occurrence of liquid water, thus its availability has long been recognised as a first order requirement for habitability. Many other factors however, including salt stress, coincide to determine the permissibility of aqueous environments. The evolution of Mars has been characterised by the prominent occurrence of concentrated saline brines, and present-day surface conditions favour their formation. Here we have used diverse, salt-tolerant microbial communities to dissect the habitability of simulated Mars-relevant brines. Our brines reflect the diversity of saline environments on Mars and the unique combinations of stresses they impose. These data show that despite current paradigms, water activity is not a sufficient predictor of habitability; indeed ionic strength may more suitably describe biological limits. Furthermore, we propose that the dominance of NaCl-enriched environments on Earth has biased our understanding of the capacity for life in high salt, and show that communities inhabiting other salts differ fundamentally from those in NaCl.