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Abstract Protected from harmful radiation, subfreezing temperatures, and low pressures, subsurface rock-hosted habitats provide potentially sustainable refugia for microbial ecosystems inside small rocky planets, such as Mars. For many chemolithotrophic communities on Earth, water–rock alteration reactions have been shown to produce the key electron donors and acceptors necessary to sustain microbial life on geologic timescales. Here we quantitatively demonstrate that radiolysis likely generated concentrations of dissolved H 2 capable of sustaining microbial communities in the subsurface of Noachian Mars (3.7–4.1 Gyr ago). When considering an environment with H 2 O groundwater, dissolved H 2 concentrations reach up to ∼55 mM in a cold early Mars climate scenario and ∼35 mM in a warm early Mars climate scenario; whereas when considering an environment with eutectic NaCl brine groundwater, dissolved H 2 concentrations reach up to ∼85 mM in a cold early Mars climate scenario and ∼45 mM in a warm early Mars climate scenario. Specifically within the subsurface habitable zone, dissolved H 2 concentrations range from ∼50–55 mM for a cold climate scenario with H 2 O groundwater. For a warm climate scenario with H 2 O groundwater, dissolved H 2 concentrations within the subsurface habitable zone range from ∼1–30 mM. For a cold climate scenario with eutectic NaCl brine groundwater, dissolved H 2 concentrations within the subsurface habitable zone range from ∼65–85 mM. For a warm climate scenario with eutectic NaCl brine groundwater, dissolved H 2 concentrations within the subsurface habitable zone range from ∼1-40 mM. Radiolysis likely produced [1.3–4.8] × 1010 moles H 2 per year globally during the Noachian depending on the assumed porosity and groundwater composition. Radiolytic H 2 , and CH 4 derived from radiolytic H 2 , can be locked in hybrid clathrate hydrates within the cryosphere and released by large impacts, volcanism, or obliquity variations. This process could warm the Noachian climate to above-freezing temperatures, and we predict that ∼1–8 warming events would be possible during the Noachian and Hesperian solely from radiolytically produced H 2. We demonstrate that the region immediately beneath the cryosphere, termed the subcryospheric highly-fractured zone (SHZ), likely contained dissolved H 2 concentrations and temperatures suitable for life regardless of the background climate scenario, making it the most consistently habitable environment on ancient Mars in terms of reductant availability. Material from this zone can be exposed by faulting and in the ejecta and uplifts of impacts, making the SHZ a crucial astrobiological target for testing the subsurface biosphere hypothesis. Highlights • Radiolysis produced enough H 2 to support subsurface life on Mars during Noachian. • Region immediately beneath cryosphere was longest-lived habitable zone on Mars. • Biosignature-hosting material from habitable zone can be excavated by impacts. • Radiolytic H 2 could contribute to transient warming during Noachian. [ABSTRACT FROM AUTHOR]

Abstract Outcrop mapping of a chalk cliff and wavecut platform in Thanet, Southeast England show a complex fracture pattern that seems to be controlled by meso-scale strike-slip faults within the chalk. The response of these faults to changes to in situ stress and fluid pressure is thought to control the nucleation and propagation of fractures in the chalk. In this study the DEM (Discrete Element Method) technique has been employed as a follow up to previous field and numerical (boundary and finite element method) investigations to ascertain the role of the faults in the initiation and nucleation of fractures The role of fluid pressure, in-situ stress, and fault geometry are recognised as focal factors. The generation of localised areas of tensile stresses due to fluid pressure and stress perturbations have been shown to cause the initiation of fractures around the fault bends. For releasing bends, localised tensile stresses tend to occur along the central segment of the fault bend, whereas for restraining bends, tensile stresses are more likely to develop on the outer edges of the fault bend. The dissimilarity in the fracturing process due to differences in the geometry of pre-existing faults demonstrates the significance of both fault geometry and fluid behaviour in controlling fracturing. Highlights • We investigated the behaviour of faults at a chalk cliff and wavecut platform at Thanet, UK. • The discrete element numerical technique was implemented. • Fluid pressure, in-situ stresses and fault geometry greatly influence fracture proliferation. • The orientation of propagating fractures is found to be perpendicular to the fault plane. • Fluid pressure and stress perturbations are shown to initiate fracturing around fault bends. [ABSTRACT FROM AUTHOR]

Today, there is an increasing need to understand how to link the management of the surface and subsurface to avoid disasters in many urban areas and/or reduce the likelihood of future risks. There is a need for thorough investigation of subsurface processes. This investigation should entail an analysis of water security, flood risks, and drought hazards in urban areas that may affect long-term sustainability and the ability to recover from disturbance, e.g., a capacity for resilience. In this context, as part of this analysis, potential biophysical and hydro-meteorological hazards need to be studied and subdivided according to geological, hydrogeological, man-made, and climatic origin, and by their characteristic temporal scales and site specific characteristics. The introduction of adaptive design and resilience in urban and suburban planning and management requires a shift towards more organic, adaptive, and flexible design and management strategies. This leads to the use of a complex cross-disciplinary methodology. We consider data collation, modelling, and monitoring designed to fit typical urban situations and complexity. Furthermore, implementation of strategic planning, decision-making to manage the consequences of future infrastructure and constructions are considered. The case studies presented are experiences from different hydrogeological studies performed in Odense, Denmark. Rising population and densification is affecting Odense, and there is risk of raised seawater level, groundwater, and surface-water flooding. The anthropogenic modification of subsurface structures and increased climate changes enhance the risk of hazards and the risk of coinciding impacts.

• Six gas diffusivity parametric functions were compared against experimental data. • Soil-type and structure-dependent functions outperformed traditional predictions. • Parameterizations were incorporated in field-scale multiphase flow models. • Results show the importance of diffusivity function selection on model accuracy. A proper understanding of subsurface gas migration patterns resulting from belowground natural gas (NG) infrastructure is essential for safe and efficient first responder operations and to improve leak repair actions. Subsurface migration of leaked gas occurs predominately by pressure-driven advection close to the leak. Far migration occurs by concentration-driven diffusion and eventually defines the plume's edge. Therefore, proper formulation of the diffusion behavior in mathematical models of gas transport is critical to safety and response. This study evaluated six diffusivity parametric functions (DPFs) for an accurate representation of soil gas diffusivity through soils by comparing predicted and measured values for mineral, clay, and sand soils. DPFs were further tested by implementing them in a multiphase flow model previously developed for belowground NG transport. DPFs were also tested by comparing with field scale NG leakage experiments conducted in varying soil conditions (mineral soil, sand, and sand/clay) at controlled NG leak rates ranging from 5 to 21 slpm. Results showed that soil-type and structure-dependent DPFs performed better than soil-type and structure-independent DPFs in simulating the diffusive transport of NG. Further, results showed that DPF selection does not significantly influence NG migration simulations under dry soil conditions during the leak period. However, when the leak is terminated, simulation results followed the same trend of over- and under-predictions as of D p /D o parameterization. Further, wet soil simulations show more than a 10% CH 4 composition deviation and 2.5-day time differences among DPFs in reaching explosive limits. Unique to this work, the selection of a DPF can result in large differences in flux estimates in transport models, masking the effects of the gas behavior, especially in cases where gas diffusion to outer boundaries is of interest. The findings, therefore, support the idea that careful selection of DPFs based on soil-type, structure and moisture condition is essential to predict NG plume behavior during a leak event and after repair. [ABSTRACT FROM AUTHOR]

• Urban design recommendations for sustainable UUS are identified and illustrated. • An integrative literature review combined with expert discussions led to novel findings. • Livability, connectivity, temporality and interdependency are key to designing UUS. • A new frame for the complex relationship between UUS and sustainability is discussed. Considerable research has been devoted to sustainable urban underground space use, with the importance of urban underground spaces (UUS) for urban sustainability having been well documented. However, the focus of most research stems from an engineering or urban planning point of view, overlooking ways in which urban design can contribute to sustainable UUS. This paper aims to synthesize and illustrate urban design recommendations for sustainable UUS in an integrative and holistic manner, taking elements identified by other disciplines into account and reinterpreting them from an urban design point of view. The findings allow us to suggest recommendations on livability, connectivity, temporality and interdependency. By renewing the focus on urban design and making the recommendations explicit, the complexity of the subsurface is acknowledged. This means designers will become better able to master this complexity and thus design more examples of good practice and further enhance the added value of the subsurface for sustainable cities. [ABSTRACT FROM AUTHOR]

Microbial communities play a key role for central biogeochemical cycles in the subsurface. Little is known about whether short-term seasonal drought and rewetting events influence the dominant microbes involved in C- and N-cycles. Here, we applied metaproteomics at different subsurface sites in winter, summer and autumn from surface litter layer, seepage water at increasing subsoil depths and remote located groundwater from two wells within the Hainich Critical Zone Exploratory, Germany. We observed changes in the dominance of microbial families at subsurface sampling sites with increasing distances, i.e., Microcoleaceae dominated in topsoil seepage, while Candidatus Brocadiaceae dominated at deeper and more distant groundwater wells. Nitrifying bacteria showed a shift in dominance from drought to rewetting events from summer by Nitrosomandaceae to autumn by Candidatus Brocadiaceae. We further observed that the reductive pentose phosphate pathway was a prominent CO2-fixation strategy, dominated by Woeseiaceae in wet early winter, which decreased under drought conditions and changed to a dominance of Sphingobacteriaceae under rewetting conditions. This study shows that increasing subsurface sites and rewetting event after drought alter the dominances of key subsurface microbes. This helps to predict the consequences of annual seasonal dynamics on the nutrient cycling microbes that contribute to ecosystem functioning.

The Kuiseb River is one of the major ephemeral rivers of Western Namibia, setting the northern limit of the Namib Sand Sea and outflowing in the Atlantic Ocean at Walvis Bay. Such ephemeral rivers are of the highest importance for the country since they are related both to recent past climatic conditions and to potential water resources. Using high-resolution radar images from the Japanese ALOS-2 satellite, we mapped for the first time the numerous channels hidden under the surface aeolian sediments: while the non-permanent tributaries of the Kuiseb River appear north of its present-day bed, a wide paleochannel system running westward, assumed by previous studies, could be clearly observed in the interdune valleys in the south. Radar-detected channels were studied during fieldwork in May 2019, which produced both subsurface ground-penetrating radar profiles and high-resolution drone-generated digital elevation models. It allowed us to confirm the existence of the “Paleo–Kuiseb” drainage system, a remnant of the Holocene history of the Kuiseb River, moving northward under the progression of the Namib Sand Sea. Our observations also contribute to the explanation of the young age of the linear dunes at the northern edge of the Namib Sand Sea, which are currently active and are pushing the Kuiseb River course toward the north.

Olkiluoto, an island on the south-west coast of Finland, will host a deep geological repository for the storage of spent nuclear fuel. Microbially induced corrosion from the generation of sulphide is therefore a concern as it could potentially compromise the longevity of the copper waste canisters. Groundwater at Olkiluoto is geochemically stratified with depth and elevated concentrations of sulphide are observed when sulphate-rich and methane-rich groundwaters mix. Particularly high sulphide is observed in methane-rich groundwater from a fracture at 530.6 mbsl, where mixing with sulphate-rich groundwater occurred as the result of an open drill hole connecting two different fractures at different depths. To determine the electron donors fuelling sulphidogenesis, we combined geochemical, isotopic, metagenomic and metaproteomic analyses. This revealed a low diversity microbial community fuelled by hydrogen and organic carbon. Sulphur and carbon isotopes of sulphate and dissolved inorganic carbon, respectively, confirmed that sulphate reduction was ongoing and that CO2 came from the degradation of organic matter. The results demonstrate the impact of introducing sulphate to a methane-rich groundwater with limited electron acceptors and provide insight into extant metabolisms in the terrestrial subsurface.

Twelve evolutionarily unrelated oxidoreductases form enzyme complexes that catalyze the simultaneous coupling of exergonic and endergonic oxidation–reduction reactions to circumvent thermodynamic barriers and minimize free energy loss in a process known as flavin-based electron bifurcation. Common to these 12 bifurcating (Bf) enzymes are protein-bound flavin, the proposed site of bifurcation, and the electron carrier ferredoxin. Despite the documented role of Bf enzymes in balancing the redox state of intracellular electron carriers and in improving the efficiency of cellular metabolism, a comprehensive description of the diversity and evolutionary history of Bf enzymes is lacking. Here, we report the taxonomic distribution, functional diversity, and evolutionary history of Bf enzyme homologs in 4,588 archaeal, bacterial, and eukaryal genomes and 3,136 community metagenomes. Bf homologs were primarily detected in the genomes of anaerobes, including those of sulfate-reducers, acetogens, fermenters, and methanogens. Phylogenetic analyses of Bf enzyme catalytic subunits (oxidoreductases) suggest they were not a property of the Last Universal Common Ancestor of Archaea and Bacteria, which is consistent with the limited and unique taxonomic distributions of enzyme homologs among genomes. Further, phylogenetic analyses of oxidoreductase subunits reveal that non-Bf homologs predate Bf homologs. These observations indicate that multiple independent recruitments of flavoproteins to existing oxidoreductases enabled coupling of numerous new electron Bf reactions. Consistent with the role of these enzymes in the energy metabolism of anaerobes, homologs of Bf enzymes were enriched in metagenomes from subsurface environments relative to those from surface environments. Phylogenetic analyses of homologs from metagenomes reveal that the earliest evolving homologs of most Bf enzymes are from subsurface environments, including fluids from subsurface rock fractures and hydrothermal systems. Collectively, these data suggest strong selective pressures drove the emergence of Bf enzyme complexes via recruitment of flavoproteins that allowed for an increase in the efficiency of cellular metabolism and improvement in energy capture in anaerobes inhabiting a variety of subsurface anoxic habitats where the energy yield of oxidation-reduction reactions is generally low.

Earth’s subsurface is often isolated from phototrophic energy sources and characterized by chemotrophic modes of life. These environments are often oligotrophic and limited in electron donors or electron acceptors, and include continental crust, subseafloor oceanic crust, and marine sediment as well as subglacial lakes and the subsurface of polar desert soils. These low energy subsurface environments are therefore uniquely positioned for examining minimum energetic requirements and adaptations for chemotrophic life. Current targets for astrobiology investigations of extant life are planetary bodies with largely inhospitable surfaces, such as Mars, Europa, and Enceladus. Subsurface environments on Earth thus serve as analogs to explore possibilities of subsurface life on extraterrestrial bodies. The purpose of this review is to provide an overview of subsurface environments as potential analogs, and the features of microbial communities existing in these low energy environments, with particular emphasis on how they inform the study of energetic limits required for life. The thermodynamic energetic calculations presented here suggest that free energy yields of reactions and energy density of some metabolic redox reactions on Mars, Europa, Enceladus, and Titan could be comparable to analog environments in Earth’s low energy subsurface habitats.

Due to the limited cultivability of the vast majority of microorganisms, researchers have applied environmental genomics and other state-of-the-art technologies to gain insights into the biology of uncultivated Archaea and bacteria in their natural biotope. In this review, we summarize the scientific findings on a recently proposed order-level lineage of uncultivated Archaea called Altiarchaeales, which includes “Candidatus Altiarchaeum hamiconexum” as the most well-described representative. Ca. A. hamiconexum possesses a complex biology: thriving strictly anaerobically, this microorganism is capable of forming highly-pure biofilms, connecting the cells by extraordinary cell surface appendages (the “hami”) and has other highly unusual traits, such as a double-membrane-based cell wall. Indicated by genomic information from different biotopes, the Altiarchaeales seem to proliferate in deep, anoxic groundwater of Earth’s crust bearing a potentially very important function: carbon fixation. Although their net carbon fixation rate has not yet been determined, they appear as highly abundant organisms in their biotopes and may thus represent an important primary producer in the subsurface. In sum, the research over more than a decade on Ca. A. hamiconexum has revealed many interesting features of its lifestyle, its genomic information, metabolism and ultrastructure, making this archaeon one of the best-studied uncultivated Archaea in the literature.

Background: The terrestrial subsurface is home to a significant proportion of the Earth's microbial biomass. Our understanding about terrestrial subsurface microbiomes is almost exclusively derived from groundwater and porous sediments mainly by using 16S rRNA gene surveys. To obtain more insights about biomass of consolidated rocks and the metabolic status of endolithic microbiomes, we investigated interbedded limestone and mudstone from the vadose zone, fractured aquifers, and deep aquitards., Results: By adapting methods from microbial archaeology and paleogenomics, we could recover sufficient DNA for downstream metagenomic analysis from seven rock specimens independent of porosity, lithology, and depth. Based on the extracted DNA, we estimated between 2.81 and 4.25 × 10 5 cells × g -1 rock. Analyzing DNA damage patterns revealed paleome signatures (genetic records of past microbial communities) for three rock specimens, all obtained from the vadose zone. DNA obtained from deep aquitards isolated from surface input was not affected by DNA decay indicating that water saturation and not flow is controlling subsurface microbial survival. Decoding the taxonomy and functional potential of paleome communities revealed increased abundances for sequences affiliated with chemolithoautotrophs and taxa such as Cand. Rokubacteria. We also found a broader metabolic potential in terms of aromatic hydrocarbon breakdown, suggesting a preferred utilization of sedimentary organic matter in the past., Conclusions: Our study suggests that limestones function as archives for genetic records of past microbial communities including those sensitive to environmental stress at modern times, due to their specific conditions facilitating long-term DNA preservation. Video Abstract., (© 2023. BioMed Central Ltd., part of Springer Nature.)

Based on a field expedition to the Dalangtan (DLT) saline playa located in a hyperarid region (Qaidam Basin) on the Tibet Plateau and follow-up investigations, we report the mineralogy and geochemistry of the salt layers in two vertical stratigraphic cross sections in the DLT playa. Na-, Ca-, Mg-, KCaMg-sulfates; Na-, K-, KMg-chlorides; mixed (K, Mg)-chloride–sulfate; and chlorate and perchlorate were identified in the collected samples. This mineral assemblage represents the last-stage precipitation products from Na-K-Mg-Ca-Cl-SO4 brine and the oxychlorine formation from photochemistry reaction similar to other hyperarid regions on Earth. The spatial distributions of these salts in both stratigraphic cross sections suggest very limited brine volumes during the precipitation episodes in the Holocene era. More importantly, sulfates and chlorides with a high degree of hydrations were found preserved within the subsurface salt-rich layers of DLT saline playa, where the environmental conditions at the surface are controlled by the hyperaridity in the Qaidam Basin on the Tibet Plateau. Our findings suggest a very different temperature and relative humidity environment maintained by the hydrous salts in a subsurface salty layer, where the climatic conditions at surface have very little or no influence. This observation bears some similarities with four observations on Mars, which implies not only a large humidity reservoir in midlatitude and equatorial regions on Mars but also habitability potential that warrants further investigation. [ABSTRACT FROM AUTHOR]

Olkiluoto, an island on the south-west coast of Finland, will host a deep geological repository for the storage of spent nuclear fuel. Microbially induced corrosion from the generation of sulphide is therefore a concern as it could potentially compromise the longevity of the copper waste canisters. Groundwater at Olkiluoto is geochemically stratified with depth and elevated concentrations of sulphide are observed when sulphate-rich and methane-rich groundwaters mix. Particularly high sulphide is observed in methane-rich groundwater from a fracture at 530.6 mbsl, where mixing with sulphate-rich groundwater occurred as the result of an open drill hole connecting two different fractures at different depths. To determine the electron donors fuelling sulphidogenesis, we combined geochemical, isotopic, metagenomic and metaproteomic analyses. This revealed a low diversity microbial community fuelled by hydrogen and organic carbon. Sulphur and carbon isotopes of sulphate and dissolved inorganic carbon, respectively, confirmed that sulphate reduction was ongoing and that CO2 came from the degradation of organic matter. The results demonstrate the impact of introducing sulphate to a methane-rich groundwater with limited electron acceptors and provide insight into extant metabolisms in the terrestrial subsurface. [ABSTRACT FROM AUTHOR]

Paleoslope of ancient river systems is a fundamental parameter needed to reconstruct paleohydrology and paleoclimate from the fluvial sedimentary record. The shape of braid-bars in 22 modern rivers yields a relationship between average bar elongation (length/width) and river slope. Steep rivers display more elongated bars than gently dipping reaches. This relationship has potential application to ancient braided systems preserved in the subsurface and imaged by 3D seismic data. The average elongation of bars identified on a stratal-slice through the DaGang river deposits in North-East China yields a slope prediction which, combined with empirical data on grain-size and slope of alluvial rivers, predicts a median grain-size of between 0.1 and 1 mm for the considered river sediment, in agreement with grain-size data in an adjacent isochronous belt. This study provides an additional approach for paleohydraulics in sedimentary basins and grain-size prediction in exploration. [ABSTRACT FROM AUTHOR]

The subsurface is a technical space, the “engine room of the city,” that incorporates the vital functions of water and energy supply, communication systems, sewers and drainage. Natural systems too - crucial for stable, dry, cool and nature inclusive cities - are also largely dependent on the quality of the subsoil. The subsurface is critical in an era of climate and demographic changes, the energy transition and economic uncertainty and constraints. However, due to the domain’s current segregation and a weak urban design and planning connection, crucial design potential, benefits and innovations, remain unexploited. This paper first introduces an innovative systems approach, the System Exploration Environment and Subsurface (SEES), to recognise the subsurface as an “under-arching” domain for urban planning and design. The physical impact of the subsurface on the surface quality is described for the categories: civil constructions, water, energy and soil/ecology. After setting understanding of the surface and subsurface as one united space, the paper will go into using the SEES as knowledge brokerage tool, integrating the subsurface artefacts into the design process and how the concept of Reversed Engineering with Nature is useful to uncover synergies between subsurface technologies, new urban maintenance regimes and scripts for design. [ABSTRACT FROM AUTHOR]

Machining of ceramic composites in electrical discharge machining process is often associated with high roughness, unwanted surface and subsurface defects. This situation is generally not desired as it will reduce the life of the ceramic components fabricated in this machining process. Thus, an improvement towards enhancement of surface characteristics is desired. Multi-pass wire electrical discharge machining process can serve as an effective means of enhancing the surface characteristics in machining of ceramic composite. The current work utilizes the multi-pass concept to improve the surface properties of multi-walled carbon nanotubes filled alumina composites. A decreasing nature of surface roughness is observed with increase in the number of machining passes. A lesser appearance of unwanted debris and cracks is observed at the end of the final pass on the machined surface. The spalling effect is found to be more dominant at the initial first pass while melting and evaporation effect is more dominating in higher passes. Also, a better subsurface characteristic with lesser porous recast layer is observed with increase in the number of passes. [ABSTRACT FROM AUTHOR]

Twelve evolutionarily unrelated oxidoreductases form enzyme complexes that catalyze the simultaneous coupling of exergonic and endergonic oxidation-reduction reactions to circumvent thermodynamic barriers and minimize free energy loss in a process known as flavin-based electron bifurcation. Common to these 12 bifurcating (Bf) enzymes are protein-bound flavin, the proposed site of bifurcation, and the electron carrier ferredoxin. Despite the documented role of Bf enzymes in balancing the redox state of intracellular electron carriers and in improving the efficiency of cellular metabolism, a comprehensive description of the diversity and evolutionary history of Bf enzymes is lacking. Here, we report the taxonomic distribution, functional diversity, and evolutionary history of Bf enzyme homologs in 4,588 archaeal, bacterial, and eukaryal genomes and 3,136 community metagenomes. Bf homologs were primarily detected in the genomes of anaerobes, including those of sulfate-reducers, acetogens, fermenters, and methanogens. Phylogenetic analyses of Bf enzyme catalytic subunits (oxidoreductases) suggest they were not a property of the Last Universal Common Ancestor of Archaea and Bacteria, which is consistent with the limited and unique taxonomic distributions of enzyme homologs among genomes. Further, phylogenetic analyses of oxidoreductase subunits reveal that non-Bf homologs predate Bf homologs. These observations indicate that multiple independent recruitments of flavoproteins to existing oxidoreductases enabled coupling of numerous new electron Bf reactions. Consistent with the role of these enzymes in the energy metabolism of anaerobes, homologs of Bf enzymes were enriched in metagenomes from subsurface environments relative to those from surface environments. Phylogenetic analyses of homologs from metagenomes reveal that the earliest evolving homologs of most Bf enzymes are from subsurface environments, including fluids from subsurface rock fractures and hydrothermal systems. Collectively, these data suggest strong selective pressures drove the emergence of Bf enzyme complexes via recruitment of flavoproteins that allowed for an increase in the efficiency of cellular metabolism and improvement in energy capture in anaerobes inhabiting a variety of subsurface anoxic habitats where the energy yield of oxidation-reduction reactions is generally low. [ABSTRACT FROM AUTHOR]

Earth’s subsurface is often isolated from phototrophic energy sources and characterized by chemotrophic modes of life. These environments are often oligotrophic and limited in electron donors or electron acceptors, and include continental crust, subseafloor oceanic crust, and marine sediment as well as subglacial lakes and the subsurface of polar desert soils. These low energy subsurface environments are therefore uniquely positioned for examining minimum energetic requirements and adaptations for chemotrophic life. Current targets for astrobiology investigations of extant life are planetary bodies with largely inhospitable surfaces, such as Mars, Europa, and Enceladus. Subsurface environments on Earth thus serve as analogs to explore possibilities of subsurface life on extraterrestrial bodies. The purpose of this review is to provide an overview of subsurface environments as potential analogs, and the features of microbial communities existing in these low energy environments, with particular emphasis on how they inform the study of energetic limits required for life. The thermodynamic energetic calculations presented here suggest that free energy yields of reactions and energy density of some metabolic redox reactions on Mars, Europa, Enceladus, and Titan could be comparable to analog environments in Earth’s low energy subsurface habitats. [ABSTRACT FROM AUTHOR]

The concentrations of electron donors and acceptors in the terrestrial subsurface biosphere fluctuate due to migration and mixing of subsurface fluids, but the mechanisms and rates at which microbial communities respond to these changes are largely unknown. Subsurface microbial communities exhibit long cellular turnover times and are often considered relatively static--generating just enough ATP for cellular maintenance. Here, we investigated how subsurface populations of CH4 oxidizers respond to changes in electron acceptor availability by monitoring the biological and geochemical composition in a 1339 m-below-land-surface (mbls) fluid-filled fracture over the course of both longer (2.5 year) and shorter (2-week) time scales. Using a combination of metagenomic, metatranscriptomic, and metaproteomic analyses, we observe that the CH4 oxidizers within the subsurface microbial community change in coordination with electron acceptor availability over time. We then validate these findings through a series of 13C-CH4 laboratory incubation experiments, highlighting a connection between composition of subsurface CH4 oxidizing communities and electron acceptor availability. [ABSTRACT FROM AUTHOR]

Abstract: We present an information‐theoretic approach for integration of multiresolution data into multiscale simulations. This general framework is used to upscale and downscale equations of fluid flow in heterogeneous porous media. Fine‐scale information can comprise observational data and/or simulation results related to both system states and system parameters. It is aggregated into its coarse‐scale representation by setting a probabilistic equivalence between the two scales, with parameters that are determined via minimization of observable error and mutual information across scales. The same quantities facilitate the use of coarse‐scale data to constrain compatible fine‐scale distributions. [ABSTRACT FROM AUTHOR]