Transitory Microbial Habitat in the Hyperarid Atacama Desert

Traces of life are nearly ubiquitous on Earth. However, a central unresolved question is whether these traces always indicate an active microbial community or whether, in extreme environments, such as hyperarid deserts, they instead reflect just dormant or dead cells. Although microbial biomass and diversity decrease with increasing aridity in the Atacama Desert, we provide multiple lines of evidence for the presence of an at times metabolically active, microbial community in one of the driest places on Earth. We base this observation on four major lines of evidence: a physico-chemical characterization of the soil habitability after an exceptional rain event, identified biomolecules indicative of potentially active cells [e.g., presence of ATP, phospholipid fatty acids (PLFAs), metabolites, and enzymatic activity], measurements of in situ replication rates of genomes of uncultivated bacteria reconstructed from selected samples, and microbial community patterns specific to soil parameters and depths. We infer that the microbial populations have undergone selection and adaptation in response to their specific soil microenvironment and in particular to the degree of aridity. Collectively, our results highlight that even the hyperarid Atacama Desert can provide a habitable environment for microorganisms that allows them to become metabolically active following an episodic increase in moisture and that once it decreases, so does the activity of the microbiota. These results have implications for the prospect of life on other planets such as Mars, which has transitioned from an earlier wetter environment to today's extreme hyperaridity. [Abstract copyright: Copyright © 2018 the Author(s). Published by PNAS.

Microbial Hotspots in Lithic Macrohabitats Inferred from DNA Fractionation and Metagenomics in the Atacama Desert

The existence of microbial activity hotspots in temperate regions of Earth is driven by soil heterogeneities, especially the temporal and spatial availability of nutrients. Here we investigate whether microbial activity hotspots also exist in lithic microhabitats in one of the most arid regions of the world, the Atacama Desert in Chile. While previous studies evaluated the total DNA fraction to elucidate the microbial communities, we here for the first time use a DNA separation approach on lithic microhabitats, together with metagenomics and other analysis methods (i.e., ATP, PLFA, and metabolite analysis) to specifically gain insights on the living and potentially active microbial community. Our results show that hypolith colonized rocks are microbial hotspots in the desert environment. In contrast, our data do not support such a conclusion for gypsum crust and salt rockenvironments, because only limited microbial activity could be observed. The hypolith community is dominated by phototrophs, mostly Cyanobacteria and Chloroflexi, at both study sites. The gypsum crusts are dominated by methylotrophs and heterotrophic phototrophs, mostly Chloroflexi, and the salt rocks (halite nodules) by phototrophic and halotolerant endoliths, mostly Cyanobacteria and Archaea. The major environmental constraints in the organic-poor arid and hyperarid Atacama Desert are water availability and UV irradiation, allowing phototrophs and other extremophiles to play a key role in desert ecology

Enhanced Microbial Survivability in Subzero Brines.

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. Key Words: Brines-Halophile-Mars-Perchlorate-Subzero-Survival. Astrobiology 18, xxx-xxx

Methanogenic archaea can produce methane in deliquescence-driven Mars analog environments

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

Laser spectroscopic real time measurements of methanogenic activity under simulated Martian subsurface analogue conditions

On Earth, chemolithoautothrophic and anaerobic microorganisms such as methanogenic archaea are regarded as model organisms for possible subsurface life on Mars. For this reason, the methanogenic strain Methanosarcina soligelidi (formerly called Methanosarcina spec. SMA-21), isolated from permafrost-affected soil in northeast Siberia, has been tested under Martian thermo-physical conditions. In previous studies under simulated Martian conditions, high survival rates of these microorganisms were observed. In our study we present a method to measure methane production as first attempt to study metabolic activity of methanogenic archaea during simulated conditions, which are approaching conditions of Mars-like environments. To determine methanogenic activity, a measurement technique which is capable to measure the produced methane concentration with high precision and with high temporal resolution is needed. Although there are several methods to detect methane, only a few fulfill all the needed requirements to work within simulated extraterrestrial environments. We have chosen laser spectroscopy, which is a non-destructive technique that measures the methane concentration without sample taking and also can be run continuously. In our simulation, we detected methane production at temperatures down to -5°C, which would be found on Mars either temporarily in the shallow subsurface or continually in the deep subsurface. The pressure of 50 kPa which we used in our experiments, corresponds to the expected pressure in the Martian near subsurface. Our new device proved to be fully functional and the results indicate that the possible existence of methanogenic archaea in Martian subsurface habitats cannot be ruled out

Raman spectroscopy for detection of biological matter in Mars analogue material

Introduction Raman spectra will be measured with the Raman Laser Spectrometer (RLS) onboard ExoMars in 2018 to identify organic compounds and mineral products as an indication of former or recent biological activi-ty. Investigation with the same specifications as those onboard the ExoMars mission is conducted to test the potential of identifying biological material on martian analogue material with Raman spectroscopy. Appropriate parameters concerning integration time and number of repititions for the detection of biological matter as well as for the determination of the mineral composition will be derived. In addition, problems are reported on using Raman spectroscopy to discriminate the microorganisms from the mineral background. Biological sample Cyanobacteria and methane producing archaea are chosen to represent potential life on Mars. Prokaryotes like archaea and bacteria appeared on early Earth at least 3.8 to 3.5 billion years ago (Gya). Life might have developed under similar conditions on Mars as on Earth or might have been transferred from Earth (or vice versa). At that time on Mars the climate was more temperate and wet compared to the present day as inferred from geological evidence for liquid water on the ancient martian surface. Methane is known to be present on Mars. A source is still unknown. Methane might originate from geothermal or biological activities nearby the surface of the red planet. Cyanobacteria and prokaryotes using photosystem I use pigments such as scytonemin and beta-carotene as UV protection. Especially beta - carotene emits a strong Raman signal at the applied laser excitation wavelength. Raman measurements are used for detection of coccid, chain, and biofilm forming cyanobacteria Nostoc commune strain 231-06 (Fraunhofer IMBT CCCryo) on the below described Mars analogue mineral mixtures. Nostoc commune is known to be resistant to desiccation, UV B radiation and low temperatures, and thus suitable as a candidate for a potential life form on Mars. Furthermore, the Raman technique is applied on samples of the methane producing archaea candidatus Methanosarcina gelisolum (strain SMA 21) isolated from Siberian permafrost affected soils and on these archaea embedded in the martian analogue material. Martian analogue material In this investigation two different Mars analogue materials prepared from mineral and rock mixtures are used. The (1) Phyllosilicatic Mars Regolith Simulant (P-MRS) and (2) Sulfatic Mars Reg-olith Simulant (S-MRS) reflect the current understanding regarding environmental changes on Mars. Weathering or hydrothermal alteration of crustal rocks and of secondary mineralization during part of the Noachian and Hesperian epoch followed by the prevailing cold and dry oxidising condition with formation of anhydrous iron oxides. The use of two different mixtures accounts for the observations that phyllosilicatic deposits do not occur together with sulphatic deposits. P-MRS and S-MRS serve as the analogue geomaterials in which the cells of cyanobacteria and of methanogenes are embedded. Results Varying periods of measurement time and number of repetitions are used to get optimal Raman spectra for cyanobacteria and methanogenes. If cyanobacteria are present, beta-carotene is the dominant feature in the spectrum. Measurement times need to be adjusted to obtain optimal spectra of the P-MRS and S-MRS with cyanobacteria. Measurements performed with various values of measurement time and number of measurements show clearly the improvement achieved by increasing the time per spectrum from 1s to 20s. But it is desirable to find a set of small values of measurement time and number of repetitions in order to optimize the detection of minerals and biological markers and to reduce the disturbing effect of cosmic rays. A measurement regime is proposed for mineral mixtures with cyanobacteria on the basis of the RLS instrument characteristics: A procedure on ExoMars should start with a measurement time of only a few seconds to identify both biomarkers and minerals. If no biomarkers can be identified the time and number of measurements need to be increased until spectra of minerals are obtained. The measurement time should be selected between 1s (for b-carotene) and 20s (for minerals) for a laser power of 1mW (spot diameter < 2 µm). Future investigations of Raman measurement parameters should consider the different environmental parameters on Mars like atmospheric pressure, composition and temperature. For methanogens a different measurement regime needs to be developed. Raman analytics are capable to identify biosignatures like beta – carotene on a multi-mineral mixture similar to those expected to be encountered during the ExoMars mission