Transformation of vivianite by anaerobic nitrate-reducing iron-oxidizing bacteria

Biological redox cycling of iron in nontronite and its potential application in nitrate removal

Biological redox cycling of structural Fe in phyllosilicates is an important but poorly understood process. The objective of this research was to study microbially mediated redox cycles of Fe in nontronite (NAu-2). During the reduction phase, structural Fe(III) in NAu-2 served as electron acceptor, lactate as electron donor, AQDS as electron shuttle, and dissimilatory Fe(III)-reducing bacterium Shewanella putrefaciens CN32 as mediator in bicarbonate- and PIPES-buffered media. During the oxidation phase, biogenic Fe(II) served as electron donor and nitrate as electron acceptor. Nitrate-dependent Fe(II)-oxidizing bacterium Pseudogulbenkiania sp. strain 2002 was added as mediator in the same media. For all three cycles, structural Fe in NAu-2 was able to reversibly undergo three redox cycles without significant dissolution. Fe(II) in bioreduced samples occurred in two distinct environments, at edges and in the interior of the NAu-2 structure. Nitrate reduction to nitrogen gas was coupled with oxidation of edge-Fe(II) and part of interior-Fe(II) under both buffer conditions, and its extent and rate did not change with Fe redox cycles. These results suggest that biological redox cycling of structural Fe in phyllosilicates is a reversible process and has important implications for biogeochemical cycles of carbon, nitrogen, and other nutrients in natural environments.

Oxidation of Fe(II)-EDTA by nitrite and by two nitrate-reducing Fe(II)-oxidizing Acidovorax strains

The enzymatic oxidation of Fe(II) by nitrate-reducing bacteria was first suggested about two decades ago. It has since been found that most strains are mixotrophic and need an additional organic co-substrate for complete and prolonged Fe(II) oxidation. Research during the last few years has tried to determine to what extent the observed Fe(II) oxidation is driven enzymatically, or abiotically by nitrite produced during heterotrophic denitrification. A recent study reported that nitrite was not able to oxidize Fe(II)-EDTA abiotically, but the addition of the mixotrophic nitrate-reducing Fe(II)-oxidizer, Acidovorax sp. strain 2AN, led to Fe(II) oxidation (Chakraborty & Picardal, 2013). This, along with other results of that study, was used to argue that Fe(II) oxidation in strain 2AN was enzymatically catalyzed. However, the absence of abiotic Fe(II)-EDTA oxidation by nitrite reported in that study contrasts with previously published data. We have repeated the abiotic and biotic experiments and observed rapid abiotic oxidation of Fe(II)-EDTA by nitrite, resulting in the formation of Fe(III)-EDTA and the green Fe(II)-EDTA-NO complex. Additionally, we found that cultivating the Acidovorax strains BoFeN1 and 2AN with 10 mM nitrate, 5 mm acetate, and approximately 10 mM Fe(II)-EDTA resulted only in incomplete Fe(II)-EDTA oxidation of 47-71%. Cultures of strain BoFeN1 turned green (due to the presence of Fe(II)-EDTA-NO) and the green color persisted over the course of the experiments, whereas strain 2AN was able to further oxidize the Fe(II)-EDTA-NO complex. Our work shows that the two used Acidovorax strains behave very differently in their ability to deal with toxic effects of Fe-EDTA species and the further reduction of the Fe(II)-EDTA-NO nitrosyl complex. Although the enzymatic oxidation of Fe(II) cannot be ruled out, this study underlines the importance of nitrite in nitrate-reducing Fe(II)- and Fe(II)-EDTA-oxidizing cultures and demonstrates that Fe(II)-EDTA cannot be used to demonstrate unequivocally the enzymatic oxidation of Fe(II) by mixotrophic Fe(II)-oxidizers.

3-D analysis of bacterial cell-(iron)mineral aggregates formed during Fe(II) oxidation by the nitrate-reducing Acidovorax sp. strain BoFeN1 using complementary microscopy tomography approaches

The formation of cell-(iron)mineral aggregates as a consequence of bacterial iron oxidation is an environmentally widespread process with a number of implications for processes such as sorption and coprecipitation of contaminants and nutrients. Whereas the overall appearance of such aggregates is easily accessible using 2-D microscopy techniques, the 3-D and internal structure remain obscure. In this study, we examined the 3-D structure of cell-(iron)mineral aggregates formed during Fe(II) oxidation by the nitrate-reducing Acidovorax sp. strain BoFeN1 using a combination of advanced 3-D microscopy techniques. We obtained 3-D structural and chemical information on different cellular encrustation patterns at high spatial resolution (4-200 nm, depending on the method): more specifically, (1) cells free of iron minerals, (2) periplasm filled with iron minerals, (3) spike- or platelet-shaped iron mineral structures, (4) bulky structures on the cell surface, (5) extracellular iron mineral shell structures, (6) cells with iron mineral filled cytoplasm, and (7) agglomerations of extracellular globular structures. In addition to structural information, chemical nanotomography suggests a dominant role of extracellular polymeric substances (EPS) in controlling the formation of cell-(iron)mineral aggregates. Furthermore, samples in their hydrated state showed cell-(iron)mineral aggregates in pristine conditions free of preparation (i.e., drying/dehydration) artifacts. All these results were obtained using 3-D microscopy techniques such as focused ion beam (FIB)/scanning electron microscopy (SEM) tomography, transmission electron microscopy (TEM) tomography, scanning transmission (soft) X-ray microscopy (STXM) tomography, and confocal laser scanning microscopy (CLSM). It turned out that, due to the various different contrast mechanisms of the individual approaches, and due to the required sample preparation steps, only the combination of these techniques was able to provide a comprehensive understanding of structure and composition of the various Fe-precipitates and their association with bacterial cells and EPS.

Abiotic process for Fe(II) oxidation and green rust mineralization driven by a heterotrophic nitrate reducing bacteria (Klebsiella mobilis)

Green rusts (GRs) are mixed Fe(II)-Fe(III) hydroxides with a high reactivity toward organic and inorganic pollutants. GRs can be produced from ferric reducing or ferrous oxidizing bacterial activities. In this study, we investigated the capability of Klebsiella mobilis to produce iron minerals in the presence of nitrate and ferrous iron. This bacterium is well-known to reduce nitrate using an organic carbon source as electron donor but is unable to enzymatically oxidize Fe(II) species. During incubation, GR formation occurred as a secondary iron mineral precipitating on cell surfaces, resulting from Fe(II) oxidation by nitrite produced via bacterial respiration of nitrate. For the first time, we demonstrate GR formation by indirect microbial oxidation of Fe(II) (i.e., a combination of biotic/abiotic processes). These results therefore suggest that nitrate-reducing bacteria can potentially contribute to the formation of GR in natural environments. In addition, the chemical reduction of nitrite to ammonium by GR is observed, which gradually turns the GR into the end-product goethite. The nitrogen mass-balance clearly demonstrates that the total amount of ammonium produced corresponds to the quantity of bioreduced nitrate. These findings demonstrate how the activity of nitrate-reducing bacteria in ferrous environments may provide a direct link between the biogeochemical cycles of nitrogen and iron.

Characterization of the physiology and cell-mineral interactions of the marine anoxygenic phototrophic Fe(II) oxidizer Rhodovulum iodosum--implications for Precambrian Fe(II) oxidation

Anoxygenic phototrophic Fe(II)-oxidizing bacteria (photoferrotrophs) are suggested to have contributed to the deposition of banded iron formations (BIFs) from oxygen-poor seawater. However, most studies evaluating the contribution of photoferrotrophs to Precambrian Fe(II) oxidation have used freshwater and not marine strains. Therefore, we investigated the physiology and mineral products of Fe(II) oxidation by the marine photoferrotroph Rhodovulum iodosum. Poorly crystalline Fe(III) minerals formed initially and transformed to more crystalline goethite over time. During Fe(II) oxidation, cell surfaces were largely free of minerals. Instead, the minerals were co-localized with EPS suggesting that EPS plays a critical role in preventing cell encrustation, likely by binding Fe(III) and directing precipitation away from cell surfaces. Fe(II) oxidation rates increased with increasing initial Fe(II) concentration (0.43-4.07 mM) under a light intensity of 12 μmol quanta m(-2) s(-1). Rates also increased as light intensity increased (from 3 to 20 μmol quanta m(-2) s(-1)), while the addition of Si did not significantly change Fe(II) oxidation rates. These results elaborate on how the physical and chemical conditions present in the Precambrian ocean controlled the activity of marine photoferrotrophs and confirm the possibility that such microorganisms could have oxidized Fe(II), generating the primary Fe(III) minerals that were then deposited to some Precambrian BIFs.

Fe(III) reduction-mediated phosphate removal as vivianite (Fe3(PO4)2⋅8H2O) in septic system wastewater

Phosphate is a water contaminant from fertilizers, soaps, and detergents that enters municipal and onsite wastewater from households, businesses, and other commercial operations. Phosphate is a limiting nutrient for algae, and is one of the molecules that promotes eutrophication of water bodies. Phosphate is especially problematic in onsite wastewater because there are few removal mechanisms under normal operating conditions; a system must be amended specifically with compounds to bond to or adsorb phosphate in the septic tank or within the leach field. Vivianite (Fe3(PO4)2⋅8H2O) is a stable mineral formed from ferrous iron and phosphate, often as the result of Fe(III) reducing microbial activity. What was unknown was the concentration of phosphate that could be removed by this process, and whether it was relevant to mixed microbial systems like septic tank wastewater. Data presented here demonstrate that significant concentrations of phosphate (12-14mM) were removed as vivianite in growing cultures of Geobacter metallireducens strain GS-15. Vivianite precipitates were identified on the cell surfaces and within multi cell clusters using TEM-EDX; the mineral phases were directly characterized using XRD. Phosphate was also removed in dilute and raw (undiluted) septic wastewater amended with different forms of Fe(III) including solid phase and soluble Fe(III). Vivianite precipitates were recovered and identified using XRD, along with siderite (ferrous carbonate), which was expected given that the systems were likely bicarbonate buffered. These data demonstrate that ferric iron amendments in septic wastewater increase phosphate removal as the mineral vivianite, and this may be a good strategy for phosphate attenuation in the septic tank portion of onsite wastewater systems.

Bench-scale evaluation of ferrous iron oxidation kinetics in drinking water: effect of corrosion control and dissolved organic matter

Corrosion control strategies are important for many utilities in maintaining water quality from the water treatment plant to the customers' tap. In drinking water with low alkalinity, water quality can become significantly degraded in iron-based pipes if water utilities are not diligent in maintaining proper corrosion control. This article reports on experiments conducted in bicarbonate buffered (5 mg-C/L) synthetic water to determine the effects of corrosion control (pH and phosphate) and dissolved organic matter (DOM) on the rate constants of the Fe(II) oxidation process. A factorial design approach elucidated that pH (P = 0.007, contribution: 42.5%) and phosphate (P = 0.025, contribution: 22.7%) were the statistically significant factors in the Fe(II) oxidation process at a 95% confidence level. The comprehensive study revealed a significant dependency relationship between the Fe(II) oxidation rate constants (k) and phosphate-to- Fe(II) mole ratio. At pH 6.5, the optimum mole ratio was found to be 0.3 to reduce the k values. Conversely, the k values were observed to increase for the phosphate-to- Fe(II) mole ratio > 1. The factorial design approach revealed that chlorine and DOM for the designated dosages did not cause a statistically significant (α = 0.05, P > 0.05)change in rate constants. However, an increment of the chlorine to ferrous iron mole ratio by a factor of ∼ 2.5 resulted in an increase k values by a factor of ∼ 10. This study conclusively demonstrated that the lowest Fe(II) oxidation rate constant was obtained under low pH conditions (pH ≤ 6.5), with chlorine doses less than 2.2 mg/L and with a phosphate-to-Fe(II) mole ratio ≈ 0.3 in the iron water systems.

Potential role of nitrite for abiotic Fe(II) oxidation and cell encrustation during nitrate reduction by denitrifying bacteria

Microorganisms have been observed to oxidize Fe(II) at neutral pH under anoxic and microoxic conditions. While most of the mixotrophic nitrate-reducing Fe(II)-oxidizing bacteria become encrusted with Fe(III)-rich minerals, photoautotrophic and microaerophilic Fe(II) oxidizers avoid cell encrustation. The Fe(II) oxidation mechanisms and the reasons for encrustation remain largely unresolved. Here we used cultivation-based methods and electron microscopy to compare two previously described nitrate-reducing Fe(II) oxidizers ( Acidovorax sp. strain BoFeN1 and Pseudogulbenkiania sp. strain 2002) and two heterotrophic nitrate reducers (Paracoccus denitrificans ATCC 19367 and P. denitrificans Pd 1222). All four strains oxidized ∼8 mM Fe(II) within 5 days in the presence of 5 mM acetate and accumulated nitrite (maximum concentrations of 0.8 to 1.0 mM) in the culture media. Iron(III) minerals, mainly goethite, formed and precipitated extracellularly in close proximity to the cell surface. Interestingly, mineral formation was also observed within the periplasm and cytoplasm; intracellular mineralization is expected to be physiologically disadvantageous, yet acetate consumption continued to be observed even at an advanced stage of Fe(II) oxidation. Extracellular polymeric substances (EPS) were detected by lectin staining with fluorescence microscopy, particularly in the presence of Fe(II), suggesting that EPS production is a response to Fe(II) toxicity or a strategy to decrease encrustation. Based on the data presented here, we propose a nitrite-driven, indirect mechanism of cell encrustation whereby nitrite forms during heterotrophic denitrification and abiotically oxidizes Fe(II). This work adds to the known assemblage of Fe(II)-oxidizing bacteria in nature and complicates our ability to delineate microbial Fe(II) oxidation in ancient microbes preserved as fossils in the geological record.

Uranium association with iron-bearing phases in mill tailings from Gunnar, Canada

The speciation of uranium was studied in the mill tailings of the Gunnar uranium mine (Saskatchewan, Canada), which operated in the 1950s and 1960s. The nature, quantification, and spatial distribution of uranium-bearing phases were investigated by chemical and mineralogical analyses, fission track mapping, electron microscopy, and X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies at the U LIII-edge and Fe K-edge. In addition to uranium-containing phases from the ore, uranium is mostly associated with iron-bearing minerals in all tailing sites. XANES and EXAFS data and transmission electron microscopy analyses of the samples with the highest uranium concentrations (∼400-700 mg kg(-1) of U) demonstrate that uranium primarily occurs as monomeric uranyl ions (UO2(2+)), forming inner-sphere surface complexes bound to ferrihydrite (50-70% of the total U) and to a lesser extent to chlorite (30-40% of the total U). Thus, the stability and mobility of uranium at the Gunnar site are mainly influenced by sorption/desorption processes. In this context, acidic pH or alkaline pH with the presence of UO2(2+)- and/or Fe(3+)-complexing agents (e.g., carbonate) could potentially solubilize U in the tailings pore waters.

Ligand-enhanced abiotic iron oxidation and the effects of chemical versus biological iron cycling in anoxic environments

This study introduces a newly isolated, genetically tractable bacterium ( Pseudogulbenkiania sp. strain MAI-1) and explores the extent to which its nitrate-dependent iron-oxidation activity is directly biologically catalyzed. Specifically, we focused on the role of iron chelating ligands in promoting chemical oxidation of Fe(II) by nitrite under anoxic conditions. Strong organic ligands such as nitrilotriacetate and citrate can substantially enhance chemical oxidation of Fe(II) by nitrite at circumneutral pH. We show that strain MAI-1 exhibits unambiguous biological Fe(II) oxidation despite a significant contribution (∼30-35%) from ligand-enhanced chemical oxidation. Our work with the model denitrifying strain Paracoccus denitrificans further shows that ligand-enhanced chemical oxidation of Fe(II) by microbially produced nitrite can be an important general side effect of biological denitrification. Our assessment of reaction rates derived from literature reports of anaerobic Fe(II) oxidation, both chemical and biological, highlights the potential competition and likely co-occurrence of chemical Fe(II) oxidation (mediated by microbial production of nitrite) and truly biological Fe(II) oxidation.

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.

Potential function of added minerals as nucleation sites and effect of humic substances on mineral formation by the nitrate-reducing Fe(II)-oxidizer Acidovorax sp. BoFeN1

The mobility of toxic metals and the transformation of organic pollutants in the environment are influenced and in many cases even controlled by iron minerals. Therefore knowing the factors influencing iron mineral formation and transformation by Fe(II)-oxidizing and Fe(III)-reducing bacteria is crucial for understanding the fate of contaminants and for the development of remediation technologies. In this study we followed mineral formation by the nitrate-reducing Fe(II)-oxidizing strain Acidovorax sp. BoFeN1 in the presence of the crystalline Fe(III) (oxyhydr)oxides goethite, magnetite and hematite added as potential nucleation sites. Mössbauer spectroscopy analysis of minerals precipitated by BoFeN1 in (57)Fe(II)-spiked microbial growth medium showed that goethite was formed in the absence of mineral additions as well as in the presence of goethite or hematite. The presence of magnetite minerals during Fe(II) oxidation induced the formation of magnetite in addition to goethite, while the addition of humic substances along with magnetite also led to goethite but no magnetite. This study showed that mineral formation not only depends on the aqueous geochemical conditions but can also be affected by the presence of mineral nucleation sites that initiate precipitation of the same underlying mineral phases.

Vivianite precipitation and phosphate sorption following iron reduction in anoxic soils

Phosphorus retention in lowland soils depends on redox conditions. The aim of this study was to evaluate how the Fe(III) reduction degree affects phosphate adsorption and precipitation. Two similarly P-saturated, ferric Fe-rich lowland soils, a sandy and a peat soil, were incubated under anaerobic conditions. Mössbauer spectroscopy demonstrated that Fe(III) in the sandy soil was present as goethite and phyllosilicates, whereas Fe(III) in the peat soil was mainly present as polynuclear, Fe-humic complexes. Following anoxic incubation, extensive formation of Fe(II) in the solids occurred. After 100 d, the Fe(II) production reached its maximum and 34% of the citrate-bicarbonate-dithionite extractable Fe (Fe(CBD)) was reduced to Fe(II) in the sandy soil. The peat soil showed a much faster reduction of Fe(III) and the maximum reduction of 89% of Fe(CBD) was reached after 200 d. Neoformation of a metavivianite/vivianite phase under anoxic conditions was identified by X-ray diffraction in the peat. The sandy soil exhibited small changes in the point of zero net sorption (EPC₀) and P(i) desorption with increasing Fe(III) reduction, whereas in the peat soil P desorption increased from 80 to 3100 μmol kg⁻¹ and EPC₀ increased from 1.7 to 83 μM, after 322 d of anoxic incubation. The fast Fe(III) reduction made the peat soils particularly vulnerable to changes in redox conditions. However, the precipitation of vivianite/metavivianite minerals may control soluble P(i) concentrations to between 2 and 3 μM in the long term if the soil is not disturbed.

Toward a mechanistic understanding of anaerobic nitrate-dependent iron oxidation: balancing electron uptake and detoxification

The anaerobic oxidation of Fe(II) by subsurface microorganisms is an important part of biogeochemical cycling in the environment, but the biochemical mechanisms used to couple iron oxidation to nitrate respiration are not well understood. Based on our own work and the evidence available in the literature, we propose a mechanistic model for anaerobic nitrate-dependent iron oxidation. We suggest that anaerobic iron-oxidizing microorganisms likely exist along a continuum including: (1) bacteria that inadvertently oxidize Fe(II) by abiotic or biotic reactions with enzymes or chemical intermediates in their metabolic pathways (e.g., denitrification) and suffer from toxicity or energetic penalty, (2) Fe(II) tolerant bacteria that gain little or no growth benefit from iron oxidation but can manage the toxic reactions, and (3) bacteria that efficiently accept electrons from Fe(II) to gain a growth advantage while preventing or mitigating the toxic reactions. Predictions of the proposed model are highlighted and experimental approaches are discussed.

Green rust formation during Fe(II) oxidation by the nitrate-reducing Acidovorax sp. strain BoFeN1

Green rust (GR) as highly reactive iron mineral potentially plays a key role for the fate of (in)organic contaminants, such as chromium or arsenic, and nitroaromatic compounds functioning both as sorbent and reductant. GR forms as corrosion product of steel but is also naturally present in hydromorphic soils and sediments forming as metastable intermediate during microbial Fe(III) reduction. Although already suggested to form during microbial Fe(II) oxidation, clear evidence for GR formation during microbial Fe(II) oxidation was lacking. In the present study, powder XRD, synchrotron-based XAS, Mössbauer spectroscopy, and TEM demonstrated unambiguously the formation of GR as an intermediate product during Fe(II) oxidation by the nitrate-reducing Fe(II)-oxidizer Acidovorax sp. strain BoFeN1. The spatial distribution and Fe redox-state of the precipitates associated with the cells were visualized by STXM. It showed the presence of extracellular Fe(III), which can be explained by Fe(III) export from the cells or extracellular Fe(II) oxidation by an oxidant diffusing from the cells. Moreover, GR can be oxidized by nitrate/nitrite and is known as a catalyst for oxidation of dissolved Fe(II) by nitrite/nitrate and may thus contribute to the production of extracellular Fe(III). As a result, strain BoFeN1 may contribute to Fe(II) oxidation and nitrate reduction both by an direct enzymatic pathway and an indirect GR-mediated process.

Preservation of protein globules and peptidoglycan in the mineralized cell wall of nitrate-reducing, iron(II)-oxidizing bacteria: a cryo-electron microscopy study

Iron-oxidizing bacteria are important actors of the geochemical cycle of iron in modern environments and may have played a key role all over Earth's history. However, in order to better assess that role on the modern and the past Earth, there is a need for better understanding the mechanisms of bacterial iron oxidation and for defining potential biosignatures to be looked for in the geologic record. In this study, we investigated experimentally and at the nanometre scale the mineralization of iron-oxidizing bacteria with a combination of synchrotron-based scanning transmission X-ray microscopy (STXM), scanning transmission electron microscopy (STEM) and cryo-transmission electron microscopy (cryo-TEM). We show that the use of cryo-TEM instead of conventional microscopy provides detailed information of the successive iron biomineralization stages in anaerobic nitrate-reducing iron-oxidizing bacteria. These results suggest the existence of preferential Fe-binding and Fe-oxidizing sites on the outer face of the plasma membrane leading to the nucleation and growth of Fe minerals within the periplasm of these cells that eventually become completely encrusted. In contrast, the septa of dividing cells remain nonmineralized. In addition, the use of cryo-TEM offers a detailed view of the exceptional preservation of protein globules and the peptidoglycan within the Fe-mineralized cell walls of these bacteria. These organic molecules and ultrastructural details might be protected from further degradation by entrapment in the mineral matrix down to the nanometre scale. This is discussed in the light of previous studies on the properties of Fe-organic interactions and more generally on the fossilization of mineral-organic assemblies.

Internal loading of phosphorus in a sedimentation pond of a treatment wetland: effect of a phytoplankton crash

Sedimentation ponds are widely believed to act as a primary removal process for phosphorus (P) in nutrient treatment wetlands. High frequency in-situ P, ammonium (NH(4)(+)) and dissolved oxygen measurements, alongside occasional water quality measurements, assessed changes in nutrient concentrations and productivity in the sedimentation pond of a treatment wetland between March and June. Diffusive equilibrium in thin films (DET) probes were used to measure in-situ nutrient and chemistry pore-water profiles. Diffusive fluxes across the sediment-water interface were calculated from the pore-water profiles, and dissolved oxygen was used to calculate rates of primary productivity and respiration. The sedimentation pond was a net sink for total P (TP), soluble reactive P (SRP) and NH(4)(+) in March, but became subject to a net internal loading of TP, SRP and NH(4)(+) in May, with SRP concentrations increasing by up to 41μM (1300μl(-1)). Reductions in chlorophyll a and dissolved oxygen concentrations also occurred at this time. The sediment changed from a small net sink of SRP in March (average diffusive flux: -8.2μmolm(-2)day(-1)) to a net source of SRP in June (average diffusive flux: +1324μmolm(-2)day(-1)). A diurnal pattern in water column P concentrations, with maxima in the early hours of the morning, and minima in the afternoon, occurred during May. The diurnal pattern and release of SRP from the sediment were attributed to microbial degradation of diatom biomass, causing reduction of the dissolved oxygen concentration and leading to redox-dependent release of P from the sediment. In June, 2.7mol-Pday(-1) were removed by photosynthesis and 23mol-Pday(-1) were supplied by respiration in the lake volume. SRP was also released through microbial respiration within the water column, including the decomposition of algal matter. It is imperative that consideration to internal recycling is given when maintaining sedimentation ponds, and before the installation of new ponds designed to treat nutrient waste.

Bio-reduction of Fe(III) ores using three pure strains of Aeromonas hydrophila, Serratia fonticola and Clostridium celerecrescens and a natural consortium

The present work describes a research approach to the anaerobic bioleaching of Fe(III) ores. Three strains (Serratia fonticola, Aeromonas hydrophila and Clostridium celerecrescens) isolated from an acidic abandoned mine were selected to test their ability to reduce dissimilatory Fe(III). Total iron bio-reduction was achieved after 48 h using either the consortium or the Aeromonas cultures. In the latter case, there was no evidence of precipitates and Fe(II) remained in solution at neutral pH through complex formation with citrate. None of the other cultures tested (mixed culture and the two isolates) exhibited this behaviour. Biotechnologically, this is a very promising result since it obviates the problem associated with undesirable precipitation of iron compounds in Fe(III)-reducing bacterial cultures. The performance of the Aeromonas culture was improved progressively by adaptation to moderately acidic pH values (up to 4.5) and to three different Fe(III)-oxyhydroxides as the sole source of iron: ferrihydrite, hematite and jarosite, commonly found as weathering compounds at mine sites. Dissimilatory Fe(III)-reducers for iron extraction from ores is therefore especially attractive in that acidification of the surrounding area can be minimized.

Extracellular iron biomineralization by photoautotrophic iron-oxidizing bacteria

Iron oxidation at neutral pH by the phototrophic anaerobic iron-oxidizing bacterium Rhodobacter sp. strain SW2 leads to the formation of iron-rich minerals. These minerals consist mainly of nano-goethite (alpha-FeOOH), which precipitates exclusively outside cells, mostly on polymer fibers emerging from the cells. Scanning transmission X-ray microscopy analyses performed at the C K-edge suggest that these fibers are composed of a mixture of lipids and polysaccharides or of lipopolysaccharides. The iron and the organic carbon contents of these fibers are linearly correlated at the 25-nm scale, which in addition to their texture suggests that these fibers act as a template for mineral precipitation, followed by limited crystal growth. Moreover, we evidence a gradient of the iron oxidation state along the mineralized fibers at the submicrometer scale. Fe minerals on these fibers contain a higher proportion of Fe(III) at cell contact, and the proportion of Fe(II) increases at a distance from the cells. All together, these results demonstrate the primordial role of organic polymers in iron biomineralization and provide first evidence for the existence of a redox gradient around these nonencrusting, Fe-oxidizing bacteria.