Abstract
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.
Highlights
Life as we know it requires liquid water as the principal solvent for its biochemistry, but most planetary surfaces in our Solar System never reach temperatures above the freezing point of pure water, rendering these localities as likely uninhabitable compared with the benign climate conditions on Earth
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
Its growth curve at 25°C was determined via colony forming units (CFUs), and cell suspensions used for inoculating the experiments were either retrieved after 4 days or 7 days of growth (Fig. 1)
Summary
Life as we know it requires liquid water as the principal solvent for its biochemistry, but most planetary surfaces in our Solar System never reach temperatures above the freezing point of pure water, rendering these localities as likely uninhabitable compared with the benign climate conditions on Earth. The presence of salts can lead to a substantial freezing point depression down to the eutectic temperature of a given salt/water mixture (e.g., -50°C for a 31 wt% CaCl2 solution) and, greatly expand the temperature range for potential habitats (Mohlmann and Thomsen, 2011). On Earth, microbial organisms such as yeast can tolerate water activities (aw) down to 0.61 (Rummel et al, 2014). The lowest salt-induced water activity that halophilic microorganisms can tolerate is that of a saturated NaCl solution (aw = 0.75), while other salts (e.g., those containing Ca2+ and Mg2+ ions) are more inhibitory to cell metabolism (Rummel et al, 2014). It has been reported that certain cyanobacterial species embedded in hygroscopic sodium chloride (NaCl) deposits found in the hyperarid soils of the Atacama Desert are able to utilize water condensed from the atmosphere via deliquescence (Davila et al, 2008; Davila and Schulze-Makuch, 2016)
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