Precambrian solution photochemistry, inverse segregation, and banded iron formations

Crystallization Pathways of Iron Formations: Insights From Magnetic Properties and High-Resolution Imaging of the 2.7 Ga Carajás Formation, Brazil

Banded iron formations (BIFs) are chemical sedimentary rocks commonly utilized for exploring the chemistry and redox state of the Precambrian ocean. Despite their significance, many aspects regarding the crystallization pathways of iron oxides in BIFs remain loosely constrained. In this study, we combine magnetic properties characterization with high-resolution optical and electron imaging of finely laminated BIFs from the 2.7 Ga Carajás Formation, Brazil, to investigate their nature and potential for preserving ancient environmental conditions. Our findings reveal that magnetite, in the form of large 0.1-0.5 mm crystals, is the main iron oxide, with an overall averaged saturation magnetization (Ms) of 25 Am2/kg (corresponding to ~27 wt% of magnetite) over the studied 230 m of the sequence. Nevertheless, the non-negligible contribution of minerals with higher coercivity suggests variable proportions of hematite along the core. Additionally, we observe non-uniform behavior in magnetite grains, with distinct populations identified through low-temperature measurements of the Verwey transition. Petrographic observations indicate that the original sediment was an Fe-Si mud consisting of a ferrihydrite-silica mixture formed in the water column. This assemblage was rapidly transformed into nano-scale hematite embedded in silica as indicated by a honeycomb structure composed of Si-spherules distributed in a microscale hematite matrix. Textural relationships show that the nucleation of magnetite started during or soon after the formation of hematite, as indicated by the preservation of the Si-spherules within magnetite cores. Further magnetite overgrowth stages are characterized by inclusion-free rims, associated with continuous Si supply during the evolving diagenetic or early metamorphic stages. These findings, combined with existing literature, suggest that ferrihydrite precipitated alongside Si and organic material, later crystallizing as hematite on the seafloor. Anaerobic respiration by Fe(III)-reducing microorganisms likely contributed to early magnetite formation in a fluid-saturated, unconsolidated sediment. Subsequent low-grade metamorphism and Si mobilization led to palisade quartz precipitation and a second stage of magnetite growth likely formed at the expense of matrix hematite through thermochemical Fe(III) reduction. Low-temperature magnetic analyses revealed that the two generations of magnetite core and rim are associated with specific stoichiometry.

Nanoparticulate apatite and greenalite in oldest, well-preserved hydrothermal vent precipitates

Paleoarchean jaspilites are used to track ancient ocean chemistry and photoautotrophy because they contain hematite interpreted to have formed following biological oxidation of vent-derived Fe(II) and seawater P-scavenging. However, recent studies have triggered debate about ancient seawater Fe and P deposition. Here, we report greenalite and fluorapatite (FAP) nanoparticles in the oldest, well-preserved jaspilites from the ~3.5-billion-year Dresser Formation, Pilbara Craton, Australia. We argue that both phases are vent plume particles, whereas coexisting hematite is linked to secondary oxidation. Geochemical modeling predicts that hydrothermal alteration of seafloor basalts by anoxic, sulfate-free seawater releases Fe(II) and P that simultaneously precipitate as greenalite and FAP upon venting. The formation, transport, and preservation of FAP nanoparticles indicate that seawater P concentrations were ≥1 to 2 orders of magnitude higher than in modern deepwater. We speculate that Archean seafloor vents were nanoparticle "factories" that, on prebiotic Earth, produced countless Fe(II)- and P-rich templates available for catalysis and biosynthesis.

Dissolved silica affects the bulk iron redox state and recrystallization of minerals generated by photoferrotrophy in a simulated Archean ocean

Chemical sedimentary deposits called Banded Iron Formations (BIFs) are one of the best surviving records of ancient marine (bio)geochemistry. Many BIF precursor sediments precipitated from ferruginous, silica-rich waters prior to the Great Oxidation Event at ~2.43 Ga. Reconstructing the mineralogy of BIF precursor phases is key to understanding the coevolution of seawater chemistry and early life. Many models of BIF deposition invoke the activity of Fe(II)-oxidizing photoautotrophic bacteria as a mechanism for precipitating mixed-valence Fe(II,III) and/or fully oxidized Fe(III) minerals in the absence of molecular oxygen. Although the identity of phases produced by ancient photoferrotrophs remains debated, laboratory experiments provide a means to explore what their mineral byproducts might have been. Few studies have thoroughly characterized precipitates produced by photoferrotrophs in settings representative of Archean oceans, including investigating how residual Fe(II)aq can affect the mineralogy of expected solid phases. The concentration of dissolved silica (Si) is also an important variable to consider, as silicate species may influence the identity and reactivity of Fe(III)-bearing phases. To address these uncertainties, we cultured Rhodopseudomonas palustris TIE-1 as a photoferrotroph in synthetic Archean seawater with an initial [Fe(II)aq ] of 1 mM and [Si] spanning 0-1.5 mM. Ferrihydrite was the dominant precipitate across all Si concentrations, even with substantial Fe(II) remaining in solution. Consistent with other studies of microbial iron oxidation, no Fe-silicates were observed across the silica gradient, although Si coprecipitated with ferrihydrite via surface adsorption. More crystalline phases such as lepidocrocite and goethite were only detected at low [Si] and are likely products of Fe(II)-catalyzed ferrihydrite transformation. Finally, we observed a substantial fraction of Fe(II) in precipitates, with the proportion of Fe(II) increasing as a function of [Si]. These experimental results suggest that photoferrotrophy in a Fe(II)-buffered ocean may have exported Fe(II,III)-oxide/silica admixtures to BIF sediments, providing a more chemically diverse substrate than previously hypothesized.

The Possible Role of Anoxic Alkaline High Subcritical Water in the Formation of Ferric Minerals, Methane and Disordered Graphitic Carbon in a BARB3 Drilled Sample of the 3.4 Ga Buck Reef Chert

The present article reports Raman spectroscopic observations of siderite, hematite, disordered graphitic carbon and possibly greenalite inside the quartz matrix of a banded iron sample from the BARB3 core drilled inside the 3.4 Ga Buck Reef Chert of the Barberton Greenstone Belt in South Africa. The article also reports Raman spectroscopic observations of quartz cavities, concluding in the presence of water, methane and sodium hydroxide at high concentration leading to pH ~ 15 inside the inclusion, suggesting an Archean water which was strongly basic. FeIII-greenalite may also be present inside the inclusion. The possible role of anoxic alkaline high subcritical water in the formation of ferric minerals and the CO required for the synthesis of molecules of biological interest has been demonstrated theoretically since 2013 and summarized in the concept of Geobiotropy. The present article experimentally confirms the importance of considering water in its anoxic strongly alkaline high subcritical domain for the formation of quartz, hematite, FeIII-greenalite, methane and disordered graphitic carbon. Methane is proposed to form locally when the carbon dioxide that is dissolved in the Archean anoxic alkaline high subcritical water, interacts with the molecular hydrogen that is emitted during the anoxic alkaline oxidation of ferrous silicates. The carbon matter is proposed to form as deposition from the anoxic methane-rich fluid. A detailed study of carbon matter from diverse origins is presented in a supplementary file. The study shows that the BARB3_23B sample has been submitted to ~ 335 °C, a temperature of the high subcritical domain, and that the graphitic structure contains very low amounts of oxygen and no hydroxyl functional groups. The importance of considering the structure of water is applied to the constructions of the Neoproterozoic and Archean banded iron formations. It is proposed that their minerals are produced inside chemical reaction chambers containing ferrous silicates, and ejected from the Earth's oceanic crust or upper mantle, during processes involving subduction events or not.

The deposition and significance of an Ediacaran non-glacial iron formation

Most Neoproterozoic iron formations (NIF) are closely associated with global or near-global "Snowball Earth" glaciations. Increasingly, however, studies indicate that some NIFs show no robust evidence of glacial association. Many aspects of non-glacial NIF genesis, including the paleo-environmental setting, Fe(II) source, and oxidation mechanisms, are poorly understood. Here, we present a detailed case study of the Jiapigou NIF, a major non-glacial NIF within a Neoproterozoic volcano-sedimentary sequence in North Qilian, northwestern China. New U-Pb geochronological data place the depositional age of the Jiapigou NIF at ~600 Ma. Petrographic and geochemical evidence supports its identification as a primary chemical sediment with significant detrital input. Major and trace element concentrations, REE + Y systematics, and ε_Nd (t) values indicate that iron was sourced from mixed seawater and hydrothermal fluids. Iron isotopic values (δ⁵⁶ Fe = -0.04‰-1.43‰) are indicative of partial oxidation of an Fe(II) reservoir. We infer that the Jiapigou NIF was deposited in a redox stratified water column, where hydrothermally sourced Fe(II)-rich fluids underwent oxidation under suboxic conditions. Lastly, the Jiapigou NIF has strong phosphorous enrichments, which in other iron formations are typically interpreted as signals for high marine phosphate concentrations. This suggests that oceanic phosphorus concentrations could have been enriched throughout the Neoproterozoic, as opposed to simply during glacial intervals.

Sunlight-Induced Interfacial Electron Transfer of Ferrihydrite under Oxic Conditions: Mineral Transformation and Redox Active Species Production

Fe(II)-catalyzed ferrihydrite transformation under anoxic conditions has been intensively studied, while such mechanisms are insufficient to be applied in oxic environments with depleted Fe(II). Here, we investigated expanded pathways of sunlight-driven ferrihydrite transformation in the presence of dissolved oxygen, without initial addition of dissolved Fe(II). We found that sunlight significantly facilitated the transformation of ferrihydrite to goethite compared to that under dark conditions. Redox active species (hole-electron pairs, reactive radicals, and Fe(II)) were produced from the ferrihydrite interface via the photoinduced electron transfer processes. Experiments with systematically varied wet chemistry conditions probed the relative contributions of three pathways for the production of hydroxyl radicals: (1) oxidation of water (5.0%); (2) reduction of dissolved oxygen (40.9%); and (3) photolysis of Fe(III)-hydroxyl complexes (54.1%). Results also showed superoxide radicals as the main oxidant for Fe(II) reoxidation under acidic conditions, thus promoting the ferrihydrite transformation. The presence of inorganic ions (chloride, sulfate, and nitrate) did not only affect the hydrolysis and precipitation of Fe(III) but also the generation of radicals via photoinduced charge transfer reactions. The involvement of redox active species and the accompanying mineral transformations would exert a profound effect on the fate of multivalent elements and organic contaminants in aquatic environments.

Anoxic photochemical weathering of pyrite on Archean continents

Sulfur is an essential element of life that is assimilated by Earth's biosphere through the chemical breakdown of pyrite. On the early Earth, pyrite weathering by atmospheric oxygen was severely limited, and low marine sulfate concentrations persisted for much of the Archean eon. Here, we show an anoxic photochemical mechanism of pyrite weathering that could have provided substantial amounts of sulfate to the oceans as continents formed in the late Archean. Pyrite grains suspended in anoxic ferrous iron solutions produced millimolar sulfate concentrations when irradiated with ultraviolet light. The Fe²⁺_(aq) was photooxidized, which, in turn, led to the chemical oxidation of pyritic sulfur. Additional experiments conducted with 2.68 Ga shale demonstrated that photochemically derived ferric iron oxidizes and dissolves sedimentary pyrite during chemical weathering. The results suggest that before the rise of atmospheric oxygen, oxidative pyrite weathering on Archean continents was controlled by the exposure of land to sunlight.

Microbial processes during deposition and diagenesis of Banded Iron Formations

Banded Iron Formations (BIFs) are marine chemical sediments consisting of alternating iron (Fe)-rich and silica (Si)-rich bands which were deposited throughout much of the Precambrian era. BIFs represent important proxies for the geochemical composition of Precambrian seawater and provide evidence for early microbial life. Iron present in BIFs was likely precipitated in the form of Fe3+ (Fe(III)) minerals, such as ferrihydrite (Fe(OH)3), either through the metabolic activity of anoxygenic photoautotrophic Fe2+ (Fe(II))-oxidizing bacteria (photoferrotrophs), by microaerophilic bacteria, or by the oxidation of dissolved Fe(II) by O2 produced by early cyanobacteria. However, in addition to oxidized Fe-bearing minerals such as hematite (FeIII 2O3), (partially) reduced minerals such as magnetite (FeIIFeIII 2O4) and siderite (FeIICO3) are found in BIFs as well. The presence of reduced Fe in BIFs has been suggested to reflect the reduction of primary Fe(III) minerals by dissimilatory Fe(III)-reducing bacteria, or by metamorphic (high pressure and temperature) reactions occurring in presence of buried organic matter. Here, we present the current understanding of the role of Fe-metabolizing bacteria in the deposition of BIFs, as well as competing hypotheses that favor an abiotic model for BIF deposition. We also discuss the potential abiotic and microbial reduction of Fe(III) in BIFs after deposition. Further, we review the availability of essential nutrients (e.g. P and Ni) and their implications on early Earth biogeochemistry. Overall, the combined results of various ancient seawater analogue experiments aimed at assessing microbial iron cycling pathways, coupled with the analysis of the BIF rock record, point towards a strong biotic influence during BIF genesis.

Highly Siderophile Elements and Coupled Fe-Os Isotope Signatures in the Temagami Iron Formation, Canada: Possible Signatures of Neoarchean Seawater Chemistry and Earth's Oxygenation History

Banded iron formations (BIFs) were deposited before and concurrent with the Great Oxidation Event at ∼2.33 Ga. They provide useful archives that document the transformation of the Precambrian hydrosphere from anoxic to progressively oxygenated conditions. Their formation involves removal of oceanic Fe by either inorganic or biologically promoted Fe²⁺ oxidation, or both. To evaluate depositional settings, elemental sources that affect seawater chemistry, and oxidation pathways, we present the first combined highly siderophile element (HSE) and Fe-Os isotope study for the ∼2.7 Ga Temagami BIF, Abitibi Greenstone Belt, Ontario (Canada). HSE abundances and ¹⁸⁷Os/¹⁸⁸Os ratios show no systematic variation between alternating magnetite and (meta)chert bands of the Temagami BIF. Whereas HSE concentrations mostly resemble modern crustal values, present-day ¹⁸⁷Os/¹⁸⁸Os ratios range from ∼0.17 to ∼10.8. Magnetite samples define a regression line corresponding to an age of 2661 ± 126 Ma. A chondrite-like ¹⁸⁷Os/¹⁸⁸Os initial value is in agreement with earlier studies on Neoarchean marine sediments and is thought to reflect seawater composition, which, unlike modern oceans, is dominated by mantle-like ¹⁸⁷Os inventory most likely derived from deep-sea hydrothermal vents. Our δ⁵⁶Fe data vary from about +0.6‰ to +0.9‰ and define a sawtooth-like pattern between alternating magnetite and (meta)chert layers. Partial oxidation of hydrothermally sourced Fe(II) and a lack of microbially mediated dissimilatory iron reduction provide the most plausible explanation for the positive δ⁵⁶Fe values. Notably, our δ⁵⁶Fe data for Temagami are in accord with trends defined by literature results for other Algoma-type BIFs that were deposited throughout the Archean.

Role of the Interchangeable Cations on the Sorption of Fumaric and Succinic Acids on Montmorillonite and its Relevance in Prebiotic Chemistry

It has been proposed that clays could have served as key factors in promoting the increase in complexity of organic matter in primitive terrestrial and extraterrestrial environments. The aim of this work is to study the adsorption-desorption of two dicarboxylic acids, fumaric and succinic acids, onto clay minerals (sodium and iron montmorillonite). These two acids may have played a role in prebiotic chemistry, and in extant biochemistry, they constitute an important redox couple (e.g. in Krebs cycle) in extant biochemistry. Smectite clays might have played a key role in the origins of life. The effect of pH on sorption has been tested; the analysis was performed by UV-vis and FTIR-ATR spectroscopy, X-ray diffraction and X-ray fluorescence. The results show that chemisorption is the main responsible of the adsorption processes among the dicarboxylic acids and clays. The role of the ion, present in the clay, is fundamental in the adsorption processes of dicarboxylic acids. These ions (sodium and iron) were selected due to their relevance on the geochemical environments that possibly existed into the primitive Earth. Different mechanisms are proposed to explain the sorption of dicarboxylic acids in the clay. In this work, we propose the formation of complexes among metal cations in the clays and dicarboxylic acids. The organic complexes were probably formed in the prebiotic environments enabling chemical processes, prior to the appearance of life. Thus, the data presented here are relevant to the origin of life studies.

Quantifying Preservation Potential: Lipid Degradation in a Mars-Analog Circumneutral Iron Deposit

Comparisons between the preservation potential of Mars-analog environments have historically been qualitative rather than quantitative. Recently, however, laboratory-based artificial maturation combined with kinetic modeling techniques have emerged as a potential means by which the preservation potential of solvent-soluble organic matter can be quantified in various Mars-analog environments. These methods consider how elevated temperatures, pressures, and organic-inorganic interactions influence the degradation of organic biomarkers post-burial. We used these techniques to investigate the preservation potential of deposits from a circumneutral iron-rich groundwater system. These deposits are composed of ferrihydrite (Fe₅HO₈ · 4H₂O), an amorphous iron hydroxide mineral that is a common constituent of rocks found in ancient lacustrine environments on Mars, such as those observed in Gale Crater. Both natural and synthetic ferrihydrite samples were subjected to hydrous pyrolysis to observe the effects of long-term burial on the mineralogy and organic content of the samples. Our experiments revealed that organic-inorganic interactions in the samples are dominated by the transformation of iron minerals. As amorphous ferrihydrite transforms into more crystalline species, the decrease in surface area results in the desorption of organic matter, potentially rendering them more susceptible to degradation. We also find that circumneutral iron-rich deposits provide unfavorable conditions for the preservation of solvent-soluble organic matter. Quantitative comparisons between preservation potentials as calculated when using kinetic parameters show that circumneutral iron-rich deposits are ∼25 times less likely to preserve solvent-soluble organic matter compared with acidic, iron-rich environments. Our results suggest that circumneutral iron-rich deposits should be deprioritized in favor of acidic iron- and sulfur-rich deposits when searching for evidence of life with solvent extraction techniques.

Anoxic photogeochemical oxidation of manganese carbonate yields manganese oxide

The oxidation states of manganese minerals in the geological record have been interpreted as proxies for the evolution of molecular oxygen in the Archean eon. Here we report that an Archean manganese mineral, rhodochrosite (MnCO3), can be photochemically oxidized by light under anoxic, abiotic conditions. Rhodochrosite has a calculated bandgap of about 5.4 eV, corresponding to light energy centering around 230 nm. Light at that wavelength would have been present on Earth's surface in the Archean, prior to the formation of stratospheric ozone. We show experimentally that the photooxidation of rhodochrosite in suspension with light centered at 230 nm produced H2 gas and manganite (γ-MnOOH) with an apparent quantum yield of 1.37 × 10-3 moles hydrogen per moles incident photons. Our results suggest that manganese oxides could have formed abiotically on the surface in shallow waters and on continents during the Archean eon in the absence of molecular oxygen.

The Great Oxidation Event expanded the genetic repertoire of arsenic metabolism and cycling

The rise of oxygen on the early Earth about 2.4 billion years ago reorganized the redox cycle of harmful metal(loids), including that of arsenic, which doubtlessly imposed substantial barriers to the physiology and diversification of life. Evaluating the adaptive biological responses to these environmental challenges is inherently difficult because of the paucity of fossil records. Here we applied molecular clock analyses to 13 gene families participating in principal pathways of arsenic resistance and cycling, to explore the nature of early arsenic biogeocycles and decipher feedbacks associated with planetary oxygenation. Our results reveal the advent of nascent arsenic resistance systems under the anoxic environment predating the Great Oxidation Event (GOE), with the primary function of detoxifying reduced arsenic compounds that were abundant in Archean environments. To cope with the increased toxicity of oxidized arsenic species that occurred as oxygen built up in Earth's atmosphere, we found that parts of preexisting detoxification systems for trivalent arsenicals were merged with newly emerged pathways that originated via convergent evolution. Further expansion of arsenic resistance systems was made feasible by incorporation of oxygen-dependent enzymatic pathways into the detoxification network. These genetic innovations, together with adaptive responses to other redox-sensitive metals, provided organisms with novel mechanisms for adaption to changes in global biogeocycles that emerged as a consequence of the GOE.

Minimal biomass deposition in banded iron formations inferred from organic matter and clay relationships

The cycling of iron and organic matter (OM) is thought to have been a major biogeochemical cycle in the early ferruginous oceans which contributed to the deposition of banded iron formations (BIF). However, BIF are deficient in OM, which is postulated to be the result of near-complete oxidation of OM during iron reduction. We test this idea by documenting the prevalence of OM in clays within BIF and clays in shales associated with BIF. We find in shales >80% of OM occurs in clays, but <1% occurs in clays within BIF. Instead, in BIF OM occurs with ¹³C-depleted carbonate and apatite, implying OM oxidation occurred. Conversely, BIF which possess primary clays would be expected to preserve OM in clays, yet this is not seen. This implies OM deposition in silicate-bearing BIF would have been minimal, this consequently stifled iron-cycling and primary productivity through the retention of nutrients in the sediments.

Banded iron formations could have formed in the early oceans due to microbial metabolism. Here Dodd and colleagues find little organic carbon in these formations, indicating microbial iron cycling was minimal and could have limited the recycling of important nutrients to overlying waters.

Water near its Supercritical Point and at Alkaline pH for the Production of Ferric Oxides and Silicates in Anoxic Conditions. A New Hypothesis for the Synthesis of Minerals Observed in Banded Iron Formations and for the Related Geobiotropic Chemistry inside Fluid Inclusions

An alternative hypothesis for the origin of the banded iron formations and the synthesis of prebiotic molecules is presented here. I show the importance of considering water near its supercritical point and at alkaline pH. It is based on the chemical equation for the anoxic oxidation of ferrous iron into ferric iron at high-subcritical conditions of water and high pH, that I extract from E-pH diagrams drawn for corrosion purposes (Geophysical Research Abstracts Vol 15, EGU2013-22 Bassez 2013, Orig Life Evol Biosph 45(1):5-13, Bassez 2015, Procedia Earth Planet Sci 17, 492-495, Bassez 2017a, Orig Life Evol Biosph 47:453-480, Bassez 2017b). The sudden change in solubility of silica, SiO2, at the critical point of water is also considered. It is shown that under these temperatures and pressures, ferric oxides and ferric silicates can form in anoxic terrains. No FeII oxidation by UV light, neither by oxygen is needed to explain the minerals of the Banded Iron Formations. The intervention of any kind of microorganisms, either sulfate-reducing, or FeII-oxidizing or O2-producing, is not required. The chemical equation for the anoxic oxidation of ferrous iron is applied to the hydrolyses of fayalite, Fe2SiO4 and ferrosilite, FeSiO3. It is shown that the BIF minerals of the Hamersley Group, Western Australia, and the Transvaal Supergroup, South Africa, are those of fayalite and ferrosilite hydrolyses and carbonations. The dissolution of crustal fayalite and ferrosilite during water-rock interaction needs to occur at T&P just below the critical point of water and in a rising water which is undersaturated in SiO2. Minerals of BIFs which can then be ejected at the surface from venting arcs are ferric oxide hydroxides, hematite, FeIII-greenalite, siderite. The greenalite dehydrated product minnesotaite forms when rising water becomes supersaturated in SiO2, as also riebeckite and stilpnomelane. Long lengths of siderite without ferric oxides neither ferric silicates can occur since the exothermic siderite formation is not so much dependent in T&P. It is also shown that the H2 which is released during hydrolysis/oxidation of fayalite/ferrosilite can lead to components of life, such as macromolecules of amino acids which are synthesized from mixtures of (CO, N2, H2O) in Sabatier-Senderens/Fischer-Tropsch & Haber-Bosch reactions or microwave or gamma-ray excitation reactions. I propose that such geobiotropic synthesis may occur inside fluid inclusions of BIFs, in the silica chert, hematite, FeIII-greenalite or siderite. Therefore, the combination of high-subcritical conditions of water, high solubility of SiO2 at these T&P values, formation of CO also at these T&P, high pH and anoxic water, leads to the formation of ferric minerals and prebiotic molecules in the process of geobiotropy.

Microbial diversity and iron oxidation at Okuoku-hachikurou Onsen, a Japanese hot spring analog of Precambrian iron formations

Banded iron formations (BIFs) are rock deposits common in the Archean and Paleoproterozoic (and regionally Neoproterozoic) sedimentary successions. Multiple hypotheses for their deposition exist, principally invoking the precipitation of iron via the metabolic activities of oxygenic, photoferrotrophic, and/or aerobic iron-oxidizing bacteria. Some isolated environments support chemistry and mineralogy analogous to processes involved in BIF deposition, and their study can aid in untangling the factors that lead to iron precipitation. One such process analog system occurs at Okuoku-hachikurou (OHK) Onsen in Akita Prefecture, Japan. OHK is an iron- and CO₂ -rich, circumneutral hot spring that produces a range of precipitated mineral textures containing fine laminae of aragonite and iron oxides that resemble BIF fabrics. Here, we have performed 16S rRNA gene amplicon sequencing of microbial communities across the range of microenvironments in OHK to describe the microbial diversity present and to gain insight into the cycling of iron, oxygen, and carbon in this ecosystem. These analyses suggest that productivity at OHK is based on aerobic iron-oxidizing Gallionellaceae. In contrast to other BIF analog sites, Cyanobacteria, anoxygenic phototrophs, and iron-reducing micro-organisms are present at only low abundances. These observations support a hypothesis where low growth yields and the high stoichiometry of iron oxidized per carbon fixed by aerobic iron-oxidizing chemoautotrophs like Gallionellaceae result in accumulation of iron oxide phases without stoichiometric buildup of organic matter. This system supports little dissimilatory iron reduction, further setting OHK apart from other process analog sites where iron oxidation is primarily driven by phototrophic organisms. This positions OHK as a study area where the controls on primary productivity in iron-rich environments can be further elucidated. When compared with geological data, the metabolisms and mineralogy at OHK are most similar to specific BIF occurrences deposited after the Great Oxygenation Event, and generally discordant with those that accumulated before it.

Biosignature Preservation and Detection in Mars Analog Environments

This review of material relevant to the Conference on Biosignature Preservation and Detection in Mars Analog Environments summarizes the meeting materials and discussions and is further expanded upon by detailed references to the published literature. From this diverse source material, there is a detailed discussion on the habitability and biosignature preservation potential of five primary analog environments: hydrothermal spring systems, subaqueous environments, subaerial environments, subsurface environments, and iron-rich systems. Within the context of exploring past habitable environments on Mars, challenges common to all of these key environments are laid out, followed by a focused discussion for each environment regarding challenges to orbital and ground-based observations and sample selection. This leads into a short section on how these challenges could influence our strategies and priorities for the astrobiological exploration of Mars. Finally, a listing of urgent needs and future research highlights key elements such as development of instrumentation as well as continued exploration into how Mars may have evolved differently from Earth and what that might mean for biosignature preservation and detection. Key Words: Biosignature preservation-Biosignature detection-Mars analog environments-Conference report-Astrobiological exploration. Astrobiology 17, 363-400.

Photoferrotrophy: Remains of an Ancient Photosynthesis in Modern Environments

Photoferrotrophy, the process by which inorganic carbon is fixed into organic matter using light as an energy source and reduced iron [Fe(II)] as an electron donor, has been proposed as one of the oldest photoautotrophic metabolisms on Earth. Under the iron-rich (ferruginous) but sulfide poor conditions dominating the Archean ocean, this type of metabolism could have accounted for most of the primary production in the photic zone. Here we review the current knowledge of biogeochemical, microbial and phylogenetic aspects of photoferrotrophy, and evaluate the ecological significance of this process in ancient and modern environments. From the ferruginous conditions that prevailed during most of the Archean, the ancient ocean evolved toward euxinic (anoxic and sulfide rich) conditions and, finally, much after the advent of oxygenic photosynthesis, to a predominantly oxic environment. Under these new conditions photoferrotrophs lost importance as primary producers, and now photoferrotrophy remains as a vestige of a formerly relevant photosynthetic process. Apart from the geological record and other biogeochemical markers, modern environments resembling the redox conditions of these ancient oceans can offer insights into the past significance of photoferrotrophy and help to explain how this metabolism operated as an important source of organic carbon for the early biosphere. Iron-rich meromictic (permanently stratified) lakes can be considered as modern analogs of the ancient Archean ocean, as they present anoxic ferruginous water columns where light can still be available at the chemocline, thus offering suitable niches for photoferrotrophs. A few bacterial strains of purple bacteria as well as of green sulfur bacteria have been shown to possess photoferrotrophic capacities, and hence, could thrive in these modern Archean ocean analogs. Studies addressing the occurrence and the biogeochemical significance of photoferrotrophy in ferruginous environments have been conducted so far in lakes Matano, Pavin, La Cruz, and the Kabuno Bay of Lake Kivu. To date, only in the latter two lakes a biogeochemical role of photoferrotrophs has been confirmed. In this review we critically summarize the current knowledge on iron-driven photosynthesis, as a remains of ancient Earth biogeochemistry.

Oxygen produced by cyanobacteria in simulated Archaean conditions partly oxidizes ferrous iron but mostly escapes-conclusions about early evolution

The Earth has had a permanently oxic atmosphere only since the great oxygenation event (GOE) 2.3-2.4 billion years ago but recent geochemical research has revealed short periods of oxygen in the atmosphere up to a billion years earlier before the permanent oxygenation. If these "whiffs" of oxygen truly occurred, then oxygen-evolving (proto)cyanobacteria must have existed throughout the Archaean aeon. Trapping of oxygen by ferrous iron and other reduced substances present in Archaean oceans has often been suggested to explain why the oxygen content of the atmosphere remained negligible before the GOE although cyanobacteria produced oxygen. We tested this hypothesis by growing cyanobacteria in anaerobic high-CO₂ atmosphere in a medium with a high concentration of ferrous iron. Microcystins are known to chelate iron, which prompted us also to test the effects of microcystins and nodularins on iron tolerance. The results show that all tested cyanobacteria, especially nitrogen-fixing species grown in the absence of nitrate, and irrespective of the ability to produce cyanotoxins, were iron sensitive in aerobic conditions but tolerated high concentrations of iron in anaerobicity. This result suggests that current cyanobacteria would have tolerated the high-iron content of Archaean oceans. However, only 1 % of the oxygen produced by the cyanobacterial culture was trapped by iron, suggesting that large-scale cyanobacterial photosynthesis would have oxygenated the atmosphere even if cyanobacteria grew in a reducing ocean. Recent genomic analysis suggesting that ability to colonize seawater is a secondary trait in cyanobacteria may offer a partial explanation for the sustained inefficiency of cyanobacterial photosynthesis during the Archaean aeon, as fresh water has always covered a very small fraction of the Earth's surface. If oxygenic photosynthesis originated in fresh water, then the GOE marks the adaptation of cyanobacteria to seawater, and the late-Proterozoic increase in oxygen concentration of the atmosphere is caused by full oxidation of the oceans.

The Role of Microbial Electron Transfer in the Coevolution of the Biosphere and Geosphere

All life on Earth is dependent on biologically mediated electron transfer (i.e., redox) reactions that are far from thermodynamic equilibrium. Biological redox reactions originally evolved in prokaryotes and ultimately, over the first ∼2.5 billion years of Earth's history, formed a global electronic circuit. To maintain the circuit on a global scale requires that oxidants and reductants be transported; the two major planetary wires that connect global metabolism are geophysical fluids-the atmosphere and the oceans. Because all organisms exchange gases with the environment, the evolution of redox reactions has been a major force in modifying the chemistry at Earth's surface. Here we briefly review the discovery and consequences of redox reactions in microbes with a specific focus on the coevolution of life and geochemical phenomena.

Atmospheric hydrogen peroxide and Eoarchean iron formations

It is widely accepted that photosynthetic bacteria played a crucial role in Fe(II) oxidation and the precipitation of iron formations (IF) during the Late Archean-Early Paleoproterozoic (2.7-2.4 Ga). It is less clear whether microbes similarly caused the deposition of the oldest IF at ca. 3.8 Ga, which would imply photosynthesis having already evolved by that time. Abiological alternatives, such as the direct oxidation of dissolved Fe(II) by ultraviolet radiation may have occurred, but its importance has been discounted in environments where the injection of high concentrations of dissolved iron directly into the photic zone led to chemical precipitation reactions that overwhelmed photooxidation rates. However, an outstanding possibility remains with respect to photochemical reactions occurring in the atmosphere that might generate hydrogen peroxide (H2 O2 ), a recognized strong oxidant for ferrous iron. Here, we modeled the amount of H2 O2 that could be produced in an Eoarchean atmosphere using updated solar fluxes and plausible CO2 , O2 , and CH4 mixing ratios. Irrespective of the atmospheric simulations, the upper limit of H2 O2 rainout was calculated to be <10(6) molecules cm(-2) s(-1) . Using conservative Fe(III) sedimentation rates predicted for submarine hydrothermal settings in the Eoarchean, we demonstrate that the flux of H2 O2 was insufficient by several orders of magnitude to account for IF deposition (requiring ~10(11) H2 O2 molecules cm(-2) s(-1) ). This finding further constrains the plausible Fe(II) oxidation mechanisms in Eoarchean seawater, leaving, in our opinion, anoxygenic phototrophic Fe(II)-oxidizing micro-organisms the most likely mechanism responsible for Earth's oldest IF.

Anoxic photochemical oxidation of siderite generates molecular hydrogen and iron oxides

Photochemical reactions of minerals are underappreciated processes that can make or break chemical bonds. We report the photooxidation of siderite (FeCO3) by UV radiation to produce hydrogen gas and iron oxides via a two-photon reaction. The calculated quantum yield for the reaction suggests photooxidation of siderite would have been a significant source of molecular hydrogen for the first half of Earth's history. Further, experimental results indicate this abiotic, photochemical process may have led to the formation of iron oxides under anoxic conditions. The reaction would have continued through the Archean to at least the early phases of the Great Oxidation Event, and provided a mechanism for oxidizing the atmosphere through the loss of hydrogen to space, while simultaneously providing a key reductant for microbial metabolism. We propose that the photochemistry of Earth-abundant minerals with wide band gaps would have potentially played a critical role in shaping the biogeochemical evolution of early Earth.

Did life originate from a global chemical reactor?

Many decades of experimental and theoretical research on the origin of life have yielded important discoveries regarding the chemical and physical conditions under which organic compounds can be synthesized and polymerized. However, such conditions often seem mutually exclusive, because they are rarely encountered in a single environmental setting. As such, no convincing models explain how living cells formed from abiotic constituents. Here, we propose a new approach that considers the origin of life within the global context of the Hadean Earth. We review previous ideas and synthesize them in four central hypotheses: (i) Multiple microenvironments contributed to the building blocks of life, and these niches were not necessarily inhabitable by the first organisms; (ii) Mineral catalysts were the backbone of prebiotic reaction networks that led to modern metabolism; (iii) Multiple local and global transport processes were essential for linking reactions occurring in separate locations; (iv) Global diversity and local selection of reactants and products provided mechanisms for the generation of most of the diverse building blocks necessary for life. We conclude that no single environmental setting can offer enough chemical and physical diversity for life to originate. Instead, any plausible model for the origin of life must acknowledge the geological complexity and diversity of the Hadean Earth. Future research may therefore benefit from identifying further linkages between organic precursors, minerals, and fluids in various environmental contexts.

Microorganisms pumping iron: anaerobic microbial iron oxidation and reduction

Iron (Fe) has long been a recognized physiological requirement for life, yet for many microorganisms that persist in water, soils and sediments, its role extends well beyond that of a nutritional necessity. Fe(II) can function as an electron source for iron-oxidizing microorganisms under both oxic and anoxic conditions and Fe(III) can function as a terminal electron acceptor under anoxic conditions for iron-reducing microorganisms. Given that iron is the fourth most abundant element in the Earth's crust, iron redox reactions have the potential to support substantial microbial populations in soil and sedimentary environments. As such, biological iron apportionment has been described as one of the most ancient forms of microbial metabolism on Earth, and as a conceivable extraterrestrial metabolism on other iron-mineral-rich planets such as Mars. Furthermore, the metabolic versatility of the microorganisms involved in these reactions has resulted in the development of biotechnological applications to remediate contaminated environments and harvest energy.

The Paleoproterozoic snowball Earth: a climate disaster triggered by the evolution of oxygenic photosynthesis

Although biomarker, trace element, and isotopic evidence have been used to claim that oxygenic photosynthesis evolved by 2.8 giga-annum before present (Ga) and perhaps as early as 3.7 Ga, a skeptical examination raises considerable doubt about the presence of oxygen producers at these times. Geological features suggestive of oxygen, such as red beds, lateritic paleosols, and the return of sedimentary sulfate deposits after a approximately 900-million year hiatus, occur shortly before the approximately 2.3-2.2 Ga Makganyene "snowball Earth" (global glaciation). The massive deposition of Mn, which has a high redox potential, practically requires the presence of environmental oxygen after the snowball. New age constraints from the Transvaal Supergroup of South Africa suggest that all three glaciations in the Huronian Supergroup of Canada predate the Snowball event. A simple cyanobacterial growth model incorporating the range of C, Fe, and P fluxes expected during a partial glaciation in an anoxic world with high-Fe oceans indicates that oxygenic photosynthesis could have destroyed a methane greenhouse and triggered a snowball event on time-scales as short as 1 million years. As the geological evidence requiring oxygen does not appear during the Pongola glaciation at 2.9 Ga or during the Huronian glaciations, we argue that oxygenic cyanobacteria evolved and radiated shortly before the Makganyene snowball.

The influence of life on the evolution of the atmosphere

The early history of life on earth may have been characterized by coevolution of microbial metabolism and atmospheric composition. Metabolic developments affected the composition of the atmosphere, and the resulting changes in the atmosphere stimulated the evolution of new metabolic capabilities. The first organisms eked out an existence by deriving energy from the fermentation of organic compounds abiotically synthesized. The abiotic source was meager, however, and when autotrophy arose, life was freed from its dependence on abiotic synthesis. The expanded level of biological activity made possible by autotrophy resulted in an increased rate of burial of reduced organic matter in sea floor sediments. The resultant drain on the concentration of electron donors in the biosphere caused a decline in the hydrogen content of the atmosphere. Biological productivity was limited by the supply of reduced compounds. This paper explores the biogeochemical circulation of electron donors in the primitive anaerobic ocean, concluding that their shortage was so critical as to provide strong selective pressure for the evolution of algal photosynthesis.

Efficient near ultraviolet light induced formation of hydrogen by ferrous hydroxide

The formation of hydrogen on irradiating ferrous ion in aqueous solution or suspension was studied over a wide range of pH. In addition to the known reaction in acid solution which decreases in yield with increasing pH and required far UV light, there is an efficient reaction occurring between pH 6 and 9 which utilizes near UV light. The latter reaction is an approximately linear function of both the concentration of ferrous ion and the light intensity. The quantum yield of hydrogen from the suspension of Fe(OH)2 at pH 7.2 is very high: > or = 0.3. The quantum yield decreases by a factor of five at 1 mole percent of ferric ions. To explain these observations it is proposed that an intermediate formed on excitation of the Fe(OH)2 polymer is further reduced by a neighboring Fe(+2) to form H2. These results support the work of Braterman et al. (1983) which claimed that the near UV driven photooxidation of ferrous ions could be responsible for formation of the Banded Iron Formations on the early earth. The efficient photoreaction observed in the present work could also serve as a source of active reducing equivalents to reduce CO2 and thus provide a solution to a dilemma in the arguments on the role of reduced carbon in the origin of life.

Anaerobic oxidation of ferrous iron by purple bacteria, a new type of phototrophic metabolism

Anoxic iron-rich sediment samples that had been stored in the light showed development of brown, rusty patches. Subcultures in defined mineral media with ferrous iron (10 mmol/liter, mostly precipitated as FeCO3) yielded enrichments of anoxygenic phototrophic bacteria which used ferrous iron as the sole electron donor for photosynthesis. Two different types of purple bacteria, represented by strains L7 and SW2, were isolated which oxidized colorless ferrous iron under anoxic conditions in the light to brown ferric iron. Strain L7 had rod-shaped, nonmotile cells (1.3 by 2 to 3 microns) which frequently formed gas vesicles. In addition to ferrous iron, strain L7 used H2 + CO2, acetate, pyruvate, and glucose as substrate for phototrophic growth. Strain SW2 had small rod-shaped, nonmotile cells (0.5 by 1 to 1.5 microns). Besides ferrous iron, strain SW2 utilized H2 + CO2, monocarboxylic acids, glucose, and fructose. Neither strain utilized free sulfide; however, both strains grew on black ferrous sulfide (FeS) which was converted to ferric iron and sulfate. Strains L7 and SW2 grown photoheterotrophically without ferrous iron were purple to brownish red and yellowish brown, respectively; absorption spectra revealed peaks characteristic of bacteriochlorophyll a. The closest phototrophic relatives of strains L7 and SW2 so far examined on the basis of 16S rRNA sequences were species of the genera Chromatium (gamma subclass of proteobacteria) and Rhodobacter (alpha subclass), respectively. In mineral medium, the new isolates formed 7.6 g of cell dry mass per mol of Fe(II) oxidized, which is in good agreement with a photoautotrophic utilization of ferrous iron as electron donor for CO2 fixation. Dependence of ferrous iron oxidation on light and CO2 was also demonstrated in dense cell suspensions. In media containing both ferrous iron and an organic substrate (e.g., acetate, glucose), strain L7 utilized ferrous iron and the organic compound simultaneously; in contrast, strain SW2 started to oxidize ferrous iron only after consumption of the organic electron donor. Ferrous iron oxidation by anoxygenic phototrophs is understandable in terms of energetics. In contrast to the Fe3+/Fe2+ pair (E0 = +0.77 V) existing in acidic solutions, the relevant redox pair at pH 7 in bicarbonate-containing environments, Fe(OH)3 + HCO3-/FeCO3, has an E0' of +0.2 V. Ferrous iron at pH 7 can therefore donate electrons to the photosystem of anoxygenic phototrophs, which in purple bacteria has a midpoint potential around +0.45 V. The existence of ferrous iron-oxidizing anoxygenic phototrophs may offer an explanation for the deposition of early banded-iron formations in an assumed anoxic biosphere in Archean times.

The oldest records of photosynthesis

There is diverse, yet controversial fossil evidence for the existence of photosynthesis 3500 million years ago. Among the most persuasive evidence is the stromatolites described from low grade metasedimentary rocks in Western Australia and South Africa. Based on the understanding of the paleobiology of stromatolites and using pertinent fossil and Recent analogs, these Early Archean stromatolites suggest that phototrophs evolved by 3500 million years ago. The evidence allows further interpretation that cyanobacteria were involved. Besides stromatolites, microbial and chemical fossils are also known from the same rock units. Some microfossils morphologically resemble cyanobacteria and thus complement the adduced cyanobacterial involvement in stromatolite construction. If cyanobacteria had evolved by 3500 million years ago, this would indicate that nearly all prokaryotic phyla had already evolved and that prokaryotes diversified rapidly on the early Earth.

The photochemistry of manganese and the origin of Banded Iron Formations

The photochemical oxidation of Fe(2+) -hydroxide complexes dissolved in anoxic Precambrian oceans has been suggested as a mechanism to explain the deposition of Banded Iron Formations (BIFs). Photochemical studies have not yet addressed the low levels of manganese in many of these deposits, which probably precipitated from solutions bearing similar concentrations of Fe2+ and Mn2+. Depositional models must also explain the stratigraphic separation of iron and manganese ores in manganiferous BIFs. In this study, solutions containing 0.56 M NaCl and approximately 180 micromoles MnCl2 with or without 3 to 200 micromoles FeCl2 were irradiated with filtered and unfiltered UV light from a medium-pressure mercury-vapor lamp for up to 8 hours. The solutions were deaerated and buffered to pH approximately 7, and all experiments were conducted under O2-free (< 1 ppm) atmospheres. In experiments with NaCl + MnCl2, approximately 20% of the Mn2+ was oxidized and precipitated as birnessite in 8 hours. Manganese precipitation was only observed when light with lambda < 240 nm was used. In experiments with NaCl + MnCl2 + FeCl2, little manganese was lost from solution, while Fe2+ was rapidly oxidized to Fe3+ and precipitated as gamma-FeOOH or as amorphous ferric hydroxide. The Mn:Fe ratio of these precipitates was approximately 1:50, similar to the ratios observed in BIFs. A strong upper limit on the rate of manganese photo-oxidation during the Precambrian is estimated to be 0.1 mg cm-2 yr-1, a factor of 10(3) slower than the rate of iron photo-oxidation considered reasonable in BIF depositional basins. Thus, a photochemical model for the origin of oxide facies BIFs is consistent with field observations, although models that invoke molecular O2 as the oxidant of Fe2+ and Mn2+ are not precluded. Apparently, oxide facies BIFs could have formed under anoxic, as well as under mildly oxygenated atmospheres.

Prebiotic sugar synthesis: hexose and hydroxy acid synthesis from glyceraldehyde catalyzed by iron(III) hydroxide oxide

Iron(III) hydroxide oxide [Fe(OH)O] efficiently catalyzed the condensation of 25 mM DL-glyceraldehyde to ketohexoses at 25 degrees C (pH 5-6). At 16 days the yields were sorbose (15.2%), fructose (12.9%), psicose (6.1%), tagatose (5.6%), and dendroketose (2.5%) with 19.6% of triose unreacted. Analysis at 96 days showed no decomposition of hexoses. Under these conditions Fe(OH)O also catalyzed the isomerization and rearrangement of glyceraldehyde to dihydroxyacetone and lactic acid, respectively. In these reactions, about 10% of the glyceraldehyde was oxidized to glyceric acid with concurrent reduction of the iron(III) to iron(II). The partial reduction of Fe(OH)O did not noticeably reduce its ability to catalyze hexose synthesis. The relationship of these results to prebiotic sugar synthesis is discussed.

Some aspects of the early evolution of photosynthesis

The early evolution of a photocatalytic system of the porphyrin type, able to efficiently collect and utilize solar energy for primary electron transfer is discussed. Experimental results concerning some spectral and photochemical properties of the porphyrins, biosynthetic precursors of chlorophyll and their complexes with polymeric templates are reviewed. Protoporphyrin IX associated with pigmented proteinoid is demonstrated to be a favourable candidate for a role of a photosensitizer of the first photosynthetic reaction centers. The origin and early evolution of the photosynthetic electron transfer chain and of the phosphorylating mechanism are discussed with emphasis on the energetic mechanisms of archaebacteria.

Hematite Nanospheres of Possible Colloidal Origin from a Precambrian Banded Iron Formation

Exceptionally small spheres (nanospheres) of hematite (diameters between 120 and 200 nanometers) occur in the Marra Mamba Iron Formation of the Hamersley Basin, Australia. The nanospheres are clustered into small aggregates and may have formed by structural ordering and dehydration of colloidal iron hydroxide particles. Individual spheres consist of numerous thin, curved hematite platelets surrounding a central void that is approximately half the diamter of the sphere; this texture suggests that they formed by a volume reduction of the original colloidal particles by approximately 12.5%. The occurrence of hematite nanospheres supports the hypothesis that some ofthe iron was deposited colloidally during the development ofbanded iron formations, approximately 2.5 billion years ago.

Bio-markers and the search for extinct life on Mars

Geologic and climatologic studies suggest that conditions on early Mars were similar to early Earth. Because life on Earth is believed to have originated during this early period (3.5 billion years ago), the Martian environment could have also been conducive to the origin of life. To investigate this possibility we must first define the attributes of an early Martian biota. Then, specific geographic locations on Mars must be chosen where life may have occurred (i.e. areas which had long standing water), and within these distinct locations search for key signatures or bio-markers of a possible extinct Martian biota. Some of the key signatures or bio-markers indicative of past biological activity on Earth may be applicable to Mars including: reduced carbon and nitrogen compounds, CO3(2-), SO4(2-), NO3-, NO2- [correction of NO2(2)], Mg, Mn, Fe, and certain other metals, and the isotopic ratios of C, N and S. However, we must also be able to distinguish abiotic from biologic origins for these bio-markers. For example, abiotically fixed N2 would form deposits of NO3- and NO2-, whereas biological processes would have reduced these to ammonium containing compounds, N2O, or N2, which would then be released to the atmosphere. A fully equipped Mars Rover might be able to perform analyses to measure most of these biomarkers while on the Martian surface.