Abstract
Two anhydrobiotic strains of the cyanobacterium Chroococcidiopsis, namely CCMEE 029 and CCMEE 171, isolated from the Negev Desert in Israel and from the Dry Valleys in Antarctica, were exposed to salty-ice simulations. The aim of the experiment was to investigate the cyanobacterial capability to survive under sub-freezing temperatures in samples simulating the environment of icy worlds. The two strains were mixed with liquid solutions having sub-eutectic concentration of Na2SO4, MgSO4 and NaCl, then frozen down to different final temperatures (258 K, 233 K and 203 K) in various experimental runs. Both strains survived the exposure to 258 K in NaCl solution, probably as they migrated in the liquid veins between ice grain boundaries. However, they also survived at 258 K in Na2SO4 and MgSO4-salty-ice samples—that is, a temperature well below the eutectic temperature of the solutions, where liquid veins should not exist anymore. Moreover, both strains survived the exposure at 233 K in each salty-ice sample, with CCMEE 171 showing an enhanced survivability, whereas there were no survivors at 203 K. The survival limit at low temperature was further extended when both strains were exposed to 193 K as air-dried cells. The results suggest that vitrification might be a strategy for microbial life forms to survive in potentially habitable icy moons, for example in Europa’s icy crust. By entering a dried, frozen state, they could be transported from niches, which became non-habitable to new habitable ones, and possibly return to metabolic activity.
Highlights
Habitable worlds are usually defined in terms of three basic requirements: the presence of liquid water, the availability of biogenic elements and a free energy source [1]
No survivors occurred among cells of strains CCMEE 171 and CCMEE 029 after being exposed for 2 h to 203 K, regardless of the salt used, because no increase in cell densities was observed after transfer to liquid BG-11 medium under routine culture conditions (Figure 4A,B)
Cell densities are shown as % of control
Summary
Habitable worlds are usually defined in terms of three basic requirements: the presence of liquid water, the availability of biogenic elements and a free energy source [1]. In natural terrestrial ice many chemical species are present together, the existence and predominance of a microenvironment over the other (i.e. liquid veins or solid grains) depends on the type of chemical impurities and the temperature of the ice. On Europa’s surface, the presence of magnesium sulfate, sodium sulfate and sodium carbonate (and, potentially, of sulfuric acid) has been revealed by Galileo Near-Infrared Mapping Spectrometer [21]. The investigation of microbial survival in different solutes under freezing conditions is relevant to better define life-limiting factors and to evaluate the possibility of non-Earth icy environments being habitable [11]. CCMEE 029 isolated from the Negev Desert, Israel, was selected due to its capability of surviving, when dried, temperatures as low as 248 K, under laboratory conditions [30]. Survival was evaluated by monitoring the growth capability after transfer to standard growth medium and incubation under routine conditions
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