Abstract
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 NaClO4 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.
Highlights
Methane in the atmosphere of Mars was first detected by Formisano, et al.[1] with the Planetary Fourier Spectrometer onboard the Mars Express orbiter
In the case of S-MRA (Martian Regolith Analog containing sulfatic minerals, see Material and Methods for details), methane production was substantially higher as M. soligelidi produced 190 ppm (0.019%), M. mazei 140 ppm (0.014%), and M. barkeri 1190 ppm (0.119%) methane
If P-MRA (Martian Regolith Analog containing phyllosilicates, see Material and Methods for details) substrate was used, methane production was more than two orders of magnitude higher, reaching 22.2% (M. soligelidi), 20.1% (M. barkeri) and 2.2% (M. mazei)
Summary
Methane in the atmosphere of Mars was first detected by Formisano, et al.[1] with the Planetary Fourier Spectrometer onboard the Mars Express orbiter. 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 crater walls situated in the mid-latitudes. Extremophilic microorganisms adapted to such arid and briny environments show an enhanced survival in chloride and perchlorate-bearing brines at the respective eutectic salt concentrations when subjected at subzero temperatures[18,19] Among those microorganisms that show adaptive behavior at low temperatures and high salinities are methanogenic archaea[20]. We report here on experiments, in a Closed Deliquescence System (CDS) using a Martian regolith analog (Fig. 1), that show methanogenic archaeal strains can regain metabolic activity after having been desiccated and subsequently wetted through in vitro deliquescence
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