Secondary Mineral Products Research Articles

Geochemical Split Among the Suspended and Mud Sediments in the Nethravati River: Insights to Compositional Similarity of Peninsular Gneiss and the Deccan Basalt Derived Sediments, and Its Implications on Tracing the Provenance in the Indian Ocean

AbstractMajor, trace, and rare earth element (REE) geochemistry of sediments from a tropical mountain river is investigated to understand the behavior of chemical elements during weathering and transportation in a Peninsular Gneissic terrain. The results from this study are compared with the Deccan Basalt‐derived River sediments and the eastern Arabian Sea sediments with an intent to highlight the challenges associated with the provenance determination of sediments along the continental margin of India. The geochemistry of suspended particulate matter (SPM) and mud sediments (<63 μm) of the Nethravati River indicate that sediments are derived from a relatively homogenous lithology, characterized by intermediate rocks of tonalitic (low‐Al TTGs) composition. The tectonic origin of the source rocks discriminated using sediment geochemistry suggests an ocean island arc origin. The sediments experience intense chemical weathering in the source region. The elemental composition and their inter‐element relationships suggest differential chemical weathering of mineral phases fractionate mafic components and their secondary mineral products in SPM, and mixed sources dominated by felsic components and their secondary mineral products in mud sediments. Intense chemical weathering induces significant geochemical splits among the suspended and mud sediments. The transport of mafic‐biased sediments from Peninsular India to Oceans, and the geochemical similarity with Deccan Basalt‐derived sediments makes it challenging to track the Peninsular Gneiss‐derived sediment provenance along the continental margin of India using conventional elemental geochemistry. The inferences from this study have important implications for determining the sediment provenance along the continental margin of India.

Enhancing cadmium immobilization by AQDS-mediated dissimilatory reduction under coexisting conditions of iron and manganese oxides

The serious contamination of cadmium (Cd) in sediments has become an urgent environmental problem. Quinones mediated dissimilatory iron and manganese reduction widely occurs in sediments, this process may enhance Cd immobilization by promoting production of secondary minerals. Nevertheless, the enhancing effect of Cd immobilization is still poorly understood. Here, 9,10-anthraquinone-2,6-disulfonic acid (AQDS) was used to facilitate the microbial reduction of iron and manganese oxides by Geobacter metallireducens GS-15, and its effects on Cd species was investigated. Compared to goethite (FeOOH), microorganisms preferentially reduced birnessite (MnO2) due to more dramatic interfacial electron transfer between birnessite and AQDS. AQDS thus significantly enhanced microbial reduction of iron and manganese coexisted minerals as well as Cd2+ immobilization. After 80 days, an impressive 83% of reducible-Cd appeared in the FeOOH–MnO2-AQDS group, demonstrating the most effective Cd immobilization. Furthermore, XANES results suggested that AQDS promoted the increase of vivianite-Cd in secondary minerals in MnO2-AQDS and FeOOH–MnO2-AQDS groups. Based on the structural equation model, HCl-extracted Fe(II) indirectly affected the increase of reducible-Cd mainly through vivianite-Cd. The AQDS enhancement on microbial reduction of iron and manganese oxides promotes the recrystallization of secondary minerals and the immobilization of Cd, offering a promising strategy for the remediation of Cd-contaminated sediments.

Science and Science‐Enabling Activities of the SHERLOC and WATSON Imaging Systems in Jezero Crater, Mars

AbstractDuring its first year of operation, the Perseverance rover explored the cratered and fractured floor of Jezero crater on Mars. Here, we report the use of the Scanning Habitability Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) imaging system that includes two high‐resolution cameras, the Autofocus and Contextual Imager (ACI) and Wide Angle Topographic Sensor for Operations and eNgineering (WATSON). ACI is a fixed focus gray scale imager with a resolution of 10.1 μm/pixel whereas WATSON is a variable field of view, variable focus imager capable of resolution down to 14 μm/pixel. WATSON is a reflight of the MArs Hand Lens Imager (MAHLI) imager and has similar capabilities. During first‐time activities, WATSON was used to support both science and engineering operations related to sample and abrasion patch assessment and sample collection and caching. WATSON also documented the deployment of the Ingenuity helicopter. The Crater Floor Campaign identified two primary rock units, the Máaz formation and the Séítah formation, which have been interpreted as lava flows and an olivine cumulate, respectively. Interpretation of rock textures with WATSON and ACI images was limited to abraded surfaces because unmodified outcrop surfaces (herein termed “natural surfaces”) show high degrees of dust covering, wind abrasion, and coating by secondary mineral products. WATSON and ACI images support the hypothesis that the material of both the Máaz and Séítah formations consists of largely aqueously altered mafic materials with varying igneous origins.

Anaerobic bioreduction of elemental sulfur improves bioavailability of Fe (III) oxides for bioremediation

Fe(III) oxides are ubiquitous electron acceptors for anaerobic bioremediation, although their bioavailability was limited due to the passivation of secondary mineralization products. Here we found the solid S0 can be added to improve their bioavailability. Using lepidocrocite (γ-FeOOH), acetate and Geobacter sulfurreducens PCA as representatives of Fe(III) oxides, intermediate of pollutant degradation and microbes, a 6 times higher amount of FeOOH reduction in the presence of S0 was observed with a time needed for S0 reduction shortened by half. The bioreduction of S0 activated the reduction of FeOOH, while the product (conductive FeS) may have bridged electron transfer across the cell membrane and periplasm. The proportion of excessive Fe(II) produced from Fe(III) was quantified as a direct bioreduction (26 %), with an abiotic FeOOH reduction to FeS (20 %) and an FeS-conducted FeOOH bioreduction (54 %), which highlight the key role of gradually formed FeS from S0 in the bioreduction of FeOOH. Our results showed that S0 can be an effective additive for the bioremediation of environments with abundant Fe(III) oxides, which has broader implications for elemental biogeochemical cycling.

Exploring the secondary mineral products generated by microbial iron respiration in Archean ocean simulations.

Marine chemical sedimentary deposits known as Banded Iron Formations (BIFs) archive Archean ocean chemistry and, potentially, signs of ancient microbial life. BIFs contain a diversity of iron- and silica-rich minerals in disequilibrium, and thus many interpretations of these phases suggest they formed secondarily during early diagenetic processes. One such hypothesis posits that the early diagenetic microbial respiration of primary iron(III) oxides in BIFs resulted in the formation of other iron phases, including the iron-rich silicates, carbonates, and magnetite common in BIF assemblages. Here, we simulated this proposed pathway in laboratory incubations combining a model dissimilatory iron-reducing (DIR) bacterium, Shewanella putrefaciens CN32, and the ferric oxyhydroxide mineral ferrihydrite under conditions mimicking the predicted Archean seawater geochemistry. We assessed the impact of dissolved silica, calcium, and magnesium on the bioreduced precipitates. After harvesting the solid products from these experiments, we analyzed the reduced mineral phases using Raman spectroscopy, electron microscopy, powder x-ray diffraction, and spectrophotometric techniques to identify mineral precipitates and track the bulk distributions of Fe(II) and Fe(III). These techniques detected a diverse range of calcium carbonate morphologies and polymorphism in incubations with calcium, as well as secondary ferric oxide phases like goethite in silica-free experiments. We also identified aggregates of curling, iron- and silica-rich amorphous precipitates in all incubations amended with silica. Although ferric oxides persist even in our electron acceptor-limited incubations, our observations indicate that microbial iron reduction of ferrihydrite is a viable pathway for the formation of early iron silicate phases. This finding allows us to draw parallels between our experimental proto-silicates and the recently characterized iron silicate nanoinclusions in BIF chert deposits, suggesting that early iron silicates could possibly be signatures of iron-reducing metabolisms on early Earth.

Insights into paleoenvironment and chemical weathering as controls on arsenic accumulation in Late Pleistocene to Holocene aquifers

High As groundwater has been reported in aquifers comprising Late Pleistocene and Holocene sediments worldwide, and could be due to the depositional environment and sedimentological processes. To investigate the paleoenvironment and chemical weathering as mechanisms controlling the fate of As, this study characterized the elemental, clay mineral, and Li isotopic compositions of sediments of an As-contaminated aquifer in the Datong Basin, China. Indices for reconstructing chemical weathering conditions, for example, the Chemical Index of Alternation (CIA), indicate overall incipient to intermediate chemical weathering conditions in the Datong Basin during the Late Pleistocene to Holocene. The higher CIA values at depths of ∼20–∼85 m indicate stronger chemical weathering and a shift in climatic regime towards warm and humid conditions in Stage 2 compared to Stage 1 and Stage 3. Under such climatic conditions, intense chemical weathering favors the leaching of silicate-bound Fe and thus the production of various secondary Fe-oxide minerals. Besides, the positive relationship between CIA values and easily reducible Fe oxides further shows the enrichment of more weakly crystalline Fe-oxides during the depositional condition; this finding is also supported by the results of chemical extractions performed on the sediments. Moreover, the good correlations between total organic carbon (TOC) and both Fe/Na and As concentrations suggest the enrichment of As and OM along with Fe enrichment during chemical weathering; this is attributed to the adsorption or co-precipitation of As and OM on Fe minerals. Under warm and humid conditions, intense chemical weathering released a large amount of Fe minerals; As was also released due to stronger weathering of the terrestrial bedrock and sequestered by Fe oxides and organic matter, thus forming complexes involving As, Fe, and OM. FTIR and chemical extraction results confirm the formation of As–Fe-OM complexes during the geological history contributes to As enrichment and accumulation in the late Pleistocene sediments. These findings provide new insights into the interaction between the paleoenvironment and the accumulation of ferric oxides/hydroxides and OM, which serve as controlling mechanisms with respect to As enrichment in Late Pleistocene to Holocene sediments, and thus shed light on the sedimentogenesis of high As aquifers.

Exploring the impact of silica and divalent cations on microbial iron respiration and secondary mineral products in Archean ocean simulations

Exploring the impact of silica and divalent cations on microbial iron respiration and secondary mineral products in Archean ocean simulations

Influence of hydrothermal activity on the mineralogical-petrophysical properties of an atypical doleritic reservoir rock: A case study of the Gabes Gulf (north Africa, Tunisia)

The Gabes Gulf located in the South Mediterranean Sea (Southeastern part of Tunisia) is a prolific petroleum-producing area with several oil and gas fields and it's the subject of significant discoveries. The area is affected by intense tectonic events and several stages of hydrothermal activities. During drilling at the MX area in NE of the Gabes Gulf, a magmatic intrusion has been encountered within a reservoir rock (i.e., Douleb Formation). The objectives of the present study are (1) to investigate the impact of the hydrothermal activities on this magmatic intrusion and (2) to characterize the potential modifications in its mineralogy and petro-physical properties that can affect the migration/accumulation of hydrocarbons. To do so we combine optical microscopy, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectrometry, micro-Raman spectroscopy, electron microprobe analyses, and petro-physical measurements.Our results show that the initial magmatic intrusion (dolerite) is composed mainly of plagioclase, clinopyroxene, and olivine. Based on petrographic observations, this intrusion subjected to several stages of hydrothermal fluid alterations that modified the primary texture and mineralogy. Albite, calcite, ankerite, dolomite, pyrite, quartz, anatase, kaolinite, and chlorite are the main secondary mineral products. During this alteration process, carbonates, pyrite, and quartz precipitated in vesicles, forming amygdules structure with enveloped zones indicating a fluctuation of fluid chemistry during their formation. In addition, petrographic observations indicate an interaction between feldspars exsolution and metasomatic processes in the studied area.The dissolution mechanisms, as well as the textural modifications in the dolerite induced by the hydrothermal activity, are underlined by the presence of spherulites, pores, and micropores. Such modifications significantly improved the porosity of the dolerite body. Conversely, the new mineralization that filled the fractures and micro-fractures, have reduced the inter-pore connections, and thus reduced its permeability. The present work demonstrates the important role of hydrothermal activity on the petro-physical properties of magmatic intrusions and how it could facilitate the migration/accumulation of hydrocarbons. Our results open the door for further investigations to check the potential presence of hydrocarbons within this doleritic intrusion.

Geochemical controls of the microbially mediated redox cycling of uranium and iron

As oxidation state is often a primary determinant of the mobility of uranium in subsurface environments, a comprehensive understanding of the redox cycling of uranium is essential to predict the fate of this contaminant. The potential of iron to serve as both a reductant and an oxidant of uranium suggests that remediation strategies which primarily rely on the reduction of the uranyl ion (U(VI)) to the poorly soluble uranous ion (U(IV)) to immobilize uranium in the solid phase may be either enhanced or hindered depending on the biogeochemical transformations of iron. To identify the geochemical controls of both uranium and iron redox cycling, batch incubations with the model metal-reducing bacterium Shewanella putrefaciens were conducted with either U(VI), ferrihydrite, or both as terminal electron acceptors, and concentrations of dissolved inorganic carbon (DIC), calcium, and silica were varied to alter aqueous uranyl speciation and secondary mineralization pathways of ferrihydrite. The presence of ferrihydrite increased the pseudo-first order rate constant of initial uranium removal from solution in nearly all conditions compared to uranium-only controls due to a combination of abiotic U(VI) reduction by Fe(II) and solid-phase association of uranium with secondary mineralization products of ferrihydrite. Following an initial period of uranium reduction, U(IV) was oxidized by ferrihydrite which led to a rebound in dissolved uranium. Once ferrihydrite was completely consumed by microbial respiration and secondary mineralization, uranium was again removed from solution via reductive precipitation and association with secondary mineralization products. DIC and calcium enhanced the oxidative dissolution of U(IV) solids but did not affect the redox potential of the U(VI)/U(IV) redox couple despite the formation of aqueous uranyl carbonate and calcium-uranyl carbonate species. Instead, removal of dissolved Fe(II) from solution and the accompanied shift in the redox potential of the Fe(III)/Fe(II) redox couple was responsible for the abrupt shift in iron acting as a reductant of U(VI) to an oxidant of U(IV). The removal of dissolved Fe(II) via siderite precipitation at the highest DIC concentration and in the presence of calcium enhanced the oxidation of U(IV), and the presence of silica limited the conversion of ferrihydrite to magnetite and sustained U(IV) oxidation. A kinetic model was developed which could reproduce incubation time series of uranium and iron speciation provided the thermodynamic favorability of abiotic uranium-iron redox cycling was accounted for. The modeling exercise revealed that the non-uraninite U(IV) solids formed in the incubations have a redox potential approximately 84 mV lower than that of amorphous uraninite, highlighting the need for a thermodynamic characterization of non-uraninite U(IV) solids. The results of this study identify dissolved Fe(II) concentration as the primary geochemical control of the role of iron in uranium redox cycling at moderate to high DIC concentrations and emphasize the importance of considering this parameter when designing site-specific, in situ bioremediation strategies.

Ultrasonic P- and S-Wave Attenuation and Petrophysical Properties of Deccan Flood Basalts, India, as Revealed by Borehole Studies

Petrophysical properties and ultrasonic P- and S-wave attenuation measurements on 35 Deccan basalt core specimens, recovered from Killari borehole site in western India, provide unique reference data-sets for a lesser studied Deccan Volcanic Province. These samples represent 338-m-thick basaltic column, consisting four lava flows each of Ambenali and Poladpur Formations, belonging to Wai Subgroup of the Deccan volcanic sequence. These basalt samples are found to be iron-rich (average FeOT: 13.4 wt%), but relatively poor in silica content (average SiO2: 47.8 wt%). The saturated massive basalt cores are characterized by a mean density of 2.91 g/cm3 (range 2.80–3.01 g/cm3) and mean P- and S-wave velocities of 5.89 km/s (range 5.01–6.50 km/s) and 3.43 km/s (range 2.84–3.69 km/s), respectively. In comparison, saturated vesicular basalt cores show a wide range in density (2.40–2.79 g/cm3) as well as P-wave (3.28–4.78 km/s) and S-wave (1.70–2.95 km/s) velocities. Based on the present study, the Deccan volcanic sequence can be assigned a weighted mean density of 2.74 g/cm3 and a low Vp and Vs of 5.00 and 3.00 km/s, respectively. Such low velocities in Deccan basalts can be attributed mainly to the presence of fine-grained glassy material, high iron contents, and hydrothermally altered secondary mineral products, besides higher porosity in vesicular samples. The measured Q values in saturated massive basalt cores vary enormously (Qp: 33–1960 and Qs: 35–506), while saturated vesicular basalt samples exhibit somewhat lesser variation in Qp (6–46) as well as Qs (5–49). In general, high-porosity rocks exhibit high attenuation, but we observed the high value of attenuation in some of the massive basalt core samples also. In such cases, energy loss is mainly due to the presence of fine-grained glassy material as well as secondary alteration products like chlorophaeite, that could contribute to intrinsic attenuation. Dominance of weekly bound secondary minerals might also be responsible for the generation of microcracks, which may generate squirt flow in saturated samples. Hence, we argue that the Deccan basalts attenuate seismic energy significantly, where its composition plays a major role.

Secondary Mineralization of Ferrihydrite Affects Microbial Methanogenesis in Geobacter-Methanosarcina Cocultures.

The transformation of ferrihydrite to stable iron oxides over time has important consequences for biogeochemical cycling of many metals and nutrients. The response of methanogenic activity to the presence of iron oxides depends on the type of iron mineral, but the effects of changes in iron mineralogy on methanogenesis have not been characterized. To address these issues, we constructed methanogenic cocultures of Geobacter and Methanosarcina strains with different ferrihydrite mineralization pathways. In this system, secondary mineralization products from ferrihydrite are regulated by the presence or absence of phosphate. In cultures producing magnetite as the secondary mineralization product, the rates of methanogenesis from acetate and ethanol increased by 30.2% and 135.3%, respectively, compared with a control lacking ferrihydrite. Biogenic magnetite was proposed to promote direct interspecies electron transfer between Geobacter and Methanosarcina in a manner similar to that of c-type cytochrome and thus facilitate methanogenesis. Vivianite biomineralization from ferrihydrite in the presence of phosphate did not significantly influence the methanogenesis processes. The correlation between magnetite occurrence and facilitated methanogenesis was supported by increased rates of methane production from acetate and ethanol with magnetite supplementation in the defined cocultures. Our data provide a new perspective on the important role of iron biomineralization in biogeochemical cycling of carbon in diverse anaerobic environments. It has been found that microbial methanogenesis is affected by the presence of iron minerals, and their influences on methanogenesis are associated with the mineralogical properties of the iron minerals. However, how changes in iron mineralogy affect microbial methanogenesis has not been characterized. To address this issue, we constructed methanogenic cocultures of Geobacter and Methanosarcina strains with different ferrihydrite mineralization pathways. The experimental results led to two contributions, i.e., (i) the transformation of iron minerals might exert an important influence on methanogenesis under anaerobic conditions and (ii) both biogenic and chemical magnetite can accelerate syntrophic ethanol oxidization between Geobacter metallireducens and Methanosarcina barkeri This study sheds new light on the important role of iron biomineralization in the biogeochemical cycling of carbon in diverse anaerobic environments, particularly in iron-rich natural and agricultural wetland soils.

Uranium Reduction by Fe(II) in the Presence of Montmorillonite and Nontronite.

Uranium(VI) interactions with three smectites (one montmorillonite and two nontronites - NAu1 and NAu2) were examined with 0, 1, and 2 mM aqueous concentrations of Fe(II) over the pH range of 3-9.5 in a background electrolyte of 100 mM NaCl and 1 mM CaCl2 in equilibration with 400 ppmv CO2(g) ([U(VI)] = 4 μM and 0.5 g smectite/L). In the absence of Fe(II), no differences were observed in the U(VI) sorption curves for the three clay minerals. In the presence of 1 or 2 mM Fe(II), under anoxic conditions, U(VI) uptake by the smectites changed slightly between ∼pH 3 and 6; however, uranium uptake increased significantly above ∼pH 6 and was proportional to the concentration of Fe(II) added to the system, particularly at pH values >8. The uptake of Fe(II) showed a sharp edge starting from ∼pH 6.5 with 95%-100% uptake occurring at pH values >7.5, with no difference observed between the iron-rich nontronites and montmorillonite. After 3 days of reaction at pH 7.6 (i.e., above the Fe(II) "sorption" edge), U(VI) was transformed to a mixture of U(IV) and U(VI) sorption complexes, and after 14 days of reaction, 100% of the U was found to be reduced to U(IV) in the form of nanocrystalline uraninite. In contrast, U remained as sorbed species until 14 days of reaction at pH 6.5. Ferrihydrite (NAu1), lepidocrocite, and magnetite (NAu2) were detected as secondary mineralization products upon reaction of the nontronites with Fe(II) but appeared to have no effect on the partitioning or speciation of uranium.

Riverine dissolved lithium isotopic signatures in low‐relief central Africa and their link to weathering regimes

AbstractThe isotopic composition of dissolved lithium (δ7Li) near the Congo River mouth varied from 14‰ to 22‰ in 2010 and was negatively correlated to discharge. From the relationship between dissolved δ7Li and strontium isotopes, we suggest that this large variation is due to mixing of waters from two contrasting continental weathering regimes. One end‐member (high δ7Li ≈ 25‰) represents waters sourced from active lateritic soils covering the periphery of the basin (Li highly sequestered into secondary mineral products) and another representing blackwater rivers (low δ7Li ≈ 5.7‰) derived from the swampy central depression where high organic matter content in water leads to congruent dissolution of the Tertiary sedimentary bedrock. This suggests that the lithium isotopic signature of tropical low‐relief surfaces is not unique and traces the long‐term, large‐scale vertical motions of the continental crust that control geomorphological settings. This evolution should be recorded in the oceanic secular δ7Li curve.

Clay mineral precipitation and low silica in glacier meltwaters explored through reaction-path modelling

AbstractThe subglacial chemical weathering environment is largely controlled by low temperatures and the presence of freshly comminuted minerals with a high surface area. These characteristics are believed to promote dissolution processes that give rise to low silica and high Ca2+fluxes emanating from glacierized basins. We test an alternative hypothesis, that mineral precipitation reactions in the subglacial environment play an equally important role in controlling the water chemistry in glacierized basins. We analyze borehole and proglacial water chemistry from a subarctic polythermal glacier, complemented by mineral XRD analysis of suspended sediment, till and bedrock samples. In conjunction with a thermodynamic analysis of the water and mineral chemistry, we use reaction-path modelling to study the chemical enrichment of water through the glacier system. We find that the high pH of the subglacial environment is conducive to secondary mineral precipitation, and that it is not possible to balance the water chemistry using dissolution reactions alone. We show that low silica can be explained by standard weathering reactions without having to invoke mineral-leaching reactions. Our results suggest that subglacial weathering intensity may be significantly underestimated if the production of secondary minerals is not considered.

The unique alkaline waters in the Chulym basin (West Siberia)

This report presents the results of studies of the ionic-saline and isotope composition of the unique, quite fresh, and high-alkaline waters (0.25–0.6 g/l salinity and pH 9.1–10.3) occurring within sedimentary rocks, as well as data on the calculations of equilibriums in the water-rock system. In view of these data, the concept was developed for the formation of fresh high-alkaline waters being not associated with igneous rocks. According to this concept, the water composition as such is formed at a definite stage of the development of the water-sedimentary rock system when all the chemical elements transferring to a solution may form secondary mineral products with no concentrating in the aqueous solution. The salinity of this latter does not increase, and yet the interaction with the rock is continued and, hence, the alkalinity increases.

Pyromorphite formation in a fungal biofilm community growing on lead metal

Lead is a priority pollutant, and lead metal is widely found in the environment as a waterproofing structural component in roofing, fence post covers, venting and flashing, as well as in industrial and urban waste. However, little is known of microbial interactions with metallic lead. The objective of this research was to investigate fungal roles in transformations of lead in a surface biofilm community growing on lead sheeting. The lead surface was found to support a diverse fungal community with several members, such as Aureobasidum pullulans, Phoma macrostoma, Penicillium sp. and Botryotinia fuckeliana, probably originating from adjacent phylloplane communities. Many fungal isolates showed tolerance to lead compounds in growth inhibition assays and were able to mediate production of lead-containing secondary minerals in the presence of metallic lead. These exhibited widely differing morphologies to the lead-containing secondary minerals produced under abiotic conditions. The presence of pyromorphite (Pb5 (PO4 )3 Cl) (approximately 50 wt%) was detected in the lead sheet biofilm, and we speculate that animal (bird) faeces could be a significant source of phosphorus in this location. Pyromorphite formation represents biomineralization of mobile lead species into a very stable form, and this research provides the first demonstration of its occurrence in the natural environment.

Bacterial communities in Fe/Mn films, sulphate crusts, and aluminium glazes from Swedish Lapland: implications for astrobiology on Mars

AbstractRock coatings have been observed on Mars by Mars Pathfinder, Viking and the Mars Exploration Rovers. Although rock varnish has been studied for its potential as a biosignature, other types of rock coating have been largely ignored. In Kärkevagge, Swedish Lapland, sulphate crusts, aluminium glazes and Fe/Mn films occur with mineralogies mimicking those observed on the surface of Mars. Molecular analysis and scanning electron microscopy (SEM) were used to investigate the bacterial communities associated with these rock coatings. Molecular techniques revealed differences in community structure and metabolisms associated with the production of secondary minerals between the three coating types. SEM analysis showed evidence of encrustation in mineral coatings in the Fe/Mn films and aluminium glazes, and evidence of abundant microbial communities in all three coating types. These observations provide evidence for bacterial participation in the genesis of rock coatings. For astrobiology on Mars, rock coatings are an attractive biosignature target scientifically and logistically: they are surface environments easily accessible by rovers, endoliths are afforded protection from surface conditions, and evidence of life could potentially be preserved through biomineralization and lithification. This study describes the bacterial communities from rock coatings compatible with martian mineralogy, explores the potential for biologically facilitated rock-coating formation, and supports rock coatings as targets of astrobiological interest on Mars.

Effects of Si-bearing minerals on the nature of secondary iron mineral products from lepidocrocite bioreduction

Lepidocrocite (γ-FeOOH) bioreduction by Shewanella putrefaciens was examined in systems containing three kinds of quartz silica sand with solid contents ranging from 5 to 70 wt.%. The results notably demonstrated that the formation routes of secondary iron minerals from microbial reduction of γ-FeOOH in silica mineral assemblages are dependent on quartz sand loading and particle size distribution. Biomineralization experiments with a concentration range of aqueous silica showed that a lower Si concentration (less than 2 mM) led to magnetite or magnetite/chukanovite, while higher concentrations resulted in the formation of green rust (GR). Dissolution of very fine quartz particles may have promoted the formation of biogenic GR instead of magnetite. This behavior could be explained by the sorption of dissolved silicate on the lepidocrocite surface, which might limit the bioreduction rate and promote GR formation. As observed with the chemically synthesized GR, the interactions of dissolved or colloidal Si with the lateral faces of the GR crystals could stabilize the GR structure, thus preventing its transformation into other products. Furthermore, complex physical and chemical distribution at the surfaces and within heteroaggregates of mixed mineral phases may affect the diffusion processes of species and therefore the biomineralization mechanism.

Effects of Oxyanions, Natural Organic Matter, and Bacterial Cell Numbers on the Bioreduction of Lepidocrocite (γ-FeOOH) and the Formation of Secondary Mineralization Products

Microbial reduction of Fe(III) oxides results in the production of Fe(II) and may lead to the subsequent formation of Fe(II)-bearing secondary mineralization products including magnetite, siderite, vivianite, chukanovite (ferrous hydroxy carbonate (FHC)), and green rust; however, the factors controlling the formation of specific Fe(II) phases are often not well-defined. This study examined effects of (i) a range of inorganic oxyanions (arsenate, borate, molybdate, phosphate, silicate, and tungstate), (ii) natural organic matter (citrate, oxalate, microbial extracellular polymeric substances [EPS], and humic substances), and (iii) the type and number of dissimilatory iron-reducing bacteria on the bioreduction of lepidocrocite and formation of Fe(II)-bearing secondary mineralization products. The bioreduction kinetics clustered into two distinct Fe(II) production profiles. "Fast" Fe(II) production kinetics [19-24 mM Fe(II) d(-1)] were accompanied by formation of magnetite and FHC in the unamended control and in systems amended with borate, oxalate, gellan EPS, or Pony Lake fulvic acid or having "low" cell numbers. Systems amended with arsenate, citrate, molybdate, phosphate, silicate, tungstate, EPS from Shewanella putrefaciens CN32, or humic substances derived from terrestrial plant material or with "high" cell numbers exhibited comparatively slow Fe(II) production kinetics [1.8-4.0 mM Fe(II) d(-1)] and the formation of green rust. The results are consistent with a conceptual model whereby competitive sorption of more strongly bound anions blocks access of bacterial cells and reduced electron-shuttling compounds to sites on the iron oxide surface, thereby limiting the rate of bioreduction.

Bacterial and iron oxide aggregates mediate secondary iron mineral formation: green rust versus magnetite

In the presence of methanoate as electron donor, Shewanella putrefaciens, a Gram-negative, facultative anaerobe, is able to transform lepidocrocite (gamma-FeOOH) to secondary Fe (II-III) minerals such as carbonated green rust (GR1) and magnetite. When bacterial cells were added to a gamma-FeOOH suspension, aggregates were produced consisting of both bacteria and gamma-FeOOH particles. Recently, we showed that the production of secondary minerals (GR1 vs. magnetite) was dependent on bacterial cell density and not only on iron reduction rates. Thus, gamma-FeOOH and S. putrefaciens aggregation pattern was suggested as the main mechanism driving mineralization. In this study, lepidocrocite bioreduction experiments, in the presence of anthraquinone disulfonate, were conducted by varying the [cell]/[lepidocrocite] ratio in order to determine whether different types of aggregate are formed, which may facilitate precipitation of GR1 as opposed to magnetite. Confocal laser scanning microscopy was used to analyze the relative cell surface area and lepidocrocite concentration within the aggregates and captured images were characterized by statistical methods for spatial data (i.e. variograms). These results suggest that the [cell]/[lepidocrocite] ratio influenced both the aggregate structure and the nature of the secondary iron mineral formed. Subsequently, a [cell]/[lepidocrocite] ratio above 1 x 10(7) cells mmol(-1) leads to densely packed aggregates and to the formation of GR1. Below this ratio, looser aggregates are formed and magnetite was systematically produced. The data presented in this study bring us closer to a more comprehensive understanding of the parameters governing the formation of minerals in dense bacterial suspensions and suggest that screening mineral-bacteria aggregate structure is critical to understanding (bio)mineralization pathways.