Field-Based Planetary Protection Operations for Melt Probes: Validation of Clean Access into the Blood Falls, Antarctica, Englacial Ecosystem

Subglacial environments on Earth offer important analogs to Ocean World targets in our solar system. These unique microbial ecosystems remain understudied due to the challenges of access through thick glacial ice (tens to hundreds of meters). Additionally, sub-ice collections must be conducted in a clean manner to ensure sample integrity for downstream microbiological and geochemical analyses. We describe the field-based cleaning of a melt probe that was used to collect brine samples from within a glacier conduit at Blood Falls, Antarctica, for geomicrobiological studies. We used a thermoelectric melting probe called the IceMole that was designed to be minimally invasive in that the logistical requirements in support of drilling operations were small and the probe could be cleaned, even in a remote field setting, so as to minimize potential contamination. In our study, the exterior bioburden on the IceMole was reduced to levels measured in most clean rooms, and below that of the ice surrounding our sampling target. Potential microbial contaminants were identified during the cleaning process; however, very few were detected in the final englacial sample collected with the IceMole and were present in extremely low abundances (∼0.063% of 16S rRNA gene amplicon sequences). This cleaning protocol can help minimize contamination when working in remote field locations, support microbiological sampling of terrestrial subglacial environments using melting probes, and help inform planetary protection challenges for Ocean World analog mission concepts.

Microbial Transport by a Descending Ice Melting Probe: Implications for Subglacial and Ocean World Exploration

Ocean Worlds beneath thick ice covers in our solar system, as well as subglacial lakes on Earth, may harbor biological systems. In both cases, thick ice covers (>100 s of meters) present significant barriers to access. Melt probes are emerging as tools for reaching and sampling these realms due to their small logistical footprint, ability to transport payloads, and ease of cleaning in the field. On Earth, glaciers are immured with various abundances of microorganisms and debris. The potential for bioloads to accumulate around and be dragged by a probe during descent has not previously been investigated. Due to the pristine nature of these environments, minimizing and understanding the risk of forward contamination and considering the potential of melt probes to act as instrument-induced special regions are essential. In this study, we examined the effect that two engineering descent strategies for melt probes have on the dragging of bioloads. We also tested the ability of a field cleaning protocol to rid a common contaminant, Bacillus. These tests were conducted in a synthetic ice block immured with bioloads using the Ice Diver melt probe. Our data suggest minimal dragging of bioloads by melt probes, but conclude that modifications for further minimization and use in special regions should be made.

Sulfur-cycling chemolithoautotrophic microbial community dominates a cold, anoxic, hypersaline Arctic spring

BACKGROUND

Gypsum Hill Spring, located in Nunavut in the Canadian High Arctic, is a rare example of a cold saline spring arising through thick permafrost. It perennially discharges cold (~ 7 °C), hypersaline (7-8% salinity), anoxic (~ 0.04 ppm O2), and highly reducing (~ - 430 mV) brines rich in sulfate (2.2 g.L-1) and sulfide (9.5 ppm), making Gypsum Hill an analog to putative sulfate-rich briny habitats on extraterrestrial bodies such as Mars.

RESULTS

Genome-resolved metagenomics and metatranscriptomics were utilized to describe an active microbial community containing novel metagenome-assembled genomes and dominated by sulfur-cycling Desulfobacterota and Gammaproteobacteria. Sulfate reduction was dominated by hydrogen-oxidizing chemolithoautotrophic Desulfovibrionaceae sp. and was identified in phyla not typically associated with sulfate reduction in novel lineages of Spirochaetota and Bacteroidota. Highly abundant and active sulfur-reducing Desulfuromusa sp. highly transcribed non-coding RNAs associated with transcriptional regulation, showing potential evidence of putative metabolic flexibility in response to substrate availability. Despite low oxygen availability, sulfide oxidation was primarily attributed to aerobic chemolithoautotrophic Halothiobacillaceae. Low abundance and transcription of photoautotrophs indicated sulfur-based chemolithoautotrophy drives primary productivity even during periods of constant illumination.

CONCLUSIONS

We identified a rare surficial chemolithoautotrophic, sulfur-cycling microbial community active in a unique anoxic, cold, hypersaline Arctic spring. We detected Mars-relevant metabolisms including hydrogenotrophic sulfate reduction, sulfur reduction, and sulfide oxidation, which indicate the potential for microbial life in analogous S-rich brines on past and present Mars. Video Abstract.

Glacial Water: A Dynamic Microbial Medium

Microbial communities and nutrient dynamics in glaciers and ice sheets continuously change as the hydrological conditions within and on the ice change. Glaciers and ice sheets can be considered bioreactors as microbiomes transform nutrients that enter these icy systems and alter the meltwater chemistry. Global warming is increasing meltwater discharge, affecting nutrient and cell export, and altering proglacial systems. In this review, we integrate the current understanding of glacial hydrology, microbial activity, and nutrient and carbon dynamics to highlight their interdependence and variability on daily and seasonal time scales, as well as their impact on proglacial environments.

A possible unique ecosystem in the endoglacial hypersaline brines in Antarctica

Here, we present the results related to a new unique terrestrial ecosystem found in an englacial hypersaline brine found in Northern Victoria Land (Antarctica). Both the geochemistry and microbial (prokaryotic and fungal) diversity revealed an unicity with respect to all the other known Antarctic brines and suggested a probable ancient origin mainly due a progressive cryoconcentration of seawater. The prokaryotic community presented some peculiarities, such as the occurrence of sequences of Patescibacteria (which can thrive in nutrient-limited water environments) or few Spirochaeta, and the presence of archaeal sequences of Methanomicrobia closely related to Methanoculleus, a methanogen commonly detected in marine and estuarine environments. The high percentage (35%) of unassigned fungal taxa suggested the presence of a high degree of undiscovered diversity within a structured fungal community (including both yeast and filamentous life forms) and reinforce the hypothesis of a high degree of biological uniqueness of the habitat under study.

A microbial driver of clay mineral weathering and bioavailable Fe source under low-temperature conditions

Biotic and abiotic Fe(III) reduction of clay minerals (illite IMt-1) under low-temperature (0 and 4°C, pH 6) was studied to evaluate the effects of bioalteration on soil properties including clay structure and elemental composition. The extent of Fe reduction in bioreduced samples (∼3.8 % at 4°C and ∼3.1 % at 0°C) was lower than abiotic reduction (∼7.6 %) using dithionite as a strong reductant. However, variations in the illite crystallinity value of bioreduced samples (°Δ2θ = 0.580–0.625) were greater than those of abiotic reduced samples (°Δ2θ = 0.580–0.601), indicating that modification of crystal structure is unlikely to have occurred in abiotic reduction. Moreover, precipitation of secondary-phase minerals such as vivianite [Fe²⁺₃(PO₄)₂^⋅8H₂O] and nano-sized biogenic silica were shown as evidence of reductive dissolution of Fe-bearing minerals that is observed only in a bioreduced setting. Our observation of a previously undescribed microbe–mineral interaction at low-temperature suggests a significant implication for the microbially mediated mineral alteration in Arctic permafrost, deep sea sediments, and glaciated systems resulting in the release of bioavailable Fe with an impact on low-temperature biogeochemical cycles.

Active lithoautotrophic and methane-oxidizing microbial community in an anoxic, sub-zero, and hypersaline High Arctic spring

Lost Hammer Spring, located in the High Arctic of Nunavut, Canada, is one of the coldest and saltiest terrestrial springs discovered to date. It perennially discharges anoxic (<1 ppm dissolved oxygen), sub-zero (~-5 °C), and hypersaline (~24% salinity) brines from the subsurface through up to 600 m of permafrost. The sediment is sulfate-rich (1 M) and continually emits gases composed primarily of methane (~50%), making Lost Hammer the coldest known terrestrial methane seep and an analog to extraterrestrial habits on Mars, Europa, and Enceladus. A multi-omics approach utilizing metagenome, metatranscriptome, and single-amplified genome sequencing revealed a rare surface terrestrial habitat supporting a predominantly lithoautotrophic active microbial community driven in part by sulfide-oxidizing Gammaproteobacteria scavenging trace oxygen. Genomes from active anaerobic methane-oxidizing archaea (ANME-1) showed evidence of putative metabolic flexibility and hypersaline and cold adaptations. Evidence of anaerobic heterotrophic and fermentative lifestyles were found in candidate phyla DPANN archaea and CG03 bacteria genomes. Our results demonstrate Mars-relevant metabolisms including sulfide oxidation, sulfate reduction, anaerobic oxidation of methane, and oxidation of trace gases (H2, CO2) detected under anoxic, hypersaline, and sub-zero ambient conditions, providing evidence that similar extant microbial life could potentially survive in similar habitats on Mars.

Microbial Communities in Volcanic Glacier Ecosystems

Glaciers constitute a polyextremophilic environment characterized by low temperatures, high solar radiation, a lack of nutrients, and low water availability. However, glaciers located in volcanic regions have special characteristics, since the volcanic foci provide them with heat and nutrients that allow the growth of microbial communities highly adapted to this environment. Most of the studies on these glacial ecosystems have been carried out in volcanic environments in the northern hemisphere, including Iceland and the Pacific Northwest. To better know, the microbial diversity of the underexplored glacial ecosystems and to check what their specific characteristics were, we studied the structure of bacterial communities living in volcanic glaciers in Deception Island, Antarctica, and in the Kamchatka peninsula. In addition to geographic coordinates, many other glacier environmental factors (like volcanic activity, altitude, temperature, pH, or ice chemical composition) that can influence the diversity and distribution of microbial communities were considered in this study. Finally, using their taxonomic assignments, an attempt was made to compare how different or similar are the biogeochemical cycles in which these microbiomes are involved.

Salt to conserve: a review on the ecology and preservation of hypersaline ecosystems

When it comes to the investigation of key ecosystems in the world, we often omit salt from the ecological recipe. In fact, despite occupying almost half of the volume of inland waters and providing crucial services to humanity and nature, inland saline ecosystems are often overlooked in discussions regarding the preservation of global aquatic resources of our planet. As a result, our knowledge of the biological and geochemical dynamics shaping these environments remains incomplete and we are hesitant in framing effective protective strategies against the increasing natural and anthropogenic threats faced by such habitats. Hypersaline lakes, water bodies where the concentration of salt exceeds 35 g/l, occur mainly in arid and semiarid areas resulting from hydrological imbalances triggering the accumulation of salts over time. Often considered the 'exotic siblings' within the family of inland waters, these ecosystems host some of the most extremophile communities worldwide and provide essential habitats for waterbirds and many other organisms in already water-stressed regions. These systems are often highlighted as natural laboratories, ideal for addressing central ecological questions due to their relatively low complexity and simple food web structures. However, recent studies on the biogeochemical mechanisms framing hypersaline communities have challenged this archetype, arguing that newly discovered highly diverse communities are characterised by specific trophic interactions shaped by high levels of specialisation. The main goal of this review is to explore our current understanding of the ecological dynamics of hypersaline ecosystems by addressing four main research questions: (i) why are hypersaline lakes unique from a biological and geochemical perspective; (ii) which biota inhabit these ecosystems and how have they adapted to the high salt conditions; (iii) how do we protect biodiversity from increasing natural and anthropogenic threats; and (iv) which scientific tools will help us preserve hypersaline ecosystems in the future? First, we focus on the ecological characterisation of hypersaline ecosystems, illustrate hydrogeochemical dynamics regulating such environments, and outline key ecoregions supporting hypersaline systems across the globe. Second, we depict the diversity and functional aspects of key taxa found in hypersaline lakes, from microorganisms to plants, invertebrates, waterbirds and upper trophic levels. Next, we describe ecosystem services and discuss possible conservation guidelines. Finally, we outline how cutting-edge technologies can provide new insights into the study of hypersaline ecology. Overall, this review sheds further light onto these understudied ecosystems, largely unrecognised as important sources of unique biological and functional diversity. We provide perspectives for key future research avenues, and advocate that the conservation of hypersaline lakes should not be taken with 'a grain of salt'.

Fundamental limits on the rate of bacterial growth and their influence on proteomic composition

Despite abundant measurements of bacterial growth rate, cell size, and protein content, we lack a rigorous understanding of what sets the scale of these quantities and when protein abundances should (or should not) depend on growth rate. Here, we estimate the basic requirements and physical constraints on steady-state growth by considering key processes in cellular physiology across a collection of Escherichia coli proteomic data covering ≈4,000 proteins and 36 growth rates. Our analysis suggests that cells are predominantly tuned for the task of cell doubling across a continuum of growth rates; specific processes do not limit growth rate or dictate cell size. We present a model of proteomic regulation as a function of nutrient supply that reconciles observed interdependences between protein synthesis, cell size, and growth rate and propose that a theoretical inability to parallelize ribosomal synthesis places a firm limit on the achievable growth rate. A record of this paper's transparent peer review process is included in the supplemental information.

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.

First Insights into the Microbiology of Three Antarctic Briny Systems of the Northern Victoria Land

Different polar environments (lakes and glaciers), also in Antarctica, encapsulate brine pools characterized by a unique combination of extreme conditions, mainly in terms of high salinity and low temperature. Since 2014, we have been focusing our attention on the microbiology of brine pockets from three lakes in the Northern Victoria Land (NVL), lying in the Tarn Flat (TF) and Boulder Clay (BC) areas. The microbial communities have been analyzed for community structure by next generation sequencing, extracellular enzyme activities, metabolic potentials, and microbial abundances. In this study, we aim at reconsidering all available data to analyze the influence exerted by environmental parameters on the community composition and activities. Additionally, the prediction of metabolic functions was attempted by the phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt2) tool, highlighting that prokaryotic communities were presumably involved in methane metabolism, aromatic compound biodegradation, and organic compound (proteins, polysaccharides, and phosphates) decomposition. The analyzed cryoenvironments were different in terms of prokaryotic diversity, abundance, and retrieved metabolic pathways. By the analysis of DNA sequences, common operational taxonomic units ranged from 2.2% to 22.0%. The bacterial community was dominated by Bacteroidetes. In both BC and TF brines, sequences of the most thermally tolerant and methanogenic Archaea were detected, some of them related to hyperthermophiles.

Draft Genome Sequence of Shewanella sp. Strain BF02_Schw, Isolated from Blood Falls, a Feature Where Subglacial Brine Discharges to the Surface of Taylor Glacier, Antarctica

We report the sequencing, assembly, and draft genome of Shewanella sp. strain BF02_Schw. The assembly contains 5,304,243 bp, with a GC content of 41.43%.

Assessment of Sulfate Sources under Cold Conditions as a Geochemical Proxy for the Origin of Sulfates in the Circumpolar Dunes on Mars

Determining aqueous sulfate sources in terrestrial cold environments can provide an insight into the surface hydrological conditions and sulfur cycle on Mars. In this study, we analyzed sulfur and oxygen isotope compositions of secondary sulfate salts (e.g., gypsum, thenardite) in the surficial sediments and soils of the McMurdo Dry Valleys (MDV), Antarctica to determine contributions of sulfate from bedrock chemical weathering and atmospheric deposition under persistent dry polar conditions. The sulfate showed wider variation of δ34S (+15.8‰ to +32.5‰) compared to smaller ranges of δ18O (−8.9‰ to −4.1‰). In contrast, the δ34S of bedrock sulfide showed significantly lower and consistent values across the studied area (−0.6‰ to +3.3‰). Based on the δ34S trends, sulfide weathering may contribute up to 20–50% of secondary sulfate salts in the MDV. While the remaining 50–80% of sulfate inputs may originate from atmospheric deposition (e.g., sea aerosols, dimethulsulfide oxidation), the subglacial brines derived by relicts of seawater and/or lake/pond water influenced by microbial sulfate reduction could also be important sulfate endmembers particularly in the Antarctic lowland thaw zones. Additional field observations of frost, ponding water, and thin gypsum crusts on the terrestrial gypsum dunes at White Sands supports reactivity of gypsum on the surface of these dunes during cold winter conditions. Combined with our improved geochemical model of the sulfur cycle for cold Antarctic settings, we propose that transient liquid water or frost was available in near-surface environments at the time of gypsum formation in the north polar region on Mars. Ice and/or water interaction with basaltic sand of the basal unit (paleo-erg) would have enhanced leaching of sulfate from both sulfide oxidation and atmospheric deposition and resulted in formation of secondary gypsum salts.

Prokaryotic Diversity and Metabolically Active Communities in Brines from Two Perennially Ice-Covered Antarctic Lakes

The genomic diversity of bacteria and archaea in brines (BC1, BC2, and BC3) from two adjacent and perennially frozen Antarctic lakes (L16 and L-2) in the Boulder Clay (BC) area was investigated together with the metabolically active fraction of both communities, by analyzing the bulk rRNA as a general marker of metabolic activity. Although similar bacterial and archaeal assemblages were observed at phylum level, differences were encountered when considering the distribution in species. Overall, the total bacterial communities were dominated by Bacteroidetes. A massive occurrence of flavobacterial sequences was observed within the metabolically active bacterial communities of the BC1 brine, whereas the active fractions in BC2 and BC3 strongly differed from the bulk communities being dominated by Betaproteobacteria (mainly Hydrogenophaga members). The BC lakes also hosted sequences of the most thermally tolerant archaea, also related to well-known hyperthermophiles. Interestingly, RNA sequences of the hyperthermophilic genus Ferroglobus were retrieved in all brine samples. Finally, a high abundance of the strictly anaerobic methanogens (such as Methanosarcina members) within the active community suggests that anoxic conditions might occur in the lake brines. Our findings indicate perennially ice-covered Antarctic lakes as plausible terrestrial candidates for the study of the potential for extant life on different bodies of our solar system.

The Influence of Glacial Cover on Riverine Silicon and Iron Exports in Chilean Patagonia

Glaciated environments have been highlighted as important sources of bioavailable nutrients, with inputs of glacial meltwater potentially influencing productivity in downstream ecosystems. However, it is currently unclear how riverine nutrient concentrations vary across a spectrum of glacial cover, making it challenging to accurately predict how terrestrial fluxes will change with continued glacial retreat. Using 40 rivers in Chilean Patagonia as a unique natural laboratory, we investigate how glacial cover affects riverine Si and Fe concentrations, and infer how exports of these bioessential nutrients may change in the future. Dissolved Si (as silicic acid) and soluble Fe (<0.02 μm) concentrations were relatively low in glacier-fed rivers, whereas concentrations of colloidal-nanoparticulate (0.02-0.45 μm) Si and Fe increased significantly as a function of glacial cover. These colloidal-nanoparticulate phases were predominately composed of aluminosilicates and Fe-oxyhydroxides, highlighting the need for size-fractionated analyses and further research to quantify the lability of colloidal-nanoparticulate species. We also demonstrate the importance of reactive particulate (>0.45 μm) phases of both Si and Fe, which are not typically accounted for in terrestrial nutrient budgets but can dominate riverine exports. Dissolved Si and soluble Fe yield estimates showed no trend with glacial cover, suggesting no significant change in total exports with continued glacial retreat. However, yields of colloidal-nanoparticulate and reactive sediment-bound Si and Fe were an order of magnitude greater in highly glaciated catchments and showed significant positive correlations with glacial cover. As such, regional-scale exports of these phases are likely to decrease as glacial cover disappears across Chilean Patagonia, with potential implications for downstream ecosystems.

Microbial Metabolic Redundancy Is a Key Mechanism in a Sulfur-Rich Glacial Ecosystem

A unique environment at Borup Fiord Pass is characterized by a sulfur-enriched glacial ecosystem in the low-temperature Canadian High Arctic. BFP represents one of the best terrestrial analog sites for studying icy, sulfur-rich worlds outside our own, such as Europa and Mars. The site also allows investigation of sulfur-based microbial metabolisms in cold environments here on Earth. Here, we report whole-genome sequencing data that suggest that sulfur cycling metabolisms at BFP are more widely used across bacterial taxa than predicted. From our analyses, the metabolic capability of sulfur oxidation among multiple community members appears likely due to functional redundancy present in their genomes. Functional redundancy, with respect to sulfur-oxidation at the BFP sulfur-ice environment, may indicate that this dynamic ecosystem hosts microorganisms that are able to use multiple sulfur electron donors alongside other metabolic pathways, including those for carbon and nitrogen.

Biological sulfur cycling in polar, low-temperature ecosystems is an understudied phenomenon in part due to difficulty of access and the dynamic nature of glacial environments. One such environment where sulfur cycling is known to play an important role in microbial metabolisms is located at Borup Fiord Pass (BFP) in the Canadian High Arctic. Here, transient springs emerge from ice near the terminus of a glacier, creating a large area of proglacial aufeis (spring-derived ice) that is often covered in bright yellow/white sulfur, sulfate, and carbonate mineral precipitates accompanied by a strong odor of hydrogen sulfide. Metagenomic sequencing of samples from multiple sites and of various sample types across the BFP glacial system produced 31 metagenome-assembled genomes (MAGs) that were queried for sulfur, nitrogen, and carbon cycling/metabolism genes. An abundance of sulfur cycling genes was widespread across the isolated MAGs and sample metagenomes taxonomically associated with the bacterial classes Alphaproteobacteria and Gammaproteobacteria and Campylobacteria (formerly the Epsilonproteobacteria). This corroborates previous research from BFP implicating Campylobacteria as the primary class responsible for sulfur oxidation; however, data reported here suggested putative sulfur oxidation by organisms in both the alphaproteobacterial and gammaproteobacterial classes that was not predicted by previous work. These findings indicate that in low-temperature, sulfur-based environments, functional redundancy may be a key mechanism that microorganisms use to enable coexistence whenever energy is limited and/or focused by redox chemistry.

IMPORTANCE A unique environment at Borup Fiord Pass is characterized by a sulfur-enriched glacial ecosystem in the low-temperature Canadian High Arctic. BFP represents one of the best terrestrial analog sites for studying icy, sulfur-rich worlds outside our own, such as Europa and Mars. The site also allows investigation of sulfur-based microbial metabolisms in cold environments here on Earth. Here, we report whole-genome sequencing data that suggest that sulfur cycling metabolisms at BFP are more widely used across bacterial taxa than predicted. From our analyses, the metabolic capability of sulfur oxidation among multiple community members appears likely due to functional redundancy present in their genomes. Functional redundancy, with respect to sulfur-oxidation at the BFP sulfur-ice environment, may indicate that this dynamic ecosystem hosts microorganisms that are able to use multiple sulfur electron donors alongside other metabolic pathways, including those for carbon and nitrogen.

Distinctive microbial communities in subzero hypersaline brines from Arctic coastal sea ice and rarely sampled cryopegs

Hypersaline aqueous environments at subzero temperatures are known to be inhabited by microorganisms, yet information on community structure in subzero brines is very limited. Near Utqiaġvik, Alaska, we sampled subzero brines (–6°C, 115–140 ppt) from cryopegs, i.e. unfrozen sediments within permafrost that contain relic (late Pleistocene) seawater brine, as well as nearby sea-ice brines to examine microbial community composition and diversity using 16S rRNA gene amplicon sequencing. We also quantified the communities microscopically and assessed environmental parameters as possible determinants of community structure. The cryopeg brines harbored surprisingly dense bacterial communities (up to 10⁸ cells mL^–1) and millimolar levels of dissolved and particulate organic matter, extracellular polysaccharides and ammonia. Community composition and diversity differed between the two brine environments by alpha- and beta-diversity indices, with cryopeg brine communities appearing less diverse and dominated by one strain of the genus Marinobacter, also detected in other cold, hypersaline environments, including sea ice. The higher density and trend toward lower diversity in the cryopeg communities suggest that long-term stability and other features of a subzero brine are more important selective forces than in situ temperature or salinity, even when the latter are extreme.

Microbial Fe(III) reduction as a potential iron source from Holocene sediments beneath Larsen Ice Shelf

Recent recession of the Larsen Ice Shelf C has revealed microbial alterations of illite in marine sediments, a process typically thought to occur during low-grade metamorphism. In situ breakdown of illite provides a previously-unobserved pathway for the release of dissolved Fe²⁺ to porewaters, thus enhancing clay-rich Antarctic sub-ice shelf sediments as an important source of Fe to Fe-limited surface Southern Ocean waters during ice shelf retreat after the Last Glacial Maximum. When sediments are underneath the ice shelf, Fe²⁺ from microbial reductive dissolution of illite/Fe-oxides may be exported to the water column. However, the initiation of an oxygenated, bioturbated sediment under receding ice shelves may oxidize Fe within surface porewaters, decreasing dissolved Fe²⁺ export to the ocean. Thus, we identify another ice-sheet feedback intimately tied to iron biogeochemistry during climate transitions. Further constraints on the geographical extent of this process will impact our understanding of iron-carbon feedbacks during major deglaciations.

Recent recession of the Larsen Ice Shelf C has revealed that microbial alteration of illite can occur within marine sediments, a process previously thought to only occur abiotically during low-grade metamorphism. Here, the authors show that such microbial alteration of illite could provide a potential source of Fe release to Southern Ocean waters during Holocene glacial cycles.

Subglacial meltwater supported aerobic marine habitats during Snowball Earth

The Earth's most severe ice ages interrupted a crucial interval in eukaryotic evolution with widespread ice coverage during the Cryogenian Period (720 to 635 Ma). Aerobic eukaryotes must have survived the "Snowball Earth" glaciations, requiring the persistence of oxygenated marine habitats, yet evidence for these environments is lacking. We examine iron formations within globally distributed Cryogenian glacial successions to reconstruct the redox state of the synglacial oceans. Iron isotope ratios and cerium anomalies from a range of glaciomarine environments reveal pervasive anoxia in the ice-covered oceans but increasing oxidation with proximity to the ice shelf grounding line. We propose that the outwash of subglacial meltwater supplied oxygen to the synglacial oceans, creating glaciomarine oxygen oases. The confluence of oxygen-rich meltwater and iron-rich seawater may have provided sufficient energy to sustain chemosynthetic communities. These processes could have supplied the requisite oxygen and organic carbon source for the survival of early animals and other eukaryotic heterotrophs through these extreme glaciations.

H2 Metabolism revealed by metagenomic analysis of subglacial sediment from East Antarctica

Subglacial ecosystems harbor diverse chemoautotrophic microbial communities in areas with limited organic carbon, and lithological H₂ produced during glacial erosion has been considered an important energy source in these ecosystems. To verify the H₂-utilizing potential there and to identify the related energy-converting metabolic mechanisms of these communities, we performed metagenomic analysis on subglacial sediment samples from East Antarctica with and without H₂ supplementation. Genes coding for several [NiFe]-hydrogenases were identified in raw sediment and were enriched after H₂ incubation. All genes in the dissimilatory nitrate reduction and denitrification pathways were detected in the subglacial community, and the genes coding for these pathways became enriched after H₂ was supplied. Similarly, genes transcribing key enzymes in the Calvin cycle were detected in raw sediment and were also enriched. Moreover, key genes involved in H₂ oxidization, nitrate reduction, oxidative phosphorylation, and the Calvin cycle were identified within one metagenome-assembled genome belonging to a Polaromonas sp. As suggested by our results, the microbial community in the subglacial environment we investigated consisted of chemoautotrophic populations supported by H₂ oxidation. These results further confirm the importance of H₂ in the cryosphere.

Microbial Assemblages in Pressurized Antarctic Brine Pockets (Tarn Flat, Northern Victoria Land): A Hotspot of Biodiversity and Activity

Two distinct pressurized hypersaline brine pockets (named TF4 and TF5), separated by a thin ice layer, were detected below an ice-sealed Antarctic lake. Prokaryotic (bacterial and archaeal) diversity, abundances (including virus-like particles) and metabolic profiles were investigated by an integrated approach, including traditional and new-generation methods. Although similar diversity indices were computed for both Bacteria and Archaea, distinct bacterial and archaeal assemblages were observed. Bacteroidetes and Gammaproteobacteria were more abundant in the shallowest brine pocket, TF4, and Deltaproteobacteria, mainly represented by versatile sulphate-reducing bacteria, dominated in the deepest, TF5. The detection of sulphate-reducing bacteria and methanogenic Archaea likely reflects the presence of a distinct synthrophic consortium in TF5. Surprisingly, members assigned to hyperthermophilic Crenarchaeota and Euryarchaeota were common to both brines, indicating that these cold habitats host the most thermally tolerant Archaea. The patterns of microbial communities were different, coherently with the observed microbiological diversity between TF4 and TF5 brines. Both the influence exerted by upward movement of saline brines from a sub-surface anoxic system and the possible occurrence of an ancient ice remnant from the Ross Ice Shelf were the likely main factors shaping the microbial communities.

Taxonomic and functional characterization of a microbial community from a volcanic englacial ecosystem in Deception Island, Antarctica

Glaciers are populated by a large number of microorganisms including bacteria, archaea and microeukaryotes. Several factors such as solar radiation, nutrient availability and water content greatly determine the diversity and abundance of these microbial populations, the type of metabolism and the biogeochemical cycles. Three ecosystems can be differentiated in glaciers: supraglacial, subglacial and englacial ecosystems. Firstly, the supraglacial ecosystem, sunlit and oxygenated, is predominantly populated by photoautotrophic microorganisms. Secondly, the subglacial ecosystem contains a majority of chemoautotrophs that are fed on the mineral salts of the rocks and basal soil. Lastly, the englacial ecosystem is the least studied and the one that contains the smallest number of microorganisms. However, these unknown englacial microorganisms establish a food web and appear to have an active metabolism. In order to study their metabolic potentials, samples of englacial ice were taken from an Antarctic glacier. Microorganisms were analyzed by a polyphasic approach that combines a set of -omic techniques: 16S rRNA sequencing, culturomics and metaproteomics. This combination provides key information about diversity and functions of microbial populations, especially in rare habitats. Several whole essential proteins and enzymes related to metabolism and energy production, recombination and translation were found that demonstrate the existence of cellular activity at subzero temperatures. In this way it is shown that the englacial microorganisms are not quiescent, but that they maintain an active metabolism and play an important role in the glacial microbial community.

Ice sheets matter for the global carbon cycle

The cycling of carbon on Earth exerts a fundamental influence upon the greenhouse gas content of the atmosphere, and hence global climate over millennia. Until recently, ice sheets were viewed as inert components of this cycle and largely disregarded in global models. Research in the past decade has transformed this view, demonstrating the existence of uniquely adapted microbial communities, high rates of biogeochemical/physical weathering in ice sheets and storage and cycling of organic carbon (>104 Pg C) and nutrients. Here we assess the active role of ice sheets in the global carbon cycle and potential ramifications of enhanced melt and ice discharge in a warming world.

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.

Evidence for Pathways of Concentrated Submarine Groundwater Discharge in East Antarctica from Helicopter-Borne Electrical Resistivity Measurements

The Southern Ocean receives limited liquid surface water input from the Antarctic continent. It has been speculated, however, that significant liquid water may flow from beneath the Antarctic Ice Sheet, and that this subglacial flow carries that water along with dissolved nutrients to the coast. The delivery of solutes, particularly limiting nutrients like bioavailable iron, to the Southern Ocean may contribute to ecosystem processes including primary productivity. Using a helicopter-borne time domain electromagnetic survey along the coastal margins of the McMurdo Dry Valleys region of Southern Victoria Land, Antarctica, we detected subsurface connections between inland lakes, aquifers, and subglacial waters. These waters, which appear as electrically conductive anomalies, are saline and may contain high concentrations of biologically important ions, including iron and silica. Local hydraulic gradients may drive these waters to the coast, where we postulate they emerge as submarine groundwater discharge. This high latitude groundwater system, imaged regionally in the McMurdo Dry Valleys, may be representative of a broader system of Antarctic submarine groundwater discharge that fertilizes the Southern Ocean. In total, it has the potential to deliver tens of gigagrams of bioavailable Fe and Si to the coastal zone.

Microbial diversity of an Antarctic subglacial community and high-resolution replicate sampling inform hydrological connectivity in a polar desert

Antarctic subglacial environments host microbial ecosystems and are proving to be geochemically and biologically diverse. The Taylor Glacier, Antarctica, periodically expels iron-rich brine through a conduit sourced from a deep subglacial aquifer, creating a dramatic red surface feature known as Blood Falls. We used Illumina MiSeq sequencing to describe the core microbiome of this subglacial brine and identified previously undetected but abundant groups including the candidate bacterial phylum Atribacteria and archaeal phylum Pacearchaeota. Our work represents the first microbial characterization of samples collected from within a glacier using a melt probe, and the only Antarctic subglacial aquatic environment that, to date, has been sampled twice. A comparative analysis showed the brine community to be stable at the operational taxonomic unit level of 99% identity over a decade. Higher resolution sequencing enabled deconvolution of the microbiome of subglacial brine from mixtures of materials collected at the glacier surface. Diversity patterns between this brine and samples from the surrounding landscape provide insight into the hydrological connectivity of subglacial fluids to the surface polar desert environment. Understanding subice brines collected on the surfaces of thick ice covers has implications for analyses of expelled materials that may be sampled on icy extraterrestrial worlds.

Genomic and physiological characterization and description of Marinobacter gelidimuriae sp. nov., a psychrophilic, moderate halophile from Blood Falls, an antarctic subglacial brine

Antarctic subice environments are diverse, underexplored microbial habitats. Here, we describe the ecophysiology and annotated genome of a Marinobacter strain isolated from a cold, saline, iron-rich subglacial outflow of the Taylor Glacier, Antarctica. This strain (BF04_CF4) grows fastest at neutral pH (range 6-10), is psychrophilic (range: 0°C-20°C), moderately halophilic (range: 0.8%-15% NaCl) and hosts genes encoding potential low temperature and high salt adaptations. The predicted proteome suggests it utilizes fewer charged amino acids than a mesophilic Marinobacter strain. BF04_CF4 has increased concentrations of membrane unsaturated fatty acids including palmitoleic (33%) and oleic (27.5%) acids that may help maintain cell membrane fluidity at low temperatures. The genome encodes proteins for compatible solute biosynthesis and transport, which are known to be important for growth in saline environments. Physiological verification of predicted metabolic functions demonstrate BF04_CF4 is capable of denitrification and may facilitate iron oxidation. Our data indicate that strain BF04_CF4 represents a new Marinobacter species, Marinobacter gelidimuriae sp. nov., that appears well suited for the subglacial environment it was isolated from. Marinobacter species have been isolated from other cold, saline environments in the McMurdo Dry Valleys and permanently cold environments globally suggesting that this lineage is cosmopolitan and ecologically relevant in icy brines.

The Ladder of Life Detection

We describe the history and features of the Ladder of Life Detection, a tool intended to guide the design of investigations to detect microbial life within the practical constraints of robotic space missions. To build the Ladder, we have drawn from lessons learned from previous attempts at detecting life and derived criteria for a measurement (or suite of measurements) to constitute convincing evidence for indigenous life. We summarize features of life as we know it, how specific they are to life, and how they can be measured, and sort these features in a general sense based on their likelihood of indicating life. Because indigenous life is the hypothesis of last resort in interpreting life-detection measurements, we propose a small but expandable set of decision rules determining whether the abiotic hypothesis is disproved. In light of these rules, we evaluate past and upcoming attempts at life detection. The Ladder of Life Detection is not intended to endorse specific biosignatures or instruments for life-detection measurements, and is by no means a definitive, final product. It is intended as a starting point to stimulate discussion, debate, and further research on the characteristics of life, what constitutes a biosignature, and the means to measure them.

A thin ice layer segregates two distinct fungal communities in Antarctic brines from Tarn Flat (Northern Victoria Land)

Brines are hypersaline solutions which have been found within the Antarctic permafrost from the Tarn Flat area (Northern Victoria Land). Here, an investigation on the possible presence and diversity of fungal life within those peculiar ecosystems has been carried out for the first time. Brines samples were collected at 4- and 5-meter depths (TF1 and TF2, respectively), from two brines separated by a thin ice layer. The samples were analyzed via Illumina MiSeq targeting the ITS region specific for both yeasts and filamentous fungi. An unexpected high alpha diversity was found. Beta diversity analysis revealed that the two brines were inhabited by two phylogenetically diverse fungal communities (Unifrac value: 0.56, p value < 0.01; Martin’s P-test p-value < 0.001) characterized by several specialist taxa. The most abundant fungal genera were Candida sp., Leucosporidium sp., Naganishia sp. and Sporobolomyces sp. in TF1, and Leucosporidium sp., Malassezia sp., Naganishia sp. and Sporobolomyces sp. in TF2. A few hypotheses on such differentiation have been done: i) the different chemical and physical composition of the brines; ii) the presence in situ of a thin layer of ice, acting as a physical barrier; and iii) the diverse geological origin of the brines.

The Possible Emergence of Life and Differentiation of a Shallow Biosphere on Irradiated Icy Worlds: The Example of Europa

Irradiated ice-covered ocean worlds with rocky mafic mantles may provide the conditions needed to drive the emergence and maintenance of life. Alkaline hydrothermal springs-relieving the geophysical, thermal, and chemical disequilibria between oceans and tidally stressed crusts-could generate inorganic barriers to the otherwise uncontrolled and kinetically disfavored oxidation of hydrothermal hydrogen and methane. Ionic gradients imposed across these inorganic barriers, comprising iron oxyhydroxides and sulfides, could drive the hydrogenation of carbon dioxide and the oxidation of methane through thermodynamically favorable metabolic pathways leading to early life-forms. In such chemostatic environments, fuels may eventually outweigh oxidants. Ice-covered oceans are primarily heated from below, creating convection that could transport putative microbial cells and cellular cooperatives upward to congregate beneath an ice shell, potentially giving rise to a highly focused shallow biosphere. It is here where electron acceptors, ultimately derived from the irradiated surface, could be delivered to such life-forms through exchange with the icy surface. Such zones would act as "electron disposal units" for the biosphere, and occupants might be transferred toward the surface by buoyant diapirs and even entrained into plumes. Key Words: Biofilms-Europa-Extraterrestrial life-Hydrothermal systems. Astrobiology 17, 1265-1273.

Snowball Earth climate dynamics and Cryogenian geology-geobiology

Geological evidence indicates that grounded ice sheets reached sea level at all latitudes during two long-lived Cryogenian (58 and ≥5 My) glaciations. Combined uranium-lead and rhenium-osmium dating suggests that the older (Sturtian) glacial onset and both terminations were globally synchronous. Geochemical data imply that CO2 was 102 PAL (present atmospheric level) at the younger termination, consistent with a global ice cover. Sturtian glaciation followed breakup of a tropical supercontinent, and its onset coincided with the equatorial emplacement of a large igneous province. Modeling shows that the small thermal inertia of a globally frozen surface reverses the annual mean tropical atmospheric circulation, producing an equatorial desert and net snow and frost accumulation elsewhere. Oceanic ice thickens, forming a sea glacier that flows gravitationally toward the equator, sustained by the hydrologic cycle and by basal freezing and melting. Tropical ice sheets flow faster as CO2 rises but lose mass and become sensitive to orbital changes. Equatorial dust accumulation engenders supraglacial oligotrophic meltwater ecosystems, favorable for cyanobacteria and certain eukaryotes. Meltwater flushing through cracks enables organic burial and submarine deposition of airborne volcanic ash. The subglacial ocean is turbulent and well mixed, in response to geothermal heating and heat loss through the ice cover, increasing with latitude. Terminal carbonate deposits, unique to Cryogenian glaciations, are products of intense weathering and ocean stratification. Whole-ocean warming and collapsing peripheral bulges allow marine coastal flooding to continue long after ice-sheet disappearance. The evolutionary legacy of Snowball Earth is perceptible in fossils and living organisms.

Characterization of a cold-active bacterium isolated from the South Pole "Ice Tunnel"

Extremely cold microbial habitats on Earth (those below -30 °C) are rare and have not been surveyed for microbes as extensively as environments in the 0 to -20 °C range. Using cryoprotected growth media incubated at -5 °C, we enriched a cold-active Pseudomonas species from -50 °C ice collected from a utility tunnel for wastewater pipes under Amundsen-Scott South Pole Station, Antarctica. The isolate, strain UC-1, is related to other cold-active Pseudomonas species, most notably P. psychrophila, and grew at -5 °C to +34-37 °C; growth of UC-1 at +3 °C was significantly faster than at +34 °C. Strain UC-1 synthesized a surface exopolymer and high levels of unsaturated fatty acids under cold growth conditions. A 16S rRNA gene diversity screen of the ice sample that yielded strain UC-1 revealed over 1200 operational taxonomic units (OTUs) distributed across eight major classes of Bacteria. Many of the OTUs were Clostridia and Bacteriodia and some of these were probably of wastewater origin. However, a significant fraction of the OTUs were Proteobacteria and Actinobacteria of likely environmental origin. Our results shed light on the lower temperature limits to life and the possible existence of functional microbial communities in ultra-cold environments.

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.

The influence of Antarctic subglacial volcanism on the global iron cycle during the Last Glacial Maximum

Marine sediment records suggest that episodes of major atmospheric CO2 drawdown during the last glacial period were linked to iron (Fe) fertilization of subantarctic surface waters. The principal source of this Fe is thought to be dust transported from southern mid-latitude deserts. However, uncertainty exists over contributions to CO2 sequestration from complementary Fe sources, such as the Antarctic ice sheet, due to the difficulty of locating and interrogating suitable archives that have the potential to preserve such information. Here we present petrographic, geochemical and microbial DNA evidence preserved in precisely dated subglacial calcites from close to the East Antarctic Ice-Sheet margin, which together suggest that volcanically-induced drainage of Fe-rich waters during the Last Glacial Maximum could have reached the Southern Ocean. Our results support a significant contribution of Antarctic volcanism to subglacial transport and delivery of nutrients with implications on ocean productivity at peak glacial conditions.

Physiological Ecology of Microorganisms in Subglacial Lake Whillans

Subglacial microbial habitats are widespread in glaciated regions of our planet. Some of these environments have been isolated from the atmosphere and from sunlight for many thousands of years. Consequently, ecosystem processes must rely on energy gained from the oxidation of inorganic substrates or detrital organic matter. Subglacial Lake Whillans (SLW) is one of more than 400 subglacial lakes known to exist under the Antarctic ice sheet; however, little is known about microbial physiology and energetics in these systems. When it was sampled through its 800 m thick ice cover in 2013, the SLW water column was shallow (~2 m deep), oxygenated, and possessed sufficient concentrations of C, N, and P substrates to support microbial growth. Here, we use a combination of physiological assays and models to assess the energetics of microbial life in SLW. In general, SLW microorganisms grew slowly in this energy-limited environment. Heterotrophic cellular carbon turnover times, calculated from ³H-thymidine and ³H-leucine incorporation rates, were long (60 to 500 days) while cellular doubling times averaged 196 days. Inferred growth rates (average ~0.006 d^-1) obtained from the same incubations were at least an order of magnitude lower than those measured in Antarctic surface lakes and oligotrophic areas of the ocean. Low growth efficiency (8%) indicated that heterotrophic populations in SLW partition a majority of their carbon demand to cellular maintenance rather than growth. Chemoautotrophic CO₂-fixation exceeded heterotrophic organic C-demand by a factor of ~1.5. Aerobic respiratory activity associated with heterotrophic and chemoautotrophic metabolism surpassed the estimated supply of oxygen to SLW, implying that microbial activity could deplete the oxygenated waters, resulting in anoxia. We used thermodynamic calculations to examine the biogeochemical and energetic consequences of environmentally imposed switching between aerobic and anaerobic metabolisms in the SLW water column. Heterotrophic metabolisms utilizing acetate and formate as electron donors yielded less energy than chemolithotrophic metabolisms when calculated in terms of energy density, which supports experimental results that showed chemoautotrophic activity in excess of heterotrophic activity. The microbial communities of subglacial lake ecosystems provide important natural laboratories to study the physiological and biogeochemical behavior of microorganisms inhabiting cold, dark environments.

Evidence for marine origin and microbial-viral habitability of sub-zero hypersaline aqueous inclusions within permafrost near Barrow, Alaska

Cryopegs are sub-surface hypersaline brines at sub-zero temperatures within permafrost; their global extent and distribution are unknown. The permafrost barrier to surface and groundwater advection maintains these brines as semi-isolated systems over geological time. A cryopeg 7 m below ground near Barrow, Alaska, was sampled for geochemical and microbiological analysis. Sub-surface brines (in situtemperature of -6 °C, salinity of 115 ppt), and an associated sediment-infused ice wedge (melt salinity of 0.04 ppt) were sampled using sterile technique. Major ionic concentrations in the brine corresponded more closely to other (Siberian) cryopegs than to Standard seawater or the ice wedge. Ionic ratios and stable isotope analysis of water conformed to a marine or brackish origin with subsequent Rayleigh fractionation. The brine contained ∼1000× more bacteria than surrounding ice, relatively high viral numbers suggestive of infection and reproduction, and an unusually high ratio of particulate to dissolved extracellular polysaccharide substances. A viral metagenome indicated a high frequency of temperate viruses and limited viral diversity compared to surface environments, with closest similarity to low water activity environments. Interpretations of the results underscore the isolation of these underexplored microbial ecosystems from past and present oceans.

Preservation of ancestral Cretaceous microflora recovered from a hypersaline oil reservoir

Microbiology of a hypersaline oil reservoir located in Central Africa was investigated with molecular and culture methods applied to preserved core samples. Here we show that the community structure was partially acquired during sedimentation, as many prokaryotic 16S rRNA gene sequences retrieved from the extracted DNA are phylogenetically related to actual Archaea inhabiting surface evaporitic environments, similar to the Cretaceous sediment paleoenvironment. Results are discussed in term of microorganisms and/or DNA preservation in such hypersaline and Mg-rich solutions. High salt concentrations together with anaerobic conditions could have preserved microbial/molecular diversity originating from the ancient sediment basin wherein organic matter was deposited.

Subglacial Lake Whillans microbial biogeochemistry: a synthesis of current knowledge

Liquid water occurs below glaciers and ice sheets globally, enabling the existence of an array of aquatic microbial ecosystems. In Antarctica, large subglacial lakes are present beneath hundreds to thousands of metres of ice, and scientific interest in exploring these environments has escalated over the past decade. After years of planning, the first team of scientists and engineers cleanly accessed and retrieved pristine samples from a West Antarctic subglacial lake ecosystem in January 2013. This paper reviews the findings to date on Subglacial Lake Whillans and presents new supporting data on the carbon and energy metabolism of resident microbes. The analysis of water and sediments from the lake revealed a diverse microbial community composed of bacteria and archaea that are close relatives of species known to use reduced N, S or Fe and CH4 as energy sources. The water chemistry of Subglacial Lake Whillans was dominated by weathering products from silicate minerals with a minor influence from seawater. Contributions to water chemistry from microbial sulfide oxidation and carbonation reactions were supported by genomic data. Collectively, these results provide unequivocal evidence that subglacial environments in this region of West Antarctica host active microbial ecosystems that participate in subglacial biogeochemical cycling.

Microbial ecology of the cryosphere: sea ice and glacial habitats

The Earth's cryosphere comprises those regions that are cold enough for water to turn into ice. Recent findings show that the icy realms of polar oceans, glaciers and ice sheets are inhabited by microorganisms of all three domains of life, and that temperatures below 0 °C are an integral force in the diversification of microbial life. Cold-adapted microorganisms maintain key ecological functions in icy habitats: where sunlight penetrates the ice, photoautotrophy is the basis for complex food webs, whereas in dark subglacial habitats, chemoautotrophy reigns. This Review summarizes current knowledge of the microbial ecology of frozen waters, including the diversity of niches, the composition of microbial communities at these sites and their biogeochemical activities.

Lost cold Antarctic deserts inferred from unusual sulfate formation and isotope signatures

The Antarctic ice cap significantly affects global ocean circulation and climate. Continental glaciogenic sedimentary deposits provide direct physical evidence of the glacial history of the Antarctic interior, but these data are sparse. Here we investigate a new indicator of ice sheet evolution: sulfates within the glaciogenic deposits from the Lewis Cliff Ice Tongue of the central Transantarctic Mountains. The sulfates exhibit unique isotope signatures, including δ(34)S up to +50‰ for mirabilite evaporites, Δ(17)O up to +2.3‰ for dissolved sulfate within contemporary melt-water ponds, and extremely negative δ(18)O as low as -22.2‰. The isotopic data imply that the sulfates formed under environmental conditions similar to today's McMurdo Dry Valleys, suggesting that ice-free cold deserts may have existed between the South Pole and the Transantarctic Mountains since the Miocene during periods when the ice sheet size was smaller than today, but with an overall similar to modern global hydrological cycle.

Microbial communities of the Lemon Creek Glacier show subtle structural variation yet stable phylogenetic composition over space and time

Glaciers are geologically important yet transient ecosystems that support diverse, biogeochemically significant microbial communities. During the melt season glaciers undergo dramatic physical, geochemical, and biological changes that exert great influence on downstream biogeochemical cycles. Thus, we sought to understand the temporal melt-season dynamics of microbial communities and associated geochemistry at the terminus of Lemon Creek Glacier (LCG) in coastal southern Alaska. Due to late season snowfall, sampling of LCG occurred in three interconnected areas: proglacial Lake Thomas, the lower glacial outflow stream, and the glacier’s terminus. LCG associated microbial communities were phylogenetically diverse and varied by sampling location. However, Betaproteobacteria, Alphaproteobacteria, and Bacteroidetes dominated communities at all sampling locations. Strict anaerobic groups such as methanogens, SR1, and OP11 were also recovered from glacier outflows, indicating anoxic conditions in at least some portions of the LCG subglacial environment. Microbial community structure was significantly correlated with sampling location and sodium concentrations. Microbial communities sampled from terminus outflow waters exhibited day-to-day fluctuation in taxonomy and phylogenetic similarity. However, these communities were not significantly different from randomly constructed communities from all three sites. These results indicate that glacial outflows share a large proportion of phylogenetic overlap with downstream environments and that the observed significant shifts in community structure are driven by changes in relative abundance of different taxa, and not complete restructuring of communities. We conclude that LCG glacial discharge hosts a diverse and relatively stable microbiome that shifts at fine taxonomic scales in response to geochemistry and likely water residence time.

Deep groundwater and potential subsurface habitats beneath an Antarctic dry valley

The occurrence of groundwater in Antarctica, particularly in the ice-free regions and along the coastal margins is poorly understood. Here we use an airborne transient electromagnetic (AEM) sensor to produce extensive imagery of resistivity beneath Taylor Valley. Regional-scale zones of low subsurface resistivity were detected that are inconsistent with the high resistivity of glacier ice or dry permafrost in this region. We interpret these results as an indication that liquid, with sufficiently high solute content, exists at temperatures well below freezing and considered within the range suitable for microbial life. These inferred brines are widespread within permafrost and extend below glaciers and lakes. One system emanates from below Taylor Glacier into Lake Bonney and a second system connects the ocean with the eastern 18 km of the valley. A connection between these two basins was not detected to the depth limitation of the AEM survey (∼350 m).

Conditions below the active permafrost layer in the McMurdo Dry Valleys, Antarctica, are thought to be ice cemented. Here, the authors use an airborne electromagnetic sensor to image the resistivity beneath the valley floor, which indicates the presence of high-salinity liquids at temperatures well below freezing.

Dominance of sulfur-fueled iron oxide reduction in low-sulfate freshwater sediments

A central tenant in microbial biogeochemistry is that microbial metabolisms follow a predictable sequence of terminal electron acceptors based on the energetic yield for the reaction. It is thereby oftentimes assumed that microbial respiration of ferric iron outcompetes sulfate in all but high-sulfate systems, and thus sulfide has little influence on freshwater or terrestrial iron cycling. Observations of sulfate reduction in low-sulfate environments have been attributed to the presumed presence of highly crystalline iron oxides allowing sulfate reduction to be more energetically favored. Here we identified the iron-reducing processes under low-sulfate conditions within columns containing freshwater sediments amended with structurally diverse iron oxides and fermentation products that fuel anaerobic respiration. We show that despite low sulfate concentrations and regardless of iron oxide substrate (ferrihydrite, Al-ferrihydrite, goethite, hematite), sulfidization was a dominant pathway in iron reduction. This process was mediated by (re)cycling of sulfur upon reaction of sulfide and iron oxides to support continued sulfur-based respiration--a cryptic sulfur cycle involving generation and consumption of sulfur intermediates. Although canonical iron respiration was not observed in the sediments amended with the more crystalline iron oxides, iron respiration did become dominant in the presence of ferrihydrite once sulfate was consumed. Thus, despite more favorable energetics, ferrihydrite reduction did not precede sulfate reduction and instead an inverse redox zonation was observed. These findings indicate that sulfur (re)cycling is a dominant force in iron cycling even in low-sulfate systems and in a manner difficult to predict using the classical thermodynamic ladder.

Microbial sulfur transformations in sediments from Subglacial Lake Whillans

Diverse microbial assemblages inhabit subglacial aquatic environments. While few of these environments have been sampled, data reveal that subglacial organisms gain energy for growth from reduced minerals containing nitrogen, iron, and sulfur. Here we investigate the role of microbially mediated sulfur transformations in sediments from Subglacial Lake Whillans (SLW), Antarctica, by examining key genes involved in dissimilatory sulfur oxidation and reduction. The presence of sulfur transformation genes throughout the top 34 cm of SLW sediments changes with depth. SLW surficial sediments were dominated by genes related to known sulfur-oxidizing chemoautotrophs. Sequences encoding the adenosine-5'-phosphosulfate (APS) reductase gene, involved in both dissimilatory sulfate reduction and sulfur oxidation, were present in all samples and clustered into 16 distinct operational taxonomic units. The majority of APS reductase sequences (74%) clustered with known sulfur oxidizers including those within the "Sideroxydans" and Thiobacillus genera. Reverse-acting dissimilatory sulfite reductase (rDSR) and 16S rRNA gene sequences further support dominance of "Sideroxydans" and Thiobacillus phylotypes in the top 2 cm of SLW sediments. The SLW microbial community has the genetic potential for sulfate reduction which is supported by experimentally measured low rates (1.4 pmol cm(-3)d(-1)) of biologically mediated sulfate reduction and the presence of APS reductase and DSR gene sequences related to Desulfobacteraceae and Desulfotomaculum. Our results also infer the presence of sulfur oxidation, which can be a significant energetic pathway for chemosynthetic biosynthesis in SLW sediments. The water in SLW ultimately flows into the Ross Sea where intermediates from subglacial sulfur transformations can influence the flux of solutes to the Southern Ocean.