SignificanceIt has remained an unresolved question whether microorganisms recovered from the most arid environments on Earth are thriving under such extreme conditions or are just dead or dying vestiges of viable cells fortuitously deposited by atmospheric processes. Based on multiple lines of evidence, we show that indigenous microbial communities are present and temporally active even in the hyperarid soils of the Atacama Desert (Chile). Following extremely rare precipitation events in the driest parts of this desert, where rainfall often occurs only once per decade, we were able to detect episodic incidences of biological activity. Our findings expand the range of hyperarid environments temporarily habitable for terrestrial life, which by extension also applies to other planetary bodies like Mars.

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 saltadapted 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 highconcentrated 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.

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.

A methanogenic archaeon, strain SMA-21 T , was isolated from a permafrost-affected soil by serial dilution in liquid medium. The cells were non-motile, stained Gram-negative and grew as irregular cocci with a diameter of 1.3-2.5 mm. Optimal growth was observed at 28 6C, pH 7.8 and 0.02 M NaCl. The strain grew on H 2 /CO 2 , methanol and acetate, but not on formate, ethanol, 2-butanol, 2-propanol, monomethylamine, dimethylamine, trimethylamine or dimethyl sulfide. Major membrane lipids of strain SMA-21T were archaeol phosphatidylglycerol, archaeol phosphatidylethanolamine and the corresponding hydroxyarchaeol compounds. The G+C content of the genomic DNA was 40.9 mol%. The 16S rRNA gene sequence was closely related to those of Methanosarcina mazei DSM 2053 T (similarity 99.9 %) and Methanosarcina Permafrost and the associated soils cover more than 25 % of the land surface (Zhang et al

the current understanding of the Martian surface indicates that briny environments at the near-surface are temporarily possible, e.g. in the case of the presumably deliquescence-driven Recurring Slope Lineae (RSL). However, whether such dynamic environments are habitable for terrestrial organisms remains poorly understood. this hypothesis was tested by developing a closed Deliquescence System (CDS) consisting of a mixture of desiccated Martian Regolith Analog (MRA) substrate, salts, and microbial cells, which over the course of days became wetted through deliquescence. The methane produced via metabolic activity for three methanogenic archaea: Methanosarcina mazei, M. barkeri and M. soligelidi, was measured after exposing them to three different MRA substrates using either NaCl or naclo 4 as a hygroscopic salt. Our experiments showed that (1) M. soligelidi rapidly produced methane at 4 °C, (2) M. barkeri produced methane at 28 °C though not at 4 °C, (3) M. mazei was not metabolically reactivated through deliquescence, (4) none of the species produced methane in the presence of perchlorate, and (5) all species were metabolically most active in the phyllosilicate-containing MRA. These results emphasize the importance of the substrate, microbial species, salt, and temperature used in the experiments. Furthermore, we show here for the first time that water provided by deliquescence alone is sufficient to rehydrate methanogenic archaea and to reactivate their metabolism under conditions roughly analogous to the near-subsurface Martian environment. Methane in the atmosphere of Mars was first detected by Formisano, et al. 1 with the Planetary Fourier Spectrometer onboard the Mars Express orbiter. Recent measurements by the Tunable Laser Spectrometer onboard the Curiosity rover show that methane concentrations are currently in the parts per billion range and fluctuate repeatedly throughout the seasons 2. However, the long-term stability and presence of methane in the Martian atmosphere is considered unlikely over geological time periods due to degradation by UV radiation and/ or oxidizing compounds at the surface 3. Hence, it has been proposed that subsurface reservoirs might sporadically release methane and/or that it is actively produced through abiotic, or potentially, as it is on Earth, from microbial processes 4. On Mars, however, the environmental conditions within the surficial regolith generally do not permit a lasting presence of liquid water necessary for microbial metabolism. Nevertheless, besides surficial monolayers of liquid-like water 5 and morning frost 6 , a possible exception are near-surface environments laden with hygroscopic salts which can undergo deliquescence, a process occurring when the relative humidity (RH) exceeds the deliquescence relative humidity (DRH) of a given salt above its eutectic temperature. Such a mechanism was first hypothesized by McEwen, et al. 7 to play a role in the seasonal surface features on Mars known as Recurring Slope Lineae (RSL), dark streaks that appear on steep crat...