Chemical reactivity of microbe and mineral surfaces in hydrous ferric oxide depositing hydrothermal springs

The hot springs in Yellowstone National Park, USA, provide concentrated microbial biomass and associated mineral crusts from which surface functional group (FG) concentrations and pK_a distributions can be determined. To evaluate the importance of substratum surface reactivity for solute adsorption in a natural setting, samples of iron‐rich sediment were collected from three different springs; two of the springs were acid‐sulfate‐chloride (ASC) in composition, while the third was neutral‐chloride (NC). At one of the ASC springs, mats of S^º‐rich Hydrogenobaculum‐like streamers and green Cyanidia algae were also collected for comparison to the sediment. All samples were then titrated over a pH range of 3–11, and comparisons were made between the overall FG availability and the concentration of solutes bound to the samples under natural conditions. Sediments from ASC springs were composed of hydrous ferric oxides (HFO) that displayed surface FGs typical of synthetic HFO, while sediments from the NC spring were characterized by a lower functional group density, reflected by decreased excess charge over the titration range (i.e., lower surface reactivity). The latter also showed a lower apparent point of zero charge (PZC), likely due the presence of silica (up to 78 wt. %) in association with HFO. Variations in the overall HFO surface charge are manifest in the quantities and types of solutes complexed; the NC sediments bound more cations, while the ASC sediments retained significantly more arsenic, presumably in the form of arsenate (H₂AsO₄^?). When the microbial biomass samples were analyzed, FG concentrations summed over the titratable range were found to be an order of magnitude lower for the Sº‐rich mats, relative to the algal and HFO samples that displayed similar FG concentrations on a dry weight basis. A diffuse‐layer surface complexation model was employed to further illustrate the importance of surface chemical parameters on adsorption reactions in complex natural systems.     

Planktonic marine iron oxidizers drive iron mineralization under low‐oxygen conditions

Observations of modern microbes have led to several hypotheses on how microbes precipitated the extensive iron formations in the geologic record, but we have yet to resolve the exact microbial contributions. An initial hypothesis was that cyanobacteria produced oxygen which oxidized iron abiotically; however, in modern environments such as microbial mats, where Fe(II) and O₂ coexist, we commonly find microaerophilic chemolithotrophic iron‐oxidizing bacteria producing Fe(III) oxyhydroxides. This suggests that such iron oxidizers could have inhabited niches in ancient coastal oceans where Fe(II) and O₂ coexisted, and therefore contributed to banded iron formations (BIFs) and other ferruginous deposits. However, there is currently little evidence for planktonic marine iron oxidizers in modern analogs. Here, we demonstrate successful cultivation of planktonic microaerophilic iron‐oxidizing Zetaproteobacteria from the Chesapeake Bay during seasonal stratification. Iron oxidizers were associated with low oxygen concentrations and active iron redox cycling in the oxic–anoxic transition zone (<3 μm O₂, <0.2 μm H₂S). While cyanobacteria were also detected in this transition zone, oxygen concentrations were too low to support significant rates of abiotic iron oxidation. Cyanobacteria may be providing oxygen for microaerophilic iron oxidation through a symbiotic relationship; at high Fe(II) levels, cyanobacteria would gain protection against Fe(II) toxicity. A Zetaproteobacteria isolate from this site oxidized iron at rates sufficient to account for deposition of geologic iron formations. In sum, our results suggest that once oxygenic photosynthesis evolved, microaerophilic chemolithotrophic iron oxidizers were likely important drivers of iron mineralization in ancient oceans.     

Dependence of In Situ Bacterial Fe(II)-Oxidation and Fe(III)-Precipitation on Sequential Reactive Transport

A study was conducted to determine in situ rates of Fe(II) oxidation and Fe(III) precipitation along a 5.0 m reach of a ferruginous groundwater discharge zone under two distinct conditions; (i) the natural state featuring abundant flocculent mats of bacteriogenic iron oxides (BIOS) produced by Fe(II)-oxidizing bacteria, and (ii) after a manual washout of the streambed to remove the microbial mat. Examination of mat samples by differential interference contrast light microscopy revealed tangled meshworks of filamentous Leptothrix sheaths and helical Gallionella stalks intermixed with fine-grained hydrous ferric oxide (HFO) precipitates. The greatest accumulation of BIOS mat was 1.0 m downstream of the groundwater spring. Redox potential (Eh) increased sharply from 200 mV to over 300 mV over the last 2.0 m of the reach. Similarly, dissolved oxygen increased from < 10% saturation to almost 100% saturation over the last 2.0 m of the reach, whereas pH increased from 6.4 to 7.3. Pseudo-first-order rate constants determined on the basis of analytical solutions to sequential partial differential advection-dispersion-reaction equations for the linear Fe(II)→Fe(III)→HFO reaction network yielded in situ Fe(II) oxidation rate constants (k_ox) of 1.70 ± 0.20 min^?1 in natural conditions and 0.48 ± 0.14 min^?1 after washout. Corresponding Fe(III)-precipitation rates (k_p) before and after washout were 3.45 ± 0.10 min^?1 and 0.90 ± 0.01 min^?1, respectively. These values for k_ox and k_p are higher than estimates obtained from closed batch microcosm and laboratory experiments, underscoring the crucial dependence of in situ Fe(II) oxidation and Fe(III) precipitation rates on advective and dispersive mass transport. The results also highlight the influence that BIOS microbial mats exert on the reaction kinetics of the multiple heterogeneous reactions contributing not only to Fe(II)/Fe(III) redox transformations in groundwater discharge zones, but also the precipitation of HFO.     

Energetics of chemolithoautotrophy in the hydrothermal system of Vulcano Island, southern Italy

The hydrothermal system at Vulcano, Aeolian Islands (Italy), is home to a wide variety of thermophilic, chemolithoautotrophic archaea and bacteria. As observed in laboratory growth studies, these organisms may use an array of terminal electron acceptors (TEAs), including O₂, , Fe(III), , elemental sulphur and CO₂; electron donors include H₂, , Fe²⁺, H₂S and CH₄. Concentrations of inorganic aqueous species and gases were measured in 10 hydrothermal fluids from seeps, wells and vents on Vulcano. These data were combined with standard Gibbs free energies () to calculate overall Gibbs free energies (ΔG_r) of 90 redox reactions that involve 16 inorganic N‐, S‐, C‐, Fe‐, H‐ and O‐bearing compounds. It is shown that oxidation reactions with O₂ as the TEA release significantly more energy (normalized per electron transferred) than most anaerobic oxidation reactions, but the energy yield is comparable or even higher for several reactions in which , or Fe(III) serves as the TEA. For example, the oxidation of CH₄ to CO₂ coupled to the reduction of Fe(III) in magnetite to Fe²⁺ releases between 94 and 123 kJ/mol e^?, depending on the site. By comparison, the aerobic oxidation of H₂ or reduced inorganic N‐, S‐, C‐ and Fe‐bearing compounds generally yields between 70 and 100 kJ/mol e^?. It is further shown that the energy yield from the reduction of elemental sulphur to H₂S is relatively low (8–19 kJ/mol e^?) despite being a very common metabolism among thermophiles. In addition, for many of the 90 reactions evaluated at each of the 10 sites, values of ΔG_r tend to cluster with differences < 20 kJ/mol e^?. However, large differences in ΔG_r (up to ～ 60 kJ/mol e^?) are observed in Fe redox reactions, due largely to considerable variations in Fe²⁺, H⁺ and H₂ concentrations. In fact, at the sites investigated, most variations in ΔG_r arise from differences in composition and not in temperature.     

Carbon and arsenic metabolism in Thiomonas strains: differences revealed diverse adaptation processes

Background  

Thiomonas strains are ubiquitous in arsenic-contaminated environments. Differences between Thiomonas strains in the way they have adapted and respond to arsenic have never been studied in detail. For this purpose, five Thiomonas strains, that are interesting in terms of arsenic metabolism were selected: T. arsenivorans, Thiomonas spp. WJ68 and 3As are able to oxidise As(III), while Thiomonas sp. Ynys1 and T. perometabolis are not. Moreover, T. arsenivorans and 3As present interesting physiological traits, in particular that these strains are able to use As(III) as an electron donor.     

Temperature dependence of Fe(III) and sulfate reduction rates and its effect on growth and composition of bacterial enrichments from an acidic pit lake neutralization experiment

Microbial Fe(III) and sulfate reduction are important electron transport processes in acidic pit lakes and stimulation by the addition of organic substrates is a strategy to remove acidity, iron and sulfate. This principle was applied in a pilot‐scale enclosure in pit lake 111 (Brandenburg, Germany). Because seasonal and spatial variation of temperature may affect the performance of in situ experiments considerably, the influence of temperature on Fe(III) and sulfate reduction was investigated in surface sediments from the enclosure in the range of 4–28 °C. Potential Fe(III) reduction and sulfate reduction rates increased exponentially with temperature, and the effect was quantified in terms of the apparent activation energy E_a measuring 42–46 kJ mol^?1 and 52 kJ mol^?1, respectively. Relatively high respiration rates at 4 °C and relatively low Q₁₀ values (～2) indicated that microbial communities were well adapted to low temperatures. In order to evaluate the effect of temperature on growth and enrichment of iron and sulfate‐reducing bacterial populations, MPN (Most Probable Number) dilution series were performed in media selecting for the different bacterial groups. While the temperature response of specific growth rates of acidophilic iron reducers showed mesophilic characteristics, the relatively high specific growth rates of sulfate reducers at the lowest incubation temperature indicated the presence of moderate psychrophilic bacteria. In contrast, the low cell numbers and low specific growth rates of neutrophilic iron reducers obtained in dilution cultures suggest that these populations play a less significant role in Fe and S cycling in these sediments. SSCP (Single‐Strand Conformation Polymorphism) or DGGE (Denaturing Gradient Gel Electrophoresis) fingerprinting based on 16S rRNA genes of Bacteria indicated different bacterial populations in the MPN dilution series exhibiting different temperature ranges for growth.     

Carbon Dioxide Fixation by Metallosphaera yellowstonensis and Acidothermophilic Iron-Oxidizing Microbial Communities from Yellowstone National Park

The fixation of inorganic carbon has been documented in all three domains of life and results in the biosynthesis of diverse organic compounds that support heterotrophic organisms. The primary aim of this study was to assess carbon dioxide fixation in high-temperature Fe(III)-oxide mat communities and in pure cultures of a dominant Fe(II)-oxidizing organism (Metallosphaera yellowstonensis strain MK1) originally isolated from these environments. Protein-encoding genes of the complete 3-hydroxypropionate/4-hydroxybutyrate (3-HP/4-HB) carbon dioxide fixation pathway were identified in M. yellowstonensis strain MK1. Highly similar M. yellowstonensis genes for this pathway were identified in metagenomes of replicate Fe(III)-oxide mats, as were genes for the reductive tricarboxylic acid cycle from Hydrogenobaculum spp. (Aquificales). Stable-isotope (¹³CO₂) labeling demonstrated CO₂ fixation by M. yellowstonensis strain MK1 and in ex situ assays containing live Fe(III)-oxide microbial mats. The results showed that strain MK1 fixes CO₂ with a fractionation factor of ∼2.5‰. Analysis of the ¹³C composition of dissolved inorganic C (DIC), dissolved organic C (DOC), landscape C, and microbial mat C showed that mat C is from both DIC and non-DIC sources. An isotopic mixing model showed that biomass C contains a minimum of 42% C of DIC origin, depending on the fraction of landscape C that is present. The significance of DIC as a major carbon source for Fe(III)-oxide mat communities provides a foundation for examining microbial interactions that are dependent on the activity of autotrophic organisms (i.e., Hydrogenobaculum and Metallosphaera spp.) in simplified natural communities.     

Precipitation of iron,silica, and sulfate on bacterial cell surfaces

The present study documents the precipitation of Fe(III), silica, and sulfate in the presence of 3 different bacteria (Bacillus subtilus, Bacillus licheniformis, and Pseudomonas aeruginosa), under different total Fe(III) concentrations (10^?2 M, 10^?3 M, 10^?4 M) at constant pH (4.0). Morphology and chemical composition of the precipitates were compared with those formed in abiotic control systems, while chemical composition and precipitation of the precipitates were modeled according to solution chemistry data. Transmission electron microscopy (TEM) observations showed morphological differences between the biotic and abiotic systems. All systems contained small grains (diam. 2–50 nm), but amorphous material (i.e., material without any specific morphology) and nodules were present only in the cell systems. This is because bacterial surfaces and exopolymers provided numerous binding sites for metal and anion sorption and promoted heterogeneous nucleation of hydrous ferric oxides (HFO). The initial Fe/Si and Fe/SO₄ molar ratios of the solutions dictated the type of precipitates in most systems, since abiotic control systems were saturated to oversaturated with respect to amorphous silica, siliceous ferrihydrite, schwertmannite, ferrihydrite, goethite, or combinations of these. Of the three strains studied, B. licheniformis appeared to have the greatest influence on the chemical composition of the precipitates, especially in the presence of Si. B. licheniformis (a gram‐positive bacterium with a large capsule) favored the precipitation of HFO containing less Si than the predicted solids, because Si rather than Fe oxides was preferentially sorted to extracellular polymers (capsule). On the other hand, the formation of SO₄‐rich HFO (similar to schwertmannite) did not seem to be affected by the presence of bacteria.     

Microbial Fe(III) oxide reduction potential in Chocolate Pots hot spring,Yellowstone National Park

Chocolate Pots hot springs (CP) is a unique, circumneutral pH, iron‐rich, geothermal feature in Yellowstone National Park. Prior research at CP has focused on photosynthetically driven Fe(II) oxidation as a model for mineralization of microbial mats and deposition of Archean banded iron formations. However, geochemical and stable Fe isotopic data have suggested that dissimilatory microbial iron reduction (DIR) may be active within CP deposits. In this study, the potential for microbial reduction of native CP Fe(III) oxides was investigated, using a combination of cultivation dependent and independent approaches, to assess the potential involvement of DIR in Fe redox cycling and associated stable Fe isotope fractionation in the CP hot springs. Endogenous microbial communities were able to reduce native CP Fe(III) oxides, as documented by most probable number enumerations and enrichment culture studies. Enrichment cultures demonstrated sustained DIR driven by oxidation of acetate, lactate, and H₂. Inhibitor studies and molecular analyses indicate that sulfate reduction did not contribute to observed rates of DIR in the enrichment cultures through abiotic reaction pathways. Enrichment cultures produced isotopically light Fe(II) during DIR relative to the bulk solid‐phase Fe(III) oxides. Pyrosequencing of 16S rRNA genes from enrichment cultures showed dominant sequences closely affiliated with Geobacter metallireducens, a mesophilic Fe(III) oxide reducer. Shotgun metagenomic analysis of enrichment cultures confirmed the presence of a dominant G. metallireducens‐like population and other less dominant populations from the phylum Ignavibacteriae, which appear to be capable of DIR. Gene (protein) searches revealed the presence of heat‐shock proteins that may be involved in increased thermotolerance in the organisms present in the enrichments as well as porin–cytochrome complexes previously shown to be involved in extracellular electron transport. This analysis offers the first detailed insight into how DIR may impact the Fe geochemistry and isotope composition of a Fe‐rich, circumneutral pH geothermal environment.     

Shewanella oneidensis MR‐1 mutants selected for their inability to produce soluble organic‐Fe(III) complexes are unable to respire Fe(III) as anaerobic electron acceptor

Summary Recent voltammetric analyses indicate that Shewanella putrefaciens strain 200 produces soluble organic‐Fe(III) complexes during anaerobic respiration of sparingly soluble Fe(III) oxides. Results of the present study expand the range of Shewanella species capable of producing soluble organic‐Fe(III) complexes to include Shewanella oneidensis MR‐1. Soluble organic‐Fe(III) was produced by S. oneidensis cultures incubated anaerobically with Fe(III) oxides, or with Fe(III) oxides and the alternate electron acceptor fumarate, but not in the presence of O₂, nitrate or trimethylamine‐N‐oxide. Chemical mutagenesis procedures were combined with a novel MicroElectrode Screening Array (MESA) to identify four (designated Sol) mutants with impaired ability to produce soluble organic‐Fe(III) during anaerobic respiration of Fe(III) oxides. Two of the Sol mutants were deficient in anaerobic growth on both soluble Fe(III)‐citrate and Fe(III) oxide, yet retained the ability to grow on a suite of seven alternate electron acceptors. The rates of soluble organic‐Fe(III) production were proportional to the rates of iron reduction by the S. oneidensis wild‐type and Sol mutant strains, and all four Sol mutants retained wild‐type siderophore production capability. Results of this study indicate that the production of soluble organic‐Fe(III) may be an important intermediate step in the anaerobic respiration of both soluble and sparingly soluble forms of Fe(III) by S. oneidensis.     

In situ analysis of oxygen consumption and diffusive transport in high‐temperature acidic iron‐oxide microbial mats

The role of dissolved oxygen as a principal electron acceptor for microbial metabolism was investigated within Fe(III)‐oxide microbial mats that form in acidic geothermal springs of Yellowstone National Park (USA). Specific goals of the study were to measure and model dissolved oxygen profiles within high‐temperature (65–75°C) acidic (pH = 2.7–3.8) Fe(III)‐oxide microbial mats, and correlate the abundance of aerobic, iron‐oxidizing Metallosphaera yellowstonensis organisms and mRNA gene expression levels to Fe(II)‐oxidizing habitats shown to consume oxygen. In situ oxygen microprofiles were obtained perpendicular to the direction of convective flow across the aqueous phase/Fe(III)‐oxide microbial mat interface using oxygen microsensors. Dissolved oxygen concentrations dropped from ～ 50–60 μM in the bulk‐fluid/mat surface to below detection (< 0.3 μM) at a depth of ～ 700 μm (～ 10% of the total mat depth). Net areal oxygen fluxes into the microbial mats were estimated to range from 1.4–1.6 × 10^?4 μmol cm^?2 s^?1. Dimensionless parameters were used to model dissolved oxygen profiles and establish that mass transfer rates limit the oxygen consumption. A zone of higher dissolved oxygen at the mat surface promotes Fe(III)‐oxide biomineralization, which was supported using molecular analysis of Metallosphaera yellowstonensis 16S rRNA gene copy numbers and mRNA expression of haem Cu oxidases (FoxA) associated with Fe(II)‐oxidation.     

Linking microbial oxidation of arsenic with detection and phylogenetic analysis of arsenite oxidase genes in diverse geothermal environments

The identification and characterization of genes involved in the microbial oxidation of arsenite will contribute to our understanding of factors controlling As cycling in natural systems. Towards this goal, we recently characterized the widespread occurrence of aerobic arsenite oxidase genes (aroA‐like) from pure‐culture bacterial isolates, soils, sediments and geothermal mats, but were unable to detect these genes in all geothermal systems where we have observed microbial arsenite oxidation. Consequently, the objectives of the current study were to measure arsenite‐oxidation rates in geochemically diverse thermal habitats in Yellowstone National Park (YNP) ranging in pH from 2.6 to 8, and to identify corresponding 16S rRNA and aroA genotypes associated with these arsenite‐oxidizing environments. Geochemical analyses, including measurement of arsenite‐oxidation rates within geothermal outflow channels, were combined with 16S rRNA gene and aroA functional gene analysis using newly designed primers to capture previously undescribed aroA‐like arsenite oxidase gene diversity. The majority of bacterial 16S rRNA gene sequences found in acidic (pH 2.6–3.6) Fe‐oxyhydroxide microbial mats were closely related to Hydrogenobaculum spp. (members of the bacterial order Aquificales), while the predominant sequences from near‐neutral (pH 6.2–8) springs were affiliated with other Aquificales including Sulfurihydrogenibium spp., Thermocrinis spp. and Hydrogenobacter spp., as well as members of the Deinococci, Thermodesulfobacteria and β‐Proteobacteria. Modified primers designed around previously characterized and newly identified aroA‐like genes successfully amplified new lineages of aroA‐like genes associated with members of the Aquificales across all geothermal systems examined. The expression of Aquificales aroA‐like genes was also confirmed in situ, and the resultant cDNA sequences were consistent with aroA genotypes identified in the same environments. The aroA sequences identified in the current study expand the phylogenetic distribution of known Mo‐pterin arsenite oxidase genes, and suggest the importance of three prominent genera of the order Aquificales in arsenite oxidation across geochemically distinct geothermal habitats ranging in pH from 2.6 to 8.     

The apoplastic antioxidant enzymatic system in the wood-forming tissues of trees

The complete apoplastic enzymatic antioxidant system, composed by class I ascorbate peroxidases (class I APXs), class III ascorbate peroxidases (class III APXs), ascorbate oxidases (AAOs), and other class III peroxidases (PRX), of wood-forming tissues has been studied in Populus alba, Citrus aurantium, and Eucalyptus camaldulensis. The aim was to ascertain whether these enzymatic systems may regulate directly (in the case of APXs), or indirectly (in the case of AAOs), apoplastic H₂O₂ levels in lignifying tissues, whose capacity to produce and to accumulate H₂O₂ is demonstrated here. Although class I APXs are particularly found in the apoplastic fraction of P. alba (poplar), and class III APXs are particularly found in the apoplastic fraction of C. aurantium (bitter orange tree), the results showed that the universal presence of AAO in the extracellular cell wall matrix of these woody species provokes the partial or total dysfunction of apoplastic class I and class III APXs, and of the whole plethora of non-enzymatic redox shuttles in which ascorbic acid (ASC) is involved, by the competitive and effective removal of ASC. In fact, the redox state (ASC/ASC+DHA) in intercellular wash fluids (IWFs) of these woody species was zero, and thus strongly shifted towards DHA (dehydroascorbate), the oxidized product of ASC. This imbalance of the apoplastic antioxidant enzymatic system apparently results in the accumulation of H₂O₂ in the apoplast of secondary wood-forming tissues, as can be experimentally observed. Furthermore, it is hypothesized that since AAO uses O₂ to remove ASC, it could regulate O₂ availability in the lignifying xylem and, thorough this mechanism, AAO could also control the activity of NADPH oxidase (the enzyme responsible for H₂O₂ production in lignifying tissues) at substrate level, by controlling the tension of O₂. That is, the presence of AAO in the extracellular cell wall matrix appears to be essential for finely tuning the oxidative performance of secondary wood-forming tissues.     

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 (H₂O₂), a recognized strong oxidant for ferrous iron. Here, we modeled the amount of H₂O₂ 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 H₂O₂ rainout was calculated to be <10⁶ 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 H₂O₂ was insufficient by several orders of magnitude to account for IF deposition (requiring ~10¹¹ H₂O₂ 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.     

Characterisation of the dissimilatory reduction of Fe(III)-oxyhydroxide at the microbe-mineral interface: the application of STXM-XMCD

A combination of scanning transmission X‐ray microscopy and X‐ray magnetic circular dichroism was used to spatially resolve the distribution of different carbon and iron species associated with Shewanella oneidensis MR‐1 cells. S. oneidensis MR‐1 couples the reduction of Fe(III)‐oxyhydroxides to the oxidation of organic matter in order to conserve energy for growth. Several potential mechanisms may be used by S. oneidensis MR‐1 to facilitate Fe(III)‐reduction. These include direct contact between the cell and mineral surface, secretion of either exogenous electron shuttles or Fe‐chelating agents and the production of conductive ‘nanowires’. In this study, the protein/lipid signature of the bacterial cells was associated with areas of magnetite (Fe₃O₄), the product of dissimilatory Fe(III) reduction, which was oversaturated with Fe(II) (compared to stoichiometric magnetite). However, areas of the sample rich in polysaccharides, most likely associated with extracellular polymeric matrix and not in direct contact with the cell surface, were undersaturated with Fe(II), forming maghemite‐like (γ‐Fe₂O₃) phases compared to stoichiometric magnetite. The reduced form of magnetite will be much more effective in environmental remediation such as the immobilisation of toxic metals. These findings suggest a dominant role for surface contact‐mediated electron transfer in this study and also the inhomogeneity of magnetite species on the submicron scale present in microbial reactions. This study also illustrates the applicability of this new synchrotron‐based technique for high‐resolution characterisation of the microbe–mineral interface, which is pivotal in controlling the chemistry of the Earth’s critical zone.     

Auxin and its transport play a role in plant tolerance to arsenite‐induced oxidative stress in Arabidopsis thaliana

The role of auxin in plant development is well known; however, its possible function in root response to abiotic stress is poorly understood. In this study, we demonstrate a novel role of auxin transport in plant tolerance to oxidative stress caused by arsenite. Plant response to arsenite [As(III)] was evaluated by measuring root growth and markers for stress on seedlings treated with control or As(III)‐containing medium. Auxin transporter mutants aux1, pin1 and pin2 were significantly more sensitive to As(III) than the wild type (WT). Auxin transport inhibitors significantly reduced plant tolerance to As(III) in the WT, while exogenous supply of indole‐3‐acetic acid improved As(III) tolerance of aux1 and not that of WT. Uptake assays using H³‐IAA showed As(III) affected auxin transport in WT roots. As(III) increased the levels of H₂O₂ in WT but not in aux1, suggesting a positive role for auxin transport through AUX1 on plant tolerance to As(III) stress via reactive oxygen species (ROS)‐mediated signalling. Compared to the WT, the mutant aux1 was significantly more sensitive to high‐temperature stress and salinity, also suggesting auxin transport influences a common element shared by plant tolerance to arsenite, salinity and high‐temperature stress.     

Influence of electron donor/acceptor concentrations on hydrous ferric oxide (HFO) bioreduction

Dissimilatory metal-reducing bacteria (DMRB) facilitate the reduction of Feand Mn oxides in anoxic soils and sediments and play an important role inthe cycling of these metals and other elements such as carbon in aqueousenvironments. Previous studies investigating the reduction of Fe(III) oxidesby DMRB focused on reactions under constant initial electron donor (lactate)and electron acceptor (Fe oxide) concentrations. Because the concentrationsof these reactants can vary greatly in the environment and would be expectedto influence the rate and extent of oxide reduction, the influence of variableelectron acceptor and donor concentrations on hydrous ferric oxide (HFO)bioreduction was investigated. Batch experiments were conducted in pH 7 HCO₃– buffered media using Shewanella putrefaciens strain CN32. In general, the rate of Fe(III) reduction decreased with increasing HFO:lactateratios, resulting in a relatively greater proportion of crystalline Fe(III) oxidesof relatively low availability for DMRB. HFO was transformed to a variety ofcrystalline minerals including goethite, lepidocrocite, and siderite but was almostcompletely dissolved at high lactate to HFO ratios. These results indicate thatelectron donor and acceptor concentrations can greatly impact the bioreductionof HFO and the suite of Fe minerals formed as a result of reduction. The respirationdriven rate of Fe(II) formation from HFO is believed to be a primary factor governingthe array of ferrous and ferric iron phases formed during reduction.     

Oxidation of arsenite by Thiomonas strains and characterization of Thiomonas arsenivorans sp. nov.

A novel bacterium, strain b6^T (T=type strain), was isolated from a disused mine site by growth using arsenite [As(III)] as energy source in a simple mineral medium. Cells of strain b6^T were rod-shaped, Gram-negative, non-sporulating and motile. Optimum growth occurred at temperatures between 20 and 30 °C, and at pH between 4.0 and 7.5. Strain b6^T grew chemoautotrophically on As(III), sulphur and thiosulphate, and also heterotrophically on yeast extract and a variety of defined organic compounds. Several other Thiomonas strains, including the type species Thiomonas (Tm.) intermedia, were able to oxidize As(III), though only strain b6^T and strain NO115 could grow using As(III) as sole energy source in the absence of any organic compound. The G+C content of the DNA of strain b6^T was 65.1 mol %. Comparative small subunit (SSU) ribosomal RNA (rRNA) analysis indicated that strain b6^T belongs to the genus Thiomonas in the β-subdivision of the Proteobacteria. It was closely related to an unnamed Thiomonas strain (NO115) isolated from a Norwegian mining site, though sequence identities between strain b6^T and characterized Thiomonas species were less than 95%. DNA–DNA hybridization between strain b6^T and the type species of the genus Tm. intermedia showed less than 50% homology. On the basis of phylogenetic and phenotypic characteristics, strain b6^T (DSM 16361^T, LMG 22795^T) is proposed as the type strain of the new species Thiomonas arsenivorans, sp. nov.     

Ferribacterium limneticum, gen. nov., sp. nov., an Fe(III)-reducing microorganism isolated from mining-impacted freshwater lake sediments

A dissimilatory Fe(III)-reducing bacterium was isolated from mining-impacted lake sediments and designated strain CdA-1. The strain was isolated from a 4-month enrichment culture with acetate and Fe(III)-oxyhydroxide. Strain CdA-1 is a motile, obligately anaerobic rod, capable of coupling the oxidation of acetate and other organic acids to the reduction of ferric iron. Fe(III) reduction was not observed using methanol, ethanol, isopropanol, propionate, succinate, fumarate, H₂, citrate, glucose, or phenol as potential electron donors. With acetate as an electron donor, strain CdA-1 also grew by reducing nitrate or fumarate. Growth was not observed with acetate as electron donor and O₂, sulfoxyanions, nitrite, trimethylamine N-oxide, Mn(IV), As(V), or Se(VI) as potential terminal electron acceptors. Comparative 16 S rRNA gene sequence analyses show strain CdA-1 to be most closely related (93.6% sequence similarity) to Rhodocyclus tenuis. However, R. tenuis did not grow heterotrophically by Fe(III) reduction, nor did strain CdA-1 grow photrophically. We propose that strain CdA-1 represents a new genus and species, Ferribacterium limneticum. Strain CdA-1 represents the first dissimilatory Fe(III) reducer in the β subclass of Proteobacteria, as well as the first Fe(III) reducer isolated from mine wastes. Received: 14 July 1998 / Accepted: 14 December 1998