Electron tunneling properties of outer-membrane decaheme cytochromes from Shewanella oneidensis

We have characterized the outer-membrane decaheme cytochromes OmcA and MtrC from Shewanella oneidensis MR-1 at the single-molecule level using scanning tunneling microscopy (STM) and tunneling spectroscopy (TS). These cytochrome proteins are of great interest because they are thought to mediate bacterial electron transfer reactions in anoxic waters that control the reductive dissolution of oxide minerals. In our study, to characterize the electron transfer properties of these proteins on a model surface, the purified cytochromes were chemically immobilized as molecular monolayers on Au(111) substrates via a recombinant tetra-cysteine sequence as verified by X-ray photoelectron spectroscopy. Atomic force microscopy images confirm the monolayer films were ∼5-8 nm thick which is consistent with the apparent lateral dimensions of individual cytochrome molecules obtained with STM. Current-voltage TS of single cytochrome molecules revealed that OmcA and MtrC have different abilities to mediate tunneling current despite having otherwise very similar molecular and biochemical properties. These observations suggest that, based on their electron tunneling properties, the two cytochromes could have specific roles during bacterial metal reduction. Additionally, this study establishes single-molecule STM/TS as an effective means for revealing insights into biogeochemical redox processes in the environment.     

Bioreduction of hematite nanoparticles by the dissimilatory iron reducing bacterium Shewanella oneidensis MR-1

We examined the reduction of different size hematite (α-Fe₂O₃) nanoparticles (average diameter of 11, 12, 30, 43, and 99 nm) by the dissimilatory iron reducing bacteria (DIRB), Shewanella oneidensis MR-1, to determine how S. oneidensis MR-1 may utilize these environmentally relevant solid-phase electron acceptors. The surface-area-normalized-bacterial Fe(III) reduction rate for the larger nanoparticles (99 nm) was one order of magnitude higher than the rate observed for the smallest nanoparticles (11 nm). The Fe(III) reduction rates for the 12, 30, and 43 nm nanoparticles fell between these two extremes. Whole-cell TEM images showed that the mode of Fe₂O₃ nanoparticle attachment to bacterial cells was different for the aggregated, pseudo-hexagonal/irregular and platey 11, 12, and 99 nm nanoparticles compared to the non-aggregated 30 and 43 nm rhombohedral nanoparticles. Due to differences in aggregation, the 11, 12, and 99 nm nanoparticles exhibited less cell contact and less cell coverage than did the 30 and 43 nm nanoparticles. We hypothesize that S. oneidensis MR-1 employs both indirect and direct mechanisms of electron transfer to Fe(III)-oxide nanoparticles and that the bioreduction mechanisms employed and Fe(III) reduction rates depend on the nanoparticles’ aggregation state, size, shape and exposed crystal faces.     

Surface structure effects on direct reduction of iron oxides by Shewanella oneidensis

The atomic and electronic structure of mineral surfaces affects many environmentally important processes such as adsorption phenomena. They are however rarely considered relevant to dissimilatory bacterial reduction of iron and manganese minerals. In this regard, surface area and thermodynamics are more commonly considered. Here we take a first step towards understanding the nature of the influence of mineral surface structure upon the rate of electron transfer from Shewanella oneidensis strain MR-1 outer membrane proteins to the mineral surface and the subsequent effect upon cell “activity.” Cell accumulation has been used as a proxy for cell activity at three iron oxide single crystal faces; hematite (001), magnetite (111) and magnetite (100). Clear differences in cell accumulation at, and release from the surfaces are observed, with significantly more cells accumulating at hematite (001) compared to either magnetite face whilst relatively more cells are released into the overlying aqueous phase from the two magnetite faces than hematite. Modeling of the electron transfer process to the different mineral surfaces from a decaheme (protoporphyrin rings containing a central hexacoordinate iron atom), outer membrane-bound cytochrome of S. oneidensis has been accomplished by employing both Marcus and ab initio density functional theories. The resultant model of electron transfer to the three oxide faces predicts that over the entire range of expected electron transfer distances the highest electron transfer rates occur at the hematite (001) surface, mirroring the observed cell accumulation data. Electron transfer rates to either of the two magnetite surfaces are slower, with magnetite (111) slower than hematite (001) by approximately two orders of magnitude. A lack of knowledge regarding the structural details of the heme-mineral interface, especially in regards to atomic distances and relative orientations of hemes and surface iron atoms and the conformation of the protein envelope, precludes a more thorough analysis. However, the results of the modeling concur with the empirical observation that mineral surface structure has a clear influence on mineral surface-associated cell activity. Thus surface structure effects must be accounted for in future studies of cell-mineral interactions.     

Platinum-group mineral formation: Evidence of an interchange process from the entropy of activation values

Small grains of Ru–Os–Ir sulfides or alloys that formed in the early stages of the crystallization of basaltic magmas occur as inclusions in chromitites from many ophiolite complexes, Ural/Alaskan-type and layered intrusions. The nature of platinum-group elements is debated, i.e., whether they originally occurred in a metallic state or bonded with sulfur or other ligands. The application of kinetics and suggestions regarding the mechanisms of PGE formation may contribute to a better understanding of the genetic processes for platinum-group mineral (PGM) formation. The aim of the present study is to investigate the type of mechanism (associative, dissociative or interchange) and the possibility of direct formation of PGM from the reaction of free metals with sulfur, defining also the structure of possible intermediate species.The literature on the grain sizes (r) of platinum-group minerals (PGM) and their formation temperatures (range between 700 and 1100 °C) reveals an Arrhenius temperature dependence. The activation energy was estimated to be 450 ± 45 kJ mol⁻¹. Applying the Eyring equation to the same data, a linear relationship between 2.5ln(r) + ln(T) versus 1/T was obtained, leading to an estimation of the free energy of activation, ΔG^≠ = 440 ± 43 kJ mol⁻¹. The thermodynamic relation ΔG^≠ = ΔH^≠ − TΔS^≠, used together with the relation ΔΗ^≠ = E_act − RT leads to the calculation of ΔS^≠ at various temperatures. The estimated entropy of activation at various temperatures being almost zero, we suggest an interchange mechanism for PGM formation.The possibility of PGM formation from the direct reaction of free metals with sulfur would be inconsistent with the estimated entropy of activation values (ΔS^≠  0) at various temperatures. In the cases of PGMs of known r, the formation temperatures can be estimated, and then the free energy of activation obtained, applying 2.5ln(r) + ln(T) versus 1/T (straight line). From that value, the entropy of activation can be calculated [ΔS^≠ = −R + (E_act − ΔG^≠)/T], and an interchange mechanism is proposed. Furthermore, applying the relation of 2.5ln(r) + ln(T) versus 1/T on PGM grains of known r and T, an interchange mechanism for their formation can be suggested if they fit the above line.     

Reaction-based modeling of quinone-mediated bacterial iron(III) reduction

This paper presents and validates a new paradigm for modeling complex biogeochemical systems using a diagonalized reaction-based approach. The bioreduction kinetics of hematite (α-Fe₂O₃) by the dissimilatory metal-reducing bacterium (DMRB) Shewanella putrefaciens strain CN32 in the presence of the soluble electron shuttling compound anthraquinone-2,6-disulfonate (AQDS) is used for presentation/validation purposes. Experiments were conducted under nongrowth conditions with H₂ as the electron donor. In the presence of AQDS, both direct biological reduction and indirect chemical reduction of hematite by bioreduced anthrahydroquinone-2,6-disulfonate (AH₂DS) can produce Fe(II). Separate experiments were performed to describe the bioreduction of hematite, bioreduction of AQDS, chemical reduction of hematite by AH₂DS, Fe(II) sorption to hematite, and Fe(II) biosorption to DMRB. The independently determined rate parameters and equilibrium constants were then used to simulate the parallel kinetic reactions of Fe(II) production in the hematite-with-AQDS experiments. Previously determined rate formulations/parameters for the bioreduction of hematite and Fe(II) sorption to hematite were systematically tested by conducting experiments with different initial conditions. As a result, the rate formulation/parameter for hematite bioreduction was not modified, but the rate parameters for Fe(II) sorption to hematite were modified slightly. The hematite bioreduction rate formulation was first-order with respect to hematite ”free“ surface sites and zero-order with respect to DMRB based on experiments conducted with variable concentrations of hematite and DMRB. The AQDS bioreduction rate formulation was first-order with respect to AQDS and first-order with respect to DMRB based on experiments conducted with variable concentrations of AQDS and DMRB. The chemical reduction of hematite by AH₂DS was fast and considered to be an equilibrium reaction. The simulations of hematite-with-AQDS experiments were very sensitive to the equilibrium constant for the hematite-AH₂DS reaction. The model simulated the hematite-with-AQDS experiments well if it was assumed that the ferric oxide “surface” phase was more disordered than pure hematite. This is the first reported study where a diagonalized reaction-based model was used to simulate parallel kinetic reactions based on rate formulations/parameters independently obtained from segregated experiments.     

A comparative study on the dissolution and solubility of hydroxylapatite and fluorapatite at 25 °C and 45 °C

Dissolution of the synthetic hydroxylapatite (HAP) and fluorapatite (FAP) in pure water was studied at 25 °C and 45 °C in a series of batch experiments. The XRD, FT-IR and SEM analyses indicated that the synthetic, microcrystalline HAP and FAP with apatite structure used in the experiments were found to have no obvious variation after dissolution except that the existence of OH groups in FT-IR spectra for FAP after 2880 h dissolution was observed. During the HAP dissolution (0–4320 h), the aqueous calcium and phosphate concentrations reached the maxima after 120 h and then decreased slowly with time. For the FAP dissolution in pure water, after a transient time of 1440 h (< 60 d), element concentrations and pH became constant suggesting attainment of a steady-state between the solution and solid. During early stages of the FAP dissolution reaction (< 72–120 h), mineral components were released in non-stoichiometric ratios with reacted solution ratios of dissolved Ca:P, Ca:F and P:F being lower than mineral stoichiometric ratios of Ca₅(PO₄)₃F, i.e., 1.67, 5.0 and 3.0, respectively. This indicated that F were preferentially released compared to Ca from the mineral structure. The mean K_sp values were calculated by using PHREEQC for HAP of 10^− 53.28 (10^− 53.02–10^− 53.51) and for FAP of 10^− 55.71 (10^− 55.18–10^− 56.13) at 25 °C, the free energies of formation ΔG_f^o[HAP] and ΔG_f^o[FAP] were calculated to be − 6282.82 kJ/mol and − 6415.87 kJ/mol, respectively.     

Study on the kinetics of iron oxide leaching by oxalic acid

The presence of iron oxides in clay or silica raw materials is detrimental to the manufacturing of high quality ceramics. Although iron has been traditionally removed by physical mineral processing, acid washing has been tested as it is more effective, especially for extremely low iron (of less than 0.1% w/w). However, inorganic acids such as sulphuric or hydrochloric acids easily contaminate the clay products with SO₄²⁻ and Cl⁻, and therefore should be avoided as much as possible. On the other hand, if oxalic acid is used, any acid left behind will be destroyed during the firing of the ceramic products. The characteristics of dissolution of iron oxides were therefore investigated in this study.The dissolution of iron oxides in oxalic acid was found to be very slow at temperatures within the range 25–60 °C, but its rate increases rapidly above 90 °C. The dissolution rate also increases with increasing oxalate concentration at the constant pH values set within the optimum range of pH2.5–3.0. At this optimum pH, the dissolution of fine pure hematite (Fe₂O₃) (105–140 μm) follows a diffusion-controlled shrinking core model. The rate expression expressed as 1 − (2 / 3)x − (1 − x)^2 / 3 where x is a fraction of iron dissolution was found to be proportional to [oxalate]^1.5.The addition of magnetite to the leach liquor at 10% w/w hematite was found to enhance the dissolution rate dramatically. Such addition of magnetite allows coarser hematite in the range 0.5–1.4 mm to be leached at a reasonable rate.     

Controls on the isotopic composition of surface water and precipitation in the Northern Andes, Colombian Eastern Cordillera

Empirical datasets provide the constraints on the variability and causes of variability in stable isotope compositions (δD or δ¹⁸O) of surface water and precipitation that are essential not only for models of modern and past climate but also for investigations of paleoelevation. This study presents stable isotope data for 76 samples from four elevation transects and three IAEA GNIP stations in the Eastern Cordillera of Colombia and the northern Andean foreland. These data are largely consistent with theories of stable isotope variability developed based on a global dataset. On a monthly basis, the precipitation-amount effect exerts the dominant control on δD_p and δ¹⁸O_p values at the IAEA GNIP stations. At the Bogotá station (2547 m), the δD_p and δ¹⁸O_p values vary seasonally, with isotopic minima correlating with maxima in precipitation-amount. Although surface water samples from Eastern Cordilleran streams and rivers fall on the Global Meteoric Water Line, samples from three of four lakes (2842–3459 m) have evaporatively elevated δD_sw and δ¹⁸O_sw values. The IAEA GNIP station data averaged over multiple years, combined with stream and river water data, define vertical lapse rates of −1.8‰ km⁻¹ for Δδ¹⁸O and −14.6‰ km⁻¹ for ΔδD, and are a close fit to a common thermodynamically based Rayleigh distillation model. Elevation uncertainties for these relationships are also evaluated. Comparison of this Colombian dataset with the elevation uncertainties generated by the thermodynamically based model shows that the model underestimates uncertainty at high Δδ¹⁸O and ΔδD values while overestimating it for low Δδ¹⁸O and ΔδD values. This study presents an independent, empirical assessment of stable isotope-based elevation uncertainties for the northern Andes based on a dataset of sufficient size to ensure statistical integrity. These vertical lapse rates and associated uncertainties form the basis for stable isotope paleoelevation studies in the northern Andes.     

Mercury in itabirite-hosted soft hematite ore in the Quadrilátero Ferrífero of Minas Gerais

Mercury contents in Precambrian banded iron formation-hosted hematite ores are virtually unknown. In an attempt to provide information on the abundance and distribution of Hg in Fe ore, we present analyses for Hg in samples of high-grade soft hematite ore from Gongo Soco, Minas Gerais, Brazil. Bulk samples contain from <  5 to 25  ppb Hg without obvious correlation with major elements. Granulometric fractions of follow-up samples have amounts of Hg from 6 to 48  ppb and display positive linear correlations with total Mn as MnO (r = 0.87), LOI (r = 0.87) and SiO₂ (r = 0.76), as well as a negative linear correlation with total Fe as Fe₂O₃ (r = −  0.87). The correlations suggest that Hg is associated with a hydrated ferruginous groundmass bearing residual Mn, Al and Si, which replaced gangue minerals in itabirite in the process of formation of the Gongo Soco soft hematite ore.     

Microbial reduction of iron(III)-rich nontronite and uranium(VI)

To assess the dynamics of microbially mediated U-clay redox reactions, we examined the reduction of iron(III)-rich nontronite NAu-2 and uranium(VI) by Shewanella oneidensis MR-1. Bioreduction experiments were conducted with combinations and varied concentrations of MR-1, nontronite, U(VI) and the electron shuttle anthraquinone-2,6-disulfonate (AQDS). Abiotic experiments were conducted to quantify U(VI) sorption to NAu-2, the reduction of U(VI) by chemically-reduced nontronite-Fe(II), and the oxidation of uraninite, U^(IV)O_2(s), by nontronite-Fe(III). When we incubated S. oneidensis MR-1 at lower concentration (0.5 × 10⁸ cell mL⁻¹) with nontronite (5.0 g L⁻¹) and U(VI) (1.0 mM), little U(VI) reduction occurred compared to nontronite-free incubations, despite the production of abundant Fe(II). The addition of AQDS to U(VI)- and nontronite-containing incubations enhanced both U(VI) and nontronite-Fe(III) reduction. While U(VI) was completely reduced by S. oneidensis MR-1 at higher concentration (1.0 × 10⁸ cell mL⁻¹) in the presence of nontronite, increasing concentrations of nontronite led to progressively slower rates of U(VI) reduction. U(VI) enhanced nontronite-Fe(III) reduction and uraninite was oxidized by nontronite-Fe(III), demonstrating that U served as an effective electron shuttle from S. oneidensis MR-1 to nontronite-Fe(III). The electron-shuttling activity of U can explain the lack or delay of U(VI) reduction observed in the bulk solution. Little U(VI) reduction was observed in incubations that contained chemically-reduced nontronite-Fe(II), suggesting that biologic U(VI) reduction drove U valence cycling in these systems. Under the conditions used in these experiments, we demonstrate that iron-rich smectite may inhibit or delay U(VI) bioreduction.     

Fluid flow in the Duperow and Winnepegosis Formations, Williston Basin, Canada: Evidence from preliminary magnetic measurements

The Devonian Winnepegosis and Duperow Formations were examined in well 4-27-11-22W1, located in at the eastern edge of the Williston Basin in Manitoba. The variation in characteristic remanent magnetization (ChRM) direction and magnetic mineral carrier is obvious: the older Winnepegosis Formation has a primary or early post-depositional magnetization held in magnetite or pyrrhotite (n = 15; D = 324.1°, I = − 27.3°, α₉₅ = 10.4°, k = 15.7), whereas the younger Duperow Formation magnetizations are carried by hematite and could be as late as Early Jurassic. The variability may be attributable to the intervening Prairie Evaporite acting as an aquitard to fluid migration.     

Iron isotopes in acid mine waters and iron-rich solids from the Tinto–Odiel Basin (Iberian Pyrite Belt, Southwest Spain)

The isotopic composition of Fe was determined in water, Fe-oxides and sulfides from the Tinto and Odiel Basins (South West Spain). As a consequence of sulfide oxidation in mine tailings both rivers are acidic (1.45 < pH < 3.85) and display high concentrations of dissolved Fe (up to 420 mmol l^− 1) and sulphates (up to 1190 mmol l^− 1).The δ⁵⁶Fe of pyrite-rich samples from the Rio Tinto and from the Tharsis mine ranged from − 0.56 ± 0.08‰ to + 0.25 ± 0.1‰. δ⁵⁶Fe values for Fe-oxides precipitates that currently form in the riverbed varied from − 1.98 ± 0.10‰ to 1.57 ± 0.08‰. Comparatively narrower ranges of values (− 0.18 ± 0.08‰ and + 0.21 ± 0.14‰) were observed in their fossil analogues from the Pliocene–Pleistocene and in samples from the Gossan (the oxidized layer that formed through exposure to oxygen of the massive sulfide deposits) (− 0.36 ± 0.12‰ to 0.82 ± 0.07‰). In water, δ⁵⁶Fe values ranged from − 1.76 ± 0.10‰ to + 0.43 ± 0.05‰.At the source of the Tinto River, fractionation between aqueous Fe(III) and pyrite from the tailings was less than would be expected from a simple pyrite oxidation process. Similarly, the isotopic composition of Gossan oxides and that of pyrite was different from what would be expected from pyrite oxidation. In rivers, the precipitation of Fe-oxides (mainly jarosite and schwertmannite and lesser amounts of goethite) from water containing mainly (more than 99%) Fe(III) with concentrations up to 372 mmol l^− 1 causes variable fractionation between the solid and the aqueous phase (− 0.98‰ < Δ⁵⁶Fe_{solid–water} < 2.25‰). The significant magnitude of the positive fractionation factor observed in several Fe(III) dominated water may be related to the precipitation of Fe(III) sulphates containing phases.     

Magnetic anisotropy of calcite at room-temperature

The intrinsic room temperature magnetic properties of pure calcite were determined from a series of natural crystals, and they were found to be highly dependent on the chemical composition. In general, dia-, para-, and ferromagnetic components contribute to the magnetic susceptibility and the anisotropy of magnetic susceptibility (AMS). With a combination of magnetic measurements and chemical analysis these three contributions were determined and related to their mineralogical sources. The intrinsic diamagnetic susceptibility of pure calcite is − 4.46 ± 0.16 × 10^− 9 m³/kg (− 12.09 ± 0.5 × 10^− 6 SI) and the susceptibility difference is 4.06 ± 0.03 × 10^− 10 m³/kg (1.10 ± 0.01 × 10^− 6 SI). These diamagnetic properties are easily dominated by other components. The paramagnetic contribution is due to paramagnetic ions in the crystal lattice that substitute for calcium; these are mainly iron and manganese. The measured paramagnetic susceptibility agrees with the values calculated from the known concentration of paramagnetic ions in the crystals according to the Curie law of paramagnetic susceptibility. Substituted iron leads to an increase in the AMS. The paramagnetic susceptibility difference was found to correlate linearly with the iron content for concentrations between 500 and 10,000 ppm. An empirical relation was determined: (k₁ − k₃)^para (kg/m³) = Fe-content (ppm) × (1 ± 0.1) × 10^− 12 (kg/m³/ppm). The maximum susceptibility difference (Δk = k₁ − k₃) was found to be unaffected by iron contents below 100 ppm. Ferromagnetic contributions due to inclusions of ferromagnetic minerals can dominate the susceptibility. They were detected by acquisition of isothermal remanent magnetization (IRM) and their contribution to the AMS was separated by high-field measurements.     

Glide systems of hematite single crystals in deformation experiments

The critical resolved shear stresses (CRSSs) of hematite crystals were determined in compression tests for r-twinning, c-twinning and {a}<m>-slip in the temperature range 25 °C to 400 °C, at 400 MPa confining pressure, and a strain rate of 10^− 5 s^− 1 by Hennig-Michaeli, Ch., Siemes, H., 1982. Experimental deformation of hematile crstals betwen 25 °C and 400 °C at 400 MPa confining pressure. In: Schreyer, W. (Ed.) High Pressure Research in Geoscience, Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, p. 133–150. In the present contribution newly performed experiments on hematite single crystals at temperatures up to 800 °C at strain rates of 10^− 5 s^− 1 and 300 MPa confining pressure extends the knowledge about the CRSS of twin and slip modes. Optical observations, neutron diffraction goniometry, SEM forescatter electron images and electron backscatter diffraction are applied in order to identify the glide modes. Both twinning systems and {a}<m>-slip were confirmed by these methods. Besides the known glide systems the existence of the (c)<a>-slip system could be stated. Mechanical data establish that the CRSS of r-twinning decreases from 140 MPa at 25 °C to  5 MPa at 800 °C and for {a}<m>-slip from > 560 MPa at 25 °C to  40 MPa at 700 °C. At room temperature the CRSS for c-twinning is around 90 MPa and at 600 °C  60 MPa. The data indicate that the CRSSs above 200 °C seem to be between the values for r-twinning and {a}<m>-slip. For (c)<a>-slip only the CRSS at 600 °C could be evaluated to  60 MPa. Exact values are difficult to determine because other glide systems are always simultaneously activated.     

Bioreduction of natural specular hematite under flow conditions

Dissimilatory reduction of Fe(III) by Shewanella oneidensis MR-1 was evaluated using natural specular hematite as sole electron acceptor in an open system under dynamic flow conditions to obtain a better understanding of biologic Fe(III) reduction in the natural environment. During initial exposure to hematite under advective flow conditions, cells exhibited a transient association with the mineral characterized by a rapid rate of attachment followed by a comparable rate of detachment before entering a phase of surface colonization that was slower but steadier than that observed initially. Accumulation of cells on the hematite surface was accompanied by the release of soluble Fe(II) into the aqueous phase when no precautions were taken to remove amorphous Fe(III) from the mineral surface before colonization. During the period of surface colonization following the detachment phase, cell yield was estimated at 1.5-4 × 10⁷ cells/μmol Fe(II) produced, which is similar to that reported in studies conducted in closed systems. This yield does not take into account those cells that detached during this phase or the Fe(II) that remained associated with the hematite surface. Hematite reduction by the bacterium led to localized surface pitting and localized discrete areas where Fe (II) precipitation occurred. The cleavage plane of hematite left behind after bacterial reduction, as revealed by our results, strongly suggests, that heterogeneous energetics of the mineral surface play a strong role in this bioprocess. AQDS, an electron shuttle shown to stimulate bioreduction of Fe(III) in other studies, inhibited reduction of hematite by this bacterium under the dynamic flow conditions employed in the current study.     

Microbial reduction of chromium from the hexavalent to divalent state

We demonstrate that Shewanella oneidensis, a metal-reducing bacteria species with cytoplasmic-membrane-bound reductases and remarkably diverse respiratory capabilities, reduced Cr(VI) to Cr(II) in anaerobic cultures where chromate was the sole terminal electron acceptor. Individual cell microanalysis by transmission electron microscopy (TEM) using electron energy-loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDXS) demonstrates Cr(II) concentrated near the cytoplasmic membrane, suggesting the terminal reduction pathway is intracellularly localized. Further, estimated cellular Cr(II) concentrations are relatively high at upwards of 0.03-0.09 g Cr/g bacterium. Accumulation of Cr(II) is observed in S. oneidensis cells prior to the formation of submicron-sized precipitates of insoluble Cr(III) on their surfaces. Furthermore, under anaerobic conditions, Cr(III) precipitates that encrust cells are shown to contain Cr(II) that is likely bound in the net negatively charged extracellular biopolymers which can permeate the surfaces of the precipitates. In otherwise nearly identical incubations, Cr(III) precipitate formation was observed in cultures maintained anaerobic with bubbled nitrogen but not in three replicate cultures in an anaerobic chamber.     

Microbial remains in some earliest Earth rocks: Comparison with a potential modern analogue

Carbonaceous matter (CM) from ca. 3.5 Ga hydrothermal black cherts of the Pilbara Craton of Western Australia and the Barberton Greenstone Belt of South Africa yielded transmission electron microscopy (TEM) images that are suggestive of microbial remains and possible remnants of microbial cell walls. These are compared to a potential modern analogue, the hyperthermophilic Methanocaldococcus jannaschii, derived from an active seafloor hydrothermal environment and cultured under similar conditions. A striking resemblance to the early Archaean forms was evident in wall structure and thermal degradation mode. Cell disintegration of the cultures occurred at 100 °C marking the limits of life. Complete disintegration, deformation and shrinkage occurred at 132 °C. A multidisciplinary approach to the characterisation of the CM was undertaken using organic petrology, TEM coupled with electron dispersive spectral analysis (EDS), high resolution TEM (HRTEM) to determine molecular ordering, and elemental and carbon isotope geochemistry. Reflectance measurements of the CM to determine thermal stress yielded a range of values corresponding to several populations, and pointing to different sources and processes. The δ¹³C values of Dresser Formation CM (−36.5 to −32.1‰) are negatively correlated with TOC (0.13–0.75%) and positively correlated with C/N ratio (134–569), which is interpreted to reflect the relative abundance of high R_o/oxidised/recycled CM and preferential loss of ¹²C and N during thermal maturation. TEM observations, inferred carbon isotopic heterogeneity and isotope fractionations of −27 to −32‰ are consistent with the activity of chemosynthetic microbes in a seafloor hydrothermal system where rapid silicification at relatively low temperature preserved the CM.     

The origin of dolomite associated with salt diapirs in central Tunisia: Preliminary investigations of field relationships and geochemistry

Black and white dolomite crystals (mm to cm width) of different isotopic composition are associated with Triassic diapirism in central Tunisia, as well as with evaporite minerals and clays. The white dolomites occur mostly in the Jabal Hadifa diapir near the contact with Cretaceous limestones, whereas the smaller black dolomites occur in the Jabal Hamra diapir. The former dolomite has a narrow range of δ¹⁸O and δ¹³C values (− 3.83‰ to − 6.60‰ VPDB for δ¹⁸O; − 2.11‰ to − 2.83‰ VPDB for δ¹³C), whereas the latter dolomite has a wider range and more depleted values (− 4.92‰ to − 9.97‰ for δ¹⁸O; − 0.55‰ to − 6.08‰ for δ¹³C). However, the ⁸⁷Sr / ⁸⁶Sr ratios of most of the samples are near Triassic seawater values. Dolomite formation is due to at least two different fluids. The main fluid originated from deeper hydrothermal or basinal sources related to the Triassic saliferous rocks and ascended through faults during the diapiric intrusion. The second, less important fluid source is related to meteoric water originating from Cretaceous rocks.     

Total mercury and monomethylmercury in water, sediments, and hydrophytes from the rivers, estuary, and bay along the Bohai Sea coast, northeastern China

Mercury contamination in aquatic environments is of worldwide concern because of its high biomagnification factor in food chains and long-range transport. The rivers, estuary and the bay along the northwestern Bohai Sea coast, northeastern China have been heavily contaminated by Hg due to long-term Zn smelting and chlor-alkali production. This work investigated the distributions of total Hg (THg) and monomethylmercury (MMHg) in the water, sediment and hydrophytes from this area. Concentrations of THg in sediment (0.5–64 mg kg⁻¹) and water (39–2700 ng L⁻¹) were elevated by 1–3 orders of magnitude compared to background concentrations, which induced high concentrations of MMHg in these media. The highest concentration of MMHg in sediment reached 35 μg kg⁻¹, which was comparable to that in the Hg mining area, Wanshan, China, and the highest MMHg concentration of 3.0 ng L⁻¹ in the water sample exceeded the MMHg Chinese drinking water guideline of 1.0 ng L⁻¹. Concentrations of THg in a sediment profile from Jinzhou Bay were found to be consistent with annual Hg emission flux from a local Zn smelter (r = 0.74, p < 0.01), indicating that Hg contamination was mainly caused by Zn smelting locally. For some freshwater hydrophytes, concentrations of THg and MMHg ranged from 5.2 to 100 μg kg⁻¹ and 0.15 to 12 μg kg⁻¹, respectively. Compared to sediment, concentrations of THg in hydrophytes were 2–3 orders of magnitude lower but MMHg was comparable or higher, indicating that the bioaccumulation in plants was distinct for the two Hg species studied. The data suggest that a significant load of Hg has been released into the northwestern coastal region of the Bohai Sea.     

Zero-valent iron and organic carbon mixtures for remediation of acid mine drainage: Batch experiments

A series of laboratory batch experiments was conducted to evaluate the potential for treatment of acid mine drainage (AMD) using organic C (OC) mixtures amended by zero-valent Fe (Fe⁰). Modest increases in SO₄ reduction rates (SRRs) of up to 15% were achieved by augmenting OC materials with 5 and 10 dry wt% Fe⁰. However, OC was essential for supporting SO₄ reducing bacteria (SRB) and therefore SO₄ reduction. This observation suggests a general absence of autotrophic SRB which can utilize H₂ as an electron donor. Sulfate reduction rates (SRRs), calculated using a mass-based approach, ranged from −12.9 to −14.9 nmol L⁻¹ d⁻¹  g⁻¹ OC. Elevated populations of SRB, iron reducing bacteria (IRB), and acid producing (fermentative) bacteria (APB) were present in all mixtures containing OC. Effective removal of Fe (91.6–97.6%), Zn (>99.9%), Cd (>99.9%), Ni (>99.9%), Co (>99.9%), and Pb (>95%) was observed in all reactive mixtures containing OC. Abiotic metal removal was achieved with Fe⁰ only, however Fe, Co and Mn removal was less effective in the absence of OC. Secondary disordered mackinawite [Fe_1+xS] was observed in field-emission scanning electron microscopy (FE-SEM) backscatter electron micrographs of mixtures that generated SO₄ reduction. Energy dispersive X-ray (EDX) spectroscopy revealed that Fe–S precipitates were Fe-rich for mixtures containing OC and Fe⁰, and S-rich in the absence of Fe⁰ amendment. Sulfur K-edges determined by synchrotron-radiation based bulk X-ray absorption near-edge structure (XANES) spectroscopy indicate solid-phase S was in a reduced form in all mixtures containing OC. Pre-edge peaks on XANES spectra suggest tetragonal S coordination, which is consistent with the presence of an Fe–S phase such as mackinawite. The addition of Fe⁰ enhanced AMD remediation over the duration of these experiments, however long-term evaluation is required to identify optimal Fe⁰ and OC mixtures.