[Show abstract][Hide abstract] ABSTRACT: Microbial S oxidation is a common process in hydrothermal systems, sulfidic springs and suboxic zones. In addition, we have recently interrogated a unique microbial habitat generated by the formation and thawing of ices that contain mM levels of sulfide, giving rise to the precipitation of km-scale deposits of elemental sulfur in the Canadian High Arctic. We will discuss recent field and laboratory observations of elemental sulfur (bio)mineralization in association with pure cultures and complex consortia of S-cycling microorganisms detected at Borup Fiord Pass on Ellesmere Island. In particular, we will discuss new observations on the exsolution, oxidation, mineralization and preservation of S derived from sulfidic ices and cold spring fluids. Our investigations integrate a combination of aqueous geochemistry, voltammetry, Raman and FTIR spectroscopy, FE-SEM, sulfur K-edge X-ray absorption spectroscopy and 16S rRNA high-throughput sequencing to identify the interrelationships between microbial community dynamics and fluid/ice geochemistry as well as to determine the controls on sulfur speciation, including biologically-mediated extracellular sulfur mineralization. We will also explore how the sulfidic ice biogeochemical system is interconnected with the subsurface environment, and how biosignatures of such S dominated ecosystems can be recognized on Earth or other icy bodies. 3097
[Show abstract][Hide abstract] ABSTRACT: A variety of sulfide-oxidizing microorganisms have the ability to form and stabilize elemental sulfur (S 0). Microbial S 0 biomineralization usually results in the formation of S 0 globules that can be found inside or outside the cells. Here we describe a very original form of S 0 biomineralization by bacteria from the Flavobacteriaceae family that have been isolated from a cold sulfur-rich environment (Borup Fiord Pass, Canadian High Arctic). When cultivated in the presence of opposing gradients of sulfide (H 2 S) and oxygen, these bacteria produce a complex network of extracellular filamentous structures rigidly connecting to each other at 45° and 90° angles, and on which S 0 eventually precipitates during the oxidation of H 2 S. S 0 is also formed at the surface of large (up to 1µm) outer membrane vesicles produced by the cells. We will present data from fluorescence microscopy, electron microscopy, synchrotron-based soft x-ray spectromicroscopy (STXM), and infrared nano-spectrocopy coupled with AFM, that provide insights into the chemical composition, ultrastructure and formation mechanism of these biomineralized structures. We will also discuss their potential as biosignatures in present and ancient sulfur-rich environments, as well as their potential utility in the material sciences field.
[Show abstract][Hide abstract] ABSTRACT: The Cedars site is an exposed peridotite body in northern California undergoing active serpentinization. Alkaline spring waters (pH >11) with high concentrations of dissolved H2 and CH4 discharge at the surface in several locations. Two sets of water samples were collected during high- and low-flow periods for gas concentration and isotopic measurements to assess mechanisms of formation and whether the isotopic compositions of the gases can be used to determine serpentinization temperatures. Multiple samples were taken from each of 3 springs complexes listed in order of increasing elevation: Grotto Pool (GP), Barnes Springs (BS) and Mineral Falls (MF). Gas concentrations within each complex were similar, but gas ratios varied between complexes with the ratio of CH4 to H2 ranging from <0.4 at GP to 0.5 to 1.0 at BS, to
>2.0 at MF. The variation in δD for all H2 samples analyzed (n=14) was small (-735 to -756‰), with no differences between complexes. Using the average δD of water (-40‰), and the relationship outlined by Horita  for equilibration of water and H2 gas using a platinum catalyst, gives calculated formation temperatures averaging 24°C. These temperatures imply H2 formation or equilibration temperatures at nearsurface
conditions. Significant H2 production at low temperatures is consistent with H2 generated in abiotic
laboratory experiments with peridotite from Cedars at <100°C. Average δ13C values of CH4 vary from -56‰ at GP to -62‰ at BS to -66‰ at MP are similar to data from the same locations recently published by Morrill et al  and correlate with the observed changes in the ratios of dissolved CH4 to dissolved H2 noted above. The isotope compositions of the CH4 do not fall in the range of typical values for abiotic CH4 and no generation of CH4 was observed in the Cedars peridotite experiments, suggesting that the CH4 is not abiogenic. Although the biological mechanism is not clear at this time, it is worth exploring the role of biological processes in the trend of increasing CH4/H2 and decreasing δ13C from the lower to higher elevation sites.
 Horita (1988) Chem. Geo. 72, 89-94  Morrill et al
(2013) Geochim Cosmochim Acta 109, 222-240
[Show abstract][Hide abstract] ABSTRACT: There are numerous indications that subseafloor basalts may currently host a huge quantity of active microbial cells and contain biosignatures of ancient life in the form of physical and chemical basalt glass alteration. Unfortunately, technological challenges prevent us from observing the formation and mineralization of these alteration features in situ, or reproducing tubular basalt alteration processes in the laboratory. Therefore, comprehensive analysis of the physical and chemical traces retained in mineralized tubules is currently the best approach for deciphering a record of glass alteration. We have used a number of high-resolution spectroscopic and microscopic methods to probe the geochemical and mineralogical characteristics of tubular alteration features in basalt glasses obtained from a suite of subseafloor drill cores that covers a range of different collection locations and ages. By combining three different synchrotron-based X-ray measurements – X-ray fluorescence microprobe mapping, XANES spectroscopy, and μ-XRD – with focused ion beam milling and transmission electron microscopy, we have spatially resolved the major and trace element distributions, as well as the oxidation state of Fe, determined the coordination chemistry of Fe, Mn and Ti at the micron-scale, and constrained the secondary minerals within these features. The tubular alteration features are characterized by strong losses of Fe2+, Mn2+, and Ca2+ compared to fresh glass, oxidation of the residual Fe, and the accumulation of Ti and Cu. The predominant phases infilling the alteration regions are Fe3+-bearing silicates dominated by 2:1 clays, with secondary Fe- and Ti-oxides, and a partially oxidized Mn-silicate phase. These geochemical patterns observed within the tubular alteration features are comparable across a diverse suite of samples formed over the past 5–100 Ma, which shows that the microscale mineralization processes are common and consistent throughout the ocean basins and throughout time. The distributions of Ti and Cu are distinct between tubular mineralization and the crack-filling minerals and thus delineate sequential stages of fluid–rock interaction. The preserved chemistry of clay and oxide mineralization in the tubular alteration then represents a common precursor state (e.g. Ti accumulation), that has not yet undergone recrystallization (e.g. titanite formation) as observed in many older, metamorphosed examples of tubular alteration.
No preview · Article · Jul 2013 · Earth and Planetary Science Letters
[Show abstract][Hide abstract] ABSTRACT: Hydrogen is commonly produced during the high-temperature hydration of
mafic and ultramafic rocks, owing to the oxidation of reduced iron
present in the minerals. Hydrothermal hydrogen is known to sustain
microbial communities in submarine vent and terrestrial hot-spring
systems. However, the rates and mechanisms of hydrogen generation below
temperatures of 150°C are poorly constrained. As such, the existence
and extent of hydrogen-fuelled ecosystems in subsurface terrestrial and
oceanic aquifers has remained uncertain. Here, we report results from
laboratory experiments in which we reacted ground ultramafic and mafic
rocks and minerals--specifically peridotite, pyroxene, olivine and
magnetite--with anoxic fluids at 55 and 100°C, and monitored
hydrogen gas production. We used synchrotron-based micro-X-ray
fluorescence and X-ray absorption near-edge structure spectroscopy to
identify changes in the speciation of iron in the materials. We report a
strong correlation between molecular hydrogen generation and the
presence of spinel phases--oxide minerals with the general formula
[M2+M23+]O4 and a cubic
crystal structure--in the reactants. We also identify
Fe(III)-(hydr)oxide reaction products localized on the surface of the
spinel phases, indicative of iron oxidation. We propose that the
transfer of electrons between Fe(II) and water adsorbed to the spinel
surfaces promotes molecular hydrogen generation at low temperatures. We
suggest that these localized sites of hydrogen generation in ultramafic
aquifers in the oceanic and terrestrial crust could support
hydrogen-based microbial life.
Full-text · Article · Jun 2013 · Nature Geoscience
[Show abstract][Hide abstract] ABSTRACT: We combined free energy calculations and metagenomic analyses of an elemental sulfur (S0) deposit on the surface of Borup Fiord Pass Glacier in the Canadian High Arctic to investigate whether the energy available from different redox reactions in an environment predicts microbial metabolism. Many S, C, Fe, As, Mn and NH4+ oxidation reactions were predicted to be energetically feasible in the deposit, and aerobic oxidation of S0 was the most abundant chemical energy source. Small subunit ribosomal RNA (SSU rRNA) gene sequence data showed that the dominant phylotypes were Sulfurovum and Sulfuricurvum, both Epsilonproteobacteria known to be capable of sulfur lithotrophy. Sulfur redox genes were abundant in the metagenome, but sox genes were significantly more abundant than reverse dsr genes. Interestingly, there appeared to be habitable niches that were unoccupied at the depth of genome coverage obtained. Photosynthesis and NH4+ oxidation should both be energetically favorable, but we found few or no functional genes for oxygenic or anoxygenic photosynthesis, or for NH4+ oxidation by either oxygen (nitrification) or nitrite (anammox). The free energy, SSU rRNA gene and quantitative functional gene data are all consistent with the hypothesis that sulfur-based chemolithoautotrophy by Epsilonproteobacteria (Sulfurovum and Sulfuricurvum) is the main form of primary productivity at this site, instead of photosynthesis. This is despite the presence of 24-hour sunlight, and the fact that photosynthesis is not known to be inhibited by any of the environmental conditions present. This is the first time that Sulfurovum and Sulfuricurvum have been shown to dominate a sub-aerial environment, rather than anoxic or sulfidic settings. We also found that Flavobacteria dominate the surface of the sulfur deposits. We hypothesize that this aerobic heterotroph uses enough oxygen to create a microoxic environment in the sulfur below, where the Epsilonproteobacteria can flourish.
Full-text · Article · Apr 2013 · Frontiers in Microbiology
[Show abstract][Hide abstract] ABSTRACT: Bulk analyses of fresh and altered subseafloor basalts have elucidated the large-scale geochemical fluxes that result from water–rock interactions, which ultimately affect both seawater and mantle compositions, but these analyses do not provide insights into the physical and chemical processes occurring on the scale of microorganisms. Micron-sized tubules that may represent microbial boring into basalt glass are ubiquitous throughout the seafloor and can potentially be preserved over geological time. These putative biosignatures give clues into the history of life on Earth, and may be significant for the search for life on other planets; however, their formation and mineralization are still poorly understood. We have used a combination of focused ion beam (FIB) milling and transmission electron microscopy (TEM) to analyze the textural and geochemical characteristics of tubular alteration features from four basalt glass samples representing different sample locations, collection depths, and ages, including one sample from the Cretaceous Troodos ophiolite.
No preview · Article · Nov 2012 · Chemical Geology
[Show abstract][Hide abstract] ABSTRACT: The habit, mineralogy, crystallography, and Fe speciation of tubular and granular alteration textures in basaltic glass recovered from DSDP Hole 418A, which have previously been associated with biologically mediated alteration, were investigated using an integrated suite of microscopic and spectroscopic approaches in order to shine light on their formation and mineralization history. Two different analytical approaches were used: (1) micro scale investigations with conventional petrographic optical microcopy and microscale X-ray fluorescence mapping and X-ray absorption spectroscopy, and (2) nano scale analyses with FIB (focused ion beam milling) to prepare cross-sections for TEM (transmission electron microscopy), EELS (electron energy loss spectroscopy), and STXM (scanning transmission electron microscopy) analyses. The integrated data show that tubular and granular textures are similar in chemical, mineralogical and structural habit. Both granular and tubular alteration textures show a marked transition from ferrous iron in the glass matrix to ferric iron in the textures. Granular and tubular textures are filled with sheet silicates of similar chemistry, and both exhibit thin amorphous alteration rims ∼10–20 nm wide. The alteration rims are typically depleted in Ca and Fe. Ca is enriched at the contact between the secondary mineralization and the alteration rims, whereas Fe is enriched throughout the alteration features and is mainly present as FeIII in contrast to FeII in the host glass. Carbon is enriched only in a few areas, and could possibly be of organic origin but is not bound in carbonate. The mineralization of the features follows the sequence: dissolution of the glass; formation of a leached amorphous rim; mineralizing the cavities by smectide type clays and subsequently congruent growing of the texture diameter by diffusing of the elements through the alteration layer. None of the features could be linked solely to a biogenic origin and hence the biogenicity of the textures can neither be refuted nor supported by this micro- and nano-scale data set.
Full-text · Article · Aug 2012 · Geochimica et Cosmochimica Acta
[Show abstract][Hide abstract] ABSTRACT: Extensive mats of Fe oxyhydroxides and associated Fe-oxidizing microbial organisms form in diverse geochemical settings - freshwater seeps to deep-sea vents - where ever opposing Fe(II)-oxygen gradients prevail. The mineralogy, reactivity, and structural transformations of Fe oxyhydroxides precipitated from submarine hydrothermal fluids within microbial mats remains elusive in active and fossil systems. In response, a study of Fe microbial mat formation at the Loihi Seamount was conducted to describe the physical and chemical characteristics of Fe-phases using extended X-ray absorption fine structure spectroscopy, powder X-ray diffraction, synchrotron radiation X-ray total scattering, low-temperature magnetic measurements, and Mössbauer spectroscopy. Particle sizes of 3.5-4.6 nm were estimated from magnetism data, and coherent scattering domain (CSD) sizes as small as 1.6 nm are indicated by pair distribution function (PDF) analysis. Disorder in the nanostructured Fe-bearing phases results in limited intermediate-range structural order: less than that of standard two-line ferrihydrite (Fh), except for the Pohaku site. The short-range ordered natural Fh (Fh(SRO)) phases were stable at 4°C in the presence of oxygen for at least 1 year and during 400°C treatment. The observed stability of the Fh(SRO) is consistent with magnetic observations that point to non-interacting nanoparticles. PDF analyses of total scattering data provide further evidence for Fh(SRO) particles with a poorly ordered silica coating. The presence of coated particles explains the small CSD for the mat minerals, as well as the stability of the minerals over time and against heating. The mineral properties observed here provide a starting point from which progressively older and more extensively altered Fe deposits may be examined, with the ultimate goal of improved interpretation of past biogeochemical conditions and diagenetic processes.
Full-text · Article · Apr 2012 · Frontiers in Microbiology
[Show abstract][Hide abstract] ABSTRACT: The compelling evidence for an ocean beneath the ice shell of Europa makes it a high priority for astrobiological investigations. Future missions to the icy surface of this moon will query the plausibly sulfur-rich materials for potential indications of the presence of life carried to the surface by mobile ice or partial melt. However, the potential for generation and preservation of biosignatures under cold, sulfur-rich conditions has not previously been investigated, as there have not been suitable environments on Earth to study. Here, we describe the characterization of a range of biosignatures within potentially analogous sulfur deposits from the surface of an Arctic glacier at Borup Fiord Pass to evaluate whether evidence for microbial activities is produced and preserved within these deposits. Optical and electron microscopy revealed microorganisms and extracellular materials. Elemental sulfur (S⁰), the dominant mineralogy within field samples, is present as rhombic and needle-shaped mineral grains and spherical mineral aggregates, commonly observed in association with extracellular polymeric substances. Orthorhombic α-sulfur represents the stable form of S⁰, whereas the monoclinic (needle-shaped) γ-sulfur form rosickyite is metastable and has previously been associated with sulfide-oxidizing microbial communities. Scanning transmission electron microscopy showed mineral deposition on cellular and extracellular materials in the form of submicron-sized, needle-shaped crystals. X-ray diffraction measurements supply supporting evidence for the presence of a minor component of rosickyite. Infrared spectroscopy revealed parts-per-million level organics in the Borup sulfur deposits and organic functional groups diagnostic of biomolecules such as proteins and fatty acids. Organic components are below the detection limit for Raman spectra, which were dominated by sulfur peaks. These combined investigations indicate that sulfur mineral deposits may contain identifiable biosignatures that can be stabilized and preserved under low-temperature conditions. Borup Fiord Pass represents a useful testing ground for instruments and techniques relevant to future astrobiological exploration at Europa.
[Show abstract][Hide abstract] ABSTRACT: The existence of life in the deep terrestrial subsurface is established, yet few studies have investigated the origin of nitrogen that supports deep life. Previously, 16S rRNA gene surveys cataloged a diverse microbial community in subsurface fluids draining from boreholes 3000 feet deep at Henderson Mine, CO, USA (Sahl et al., 2008). The prior characterization of the fluid chemistry and microbial community forms the basis for the further investigation here of the source of NH4+. The reported fluid chemistry included N-2, NH4+ (5-112 mu M), NO2- (27-48 mu M), and NO3- (17-72 mu M). In this study, the correlation between low NH4+ concentrations in dominantly meteoric fluids and higher NH4+ in rock-reacted fluids is used to hypothesize that NH4+ is sourced from NH4+-bearing biotite. However, biotite samples from the host rocks and ore-body minerals were analyzed by Fourier transform infrared (FTIR) microscopy and none-contained NH4+. However, the nitrogenase-encoding gene nifH was successfully amplified from DNA of the fluid sample with high NH4+, suggesting that subsurface microbes have the capability to fix N-2. If so, unregulated nitrogen fixation may account for the relatively high NH4+ concentrations in the fluids. Additionally, the amoA and nxrB genes for archaeal ammonium monooxygenase and nitrite oxidoreductase, respectively, were amplified from the high NH4+ fluid DNA, while bacterial amoA genes were not. Putative nitrifying organisms are closely related to ammonium-oxidizing Crenarchaeota and nitrite-oxidizing Nitrospira detected in other subsurface sites based upon 16S rRNA sequence analysis. Thermodynamic calculations underscore the importance of NH4+ as an energy source in a subsurface nitrification pathway. These results suggest that the subsurface microbial community at Henderson is adapted to the low nutrient and energy environment by their capability of fixing nitrogen, and that fixed nitrogen may support subsurface biomass via nitrification.
Full-text · Article · Dec 2011 · Frontiers in Microbiology
[Show abstract][Hide abstract] ABSTRACT: Fluid-filled microfractures within mafic and ultramafic rocks, such as
basalt and peridotite, may be one of the most ubiquitous microbial
habitats on the modern and ancient earth. In seafloor and subseafloor
systems, one of the dominant energy sources is the oxidation of Fe by
numerous potential oxidants under aerobic to anaerobic conditions. In
particular, the oxidation of Fe may be directly catalyzed by microbial
organisms, or result in the production of molecular hydrogen which can
then fuel diverse lithotrophic metabolisms. However, it remains
challenging to identify the dominant metabolic activities and unravel
the microscale biogeochemical processes occuring within such rock-hosted
systems. We are investigating the mechanisms of solid-state Fe-oxidation
and biomineralization in basalt, olivine, pyroxenes and basalts, in the
presence and absence of microbial organisms that can thrive across the
full stability range of water. In this talk we will present
synchrotron-based x-ray scattering and spectroscopic analyses of Fe
speciation within secondary minerals formed during microbially-mediated
vs. abiotic water-rock interactions. Determining the valence state and
mineralogy of Fe-bearing phases is critical for determining the
water-rock reaction pathways and identifying potential biominerals that
may form; therefore, we will highlight new approaches for identifying
key Fe transformations within complex geological media. In addition,
many of our experimental studies involve the growth of lithotrophic
biofilms on well-characterized mineral surfaces in order to determine
the chemistry of the microbe-mineral interface during progressive
electron-transfer reactions. By coupling x-ray spectroscopy, x-ray
diffraction, and electron-microscopy measurements, we will also contrast
the evolution of mineral surfaces that undergo microbially-mediated
oxidative alteration against minerals surfaces that produce H2 to
sustain anaerobic microbial communities.
[Show abstract][Hide abstract] ABSTRACT: The identification of putative biosignatures in subseafloor basalt
glasses in the form of tubular alteration features, and their analogs in
ancient ophiolites, raises the intriguing possibility that this
biosphere may have persisted for most of the history of life on Earth.
Given the technological challenges in accessing the subseafloor and the
possibility that it could take thousands of years to form these
alteration features, at this point it is not possible to actually
observe their formation in situ, nor have they been successfully modeled
in the lab. Thus, we are left with inferring the formation and
preservation mechanisms of these biosignatures from physical and
chemical clues. We have used a number of high-resolution techniques to
analyze the geochemical characteristics of these features and have put
together a potential model of the stages of alteration. The original
formation of the features appears to involve congruent dissolution of
the glass, with either little precipitation of authigenic minerals, or
the formation of unstable phases that are later replaced. Successive
stages of hydrothermal and cold aqueous fluid flow then in-fill the
tubules with a variety of Fe- and Ti-rich minerals, which vary somewhat
depending on age and location of sample. The long-term preservation of
the features appears to be dependent on the stability of the Ti-bearing
[Show abstract][Hide abstract] ABSTRACT: Elemental sulfur (S(0) ) is deposited each summer onto surface ice at Borup Fiord pass on Ellesmere Island, Canada, when high concentrations of aqueous H(2) S are discharged from a supraglacial spring system. 16S rRNA gene clone libraries generated from sulfur deposits were dominated by β-Proteobacteria, particularly Ralstonia sp. Sulfur-cycling micro-organisms such as Thiomicrospira sp., and ε-Proteobacteria such as Sulfuricurvales and Sulfurovumales spp. were also abundant. Concurrent cultivation experiments isolated psychrophilic, sulfide-oxidizing consortia, which produce S(0) in opposing gradients of Na(2) S and oxygen. 16S rRNA gene analyses of sulfur precipitated in gradient tubes show stable sulfur-biomineralizing consortia dominated by Marinobacter sp. in association with Shewanella, Loktanella, Rubrobacter, Flavobacterium, and Sphingomonas spp. Organisms closely related to cultivars appear in environmental 16S rRNA clone libraries; none currently known to oxidize sulfide. Once consortia were simplified to Marinobacter and Flavobacteria spp. through dilution-to-extinction and agar removal, sulfur biomineralization continued. Shewanella, Loktanella, Sphingomonas, and Devosia spp. were also isolated on heterotrophic media, but none produced S(0) alone when reintroduced to Na(2) S gradient tubes. Tubes inoculated with a Marinobacter and Shewanella spp. co-culture did show sulfur biomineralization, suggesting that Marinobacter may be the key sulfide oxidizer in laboratory experiments. Light, florescence and scanning electron microscopy of mineral aggregates produced in Marinobacter experiments revealed abundant cells, with filaments and sheaths variably mineralized with extracellular submicron sulfur grains; similar biomineralization was not observed in abiotic controls. Detailed characterization of mineral products associated with low temperature microbial sulfur-cycling may provide biosignatures relevant to future exploration of Europa and Mars.
[Show abstract][Hide abstract] ABSTRACT: Hydrogen gas produced in the subsurface from the hydration of mafic rocks is known to be a major energy source for chemolithotrophic life in extreme environments such as hydrothermal vents. The possibility that in situ anaerobic microorganisms present in the deep subsurface are sustained by low temperature H2-generating water–rock reactions taking place around them is being investigated. Whether the growth and activity of H2-utilizing microbes directly influences aqueous geochemistry, rates of mineral dissolution, and the chemical composition of the alteration products is also being quantitatively evaluated.
No preview · Article · Jun 2011 · Applied Geochemistry
[Show abstract][Hide abstract] ABSTRACT: The authors are studying microbial sulfur redox metabolisms in a glacial environment. The energy available from sulfur redox reactions at this site has been calculated using geochemical data obtained from the site. DNA has been extracted from the same site and is being analyzed for the presence and relative quantities of sulfur redox genes, to determine whether bioenergetic calculations can predict the sulfur redox reactions that microbes are in fact utilizing.
Full-text · Article · Jun 2011 · Applied Geochemistry
[Show abstract][Hide abstract] ABSTRACT: Deep subsurface oxic/anoxic interfaces within Henderson Mine, CO were investigated for the potential to support novel metal-oxidizing microorganisms. Ralstonia sp. were isolated from Fe-oxidizing enrichments inoculated with fracture fluids released through boreholes as well as Fe-oxides mineralizing around the mouths of the boreholes. 16S rRNA clone libraries of environmental DNA revealed that closely related Ralstonia sp. were numerically-dominant in metal-rich subsurface fluids. FeCO3 gradient tubes were then utilized to demonstrate that isolate Ralstonia HM08-01 grows by oxidizing Fe(II) with O2 at circumneutral pH. These results and the geochemical data from the borehole fluids implicate Fe-oxidation as a viable subsurface lifestyle. The differential development of Fe-oxide bands in biotic vs. abiotic gradient tubes suggests that Ralstonia HM08-01 exerts spatial control over Fe oxidation and precipitation. Geochemical profiles of Fe(II), Fe(III) and O2 taken through the gradient tubes with voltammetric microelectrodes reveal that despite visual differences, similar total concentrations and distributions of aqueous Fe species were present in both systems. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy was used to characterize the mineralogy of the Fe-oxides produced in biotic vs. abiotic experiments. 2L-ferrihydrite dominated the mineral fits in both systems, and SEM revealed the ferrihydrite particles to be 50–100 nm in diameter. This mineralogical identification combined with the detection of an abundant electroactive Fe(III) species are used to infer that 2L-ferrihydrite is a long-term stabilized colloidal species. The mechanism for stabilization of this phase is the presence of PO42− and Si in growth experiments. In the Henderson fluids, PO42− is below detection, but Si is at micromolar concentrations and likely influences the formation of potentially colloidal Fe-oxides in the environment.
[Show abstract][Hide abstract] ABSTRACT: The oxidation state, speciation, and distribution of Fe are critical determinants of Fe reactivity in natural and engineered environments. However, it is challenging to follow dynamic changes in Fe speciation in environmental systems during progressive fluid-mineral interactions. Two common geological and aquifer materials-basalt and Fe(III) oxides-were incubated with saline fluids at 55 °C under highly reducing conditions maintained by the presence of Fe(0). We tracked changes in Fe speciation after 48 h (incipient water-rock reaction) and 10 months (extensive water-rock interaction) using synchrotron-radiation μXRF maps collected at multiple energies (ME) within the Fe K-edge. Immediate PCA analysis of the ME maps was used to optimize μXANES analyses; in turn, refitting the ME maps with end-member XANES spectra enabled us to detect and spatially resolve the entire variety of Fe-phases present in the system. After 48 h, we successfully identified and mapped the major Fe-bearing components of our samples (Fe(III) oxides, basalt, and rare olivine), as well as small quantities of incipient brucite associated with olivine. After 10 months, the Fe(III)-oxides remained stable in the presence of Fe(0), whereas significant alteration of basalt to minnesotaite and chlinochlore had occurred, providing new insights into heterogeneous Fe speciation in complex geological media under highly reducing conditions.
[Show abstract][Hide abstract] ABSTRACT: The rapid redox cycling of iron is one of the most pervasive geochemical processes catalyzed by microbial organisms. Numerous
microbial metabolisms rely on transferring electrons to and from iron, even in “extreme” environments considered challenging
for life due to high acidity, high alkalinity, high temperature, low organic content, or low water abundance. Recent efforts
to explore the iron biogeochemistry of extreme systems, such as hydrothermal vents, seafloor basalts, serpentinizing systems,
and acid mine drainage, have significantly expanded our expectations regarding the distribution and activity of iron-dependent
life on Earth, and potentially other iron-rich silicate planets, such as Mars.
[Show abstract][Hide abstract] ABSTRACT: Europa is a high priority for astrobiological investigations. Future missions to the icy surface of this moon will query the arguably sulfur-rich materials for potential indications of the presence of life carried to the surface by mobile ice or partial melt. Cold sulfur-rich environments are rare on the Earth, and the potential for the generation and preservation of biosignatures under these conditions remains largely unconstrained. Here we describe investigations into the biogenicity of analogous sulfur deposits from the surface of an Arctic glacier at Borup Fiord pass, Ellesmere Island. Optical and electron microscopy indicate that the sulfur in field samples is present in the form of clumps of mineral grains and spherical mineral aggregates, in close association with microbial sheaths. The morphologies of these materials are consistent with observations of the sulfur generated by sulfide-oxidizing bacteria cultivated from field samples in previous studies. X-ray diffraction measurements provide some evidence for the presence of rosickyite, a metastable form of sulfur previously recognized to be associated with the presence of life. Infrared spectroscopy reveals the presence of organics at parts per million levels, and organic functional groups diagnostic of proteins and fatty acids are identified. Organic components were below the detection limit for Raman spectra, which were dominated by sulfur peaks. These combined investigations indicate that sulfur minerals have the potential to contain identifiable biosignatures that low-temperature conditions help stabilize and preserve. Borup Fiord Pass represents a useful testing ground for instruments and techniques relevant to future astrobiological exploration at Europa.