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It is well known that dissolved salts can significantly lower the freezing point of water and thus extend habitability to subzero conditions. However, most investigations thus far have focused on sodium chloride as a solute. In this study, we report on the survivability of the bacterial strain Planococcus halocryophilus in sodium, magnesium, and calcium chloride or perchlorate solutions at temperatures ranging from +25°C to −30°C. In addition, we determined the survival rates of P. halocryophilus when subjected to multiple freeze/thaw cycles. We found that cells suspended in chloride-containing samples have markedly increased survival rates compared with those in perchlorate-containing samples. In both cases, the survival rates increase with lower temperatures; however, this effect is more pronounced in chloride-containing samples. Furthermore, we found that higher salt concentrations increase survival rates when cells are subjected to freeze/thaw cycles. Our findings have important implications not only for the habitability of cold environments on Earth but also for extraterrestrial environments such as that of Mars, where cold brines might exist in the subsurface and perhaps even appear temporarily at the surface such as at recurring slope lineae. [ABSTRACT FROM AUTHOR]

Extraterrestrial environments encompass physicochemical conditions and habitats that are unknown on Earth, such as perchlorate-rich brines that can be at least temporarily stable on the martian surface. To better understand the potential for life in these cold briny environments, we determined the maximum salt concentrations suitable for growth (MSCg) of six different chloride and perchlorate salts at 25°C and 4°C for the extremotolerant cold- and salt-adapted bacterial strain Planococcus halocryophilus. Growth was measured through colony-forming unit (CFU) counts, while cellular and colonial phenotypic stress responses were observed through visible light, fluorescence, and scanning electron microscopy. Our data show the following: (1) The tolerance to high salt concentrations can be increased through a stepwise inoculation toward higher concentrations. (2) Ion-specific factors are more relevant for the growth limitation of P. halocryophilus in saline solutions than single physicochemical parameters like ionic strength or water activity. (3) P. halocryophilus shows the highest microbial sodium perchlorate tolerance described so far. However, (4) MSCg values are higher for all chlorides compared to perchlorates. (5) The MSCg for calcium chloride was increased by lowering the temperature from 25°C to 4°C, while sodium- and magnesium-containing salts can be tolerated at 25°C to higher concentrations than at 4°C. (6) Depending on salt type and concentration, P. halocryophilus cells show distinct phenotypic stress responses such as novel types of colony morphology on agar plates and biofilm-like cell clustering, encrustation, and development of intercellular nanofilaments. This study, taken in context with previous work on the survival of extremophiles in Mars-like environments, suggests that high-concentrated perchlorate brines on Mars might not be habitable to any present organism on Earth, but extremophilic microorganisms might be able to evolve thriving in such environments. [ABSTRACT FROM AUTHOR]