The search for life on Europa: limiting environmental factors, potential habitats, and Earth analogues

Marine Science Can Contribute to the Search for Extra-Terrestrial Life

Life on our planet likely evolved in the ocean, and thus exo-oceans are key habitats to search for extraterrestrial life. We conducted a data-driven bibliographic survey on the astrobiology literature to identify emerging research trends with marine science for future synergies in the exploration for extraterrestrial life in exo-oceans. Based on search queries, we identified 2592 published items since 1963. The current literature falls into three major groups of terms focusing on (1) the search for life on Mars, (2) astrobiology within our Solar System with reference to icy moons and their exo-oceans, and (3) astronomical and biological parameters for planetary habitability. We also identified that the most prominent research keywords form three key-groups focusing on (1) using terrestrial environments as proxies for Martian environments, centred on extremophiles and biosignatures, (2) habitable zones outside of "Goldilocks" orbital ranges, centred on ice planets, and (3) the atmosphere, magnetic field, and geology in relation to planets' habitable conditions, centred on water-based oceans.

Bioenergetics of iron snow fueling life on Europa

The main sources of redox gradients supporting high-productivity life in the Europan and other icy ocean world oceans were proposed to be photolytically derived oxidants, such as reactive oxygen species (ROS) from the icy shell, and reductants (Fe(II), S(-II), CH4, H2) from bottom waters reacting with a (ultra)mafic seafloor. Important roadblocks to maintaining life, however, are that the degree of ocean mixing to combine redox species is unknown, and ROS damage biomolecules. Here, we envisage a unique solution using an acid mine drainage (AMD)-filled pit lakes analog system for the Europan ocean, which previous models predicted to be acidic. We hypothesize that surface-generated ROS oxidize dissolved Fe(II) resulting in Fe(III) (hydr)oxide precipitates, that settle to the seafloor as "iron snow." The iron snow provides a respiratory substrate for anaerobic microorganisms ("breathing iron"), and limits harmful ROS exposure since they are now neutralized at the ice-water interface. Based on this scenario, we calculated Gibbs energies and maximal biomass productivities of various anaerobic metabolisms for a range of pH, temperatures, and H2 fluxes. Productivity by iron reducers was greater for most environmental conditions considered, whereas sulfate reducers and methanogens were more favored at high pH. Participation of Fe in the metabolic redox processes is largely neglected in most models of Europan biogeochemistry. Our model overcomes important conceptual roadblocks to life in icy ocean worlds and broadens the potential metabolic diversity, thus increasing total primary productivity, the diversity and volume of habitable environmental niches and, ultimately, the probability of biosignature detection.

DNA Polymerization in Icy Moon Abyssal Pressure Conditions

Evidence of stable liquid water oceans beneath the ice crust of moons within the Solar System is of great interest for astrobiology. In particular, subglacial oceans may present hydrothermal processes in their abysses, similarly to terrestrial hydrothermal vents. Therefore, terrestrial extremophilic deep life can be considered a model for putative icy moon extraterrestrial life. However, the comparison between putative extraterrestrial abysses and their terrestrial counterparts suffers from a potentially determinant difference. Indeed, some icy moons oceans may be so deep that the hydrostatic pressure would exceed the maximal pressure at which hydrothermal vent organisms have been isolated. While terrestrial microorganisms that are able to survive in such conditions are known, the effect of high pressure on fundamental biochemical processes is still unclear. In this study, the effects of high hydrostatic pressure on DNA synthesis catalyzed by DNA polymerases are investigated for the first time. The effect on both strand displacement and primer extension activities is measured, and pressure tolerance is compared between enzymes of various thermophilic organisms isolated at different depths.

Investigating Europa's Habitability with the Europa Clipper

The habitability of Europa is a property within a system, which is driven by a multitude of physical and chemical processes and is defined by many interdependent parameters, so that its full characterization requires collaborative investigation. To explore Europa as an integrated system to yield a complete picture of its habitability, the Europa Clipper mission has three primary science objectives: (1) characterize the ice shell and ocean including their heterogeneity, properties, and the nature of surface-ice-ocean exchange; (2) characterize Europa's composition including any non-ice materials on the surface and in the atmosphere, and any carbon-containing compounds; and (3) characterize Europa's geology including surface features and localities of high science interest. The mission will also address several cross-cutting science topics including the search for any current or recent activity in the form of thermal anomalies and plumes, performing geodetic and radiation measurements, and assessing high-resolution, co-located observations at select sites to provide reconnaissance for a potential future landed mission. Synthesizing the mission's science measurements, as well as incorporating remote observations by Earth-based observatories, the James Webb Space Telescope, and other space-based resources, to constrain Europa's habitability, is a complex task and is guided by the mission's Habitability Assessment Board (HAB).

Genomic characterization and radiation tolerance of Naganishia kalamii sp. nov. and Cystobasidium onofrii sp. nov. from Mars 2020 mission assembly facilities

During the construction and assembly of the Mars 2020 mission components at two different NASA cleanrooms, several fungal strains were isolated. Based on their colony morphology, two strains that showed yeast-like appearance were further characterized for their phylogenetic position. The species-level classification of these two novel strains, using traditional colony and cell morphology methods combined with the phylogenetic reconstructions using multi-locus sequence analysis (MLSA) based on several gene loci (ITS, LSU, SSU, RPB1, RPB2, CYTB and TEF1), and whole genome sequencing (WGS) was carried out. This polyphasic taxonomic approach supported the conclusion that the two basidiomycetous yeasts belong to hitherto undescribed species. The strain FJI-L2-BK-P3T, isolated from the Jet Propulsion Laboratory Spacecraft Assembly Facility, was placed in the Naganishia albida clade (Filobasidiales, Tremellomycetes), but is genetically and physiologically different from other members of the clade. Another yeast strain FKI-L6-BK-PAB1T, isolated from the Kennedy Space Center Payload Hazardous and Servicing Facility, was placed in the genus Cystobasidium (Cystobasidiales, Cystobasidiomycetes) and is distantly related to C. benthicum. Here we propose two novel species with the type strains, Naganishia kalamii sp. nov. (FJI-L2-BK-P3T = NRRL 64466 = DSM 115730) and Cystobasidium onofrii sp. nov. (FKI-L6-BK-PAB1T = NRRL 64426 = DSM 114625). The phylogenetic analyses revealed that single gene phylogenies (ITS or LSU) were not conclusive, and MLSA and WGS-based phylogenies were more advantageous for species discrimination in the two genera. The genomic analysis predicted proteins associated with dehydration and desiccation stress-response and the presence of genes that are directly related to osmotolerance and psychrotolerance in both novel yeasts described. Cells of these two newly-described yeasts were exposed to UV-C radiation and compared with N. onofrii, an extremophilic UV-C resistant cold-adapted Alpine yeast. Both novel species were UV resistant, emphasizing the need for collecting and characterizing extremotolerant microbes, including yeasts, to improve microbial reduction techniques used in NASA planetary protection programs.

A Review on Hypothesized Metabolic Pathways on Europa and Enceladus: Space-Flight Detection Considerations

Enceladus and Europa, icy moons of Saturn and Jupiter, respectively, are believed to be habitable with liquid water oceans and therefore are of interest for future life detection missions and mission concepts. With the limited data from missions to these moons, many studies have sought to better constrain these conditions. With these constraints, researchers have, based on modeling and experimental studies, hypothesized a number of possible metabolisms that could exist on Europa and Enceladus if these worlds host life. The most often hypothesized metabolisms are methanogenesis for Enceladus and methane oxidation/sulfate reduction on Europa. Here, we outline, review, and compare the best estimated conditions of each moon's ocean. We then discuss the hypothetical metabolisms that have been suggested to be present on these moons, based on laboratory studies and Earth analogs. We also detail different detection methods that could be used to detect these hypothetical metabolic reactions and make recommendations for future research and considerations for future missions.

Modeled energetics of bacterial communities in ancient subzero brines

Cryopeg brines are isolated volumes of hypersaline water in subzero permafrost. The cryopeg system at Utqiaġvik, Alaska, is estimated to date back to 40 ka BP or earlier, a remnant of a late Pleistocene Ocean. Surprisingly, the cryopeg brines contain high concentrations of organic carbon, including extracellular polysaccharides, and high densities of bacteria. How can these physiologically extreme, old, and geologically isolated systems support such an ecosystem? This study addresses this question by examining the energetics of the Utqiaġvik cryopeg brine ecosystem. Using literature-derived assumptions and new measurements on archived borehole materials, we first estimated the quantity of organic carbon when the system formed. We then considered two bacterial growth trajectories to calculate the lower and upper bounds of the cell-specific metabolic rate of these communities. These bounds represent the first community estimates of metabolic rate in a subzero hypersaline environment. To assess the plausibility of the different growth trajectories, we developed a model of the organic carbon cycle and applied it to three borehole scenarios. We also used dissolved inorganic carbon and nitrogen measurements to independently estimate the metabolic rate. The model reconstructs the growth trajectory of the microbial community and predicts the present-day cell density and organic carbon content. Model input included measured rates of the in-situ enzymatic conversion of particulate to dissolved organic carbon under subzero brine conditions. A sensitivity analysis of model parameters was performed, revealing an interplay between growth rate, cell-specific metabolic rate, and extracellular enzyme activity. This approach allowed us to identify plausible growth trajectories consistent with the observed bacterial densities in the cryopeg brines. We found that the cell-specific metabolic rate in this system is relatively high compared to marine sediments. We attribute this finding to the need to invest energy in the production of extracellular enzymes, for generating bioavailable carbon from particulate organic carbon, and the production of extracellular polysaccharides for cryoprotection and osmoprotection. These results may be relevant to other isolated systems in the polar regions of Earth and to possible ice-bound brines on worlds such as Europa, Enceladus, and Mars.

Influence of pH and salts on DMF-DMA derivatization for future Space Applications

For gas chromatography - mass spectrometry (GC-MS) analyses performed in situ, pH and salts (e.g., chlorides, sulfates) may enhance or inhibit the detection of targeted molecules of interest for astrobiology (e.g. amino acids, fatty acids, nucleobases). Obviously, salts influence the ionic strength of the solutions, the pH value, and the salting effect. But the presence of salts may also produce complexes or mask ions in the sample (masking effect on hydroxide ion, ammonia, etc.). For future space missions, wet chemistry will be conducted before GC-MS analyses to detect the full organic content of a sample. The defined organic targets for space GC-MS instrument requirements are generally strongly polar or refractory organic compounds, such as amino acids playing a role in the protein production and metabolism regulations for life on Earth, nucleobases essential for DNA and RNA formation and mutation, and fatty acids that composed most of the eukaryote and prokaryote membranes on Earth and resist to environmental stress long enough to still be observed on Mars or ocean worlds in geological well-preserved records. The wet-chemistry chemical treatment consists of reacting an organic reagent with the sample to extract and volatilize polar or refractory organic molecules (i.e. dimethylformamide dimethyl acetal (DMF-DMA) in this study). DMF-DMA derivatizes functional groups with labile H in organics, without modifying their chiral conformation. The influence of pH and salt concentration of extraterrestrial materials on the DMF-DMA derivatization remains understudied. In this research, we studied the influence of different salts and pHs on the derivatization of organic molecules of astrobiological interest with DMF-DMA, such as amino acids, carboxylic acids, and nucleobases. Results show that salts and pH influence the derivatization yield, and that their effect depend on the nature of the organics and the salts studied. Second, monovalent salts lead to a higher or similar organic recovery compared to divalent salts regardless of pH below 8. However, a pH above 8 inhibits the DMF-DMA derivatization influencing the carboxylic acid function to become an anionic group without labile H. Overall, considering the negative effect of the salts on the detection of organic molecules, future space missions may have to consider a desalting step prior to derivatization and GC-MS analyses.

Traces of Ancient Life in Oceanic Basalt Preserved as Iron-Mineralized Ultrastructures: Implications for Detecting Extraterrestrial Biosignatures

Benefiting from their adaptability to extreme environments, subsurface microorganisms have been discovered in sedimentary and igneous rock environments on Earth and have been advocated as candidates in the search for extraterrestrial life. In this article, we study iron-mineralized microstructures in calcite-filled veins within basaltic pillows of the late Ladinian Fernazza group (Middle Triassic, 239 Ma) in Italy. These microstructures represent diverse morphologies, including filaments, globules, nodules, and micro-digitate stromatolites, which are similar to extant iron-oxidizing bacterial communities. In situ analyses including Raman spectroscopy have been used to investigate the morphological, elemental, mineralogical, and bond-vibrational modes of the microstructures. According to the Raman spectral parameters, iron minerals preserve heterogeneous ultrastructures and crystallinities, coinciding with the morphologies and precursor microbial activities. The degree of crystallinity usually represents a microscale gradient decreasing toward previously existing microbial cells, revealing a decline of mineralization due to microbial activities. This study provides an analog of possible rock-dwelling subsurface life on Mars or icy moons and advocates Raman spectroscopy as an efficient tool for in situ analyses. We put forward the concept that ultrastructural characteristics of minerals described by Raman spectral parameters corresponding to microscale morphologies could be employed as carbon-lean biosignatures in future space missions.

Physiological and genomic insights into abiotic stress of halophilic archaeon Natrinema altunense 4.1R isolated from a saline ecosystem of Tunisian desert

Halophilic archaea are polyextremophiles with the ability to withstand fluctuations in salinity, high levels of ultraviolet radiation, and oxidative stress, allowing them to survive in a wide range of environments and making them an excellent model for astrobiological research. Natrinema altunense 4.1R is a halophilic archaeon isolated from the endorheic saline lake systems, Sebkhas, located in arid and semi-arid regions of Tunisia. It is an ecosystem characterized by periodic flooding from subsurface groundwater and fluctuating salinities. Here, we assess the physiological responses and genomic characterization of N. altunense 4.1R to UV-C radiation, as well as osmotic and oxidative stresses. Results showed that the 4.1R strain is able to survive up to 36% of salinity, up to 180 J/m² to UV-C radiation, and at 50 mM of H₂O₂, a resistance profile similar to Halobacterium salinarum, a strain often used as UV-C resistant model. In order to understand the genetic determinants of N. altunense 4.1R survival strategy, we sequenced and analyzed its genome. Results showed multiple gene copies of osmotic stress, oxidative stress, and DNA repair response mechanisms supporting its survivability at extreme salinities and radiations. Indeed, the 3D molecular structures of seven proteins related to responses to UV-C radiation (excinucleases UvrA, UvrB, and UvrC, and photolyase), saline stress (trehalose-6-phosphate synthase OtsA and trehalose-phosphatase OtsB), and oxidative stress (superoxide dismutase SOD) were constructed by homology modeling. This study extends the abiotic stress range for the species N. altunense and adds to the repertoire of UV and oxidative stress resistance genes generally known from haloarchaeon.

Eurypsychrophilic acidophiles: From (meta)genomes to low-temperature biotechnologies

Low temperature and acidic environments encompass natural milieus such as acid rock drainage in Antarctica and anthropogenic sites including drained sulfidic sediments in Scandinavia. The microorganisms inhabiting these environments include polyextremophiles that are both extreme acidophiles (defined as having an optimum growth pH < 3), and eurypsychrophiles that grow at low temperatures down to approximately 4°C but have an optimum temperature for growth above 15°C. Eurypsychrophilic acidophiles have important roles in natural biogeochemical cycling on earth and potentially on other planetary bodies and moons along with biotechnological applications in, for instance, low-temperature metal dissolution from metal sulfides. Five low-temperature acidophiles are characterized, namely, Acidithiobacillus ferriphilus, Acidithiobacillus ferrivorans, Acidithiobacillus ferrooxidans, “Ferrovum myxofaciens,” and Alicyclobacillus disulfidooxidans, and their characteristics are reviewed. Our understanding of characterized and environmental eurypsychrophilic acidophiles has been accelerated by the application of “omics” techniques that have aided in revealing adaptations to low pH and temperature that can be synergistic, while other adaptations are potentially antagonistic. The lack of known acidophiles that exclusively grow below 15°C may be due to the antagonistic nature of adaptations in this polyextremophile. In conclusion, this review summarizes the knowledge of eurypsychrophilic acidophiles and places the information in evolutionary, environmental, biotechnological, and exobiology perspectives.

Carnobacterium Species Capable of Growth at Pressures Ranging Over 5 Orders of Magnitude, from the Surface of Mars (103 Pa) to Deep Oceans (107 Pa) in the Solar System

Several permanently cold solar system bodies are being investigated with regard to their potential habitability, including Mars and icy moons. In such locations, microbial life would have to cope with low temperatures and both high and low pressures, ranging from ∼10² to 10³ Pa on the surface of Mars to upward of ∼10⁸-10⁹ Pa in the subsurface oceans of icy moons. The bacterial genus Carnobacterium consists of species that were previously shown to be capable of growth in the absence of oxygen at low temperatures and at either low pressure or high pressure, but to date the entire pressure range of the genus has not been explored. In the present study, we subjected 14 Carnobacterium strains representing 11 species to cultivation in a complex liquid medium under anaerobic conditions at 2°C and at a range of pressures spanning 5 orders of magnitude, from 10³ to 10⁷ Pa. Eleven of the 14 strains showed measurable growth rates at all pressures tested, representing the first demonstration of terrestrial life forms capable of growth under such a wide range of pressures. These findings expand the physical boundaries of the capabilities of life to occur in extreme extraterrestrial environments.

Snow Surface Microbial Diversity at the Detection Limit within the Vicinity of the Concordia Station, Antarctica

The Concordia Research Station provides a unique location for preparatory activities for future human journey to Mars, to explore microbial diversity at subzero temperatures, and monitor the dissemination of human-associated microorganisms within the pristine surrounding environment. Amplicon sequencing was leveraged to investigate the microbial diversity of surface snow samples collected monthly over a two-year period, at three distances from the Station (10, 500, and 1000 m). Even when the extracted total DNA was below the detection limit, 16S rRNA gene sequencing was successfully performed on all samples, while 18S rRNA was amplified on 19 samples out of 51. No significant relationships were observed between microbial diversity and seasonality (summer or winter) or distance from the Concordia base. This suggested that if present, the anthropogenic impact should have been below the detectable limit. While harboring low microbial diversity, the surface snow samples were characterized by heterogeneous microbiomes. Ultimately, our study corroborated the use of DNA sequencing-based techniques for revealing microbial presence in remote and hostile environments, with implications for Planetary Protection during space missions and for life-detection in astrobiology relevant targets.

Cultivable bacteria in the supraglacial lake formed after a glacial lake outburst flood in northern Pakistan

Recently, a supraglacial lake formed as a result of a glacial lake outburst flood (GLOF) in the Dook Pal Glacier. Lake debris and meltwater samples were collected from the supraglacial lake to determine bacterial diversity. Geochemical analyses of samples showed free amino acids (FAAs), anions, cations, and heavy metals. Comparable viable bacterial counts were observed in meltwater and debris samples. Using R₂A media, a total of 52 bacterial isolates were identified: 40 from debris and 12 from meltwater. The relative abundance of Gram-positive (80.8%) bacteria was greater than Gram-negative (19.2%). Molecular identification of these isolates revealed that meltwater was dominated by Firmicutes (41.6%) and Proteobacteria (41.6%), while lake debris was dominated by Firmicutes (65.0%). The isolates belonged to 14 genera with the greatest relative abundance in Bacillus. Tolerance level of isolates to salts was high. Most of the Gram-positive bacteria were eurypsychrophiles, while most of the Gram-negative bacteria were stenopsychrophiles. Gram-negative bacteria displayed a higher minimum inhibitory concentration of selected heavy metals and antibiotics than Gram-positive. This first-ever study of culturable bacteria from a freshly formed supraglacial lake improves our understanding of the bacterial diversity and antibiotic resistance released from the glaciers as a result of GLOF.

"Freezing" Thermophiles: From One Temperature Extreme to Another

New detections of thermophiles in psychrobiotic (i.e., bearing cold-tolerant life forms) marine and terrestrial habitats including Arctic marine sediments, Antarctic accretion ice, permafrost, and elsewhere are continually being reported. These microorganisms present great opportunities for microbial ecologists to examine biogeographical processes for spore-formers and non-spore-formers alike, including dispersal histories connecting warm and cold biospheres. In this review, we examine different examples of thermophiles in cryobiotic locations, and highlight exploration of thermophiles at cold temperatures under laboratory conditions. The survival of thermophiles in psychrobiotic environments provokes novel considerations of physiological and molecular mechanisms underlying natural cryopreservation of microorganisms. Cultures of thermophiles maintained at low temperature may serve as a non-sporulating laboratory model for further exploration of metabolic potential of thermophiles at psychrobiotic temperatures, as well as for elucidating molecular mechanisms behind natural preservation and adaptation to psychrobiotic environments. These investigations are highly relevant for the search for life on other cold and icy planets in the Solar System, such as Mars, Europa and Enceladus.

The archaeal class Halobacteria and astrobiology: Knowledge gaps and research opportunities

Water bodies on Mars and the icy moons of the outer solar system are now recognized as likely being associated with high levels of salt. Therefore, the study of high salinity environments and their inhabitants has become increasingly relevant for Astrobiology. Members of the archaeal class Halobacteria are the most successful microbial group living in hypersaline conditions and are recognized as key model organisms for exposure experiments. Despite this, data for the class is uneven across taxa and widely dispersed across the literature, which has made it difficult to properly assess the potential for species of Halobacteria to survive under the polyextreme conditions found beyond Earth. Here we provide an overview of published data on astrobiology-linked exposure experiments performed with members of the Halobacteria, identifying clear knowledge gaps and research opportunities.

Science Autonomy for Ocean Worlds Astrobiology: A Perspective

Astrobiology missions to ocean worlds in our solar system must overcome both scientific and technological challenges due to extreme temperature and radiation conditions, long communication times, and limited bandwidth. While such tools could not replace ground-based analysis by science and engineering teams, machine learning algorithms could enhance the science return of these missions through development of autonomous science capabilities. Examples of science autonomy include onboard data analysis and subsequent instrument optimization, data prioritization (for transmission), and real-time decision-making based on data analysis. Similar advances could be made to develop streamlined data processing software for rapid ground-based analyses. Here we discuss several ways machine learning and autonomy could be used for astrobiology missions, including landing site selection, prioritization and targeting of samples, classification of "features" (e.g., proposed biosignatures) and novelties (uncharacterized, "new" features, which may be of most interest to agnostic astrobiological investigations), and data transmission.

The Bioburden and Ionic Composition of Hypersaline Lake Ices: Novel Habitats on Earth and Their Astrobiological Implications

We present thermophysical, biological, and chemical observations of ice and brine samples from five compositionally diverse hypersaline lakes in British Columbia's interior plateau. Possessing a spectrum of magnesium, sodium, sulfate, carbonate, and chloride salts, these low-temperature high-salinity lakes are analogs for planetary ice-brine environments, including the ice shells of Europa and Enceladus and ice-brine systems on Mars. As such, understanding the thermodynamics and biogeochemistry of these systems can provide insights into the evolution, habitability, and detectability of high-priority astrobiology targets. We show that biomass is typically concentrated in a layer near the base of the ice cover, but that chemical and biological impurities are present throughout the ice. Coupling bioburden, ionic concentration, and seasonal temperature measurements, we demonstrate that impurity entrainment in the ice is directly correlated to ice formation rate and parent fluid composition. We highlight unique phenomena, including brine supercooling, salt hydrate precipitation, and internal brine layers in the ice cover, important processes to be considered for planetary ice-brine environments. These systems can be leveraged to constrain the distribution, longevity, and habitability of low-temperature solar system brines-relevant to interpreting spacecraft data and planning future missions in the lens of both planetary exploration and planetary protection.

Laboratory exploration of mineral precipitates from Europa's subsurface ocean

The precipitation of hydrated phases from a chondrite-like Na-Mg-Ca-SO4-Cl solution is studied using in situ synchrotron X-ray powder diffraction, under rapid- (360 K h-1, T = 250-80 K, t = 3 h) and ultra-slow-freezing (0.3 K day-1, T = 273-245 K, t = 242 days) conditions. The precipitation sequence under slow cooling initially follows the predictions of equilibrium thermodynamics models. However, after ∼50 days at 245 K, the formation of the highly hydrated sulfate phase Na2Mg(SO4)2·16H2O, a relatively recent discovery in the Na2Mg(SO4)2-H2O system, was observed. Rapid freezing, on the other hand, produced an assemblage of multiple phases which formed within a very short timescale (≤4 min, ΔT = 2 K) and, although remaining present throughout, varied in their relative proportions with decreasing temperature. Mirabilite and meridianiite were the major phases, with pentahydrite, epsomite, hydrohalite, gypsum, blödite, konyaite and loweite also observed. Na2Mg(SO4)2·16H2O was again found to be present and increased in proportion relative to other phases as the temperature decreased. The results are discussed in relation to possible implications for life on Europa and application to other icy ocean worlds.

Complex Brines and Their Implications for Habitability

There is evidence that life on Earth originated in cold saline waters around scorching hydrothermal vents, and that similar conditions might exist or have existed on Mars, Europa, Ganymede, Enceladus, and other worlds. Could potentially habitable complex brines with extremely low freezing temperatures exist in the shallow subsurface of these frigid worlds? Earth, Mars, and carbonaceous chondrites have similar bulk elemental abundances, but while the Earth is depleted in the most volatile elements, the Icy Worlds of the outer solar system are expected to be rich in them. The cooling of ionic solutions containing substances that likely exist in the Icy Worlds could form complex brines with the lowest eutectic temperature possible for the compounds available in them. Indeed, here, we show observational and theoretical evidence that even elements present in trace amounts in nature are concentrated by freeze–thaw cycles, and therefore contribute significantly to the formation of brine reservoirs that remain liquid throughout the year in some of the coldest places on Earth. This is interesting because the eutectic temperature of water–ammonia solutions can be as low as ~160 K, and significant fractions of the mass of the Icy Worlds are estimated to be water substance and ammonia. Thus, briny solutions with eutectic temperature of at least ~160 K could have formed where, historically, temperature have oscillated above and below ~160 K. We conclude that complex brines must exist in the shallow subsurface of Mars and the Icy Worlds, and that liquid saline water should be present where ice has existed, the temperature is above ~160 K, and evaporation and sublimation have been inhibited.

Genomic reconstruction of fossil and living microorganisms in ancient Siberian permafrost

BACKGROUND

Total DNA (intracellular, iDNA and extracellular, eDNA) from ancient permafrost records the mixed genetic repository of the past and present microbial populations through geological time. Given the exceptional preservation of eDNA under perennial frozen conditions, typical metagenomic sequencing of total DNA precludes the discrimination between fossil and living microorganisms in ancient cryogenic environments. DNA repair protocols were combined with high throughput sequencing (HTS) of separate iDNA and eDNA fraction to reconstruct metagenome-assembled genomes (MAGs) from ancient microbial DNA entrapped in Siberian coastal permafrost.

RESULTS

Despite the severe DNA damage in ancient permafrost, the coupling of DNA repair and HTS resulted in a total of 52 MAGs from sediments across a chronosequence (26-120 kyr). These MAGs were compared with those derived from the same samples but without utilizing DNA repair protocols. The MAGs from the youngest stratum showed minimal DNA damage and thus likely originated from viable, active microbial species. Many MAGs from the older and deeper sediment appear related to past aerobic microbial populations that had died upon freezing. MAGs from anaerobic lineages, including Asgard archaea, however exhibited minimal DNA damage and likely represent extant living microorganisms that have become adapted to the cryogenic and anoxic environments. The integration of aspartic acid racemization modeling and metaproteomics further constrained the metabolic status of the living microbial populations. Collectively, combining DNA repair protocols with HTS unveiled the adaptive strategies of microbes to long-term survivability in ancient permafrost.

CONCLUSIONS

Our results indicated that coupling of DNA repair protocols with simultaneous sequencing of iDNA and eDNA fractions enabled the assembly of MAGs from past and living microorganisms in ancient permafrost. The genomic reconstruction from the past and extant microbial populations expanded our understanding about the microbial successions and biogeochemical alterations from the past paleoenvironment to the present-day frozen state. Furthermore, we provided genomic insights into long-term survival mechanisms of microorganisms under cryogenic conditions through geological time. The combined strategies in this study can be extrapolated to examine other ancient non-permafrost environments and constrain the search for past and extant extraterrestrial life in permafrost and ice deposits on Mars. Video abstract.

Subsurface In Situ Detection of Microbes and Diverse Organic Matter Hotspots in the Greenland Ice Sheet

We used a deep-ultraviolet fluorescence mapping spectrometer, coupled to a drill system, to scan from the surface to 105 m depth into the Greenland ice sheet. The scan included firn and glacial ice and demonstrated that the instrument is able to determine small (mm) and large (cm) scale regions of organic matter concentration and discriminate spectral types of organic matter at high resolution. Both a linear point cloud scanning mode and a raster mapping mode were used to detect and localize microbial and organic matter "hotspots" embedded in the ice. Our instrument revealed diverse spectral signatures. Most hotspots were <20 mm in diameter, clearly isolated from other hotspots, and distributed stochastically; there was no evidence of layering in the ice at the fine scales examined (100 μm per pixel). The spectral signatures were consistent with organic matter fluorescence from microbes, lignins, fused-ring aromatic molecules, including polycyclic aromatic hydrocarbons, and biologically derived materials such as fulvic acids. In situ detection of organic matter hotspots in ice prevents loss of spatial information and signal dilution when compared with traditional bulk analysis of ice core meltwaters. Our methodology could be useful for detecting microbial and organic hotspots in terrestrial icy environments and on future missions to the Ocean Worlds of our Solar System.

Survival of the Halophilic Archaeon Halovarius luteus after Desiccation, Simulated Martian UV Radiation and Vacuum in Comparison to Bacillus atrophaeus

Extraterrestrial environments influence the biochemistry of organisms through a variety of factors, including high levels of radiation and vacuum, temperature extremes and a lack of water and nutrients. A wide variety of terrestrial microorganisms, including those counted amongst the most ancient inhabitants of Earth, can cope with high levels of salinity, extreme temperatures, desiccation and high levels of radiation. Key among these are the haloarchaea, considered particularly relevant for astrobiological studies due to their ability to thrive in hypersaline environments. In this study, a novel haloarchaea isolated from Urmia Salt Lake, Iran, Halovarius luteus strain DA50^T, was exposed to varying levels of simulated extraterrestrial conditions and compared to that of the bacteria Bacillus atrophaeus. Bacillus atrophaeus was selected for comparison due to its well-described resistance to extreme conditions and its ability to produce strong spore structures. Thin films were produced to investigate viability without the protective influence of cell multi-layers. Late exponential phase cultures of Hvr. luteus and B. atrophaeus were placed in brine and phosphate buffered saline media, respectively. The solutions were allowed to evaporate and cells were encapsulated and exposed to radiation, desiccation and vacuum conditions, and their post-exposure viability was studied by the Most Probable Number method. The protein profile using High Performance Liquid Chromatography and Matrix Assisted Laser Desorption/Ionization bench top reflector time-of-flight are explored after vacuum and UV-radiation exposure. Results showed that the change in viability of the spore-forming bacteria B. atrophaeus was only minor whereas Hvr. luteus demonstrated a range of viability under different conditions. At the peak radiation flux of 10⁵ J/m² under nitrogen flow and after two weeks of desiccation, Hvr. luteus demonstrated the greatest decrease in viability. This study further expands our understanding of the boundary conditions of astrobiologically relevant organisms in the harsh space environment.

Growth, Cell Division, and Gene Expression of Escherichia coli at Elevated Concentrations of Magnesium Sulfate: Implications for Habitability of Europa and Mars

We perform quantitative studies of the growth, death, and gene expression of Escherichia coli in a wide range of magnesium sulfate (MgSO 4 ) concentrations (0-2.5 M). Elevated concentration of MgSO 4 causes the inhibition of cell growth, leading to an increase in the population doubling time. We find that cells exhibit three distinct morphological phenotypes-(i) normal, (ii) filamentous, and (iii) small cells at 1 . 25 M MgSO 4 . Filamentous cells arise due to the lack of cell division, while the small cells arise due to the partial plasmolysis of the cells. We further find that cell death starts for salt concentrations >1.25 M and increases with an increasing concentration of MgSO 4 . For salt concentrations ≥1.66 M, the growth of cells stops and all the cells become smaller than the control cells, suggesting the plasmolysis of the population. Cells grown at salt concentration up to 2 . 07 M are reversible in both the growth rate and morphology upon the removal of the salt stress. The time scale of reversibility increases with increasing salt concentration. Finally, we investigate the expression of an osmotically inducible gene (osmC), genes involved in magnesium transport (corA), sulfate transport (cysP), and osmotically driven transport of water (aqpZ). We find that a high concentration of magnesium sulfate leads to the upregulation of cysP and osmC.

Factoring Origin of Life Hypotheses into the Search for Life in the Solar System and Beyond

Two widely-cited alternative hypotheses propose geological localities and biochemical mechanisms for life's origins. The first states that chemical energy available in submarine hydrothermal vents supported the formation of organic compounds and initiated primitive metabolic pathways which became incorporated in the earliest cells; the second proposes that protocells self-assembled from exogenous and geothermally-delivered monomers in freshwater hot springs. These alternative hypotheses are relevant to the fossil record of early life on Earth, and can be factored into the search for life elsewhere in the Solar System. This review summarizes the evidence supporting and challenging these hypotheses, and considers their implications for the search for life on various habitable worlds. It will discuss the relative probability that life could have emerged in environments on early Mars, on the icy moons of Jupiter and Saturn, and also the degree to which prebiotic chemistry could have advanced on Titan. These environments will be compared to ancient and modern terrestrial analogs to assess their habitability and biopreservation potential. Origins of life approaches can guide the biosignature detection strategies of the next generation of planetary science missions, which could in turn advance one or both of the leading alternative abiogenesis hypotheses.

Culture-dependent diversity of bacteria from Laohugou glacier, Qilian Mts., China and their resistance against metals

In the current study, psychrophilic, endolithic, and epilithic bacterial strains were isolated and characterized from the nonpolar Laohugou glacier (LHG) no. 12, the largest valley glacier in the western Qilian Mts. located on the northeastern edge of the Tibetan Plateau. Five different types of samples, rocks, soil, glacial water, ice/snow, and cryoconite, were collected. A total of 48 bacterial strains were isolated by using the R2A bacterial cultural medium. The findings revealed that the Gram-positive bacteria 41 (85.4%) dominated the Gram-negative bacteria 7 (14.6%) in this extremely harsh environment. Molecular characterization based on 16S ribosomal RNA gene sequencing exhibited that the obtained isolates belong to four phyla, among which the diversity of Firmicutes (58.33%) was higher followed by Actinobacteria (23.0%), Proteobacteria (14.6%), and least diversity was reported in Euryarchaeota (4.2%). The bacterial communities were most dominant in soil samples followed by cryoconite sample and least dominant in the ice and snow samples. Moreover, the obtained bacterial isolates were found resistant to high concentrations of heavy metals (Cr³⁺ , Cd²⁺ , Hg²⁺ , and Ar³⁺ ) and sodium chloride, and, therefore, exhibited polyextremophilic characteristics. LHG no. 12 is rich in bacterial and archaeal diversities and provides a potentially curious site for further in-depth exploration of microbial diversity and their biotechnological applications.

Insights Into the Microbiology of the Chaotropic Brines of Salar de Atacama, Chile

Microbial life inhabiting hypersaline environments belong to a limited group of extremophile or extremotolerant taxa. Natural or artificial hypersaline environments are not limited to high concentrations of NaCl, and under such conditions, specific adaptation mechanisms are necessary to permit microbial survival and growth. Argentina, Bolivia, and Chile include three large salars (salt flats) which globally, represent the largest lithium reserves, and are commonly referred to as the Lithium Triangle Zone. To date, a large amount of information has been generated regarding chemical, geological, meteorological and economical perspectives of these salars. However, there is a remarkable lack of information regarding the biology of these unique environments. Here, we report the presence of two bacterial strains (isolates LIBR002 and LIBR003) from one of the most hypersaline lithium-dominated man-made environments (total salinity 556 g/L; 11.7 M LiCl) reported to date. Both isolates were classified to the Bacillus genera, but displayed differences in 16S rRNA gene and fatty acid profiles. Our results also revealed that the isolates are lithium-tolerant and that they are phylogenetically differentiated from those Bacillus associated with high NaCl concentration environments, and form a new clade from the Lithium Triangle Zone. To determine osmoadaptation strategies in these microorganisms, both isolates were characterized using morphological, metabolic and physiological attributes. We suggest that our characterization of bacterial isolates from a highly lithium-enriched environment has revealed that even at such extreme salinities with high concentrations of chaotropic solutes, scope for microbial life exists. These conditions have previously been considered to limit the development of life, and our work extends the window of life beyond high concentrations of MgCl2, as previously reported, to LiCl. Our results can be used to further the understanding of salt tolerance, most especially for LiCl-dominated brines, and likely have value as models for the understanding of putative extra-terrestrial (e.g., Martian) life.

A Simple Instrument Suite for Characterizing Habitability and Weathering: The Modern Aqueous Habitat Reconnaissance Suite (MAHRS)

The shallow subsurface of Mars is extremely interesting as a possible microbial habitat because it becomes temporarily wet, it is shielded from radiation, and mixing by aeolian processes could provide the sources of energy and nutrients necessary for sustaining microbial life in it. The Modern Aqueous Habitat Reconnaissance Suite (MAHRS) was developed primarily to search for potentially habitable environments in the shallow subsurface of Mars and to study weathering, but it can also be used to search for potentially habitable environments in the shallow subsurface of other planetary bodies such as the Icy Worlds. MAHRS includes an instrument developed to measure regolith wetness and search for brine in the shallow subsurface of Mars, where it is most likely to be found. The detection of brine can aid in our understanding not only of habitability but also of geochemistry and aqueous weathering processes. Besides the regolith wetness sensor, MAHRS includes an electric field sensor, an optical microscope, and a radiometer developed to characterize the near-surface environment and study mixing by aeolian processes. MAHRS was designed to aid in the selection of optimum areas for sample collection for return to Earth.

Cultivation and characterization of the bacterial assemblage of epsomic Basque Lake, BC

Athalassohaline waters that are rich in divalent ions are good analogues for the chemical environments of Mars and the ocean worlds. Sulfate salts, along with chlorides, are important in Mars regolith with Ca, Fe, Mg, and Na counterions. Certain lakes in the Pacific Northwest are saturated with MgSO4 as epsomite. Here we report on the microbial community of Basque Lake, BC, a group of playas that is saturated with MgSO4. More than 60 bacterial isolates were obtained from Basque Lake soils by enrichment culture and repetitive streak-plating using media containing 10% (~ 1.7 M) NaCl or 50% (~ 2 M) MgSO4. Most of the isolates (~ 75%) were Gram-positive, motile, and produced endospores. Isolates related to Marinococcus halophilus and Virgibacillus marismortui dominated the collection. Halomonas and Salinivibrio were Gram-negative genera found at Basque Lake. Nearly all of the Basque Lake isolates grew at 50% MgSO4, with 65% growing at 60% MgSO4. Several isolates could grow in saturated (67%) MgSO4 (aw = 0.90). All of the isolates grew at 10% NaCl with 70% growing at 20% salinity (~ 3.5 M NaCl; aw = 0.82). Basque Lake isolates grew better at basic pH than acidic pH, with 80% growing at pH 9 and 30% growing at pH 10. Only 20% of the isolates grew at pH 5. Numerical taxonomy dendrograms based on 44 phenetic characteristics showed a strong correspondence to phylogenetic trees constructed from 16S rRNA gene sequences. Pyrosequencing of 16S rRNA gene sequences from direct DNA extracts of Basque Lake soils recovered predominantly Proteobacteria (60%), Firmicutes (11%), and unclassified bacteria (27%). Microbes capable of growth under the extreme chemical conditions of Mars are a particular concern for forward planetary protection should they contaminate a spacecraft.

Survival of Radioresistant Bacteria on Europa’s Surface after Pulse Ejection of Subsurface Ocean Water

We briefly present preliminary results of our study of the radioresistant bacteria in a low temperature and pressure and high-radiation environment and hypothesize the ability of microorganisms to survive extraterrestrial high-radiation environments, such as the icy surface of Jupiter’s moon, Europa. In this study, samples containing a strain of Deinococcus radiodurans VKM B-1422T embedded into a simulated version of Europa’s ice were put under extreme environmental (−130 °C, 0.01 mbar) and radiation conditions using a specially designed experimental vacuum chamber. The samples were irradiated with 5, 10, 50, and 100 kGy doses and subsequently studied for residual viable cells. We estimate the limit of the accumulated dose that viable cells in those conditions could withstand at 50 kGy. Combining our numerical modelling of the accumulated dose in ice with observations of water eruption events on Europa, we hypothesize that in the case of such events, it is possible that putative extraterrestrial organisms might retain viability in a dormant state for up to 10,000 years, and could be sampled and studied by future probe missions.

Watershed-Induced Limnological and Microbial Status in Two Oligotrophic Andean Lakes Exposed to the Same Climatic Scenario

Laguna Negra and Lo Encañado are two oligotrophic Andean lakes forming part of the system fed by meltwater from distinct glacial tongues of the Echaurren glacier in central Chile, which is in a recession period. The recent increase in temperature and decline in precipitation have led to an increase of glacial meltwater and sediments entering these lakes. Although the lacustrine systems are also hydrogeologically connected, the limnology of the lakes is strongly controlled by the surface processes related to the respective sub-watersheds and hydrology. Watershed characteristics (area and length, slope, lithology, resistance to erosion, among others) affect the chemical and physical characteristics of both lakes (e.g., nutrient concentration and turbidity). We studied physical and chemical variables and performed 16S rRNA amplicon sequencing to determine the specific microbial signature of the lakes. The transparency, temperature, turbidity and concentrations of chlorophyll-a, dissolved organic matter, nutrients and the total number of cells, revealed the different status of both lakes at the time of sampling. The predominant bacterial groups in both lakes were Proteobacteria, Verrucomicrobia, and Bacteroidetes. Interestingly, the contribution of phototrophs was significantly higher in LN compared to LE (13 and 4% respectively) and the major fraction corresponded to Anoxygenic Phototrophs (AP) represented by Chloroflexi, Alpha, and Betaproteobacteria. Multivariate analyses showed that the nutrient levels and the light availability of both lakes, which finally depend on the hydrological characteristics of the respective watersheds, explain the differential community composition/function. The abundance of a diverse photoheterotrophic bacterioplankton community suggests that the ability to utilize solar energy along with organic and inorganic substrates is a key function in these oligotrophic mountain lakes.

Microbial habitability of Europa sustained by radioactive sources

There is an increasing interest in the icy moons of the Solar System due to their potential habitability and as targets for future exploratory missions, which include astrobiological goals. Several studies have reported new results describing the details of these moons' geological settings; however, there is still a lack of information regarding the deep subsurface environment of the moons. The purpose of this article is to evaluate the microbial habitability of Europa constrained by terrestrial analogue environments and sustained by radioactive energy provided by natural unstable isotopes. The geological scenarios are based on known deep environments on Earth, and the bacterial ecosystem is based on a sulfate-reducing bacterial ecosystem found 2.8 km below the surface in a basin in South Africa. The results show the possibility of maintaining the modeled ecosystem based on the proposed scenarios and provides directions for future models and exploration missions for a more complete evaluation of the habitability of Europa and of icy moons in general.

Glaciers and Ice Sheets As Analog Environments of Potentially Habitable Icy Worlds

Icy worlds in the solar system and beyond have attracted a remarkable attention as possible habitats for life. The current consideration about whether life exists beyond Earth is based on our knowledge of life in terrestrial cold environments. On Earth, glaciers and ice sheets have been considered uninhabited for a long time as they seemed too hostile to harbor life. However, these environments are unique biomes dominated by microbial communities which maintain active biochemical routes. Thanks to techniques such as microscopy and more recently DNA sequencing methods, a great biodiversity of prokaryote and eukaryote microorganisms have been discovered. These microorganisms are adapted to a harsh environment, in which the most extreme features are the lack of liquid water, extremely cold temperatures, high solar radiation and nutrient shortage. Here we compare the environmental characteristics of icy worlds, and the environmental characteristics of terrestrial glaciers and ice sheets in order to address some interesting questions: (i) which are the characteristics of habitability known for the frozen worlds, and which could be compatible with life, (ii) what are the environmental characteristics of terrestrial glaciers and ice sheets that can be life-limiting, (iii) What are the microbial communities of prokaryotic and eukaryotic microorganisms that can live in them, and (iv) taking into account these observations, could any of these planets or satellites meet the conditions of habitability? In this review, the icy worlds are considered from the point of view of astrobiological exploration. With the aim of determining whether icy worlds could be potentially habitable, they have been compared with the environmental features of glaciers and ice sheets on Earth. We also reviewed some field and laboratory investigations about microorganisms that live in analog environments of icy worlds, where they are not only viable but also metabolically active.

Microbial Morphology and Motility as Biosignatures for Outer Planet Missions

Meaningful motion is an unambiguous biosignature, but because life in the Solar System is most likely to be microbial, the question is whether such motion may be detected effectively on the micrometer scale. Recent results on microbial motility in various Earth environments have provided insight into the physics and biology that determine whether and how microorganisms as small as bacteria and archaea swim, under which conditions, and at which speeds. These discoveries have not yet been reviewed in an astrobiological context. This paper discusses these findings in the context of Earth analog environments and environments expected to be encountered in the outer Solar System, particularly the jovian and saturnian moons. We also review the imaging technologies capable of recording motility of submicrometer-sized organisms and discuss how an instrument would interface with several types of sample-collection strategies. Key Words: In situ measurement-Biosignatures-Microbiology-Europa-Ice. Astrobiology 16, 755-774.

Germination of Spores of Astrobiologically Relevant Bacillus Species in High-Salinity Environments

In times of increasing space exploration and search for extraterrestrial life, new questions and challenges for planetary protection, aiming to avoid forward contamination of different planets or moons with terrestrial life, are emerging. Spore-forming bacteria such as Bacillus species have a high contamination potential due to their spores' extreme resistance, enabling them to withstand space conditions. Spores require liquid water for their conversion into a growing cell (i.e., spore germination and subsequent growth). If present, water on extraterrestrial planets or moons is likely to be closely associated with salts (e.g., in salty oceans or brines), thus constituting high-salinity environments. Spores of Bacillus subtilis can germinate despite very high salt concentrations, although salt stress does exert negative effects on this process. In this study, germination and metabolic reactivation ("outgrowth") of spores of five astrobiologically relevant Bacillus species (B. megaterium, B. pumilus SAFR-032, B. nealsonii, B. mojavensis, and B. vallismortis) in high salinity (≤3.6 M NaCl) were investigated. Spores of different species exhibited different germination and outgrowth capabilities in high salinity, which strongly depended on germination conditions, especially the exact composition of the medium. In this context, a new "universal" germination trigger for Bacillus spores, named KAGE (KCl, L-alanine, D-glucose, ectoine), was identified, which will be very useful for future comparative germination and outgrowth studies on different Bacillus species. Overall, this study yielded interesting new insights on salt stress effects on spore germination and points out the difficulty of predicting the potential of spores to contaminate salty environments on extraterrestrial celestial bodies.

KEY WORDS

Bacillus species-Spores-Germination-High salinity-Salt stress-NaCl-Inhibition. Astrobiology 16, 500-512.

Technologies for retrieving sediment cores in Antarctic subglacial settings

Accumulations of sediment beneath the Antarctic Ice Sheet contain a range of physical and chemical proxies with the potential to document changes in ice sheet history and to identify and characterize life in subglacial settings. Retrieving subglacial sediments and sediment cores presents several unique challenges to existing technologies. This paper briefly reviews the history of sediment sampling in subglacial environments. It then outlines some of the technological challenges and constraints in developing the corers being used in sub-ice shelf settings (e.g. George VI Ice Shelf and Larsen Ice Shelf), under ice streams (e.g. Rutford Ice Stream), at or close to the grounding line (e.g. Whillans Ice Stream) and in subglacial lakes deep under the ice sheet (e.g. Lake Ellsworth). The key features of the corers designed to operate in each of these subglacial settings are described and illustrated together with comments on their deployment procedures.

Descent without Modification? The Thermal Chemistry of H2O2 on Europa and Other Icy Worlds

The strong oxidant H2O2 is known to exist in solid form on Europa and is suspected to exist on several other Solar System worlds at temperatures below 200 K. However, little is known of the thermal chemistry that H2O2 might induce under these conditions. Here, we report new laboratory results on the reactivity of solid H2O2 with eight different compounds in H2O-rich ices. Using infrared spectroscopy, we monitored compositional changes in ice mixtures during warming. The compounds CH4 (methane), C3H4 (propyne), CH3OH (methanol), and CH3CN (acetonitrile) were unaltered by the presence of H2O2 in ices, showing that exposure to either solid H2O2 or frozen H2O+H2O2 at cryogenic temperatures will not oxidize these organics, much less convert them to CO2. This contrasts strongly with the much greater reactivity of organics with H2O2 at higher temperatures, and particularly in the liquid and gas phases. Of the four inorganic compounds studied, CO, H2S, NH3, and SO2, only the last two reacted in ices containing H2O2, NH3 making NH4+ and SO2 making SO(4)2- by H+ and e- transfer, respectively. An important astrobiological conclusion is that formation of surface H2O2 on Europa and that molecule's downward movement with H2O-ice do not necessarily mean that all organics encountered in icy subsurface regions will be destroyed by H2O2 oxidation.

Molecular and Phenetic Characterization of the Bacterial Assemblage of Hot Lake, WA, an Environment with High Concentrations of Magnesium Sulfate, and Its Relevance to Mars

Hot Lake (Oroville, WA) is an athalassohaline epsomite lake that can have precipitating concentrations of MgSO4 salts, mainly epsomite. Little biotic study has been done on epsomite lakes and it was unclear whether microbes isolated from epsomite lakes and their margins would fall within recognized halotolerant genera, common soil genera, or novel phyla. Our initial study cultivated and characterized epsotolerant bacteria from the lake and its margins. Approximately 100 aerobic heterotrophic microbial isolates were obtained by repetitive streak-plating in high-salt media including either 10% NaCl or 2 M MgSO4. The collected isolates were all bacteria, nearly evenly divided between Gram-positive and Gram-negative clades, the most abundant genera being Halomonas, Idiomarina, Marinobacter, Marinococcus, Nesterenkonia, Nocardiopsis, and Planococcus. Bacillus, Corynebacterium, Exiguobacterium, Kocuria, and Staphylococcus also were cultured. This initial study included culture-independent community analysis of direct DNA extracts of lake margin soil using PCR-based clone libraries and 16S rRNA gene phylogeny. Clones assigned Gram-positive bacterial clades (70% of total clones) were dominated by sequences related to uncultured actinobacteria. There were abundant Deltaproteobacteria clones related to bacterial sulfur metabolisms and clones of Legionella and Coxiella. These epsomite lake microbial communities seem to be divided between bacteria primarily associated with hyperhaline environments rich in NaCl and salinotolerant relatives of common soil organisms. Archaea appear to be in low abundance and none were isolated, despite near-saturated salinities. Growth of microbes at very high concentrations of magnesium and other sulfates has relevance to planetary protection and life-detection missions to Mars, where scant liquid water may form as deliquescent brines and appear as eutectic liquids.

Science potential from a Europa lander

The prospect of a future soft landing on the surface of Europa is enticing, as it would create science opportunities that could not be achieved through flyby or orbital remote sensing, with direct relevance to Europa's potential habitability. Here, we summarize the science of a Europa lander concept, as developed by our NASA-commissioned Science Definition Team. The science concept concentrates on observations that can best be achieved by in situ examination of Europa from its surface. We discuss the suggested science objectives and investigations for a Europa lander mission, along with a model planning payload of instruments that could address these objectives. The highest priority is active sampling of Europa's non-ice material from at least two different depths (0.5-2 cm and 5-10 cm) to understand its detailed composition and chemistry and the specific nature of salts, any organic materials, and other contaminants. A secondary focus is geophysical prospecting of Europa, through seismology and magnetometry, to probe the satellite's ice shell and ocean. Finally, the surface geology can be characterized in situ at a human scale. A Europa lander could take advantage of the complex radiation environment of the satellite, landing where modeling suggests that radiation is about an order of magnitude less intense than in other regions. However, to choose a landing site that is safe and would yield the maximum science return, thorough reconnaissance of Europa would be required prior to selecting a scientifically optimized landing site.

Amino acid substitutions in cold-adapted proteins from Halorubrum lacusprofundi, an extremely halophilic microbe from antarctica

The halophilic Archaeon Halorubrum lacusprofundi, isolated from the perennially cold and hypersaline Deep Lake in Antarctica, was recently sequenced and compared to 12 Haloarchaea from temperate climates by comparative genomics. Amino acid substitutions for 604 H. lacusprofundi proteins belonging to conserved haloarchaeal orthologous groups (cHOGs) were determined and found to occur at 7.85% of positions invariant in proteins from mesophilic Haloarchaea. The following substitutions were observed most frequently: (a) glutamic acid with aspartic acid or alanine; (b) small polar residues with other small polar or non-polar amino acids; (c) small non-polar residues with other small non-polar residues; (d) aromatic residues, especially tryptophan, with other aromatic residues; and (e) some larger polar residues with other similar residues. Amino acid substitutions for a cold-active H. lacusprofundi β-galactosidase were then examined in the context of a homology modeled structure at residues invariant in homologous enzymes from mesophilic Haloarchaea. Similar substitutions were observed as in the genome-wide approach, with the surface accessible regions of β-galactosidase displaying reduced acidity and increased hydrophobicity, and internal regions displaying mainly subtle changes among smaller non-polar and polar residues. These findings are consistent with H. lacusprofundi proteins displaying amino acid substitutions that increase structural flexibility and protein function at low temperature. We discuss the likely mechanisms of protein adaptation to a cold, hypersaline environment on Earth, with possible relevance to life elsewhere.

Cloning, overexpression, purification, and characterization of a polyextremophilic β-galactosidase from the Antarctic haloarchaeon Halorubrum lacusprofundi

Background

Halorubrum lacusprofundi is a cold-adapted halophilic archaeon isolated from Deep Lake, a perennially cold and hypersaline lake in Antarctica. Its genome sequencing project was recently completed, providing access to many genes predicted to encode polyextremophilic enzymes active in both extremely high salinity and cold temperatures.

Results

Analysis of the genome sequence of H. lacusprofundi showed a gene cluster for carbohydrate utilization containing a glycoside hydrolase family 42 β-galactosidase gene, named bga. In order to study the biochemical properties of the β-galactosidase enzyme, the bga gene was PCR amplified, cloned, and expressed in the genetically tractable haloarchaeon Halobacterium sp. NRC-1 under the control of a cold shock protein (cspD2) gene promoter. The recombinant β-galactosidase protein was produced at 20-fold higher levels compared to H. lacusprofundi, purified using gel filtration and hydrophobic interaction chromatography, and identified by SDS-PAGE, LC-MS/MS, and ONPG hydrolysis activity. The purified enzyme was found to be active over a wide temperature range (−5 to 60°C) with an optimum of 50°C, and 10% of its maximum activity at 4°C. The enzyme also exhibited extremely halophilic character, with maximal activity in either 4 M NaCl or KCl. The polyextremophilic β-galactosidase was also stable and active in 10–20% alcohol-aqueous solutions, containing methanol, ethanol, n-butanol, or isoamyl alcohol.

Conclusion

The H. lacusprofundi β-galactosidase is a polyextremophilic enzyme active in high salt concentrations and low and high temperature. The enzyme is also active in aqueous-organic mixed solvents, with potential applications in synthetic chemistry. H. lacuprofundi proteins represent a significant biotechnology resource and for developing insights into enzyme catalysis under water limiting conditions. This study provides a system for better understanding how H. lacusprofundi is successful in a perennially cold, hypersaline environment, with relevance to astrobiology.

Colors of extreme exo-Earth environments

The search for extrasolar planets has already detected rocky planets and several planetary candidates with minimum masses that are consistent with rocky planets in the habitable zone of their host stars. A low-resolution spectrum in the form of a color-color diagram of an exoplanet is likely to be one of the first post-detection quantities to be measured for the case of direct detection. In this paper, we explore potentially detectable surface features on rocky exoplanets and their connection to, and importance as, a habitat for extremophiles, as known on Earth. Extremophiles provide us with the minimum known envelope of environmental limits for life on our planet. The color of a planet reveals information on its properties, especially for surface features of rocky planets with clear atmospheres. We use filter photometry in the visible as a first step in the characterization of rocky exoplanets to prioritize targets for follow-up spectroscopy. Many surface environments on Earth have characteristic albedos and occupy a different color space in the visible waveband (0.4-0.9 μm) that can be distinguished remotely. These detectable surface features can be linked to the extreme niches that support extremophiles on Earth and provide a link between geomicrobiology and observational astronomy. This paper explores how filter photometry can serve as a first step in characterizing Earth-like exoplanets for an aerobic as well as an anaerobic atmosphere, thereby prioritizing targets to search for atmospheric biosignatures.

Evaluation of MODIS surrogates for meteorological humidity data in east Africa

Satellite remote sensing technology has shown promising results in characterizing the environment in which plants and animals thrive. Remote sensing scientists, biologists, and epidemiologists are adopting remotely sensed imagery to compensate for the paucity of weather information measured by weather stations. With measured humidity from three stations as baselines, our study reveals that Normalised Difference Vegetation Index (NDVI) and atmosphere saturation deficits at the 780 hPa pressure level (DMODIS), both of which were derived from the MODerate-resolution Imaging Spectroradiometer (MODIS) sensor, were significantly correlated with station saturation deficits (Dstn)(ȣrȣ = 0.42-0.63, p < 0.001). These metrics have the potential to estimate saturation deficits over east Africa. Four to nine days of lags were found in the NDVI responding to Dstn. For the daily estimations of Dstn, DMODIS had a better performance than the NDVI. However, both of them poorly explained the variances in daily Dstn using simple regression models (adj. R2 = 0.17-0.39). When the estimation temporal scale was changed to 16-day, their performances were similar, and both were better than daily estimations. For Dstn estimations at coarser geographic scales, given that many factors such as soil, vegetation, slope, aspect, and wind speed might complicate the NDVI response lags and model construction, DMODIS is more favourable as a proxy of the saturation deficit over ground due to its simple relationship with Dstn.