Journal of Geophysical Research: Biogeosciences

Putative alkaline hydrothermal systems on Noachian Mars were potentially habitable environments for microorganisms. However, the types of reactions that could have fueled microbial life in such systems and the amount of energy available from them have not been quantitatively constrained. In this study, we use thermodynamic modeling to calculate which catabolic reactions could have supported ancient life in a saponite-precipitating hydrothermal vent system in the Eridania basin on Mars. To further evaluate what this could mean for microbial life, we evaluated the energy potential of an analogue site in Iceland, the Strytan Hydrothermal Field (SHF). Results show that out of the 85 relevant redox reactions that were considered, the highest energy-yielding reactions in the Eridania hydrothermal system were dominated by methane formation. By contrast, Gibbs energy calculations carried out for Strytan indicate that the most energetically favorable reactions are CO2 and O2 reduction coupled to H2 oxidation. In particular, our calculations indicate that an ancient hydrothermal system within the Eridania basin could have been a habitable environment for methanogens using NH4+ as an electron acceptor. Differences in Gibbs energies between the two systems were largely determined by oxygen - its presence on Earth and absence on Mars. However, Strytan can serve as a useful analogue for Eridania when studying methane producing reactions that do not involve O2.

Permafrost thaw exposes ancient organic matter to microbial degradation, which is predicted to release globally significant quantities of greenhouse gases into the atmosphere. Though microorganisms drive these processes, the relative importance of biotic (taxonomic and functional community composition) versus environmental (e.g., soil physicochemistry) drivers and their interactions are unknown. Using a novel in situ thaw experiment conducted at the Cold Regions Research and Engineering Laboratorys Permafrost Tunnel near Fairbanks, Alaska, we experimentally separated the effects of soil physicochemistry and microbial communities under "real-world" thaw conditions. To simulate thaw, active layer soil, Holocene permafrost (2 kya), and Pleistocene permafrost (40 kya) were sterilized, inoculated with microbial communities from the different soils, enclosed in 0.22 {micro}m membrane bags to prevent immigration, and buried in the active layer. We retrieved the bags after two weeks and two months of thaw and characterized microbial community structure (16S rRNA and ITS2 amplicon sequencing), functional potential (metagenome sequencing), and soil organic matter (OM) composition at the molecular level (FT-ICR MS). Soil had a stronger effect on bacterial community and gene assemblages than inoculum, and the effects of inoculum were stronger and longer-lasting on community structure than functional potential. Pleistocene permafrost initially contained approximately eleven times more dissolved organic carbon than the other soils, and was enriched in OM derived from microbial necromass and low molecular weight organic acids. This carbon was rapidly depleted during thaw and OM compositional characteristics became increasingly similar to active layer and Holocene permafrost, paralleling shifts in Pleistocene permafrost functional gene profiles and bacterial community structure towards those of other soils. Overall, this work provides new insights into the susceptibility of OM to microbial degradation in compositionally distinct permafrost soils, and ways in which Pleistocene Yedoma permafrost carbon is likely to be particularly vulnerable to permafrost thaw.

Microbial metabolism relies on redox reactions that exploit chemical disequilibria. While aerobic carbon oxidation, carbon fixation, and fermentation are well studied, the broader space of anaerobic carbon redox reactions remains underexplored. In this study, carbon comproportionation, or reverse fermentation, reactions are identified as a previously unrecognized and potentially favorable class of microbial carbon redox transformations. Particular attention is given to the reaction between methane (CH4) and carbon monoxide (CO) to form acetate, a reaction that has not previously been evaluated despite the widespread occurrence of CH4 and CO in anoxic systems. Gibbs energies ({Delta}Gr) for this reaction were calculated across broad ranges of temperature, pH, and dissolved CH4 and CO concentrations using measured physicochemical data from a wide variety of environmental systems. We show that acetogenic CH4-CO comproportionation is exergonic in all environments where both substrates were detected. The most favorable energetic conditions occur at high pH, low temperature, and high reactant concentrations, consistent with cool serpentinizing systems. In several settings, the calculated Gibbs energy yields and energy densities overlap or exceed known anaerobic metabolisms involving CH4, CO, and acetate. These results demonstrate that acetogenic CH4-CO comproportionation can support microbial energy conservation in a variety of settings. To determine if this metabolism could have operated on early Earth or Mars, modeled fluid compositions show that this reaction is also exergonic under plausible physicochemical regimes. This work broadens the suite of possible microbial energy metabolisms and provides testable criteria for evaluating carbon-based catabolic reactions on Earth and on other planetary bodies. Plain Language SummaryMicroorganisms obtain energy by catalyzing chemical reactions in their environment. The energy available from a reaction can be quantified using Gibbs energies of reaction ({Delta}Gr). When {Delta}Gr < 0, energy is released that microorganisms can use to build biomass and carry out other activities. In this study, we predicted a new energy-yielding reaction that could potentially support microbial life. In this reaction, methane (CH4) is oxidized using carbon monoxide (CO) to produce acetate. Using thermodynamic calculations and measured geochemical data from natural environments, we show that this reaction can release usable energy under a wide range of conditions, including continental serpentinizing systems, the deep continental and marine subsurface, and geothermal springs. We also predict that this reaction could support life under plausible early Earth conditions and in modeled Martian fluids. Together, these observations identify the reaction of CH4 and CO to form acetate as a potentially viable microbial energy source in anoxic environments on Earth and other planetary bodies.

Peatlands are well-known emitters of methane. European alpine peatlands share certain characteristics with boreal peatlands, despite being located at temperate latitudes, such as a strong seasonality with snowfall in winter and a short summer and growing season. Unlike boreal peatlands, they experience relatively large temperature fluctuations between day and night and are more likely to be sloping. It is unknown how these factors affect methane dynamics. Furthermore, winter methane dynamics have rarely been studied. We therefore quantified the soil-atmosphere methane flux at an alpine peatland in Austria (1700 m a.s.l), with a focus on the spatial and temporal heterogeneity in this ecosystem. In summer, methane emissions were high (49 mg m2 h-1), whereas in spring, shortly after snowmelt, both methane uptake and emissions were observed at different locations within the alpine peatland. In winter, a local snow-free patch persisted at the peatland due to the year-round influx of 5{degrees}C spring water. We compared the methane flux from this snow-free patch to another alpine peatland which also contained such a snow-free area and observed methane emissions at the one peatland (1.2 mg m2 h-1) and methane uptake at the other (-0.06 mg m2 h-1). The input of spring water in combination with the sloping nature of the peatlands resulted in a large spatial heterogeneity, likely as a result of the input of redox-active components such as sulfate by the spring water. The microbial community composition also suggested the presence of active sulfur, iron and methane cycling in the peat soil. Overall, our research shows that alpine peatlands are unique systems due to the year-round spring water throughput, altering biogeochemical cycles and creating local snow-free conditions, with implications for methane cycling.

Mars relatively moderate surface conditions, availability of solar energy, and in situ resources like water ice, carbon dioxide, and mineral-rich regolith make it a compelling target for supporting life beyond Earth. However, existing experiments testing habitability in Mars conditions generally rely on leachates of physical regolith simulants, which vary in composition across simulant types, leaching conditions, and production batches. We introduce a defined Mars media (DMM) that accurately simulates the biologically relevant nutrients (nitrogen, phosphorus, and sulfur) and stressors (perchlorates, heavy metals) in Martian regolith when it is leached in water at neutral pH. We formulated DMM by combining direct rover and lander measurements from Mars with laboratory measurements of regolith simulant leachates. We validate DMM from a lx to 20x concentrate, equivalent to 40 g/L to 800 g/L of leached regolith. Using DMM with acetate as a Mars atmosphere-derived carbon source, we grew eight heterotrophic bacteria, confirming that organisms can source all essential nutrients from Martian resources. We also show that microbial growth in DMM is robust to uncertainties in Martian regolith composition: sensitivity experiments can identify limiting trace element nutrients and toxins in DMM, and demonstrate that bacterial growth is maintained across at least an order of magnitude variation in their concentrations. This is the first defined Mars regolith media recipe containing both macro- and micro- nutrients, and designed specifically for biological experimentation. By shifting from variable leachate-based approaches to a defined aqueous analog, we enable controlled hypothesis testing of microbial survival, growth, and function. DMM will enable further research on astrobiology, biological in situ resource utilization, large-scale soil remediation, and terraforming. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=121 SRC="FIGDIR/small/719001v1_ufig1.gif" ALT="Figure 1"> View larger version (28K): org.highwire.dtl.DTLVardef@1314b20org.highwire.dtl.DTLVardef@13b57d4org.highwire.dtl.DTLVardef@103315eorg.highwire.dtl.DTLVardef@9e18fe_HPS_FORMAT_FIGEXP M_FIG C_FIG

While wetlands represent a small fraction ([~]5-10%) of the worlds land surface, it is estimated that one-third of wetlands have been lost due to human activities. Wetland habitat loss decreases ecosystem benefits, including improved water quality and climate change mitigation. These microbially mediated functions are dependent on redox conditions, which are altered by soil hydrology and the presence of plants. We tested the overarching hypothesis that while microbial community composition would be resistant to change due to long-term hydrologic history, key functions like greenhouse gas production would remain plastic and responsive to short-term environmental shifts. Using a mesocosm design, we manipulated the duration of hydrologic conditions (i.e., stable dry, stable flooding, and alternating wet/dry) and the presence of plants to induce soil redox changes in wetland soils. We measured soil redox status, used targeted amplicon and shotgun metagenomic sequencing to characterize microbial communities, and measured greenhouse gas production to assess microbial function. The eight-week hydrologic treatment shifted community composition but did not override the stronger effects of long-term hydrologic history. Methane and carbon dioxide fluxes were altered by short-term hydrologic treatment, with methane production favored in the wet treatment and carbon dioxide production favored in the dry treatment. Plant presence versus absence manipulation had little impact on soil microbiome composition or soil greenhouse gas production. The results highlight the resistance of microbial community structure shaped by historical hydrologic regimes, and emphasize that hydrologic conditions exert a stronger influence than plant presence on microbial composition and function. Predicting the outcomes of wetland disturbance and restoration requires an enhanced understanding of community stability and functional plasticity. Our results suggest that wetland hydrologic restoration can establish a stable microbial community that is resistant to environmental shifts, but microbial functions such as greenhouse gas emissions remain responsive to hydrologic disturbances, including flooding and drought.

Nitrogen is the most limiting nutrient to forest productivity worldwide. Recently, it has been established that diverse ecosystems source a substantial fraction of their water from weathered bedrock, leading to questions about whether root-driven nitrogen cycling extends into weathered bedrock as well. In this study, we specifically examined nitrogen dynamics using specialized instrumentation distributed across a 16 m weathered bedrock vadose zone (WBVZ) underlying an old growth forest in northern California where the rhizosphere--composed of plant roots and their associated microbiome--extends meters into rock. We documented total dissolved nitrogen (TDN), dissolved organic carbon (DOC), inorganic N (ammonium and nitrate) and CO2 and O2 gasses every 1.5 m to 16 m depth for two years. We found that biologically available nitrogen in the weathered bedrock rhizosphere was comparable in concentration to temperate forest soils and primarily organic. TDN concentrations in the WBVZ exhibited distinct patterns with depth and were correlated with periods of increased whole-ecosystem metabolic activity as well as stream discharge, suggesting competing rhizosphere and leaching processes in the fate of TDN in the WBVZ. Carbon isotope composition of the DOC suggests that dissolved organic matter in the WBVZ is primarily derived from fresh plant sources. We conclude that N cycling in the WBVZ is driven by an active rhizosphere meters below the base of soil and represents an important and overlooked component of deeply rooted ecosystems that must be incorporated into future models and theory of ecosystem function.

Warming of the Arctic enhances microbial activity and the decomposition of large stocks of organic matter retained in permafrost soil. Resulting changes in the availability of sulfur may lead to increased emissions of volatile sulfur compounds (VSCs), which impact atmospheric particle and cloud formation, terrestrial and aquatic acidification, and malodor. Marine microbial production of dimethyl sulfide (DMS) has been studied for decades but other VSCs have been largely ignored, while VSC emissions from terrestrial ecosystems are even less studied. Currently, we lack fundamental understanding of the metabolic processes behind VSC production in permafrost soil bacteria, essential for estimating how emissions may change due to thawing. To fill this knowledge gap, we measured VSC emissions from thawing permafrost and three bacterial strains isolated from Greenlandic permafrost and biological soil crust. We show that the bacterial strains produced high levels of VSCs in vitro - including hydrogen sulfide, methanethiol, DMS, dimethyl disulfide, and dimethyl trisulfide. We further show that the same VSCs were also emitted from permafrost upon thaw. Metabolic pathway mapping of the bacterial strains revealed both inorganic sulfate reduction pathways and amino acid metabolism behind bacterial VSC production. High production of VSCs in the late-active and stationary phase suggests connection to secondary metabolism, except for DMS which was linked to early growth, and possibly, primary energy metabolism. Our findings suggest that thawing increases VSC emissions from permafrost soil, possibly leading to higher input of sulfur into the atmosphere from the warming Arctic in the future.

The contribution of tree foliage to atmospheric methane (CH4) and nitrous oxide (N2O) fluxes remains a major uncertainty in global GHG budgets. We made repeated in situ measurements of foliar CH4 and N2O fluxes across 25 temperate tree species interplanted at a forest restoration site using high-resolution laser spectroscopy. Tree foliage was consistently a net CH4 sink and a net N2O source in all species. Foliar CH4 oxidation increased by [~]33% in fall relative to spring and was [~]3-fold higher in shade-tolerant than shade-intolerant angiosperm species. Species differences accounted for most of the variability in fluxes, while correlations with soil emissions were comparatively weak. Microbial DNA sequencing revealed that the highest CH4-oxidizing angiosperm species (Tilia americana) harbored abundant Type I methanotrophs, whereas the lowest-oxidizing species (Prunus virginiana) had nearly 100-fold lower methanotroph abundance, with a foliar microbial community dominated by facultative methylotrophs. Global warming potential (GWP) scaling indicates that foliar CH4 uptake overwhelmingly dominates the net climate forcing effect. Our results suggest that the large and predictable differences in foliar CH4 uptake among tree species and associated differences in foliar microbial communities are of importance in understanding and potentially enhancing the global terrestrial CH4 sink.

Snow algae darken snowpacks and accelerate melt worldwide. Although elevation strongly structures the physical conditions of mountain snowfields, its influence on snow algal traits and their effects on snowpack reflectance remains unclear. Here, we investigated snow algal composition, cellular traits, and optical properties in summer blooms across an elevational range of 1,059-3,423 m a.s.l. in the western United States, spanning two elevational gradients in the Cascade Range (CA, OR, WA) and the Rocky Mountains (UT, WY, MT). Across all samples (n = 294), snow albedo declined strongly with increasing algal cell density, indicating that total biomass, rather than pigment composition, is the dominant driver of albedo reduction. However, within Sanguina-dominated blooms (117 of 206 samples bloom samples identified across the dataset), neither relative abundance nor algal cell density varied systematically with elevation. Instead, mean cell size increased with elevation, while per-cell pigment concentrations declined, leading to higher astaxanthin:chlorophyll-a ratios driven primarily by reductions in chlorophyll-a per cell. These elevation-dependent shifts in cell size and pigment balance were consistent across both mountain ranges, indicating phenotypic acclimation to increasing environmental stress with elevation. Together, these findings link cellular-scale acclimation of a widespread snow alga to radiative processes shaping mountain snowpacks.

Northern peatlands store large carbon stocks but are sensitive to disturbance. Hydrology, vegetation, herbivory and snow conditions may affect the soil microorganisms driving methane (CH) and nitrous oxide (N2O) cycling. We investigated how reindeer exclusion and snow depth (increased and reduced relative to ambient) manipulations (ongoing for three seasons) influenced archaeal and bacterial communities in a boreal rich fen. Metagenomic (MG) and metatranscriptomic (MT) sequencing were combined with pore-water chemistry and CH flux measurements to link the microbiome to ecosystem processes. Microbial communities differed between outside and inside the exclosure. However, these patterns primarily reflected underlying hydrological variation. Slightly wetter inside plots showed higher expression of denitrification genes (norB, nosZ) and lower (nirS+nirK)/nosZ ratios, indicating greater potential for complete denitrification to N2 instead of N2O. Methane dynamics were mainly associated with vegetation: plots associated with Carex rostrata exhibited lower pmoA/mcrA ratios and elevated CH fluxes. Snow manipulations had subtle effects: reduced snow depth decreased the expression of taxa dependent on microbial interactions, while the effect to the investigated metabolic marker genes was small. Overall hydrology, leading to variations in redox conditions and nutrient availability, together with vegetation appeared as the primary drivers on microbial greenhouse gas processes in this peatland.

Arctic winters are long and cold and have traditionally been considered a period of limited biological activity. However, the seasonal dynamics of microbial community composition and functional potential during winter remain poorly understood. Here, we investigated taxonomic (bacteria, fungi, archaea) and functional (fungal guilds and nitrogen cycling genes) dynamics throughout a full year at two Arctic tundra heath sites with contrasting snow regimes. A steep drop in microbial abundances in early to mid-winter, likely linked to freeze-thaw events, coincided with shifts in soil pH and elevated community turnover. Saprotrophic and root-associated fungi were more abundant in the cold-season, while inorganic nitrogen cycling groups were more abundant in summer and declined toward winter despite high bacterial abundance. This indicates sustained organic matter cycling during the winter and expanded inorganic nitrogen cycling in the summer. Functional gene ratios further suggested a higher early-winter nitrogen loss potential via nitrous oxide and greater late-winter nitrogen retention. Site-specific differences in snow regime altered the timing and magnitude of these dynamics. Together, our results demonstrate that winter represents a critical and dynamic period for microbial community restructuring with important implications for nitrogen turnover in Arctic tundra soils.

Quantifying the lipid biosynthesis rate of archaea in hot spring sediments is necessary to interpret the abundance, isotopic patterns, and environmental significance of archaeal lipid biosignatures, with implications for modern biogeochemical cycling and astrobiology. Here, we performed lipid hydrogen stable isotope probing (LH-SIP) experiments on whole sediments collected from two high-temperature, suboxic, circumneutral hot springs in Yellowstone National Park (USA) and El Tatio Geyserfield (Chile). We determined the incorporation of 2H2O into intact polar lipids (IPLs) which provides a taxon- and metabolism-agnostic quantification of biosynthesis under near-natural conditions. We targeted isoprenoid glycerol dialkyl glycerol tetraether lipids (IPL iGDGTs) and recovered structures with 0 to 7 cyclopentyl rings from both springs. We observed minor 2H-uptake into archaeal IPLs in spring sediments in Yellowstone, corresponding to decadal-scale apparent generation times (16 {+/-} 7 years), and no uptake in El Tatio sediments (consistent with minimum generation times of 35 {+/-} 5 years). We infer that net production of sedimentary IPL-iGDGTs is very slow, consistent with a combination of slow archaeal growth, persistence of older IPLs, lipid recycling, and/or contributions from recently sedimented planktonic biomass. These are the first direct, ex situ estimates of archaeal lipid production rates in terrestrial hydrothermal systems using LH-SIP incubations and provide critical constraints for interpreting archaeal lipids in ancient hot spring deposits. This research establishes a framework for assessing activity by slow-growing extremophilic archaea in hydrothermal environments and provides support for targeting hydrothermal deposits on Mars for biosignature detection efforts. Plain Language SummaryHot springs on Earth are important natural laboratories for understanding how signs of life might form and be preserved in hydrothermal environments on early Earth or Mars. In this study, we examine the rate of archaeal lipid biosignature production in sediments from two hot springs in Yellowstone National Park and the El Tatio Geyserfield in Chile. We used a method that measures new microbial production by tracing heavy hydrogen from labeled water as microbes incorporate that hydrogen into newly made lipids in their cell membranes. We found that archaeal lipids in hot spring sediments are produced very slowly, on timescales of decades. This result, along with the chemical stability of lipids and the rapid mineralization rate in hot springs, may allow these molecular biosignatures to be entombed and preserved in hot spring mineral deposits. These results help us better interpret ancient hydrothermal deposits on Earth and support the idea that slowly growing microbial communities could still leave detectable molecular traces in similar environments on Mars and other rocky planets. Key PointsO_LILipid hydrogen stable isotope probing is applied to high temperature hot spring sediments for the first time C_LIO_LIIn hot spring sediments, archaeal lipid production occurs on decadal timescales comparable to some marine sediments, but are much faster than the century- to millennia-scale rates observed in the deep subsurface C_LIO_LIConfirmation of archaeal lipid synthesis in hot spring sediments adds additional support for targeting Martian hydrothermal deposits for biosignature detection efforts C_LI

Serpentinization produces hyperalkaline, H2- and CH4-rich fluids that support microbial life in extreme conditions and serve as analogs for ocean worlds such as Enceladus. While methane production in these systems has been well studied, methane consumption--especially under high pH--remains poorly understood. Here, we present isotopic, geochemical, and genomic evidence for hyperalkaliphilic (pH > 11) methanotrophy in the Samail Ophiolite in Oman. Using models that account for fluid mixing and gas exsolution, we identify {delta}13CH4 enrichment that cannot be explained by abiotic processes alone. The enrichment of 13CH4 co-occurs with methanotroph 16S rRNA gene sequences, particularly in fluids formed by mixing CH4-rich, anoxic fluids with oxidant-rich surface waters. Shotgun metagenomics reveals a metagenome-assembled genome affiliated with Methylovulum, encoding a complete methane oxidation pathway, multiple carbon assimilation routes, and Na+/H+ antiporters--adaptations likely enabling growth above pH 11. Methanotroph diversity and abundance peak in mixed fluids but are suppressed at total ammonia nitrogen concentrations >20 M. Anaerobic methane-oxidizing archaea (ANME) may also contribute to CH4 oxidation in the deep subsurface. Our findings highlight the viability of methanotrophy under extreme alkaline conditions and provide a framework for interpreting {delta}13CH4 signals in serpentinizing environments on Earth and beyond.

The Arctic is warming rapidly, causing permafrost thaw and accelerating the release of greenhouse gases. Rapid thaw features such as retrogressive thaw slumps are increasing in frequency and severity across the Arctic; however, their associated greenhouse gas emissions are poorly constrained. Current estimates of emissions from retrogressive thaw slumps rely largely on laboratory incubations and carbon stock estimates rather than in-situ field measurements. Here we directly quantify methane and carbon dioxide fluxes from the exposed headwall of an active retrogressive thaw slump. We show that thaw immediately releases biogenic methane and carbon dioxide, originating from gases trapped within the frozen soil matrix. Microbial transcription of methyl-coenzyme M reductase suggests archaea carrying out methanogenesis at subzero temperatures are the source of trapped methane. Carbon emissions varied by an order of magnitude among cryostratigraphic units, reflecting differences in geomorphologic history, organic carbon and nitrogen content, and microbial community composition. Carbon emissions were highest from organic-rich paleo cryosols from the Late Holocene that contained abundant methanogenic archaea. We estimate that [~]300 kg C (CO2 equivalents) is emitted annually from the headwall of this small thaw slump (surface area of [~]1200 m2). Considering the thousands of active slumps and extensive coastal permafrost erosion across the northern continuous permafrost zone, such features may represent a growing natural source of GHG emissions. These findings indicate that current permafrost carbon feedback models underestimate GHG release by omitting the direct release of trapped gases stored in permafrost.

In this study, we examined the reflectance, pigment composition, and community composition of three snow algae blooms showing distinct colors in the same snowfield in Glacier National Park (USA). Each color bloom was dominated by a different algae, each exhibiting a unique pigment signature but with astaxanthin as the predominant pigment across all three blooms. The spectral reflectance of red snow algae was consistently lower than that of green algae, while orange algae had intermediate reflectance values. Specifically, red algae reduced reflectance by approximately 55% across the PAR range, while green algae reduced reflectance by 25%. Red algae also demonstrated the highest radiative forcing, double that of green algae, leading to increased energy re-emission into the surrounding environment, which likely contributes to the localized melting of adjacent ice crystals. The high absorbance around 680 nm in cells with high astaxanthin content, such as the orange algae, suggests that semi-automatic detection methods could effectively identify these algae, as their spectral features remain distinct despite the presence of secondary carotenoids. Our data demonstrate the impact of snow algae taxonomic and pigment composition on the radiative balance of snowfields, underscoring taxonomy as a key determinant of bloom color under similar environmental conditions

Evidence from the Cassini mission confirmed that Saturn's moon Enceladus hosts a subsurface alkaline ocean where rock-water reactions may generate redox disequilibria capable of supporting microbial metabolisms. To investigate potential microbial survival under simulated Enceladus ocean conditions, we used thermodynamic modeling to develop a salt formulation consistent with one possible Enceladus ocean composition and supplemented it with putative microbial energy sources to create a growth medium. The medium was inoculated with samples from diverse ocean world analog environments on Earth to determine which microorganisms could persist under Enceladus-like conditions. The microorganisms persisting in this geochemically bounded medium were heterotrophic, metabolically versatile bacteria with low carbon requirements. Genomic and physiological analyses further showed the presence of multiple stress-response pathways, sodium- based bioenergetic systems, osmoregulation strategies, and other adaptations consistent with survival in alkaline, low-nutrient settings. These results suggest that some stress-tolerant heterotrophic bacteria may serve as useful model organisms for life in Enceladus' subsurface ocean. These findings demonstrate the value of geochemically modeled media as a framework for constraining habitability, identifying relevant biosignatures, and probing potential microbial survival strategies beyond Earth.

Accurate inventories of terrestrial carbon pools and fluxes are crucial for understanding ecosystem processes, tracking climate change impacts, and meeting the monitoring, reporting, and verification (MRV) requirements in international treaties and voluntary carbon markets. In meeting this need, the fusion of process-based modeling, field data, and remote sensing observations has the potential to provide more accurate and precise estimates than each alone. However, as the number of data constraints on a system increases, different sources of information can interact with each other in complex ways across space, time, and processes. In this study, we undertake a value-of-information analysis to assess the contribution of different observations to reducing carbon cycle uncertainties across pools, fluxes, and spatial domains within the PEcAn carbon cycle data assimilation system. We used a novel block-based Tobit Gamma Ensemble Filter to assimilate four synergistic data constraints, MODIS leaf area index, Landtrendr aboveground biomass, SMAP soil moisture, and SoilGrids soil organic C, into a process-based ecosystem model (SIPNET) at 39 National Ecological Observatory Network sites across the contiguous U.S. from 2012 to 2021. Results showed that not only did we greatly reduce uncertainty among the directly constrained pools but many observations were able to share information across variables and space. These indirect constraints helped identify synergies and conflicts among data streams and across space, which provides insights for further constraining carbon inventories. Overall, soil carbon remains the largest source of uncertainty in the overall carbon budget due to both its large size and limited observational constraints.

Snow is a critical component of the Earth system. High elevation snow can persist into the melt season and hosts a diverse array of life including snow algae. Due in part to the presence of pigments, snow algae lower albedo and accelerate snow melt which has led to increasing interest in identifying and quantifying the environmental factors that constrain their distribution. Dissolved inorganic carbon (DIC) concentration is low in supraglacial snow on Cascade stratovolcanoes and snow algae primary productivity can be stimulated through DIC addition. Here we asked if CO2 would still be a limiting nutrient for snow hosted on glacially eroded carbonate bedrock (which could provide an additional source of DIC). We assayed snow algae communities for nutrient and DIC limitation on two seasonal snowfields on glacially eroded carbonate bedrock in the Snowy Range of the Medicine Bow Mountains, Wyoming, USA. DIC stimulated snow algae primary productivity in snow with lower DIC concentration despite the presence of carbonate bedrock, which alleviated DIC limitation in the other site. Our results support the hypothesis that increased atmospheric CO2 concentrations may lead to larger and more robust snow algae blooms globally, even for sites with carbonate bedrock.

The eddy covariance technique has revolutionized our understanding of ecosystem-atmosphere interactions. Eddy covariance studies often use a "paired" tower design in which observations from nearby towers are used to understand how different vegetation, soils, hydrology, or experimental treatment shape ecosystem function and surface-atmosphere exchange. Paired towers have never been formally defined and their global distribution has not been quantified. We compiled eddy covariance tower information to find towers that could be considered paired. Of 1233 global eddy covariance towers, 692 (56%) were identified as paired by our criteria. Paired towers had cooler mean annual temperature (mean = 9.9 {degrees}C) than the entire eddy covariance network (10.5 {degrees}C) but warmer than the terrestrial surface (8.9 {degrees}C) from WorldClim 2.1, on average. The paired and entire tower networks had greater average soil nitrogen (0.57-0.58 g/kg) and more silt (36.0-36.4%) than terrestrial ecosystems (0.38 g/kg and 30.5%), suggesting that eddy covariance towers sample richer soils than the terrestrial surface as a whole. Paired towers existed in a climatic space that was more different from the global climate distribution sampled by the entire eddy covariance network, as revealed by an analysis of the Kullback-Leibler divergence, but the edaphic space sampled by the entire network and paired towers was similar. The lack of paired towers with available data across much of Africa, northern, central, southern, and western Asia, and Latin America with few towers in savannas, shrublands, and evergreen broadleaf forests point to key regions, ecosystems, and ecosystem transitions in need of additional research. Few if any paired towers study the flux of ozone and other atmospherically active trace gases at the present. By studying what paired towers measure - and what they do not - we can make infrastructural investments to further enhance the value of FLUXNET as it moves toward its fourth decade.