Use of plasma-based spectroscopy and infrared microspectroscopy techniques to determine the uptake and effects of chromium(III) and chromium(VI) on Parkinsonia aculeata

Chromium uptake and tolerance by Mexican Palo Verde (Parkinsonia aculeata) (MPV) was studied in a six-month experiment with Cr(III) and Cr(VI) at 60 and 10 mg kg(-1), respectively. Chromium and nutrient uptake were determined by ICP-OES and changes in macromolecules were studied by infrared microspectroscopy (IMS). In the Cr(VI)-treated plants, chromium concentration increased in the roots only through the third month, while translocation to stems increased constantly throughout the six months. Cr(III) applications decreased the amount of Zn in leaves and stems (p < or = 0.05). Cr(VI) increased P and S in all plant tissues and increased Ca in roots, but decreased Ca in stems and leaves, and Mg in roots and stems. Cr(III) decreased P in stems and leaves, while both Cr ions decreased K in all MPV tissues. Relative to untreated plant tissue, the IMS revealed significant changes at 1730 cm(-1) and 845 cm(-1). Changes at 1730 cm(-1) indicated that the cortex and xylem of Cr-treated plants were more proteinaceous. Changes at 845 cm(-1) revealed higher lignifications in cortex. However, at the stem level, Cr(VI) decreased lignin deposition in xylem. The data showed that MPV could be useful in the phytoremediation of Cr in moderately impacted soils.     

Biological removal of carcinogenic chromium(VI) using mixed Pseudomonas strains

The contamination of soil and wastewaters with Cr(VI) is a major problem. It has been suggested that microbial methods for Cr(VI) reduction are better than chemical methods, as they do not add other ions or toxic chemicals to the environment. In this study an aerobic reduction of Cr(VI) to Cr(III) by employing mixed Pseudomonas cultures isolated from a marshy land has been reported. The role of chromium concentration, temperature, pH and additives on the microbial reduction of Cr(VI) has been investigated. NADH was found to enhance the rate of reduction of Cr(VI). Complete reduction of chromium(VI) has been possible even at chromium(VI) concentrations of 300 ppm. Ions like SO(4)(2-) and poly-phenols inhibited the metabolic activity relating to Cr(VI) reduction. Under optimal conditions 100 mg/L of Cr(VI) was completely reduced within 180 min.     

SPECTROSCOPIC DETERMINATION OF THE TOXICITY,ABSORPTION, REDUCTION,AND TRANSLOCATION OF Cr(VI) IN TWO MAGNOLIOPSIDA SPECIES

Hexavalent chromium is a contaminant highly mobile in the environment that is toxic for plants at low concentrations. In this work, the physiological response of Convolvulus arvensis and Medicago truncatula plants to Cr(VI) treatments was compared. C. arvensis is a potential Cr hyperaccumulator well adapted to semiarid conditions that biotransform Cr(VI) to the less toxic Cr(III). M. truncatula is a model plant well adapted to semiarid conditions with a well studied genetic response to heavy metal stress. The results demonstrated that C. arvensis is more tolerant to Cr toxicity and has a higher Cr translocation to the leaves. The inductively coupled plasma optical emission spectroscopy results showed that C. arvensis plants treated with 10 mg Cr(VI) L^–1 accumulated 1512, 210, and 131 mg Cr kg^–1 in roots, stems, and leaves, respectively. While M. truncatula plants treated with the same Cr(VI) concentration accumulated 1081, 331, and 44 (mg Cr kg^–1) in roots, stems, and leaves, respectively. Enzymatic assays demonstrated that Cr(VI) decreased ascorbate peroxidase activity and increased catalase activity in M. truncatula, while an opposite response was found in C. arvensis. The x-ray absorption spectroscopy studies showed that both plant species reduced Cr(VI) to the less toxic Cr(III).     

Toxicity of arsenic (III) and (V) on plant growth, element uptake, and total amylolytic activity of mesquite (Prosopis juliflora x P. velutina)

The effects of arsenite [As(III)] and arsenate [As(V)] on the growth of roots, stems, and leaves and the uptake of arsenic (As), micro- and macronutrients, and total amylolytic activity were investigated to elucidate the phytotoxicity of As to the mesquite plant (Prosopis juliflora x P. velutina). The plant growth was evaluated by measuring the root and shoot length, and the element uptake was determined using inductively coupled plasma optical emission spectroscopy. The root and leaf elongation decreased significantly with increasing As(III) and As(V) concentrations; whereas, stem elongation remained unchanged. The As uptake increased with increasing As(III) or As(V) concentrations in the medium. Plants treated with 50 mg/L As(III) accumulated up to 920 mg/kg dry weight (d wt) in roots and 522 mg/kg d wt in leaves, while plants exposed to 50 mg/L As(V) accumulated 1980 and 210 mg/kg d wt in roots and leaves, respectively. Increasing the As(V) concentration up to 20 mg/L resulted in a decrease in the total amylolytic activity. On the contrary, total amylolytic activity in As(III)-treated plants increased with increasing As concentration up to 20 mg/L. The macro- and micronutrient concentrations changed in As-treated plants. In shoots, Mo and K were reduced but Ca was increased, while in roots Fe and Ca were increased but K was reduced. These changes reduced the size of the plants, mainly in the As(III)-treated plants; however, there were no visible sign of As toxicity.     

Uptake, distribution, and speciation of chromium in Brassica juncea

Brassica juncea (Indian mustard) has been widely used in phytoremediation because of its capacity to accumulate high levels of chromium (Cr) and other metals. The present study was conducted to investigate mechanism(s) involved in Cr binding and sequestration by B. juncea. The plants were grown under greenhouse conditions in field-moist or air-dried soils, amended with 100 mg kg(-1) of Cr (III) or VI). The plant concentrated Cr mainly in the roots. B. juncea removed an average of 48 and 58 microg Cr per plant from Cr (III) and Cr (VI)-treated soils, respectively. The uptake of Cr was not affected by the moisture status of the soils. X-ray absorption near-edge spectroscopy measurements showed only Cr (III) bound predominantly to formate and acetate ligands, in the bulk and rhizosphere soils, respectively. In the plant tissues, Cr (III) was detected, primarily as acetate in the roots and oxalate in the leaves. X-ray microprobe showed the sites of Cr localization, and probably sequestration, in epidermal and cortical cells in the roots and epidermal and spongy mesophyll cells in the leaves. These findings demonstrate the ability of B. juncea to detoxify more toxic Cr (VI), thereby making this plant a potential candidate for phytostabilization.     

Removal of chromium from industrial waste by using eucalyptus bark

Several low cost biomaterials such as baggase, charred rice husk, activated charcoal and eucalyptus bark (EB) were tested for removal of chromium. All the experiments were carried out in batch process with laboratory prepared samples and wastewater obtained from metal finishing section of auto ancillary unit. The adsorbent, which had highest chromium(VI) removal was EB. Influences of chromium concentration, pH, contact time on removal of chromium from effluent was investigated. The adsorption data were fitted well by Freundlich isotherm. The kinetic data were analyzed by using a first order Lagergren kinetic. The Gibbs free energy was obtained for each system and was found to be -1.879 kJ mol(-1) for Cr(VI) and -3.885 kJ mol(-1) for Cr(III) for removal from industrial effluent. The negative value of deltaG0 indicates the feasibility and spontaneous nature of adsorption. The maximum removal of Cr(VI) was observed at pH 2. Adsorption capacity was found to be 45 mg/g of adsorbent, at Cr(VI) concentration in the effluent being 250 mg/l. A waste water sample containing Cr(VI), Cr(III), Mg, and Ca obtained from industrial unit showed satisfactory removal of chromium. The results indicate that eucalyptus bark can be used for the removal of chromium.     

Hexavalent chromium uptake and its effects on mineral uptake, antioxidant defence system and photosynthesis in Amaranthus viridis L

The effects of different concentrations (10(-6)M, 10(-5)M and 10(-4)M) of K2Cr2O7Cr(VI) on some minerals (Mn, Fe, Cu and Zn), lipid peroxidation, activities of antioxidant enzymes, photosynthetic function, and chlorophyll fluorescence characteristics were investigated in hydroponically grown Amaranthus viridis L. Results indicated that chromium was accumulated primarily in roots. In the roots and shoots, the Cr content increased with the increasing Cr(VI) concentrations, and induced decrease of Mn, Fe, Cu and Zn. Chromium Cr(VI) induced oxidation stress and lipid peroxidation in A. viridis L. shown by the increased concentration of MDA. The increased activities of POD and SOD indicated that they could serve as important components of antioxidant defense mechanisms to minimize Cr induced oxidative injury. The net photosynthetic rate, transpiration rate, stomatal conductance and intercellular CO2 concentration were reduced only by high Cr(VI) treatments (10(-5)M and 10(-4)M). The chlorophyll fluorescence parameters Fv/Fm, Fv(')/Fm('), Phi PSII and qP, decreased in Cr(VI)-treated, but qN and NPQ showed an increase in Cr(VI) treated plants.     

Speciation dependant antioxidative response in roots and leaves of sorghum (Sorghum bicolor (L.) Moench cv CO 27) under Cr(III) and Cr(VI) stress

Growth, lipid peroxidation, H₂O₂ produciton and the response of the antioxidant enzymes and metabolites of the ascorbate glutathione pathway to oxidative stress caused by two concentrations (50 and 100 µM) of Cr(III) and Cr(VI) was studied in 15 day old seedlings of sorghum (Sorghum bicolor (L.) Moench cv CO 27) after 10 days of treatment. Cr accumulation in sorghum plants was concentration and organ dependant. There was no significant growth retardation of plants under 50 µM Cr(III) stress. 100 µM Cr(VI) was most toxic of all the treatments in terms of root and leaf growth and oxidative stress. 50 µM Cr(VI) treated roots exhibited high significant increase in superoxide dismutase (SOD), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) (p < 0.01) and significant increases in catalse (CAT), ascorbate peroxidase (APX) and monodehydroascorbate reductase (MDHAR) (p < 0.05). A high increase in ascorbic acid (AA) level was seen in roots of 50 µM Cr(VI) treated plants in comparison with control. Levels of reduced glutathione (GSH) showed a varied and complex response in all the treatments in both plant parts. GSH/GSSG ratio was not affected by Cr(III) treatment in leaves, in contrast, roots exhibited significant reduction in the ratio. Results indicate that GSH depletion increased sensitivity to oxidative stress (Cr(VI) roots and leaves and Cr(III) 100 µM roots) and AA in tandem with APX compensated for GSH depletion by acting directly on H₂O₂ and the mechanism of defensive response in roots as well as leaves varied in its degree and effectiveness due to the concentration dependant differences observed in translocation of the element itself, reactive oxygen species (ROS) generation and enzyme inhibition based on the oxidation state supplied to the plants.     

Cr Localization and Speciation in Roots of Chromate Fed Helianthus annuus L. Seedlings Using Synchrotron Techniques

In order to gain knowledge on the potential use of Helianthus annuus L. for the remediation of Cr(VI) polluted waters, hydroponics experiments were set up to determine Cr uptake and tolerance in different Cr(VI)-sulfate conditions, and Cr biotransformations. Results indicated that Cr(VI) promoted seed germination, and plant tolerance was higher at younger plant stages. Cr uptake was dependent on sulfate concentrations. The highest Cr levels in roots and shoots (13,700 and 2,500 mg kg^–1dry weight (DW), respectively) were obtained in 1 mM sulfate. The lowest Cr uptake in roots (10,600 mg kg^–1DW) was observed in seedlings treated with no sulfate. In shoots, Cr concentration was of 1,500 mg kg^–1DW for the 1 mM sulfate treatment, indicating a different level of interaction between chromate and sulfate in both tissues. For the first time, using micro X-ray florescence (μXRF), we demonstrated Cr reaches the root stele and is located in the walls of xylem vessels. Bulk and micro X-ray Absorption Near-Edge Structure (μXANES) results showed that Cr in the roots is mostly in the form of Cr(III) phosphate (80%), with the remainder complexed to organic acids. Our results suggest this plant species may serve for Cr(VI) rhizofiltration purposes.     

Simultaneous speciation of chromium by spectrophotometry and multicomponent analysis

Abstract

A simple, fast and sensitive spectrophotometric method for the simultaneous determination of Cr(III) and Cr(VI) in effluents and contaminated waters using a UV-visible spectrophotometer, which operates with an advanced software for multicomponent analysis, is proposed. The method consists in the complexation of Cr (III) with EDTA and reaction of Cr(VI) with diphenylcarbazide (DPC). Variables, such as pH and colour stability time, were studied. The effect of concomitant ions on the simultaneous Cr(III) and Cr(VI) determination was also investigated. The sums of the chromium species concentrations obtained by the proposed method were compared with the total chromium concentrations found by electrothermal atomic absorption spectrometry. Recoveries of the chromium species between 75 and 136% were obtained for spiked samples. The linear working range for Cr(III) was 0.5-30 mg L^?1, while for Cr(VI) was 0.005-0.30 mg L^?1. The detection limits were 0.3 mg L^?1 for Cr(III) and 0.003 mg L^?1 for Cr(VI) while the quantification limits were 1.0 mg L^?1 for Cr(III) and 0.01 mg L^?1 for Cr(VI).     

Distribution and speciation of chromium accumulated in Gynura pseudochina (L.) DC.

Soil and water contamination with chromium is an issue of recent concern in Thailand due to increases in industrial activity. Gynura pseudochina (L.) DC., a chromium tolerance plant, could be employed to address this problem via phytoremediation. To understand the tolerance mechanism, this study investigated the speciation and distribution of chromium accumulated in G. pseudochina (L.) DC. using AAS, XAFS, μ-XANES, μ-XRF imaging and EPR. The plants were separately treated with K₂Cr₂O₇ and Cr₂(SO₄)₃ in a hydroponic system. μ-XRF imaging clarified the distributions of Cr, Fe, Zn, Ca, Cl, K and S within the samples. In G. pseudochina (L.) DC. treated with Cr(VI) solution, the Cr was mainly distributed in the vascular bundle and periderm of the tuber, the stem xylem, the vein and the epidermis, including the trichome of the leaf tissues. This Cr distribution corresponded to those of Cu, Fe and Zn. In G. pseudochina (L.) DC. treated with Cr(III) solution, the Cr was distributed in the periderm of the tuber, the stem cortex, and the epidermis and parenchyma of the leaf tissues. μ-XANES and XAFS indicated that highly toxic Cr(VI) was reduced to the intermediate Cr(V) and accumulated as less toxic Cr(III), and EXAFS spectra showed that the reduced Cr(III) was bound to oxygen ligands. The coordination number (N) and the interatomic distance (R) to the first shell were approximately 3–4 (N) and 2 Å (R), respectively. EPR spectra of the plant samples treated with Cr(VI) revealed the presence of Cr(V) and Cr(III). Thus, Cr(III) and Cr(VI) were taken up into the vascular system and transported from the roots to the leaves. Cr(III) was distributed via the symplast system to the ground tissue and accumulated mainly in the stem cortex. Cr(VI) was transported to the xylem via the apoplast system, and the adsorption of Cr(VI) and its reduction to Cr(V) and Cr(III) occurred on oxygen ligands in the lignocellulosic structure of the xylem and vein.     

Bioremediation of chromium by the yeast Pichia guilliermondii: toxicity and accumulation of Cr (III) and Cr (VI) and the influence of riboflavin on Cr tolerance

A comparative study has been made on the sensitivity of the yeast Pichia guilliermondii to Cr (III) and Cr (VI) as well as on the Cr uptake potential at growth-inhibitory concentrations of chromium. The strains used in the study were either isolated from natural sources or obtained from a laboratory strain collection. The results show that most of the natural strains were more tolerant to chromium and were able to grow in the presence of 5 mM Cr (III) or 0.5 mM Cr (VI), that is at concentrations which substantially inhibited the growth of laboratory strains. The cellular Cr content after treatment was similar for both strain types and ranged from 1.2-4.0 mg/g d.w. and 0.4-0.9 mg/g d.w., for Cr (III) and Cr (VI) forms, respectively, however, in one case of a natural strain it reached the value of 10 mg Cr (III)/g dry mass. Natural-source strains were grouped into four groups based on the yeasts' differential response to Cr (III) and Cr (VI). Hexavalent Cr-resistant mutants of a P. giuilliermondii laboratory strain, which revealed markedly changed capabilities of chromium accumulation, were obtained by means of UV-induced mutagenesis. Cr (VI) treatment triggered oversynthesis of riboflavin and the addition of exogenous riboflavin increased P. guilliermondii resistance to both Cr (III) and Cr (VI). Electrophoretic protein profiles revealed the induction and/or suppression of several proteins in response to toxic Cr (VI) levels.     

Changes in biomass and photosynthetic parameters of tomato plants exposed to trivalent and hexavalent chromium

Tomato plants were treated for two weeks with different concentrations of Cr(III) or Cr(VI) compounds to compare their toxic effects. The concentration of total Cr in plant tissues increased linearly with its concentration in the growth medium and Cr accumulated largely in the roots, regardless of the form in which it was supplied to the plant. All measured plant growth parameters were negatively affected by Cr, but Cr(VI) showed much more pronounced toxic effects. Leaf net photosynthetic rate (P_N) was decreased by both Cr forms, and the decrease was also greater for Cr(VI). Cr(III) caused no significant effect on leaf stomatal conductance, whereas Cr(VI) reduced it. Cr(VI) also markedly reduced the variable to maximum chlorophyll a fluorescence ratio, measured in dark-adapted leaves.     

Effects of chromium(VI) and chromium(III) on energy charge and oxygen consumption in rat thymocytes

The cytotoxic effects of chromium compounds in two oxidation states have been studied in rat thymocytes. endogenous nucleotide levels and oxygen consumption were examined as relevant parameters of the physiological state of the cell. Incubation of rat thymocytes with Cr(VI) produced a marked unbalance of endogenous purine nucleotide pool and a parallel decrease in oxygen consumption. A close correlation between the reduction of oxygen consumption and ATP level in rat thymocytes treated with increasing concentrations of Cr(VI) has been found. In rat thymocytes permeabilized with digitonin and in isolated rat liver mitochondria both Cr(VI) and Cr(III) showed, at different range of concentrations, a marked inhibition of maximal oxygen consumption rate (uncoupled respiration). The effects observed were depending on chromium oxidation state and on different mitochondrial sites of substrate oxidation.     

Effect of hexavalent chromium on the growth and physiological and biochemical parameters on Brassica oleracea L. var. acephala DC

In order to determine the toxic effect of chromium Cr(VI) on the seed germination, the root and shoot length, the root-cotyledonary leaves, the fresh and dry weight in eight-day-old seedlings Brassica oleracea L. var. acephala DC (kale) were treated with various concentrations of Cr in the growth medium. The accumulation of chromium in the tissues was determined in the cotyledons and the roots of the kale seedlings. High rate of Cr uptake was observed in the roots. But the organs could not accumulate large amount Cr. The effect of Cr on B. oleracea var. acephala was evaluated by changes in chlorophyll a, b, lipid peroxidation, proline, ascorbate, protein carbonyl groups, non-protein thiols and peroxidase activity. There were significant decreases in chlorophylls a, b content of the plants treated with Cr. Chromium treated kale seedlings had higher lipid peroxidation and the protein carbonyl groups in cotyledonary leaves than the roots. The changes refer to toxic effects of Cr. There were increases in the non-protein thiol, the total ascorbate, and proline content in the cotyledons and the roots of the seedlings grown on the media containing 0.1 and 0.15 mM Cr. The guaiacol peroxidase activity was higher in the roots of the seedlings than their cotyledons.     

Effect of hexavalent chromium on the activated sludge process and on the sludge protozoan community

The objectives of this study were the determination of chromium effects to the performance of an activated sludge unit and the investigation of the response of the activated sludge protozoan community to Cr(VI). Two bench scale activated sludge reactors were supplied with synthetic sewage containing Cr(VI), at concentrations from 1 up to 50 mg L(-1). Protozoan species were identified and were related to the system efficiency. Variations in the abundance and diversity of the protozoan species were observed under various chromium concentrations. High removal rates of organics and nutrients were observed after the acclimatization of the activated sludge, which were related to the initial chromium(VI) concentration. Chromium(VI) removal efficiency was high in all cases. The protistan community was affected by the influent chromium content. Dominance of sessile species was observed in the reactor receiving 5 mg L(-1) influent chromium, whereas co-dominance of sessile and carnivorous species was observed in the reactors receiving higher chromium concentrations.     

Preliminary studies on continuous chromium(VI) biological removal from wastewater by anaerobic-aerobic activated sludge process

The long-term continuous chromium(VI) removal from synthetic wastewater affected by influent hexavalent chromium (Cr(VI)) and glucose concentrations were studied with an anaerobic-aerobic activated sludge process. It was observed that before activated sludge was acclimated, the chromium in the effluent increased immediately as the influent chromium increased. However, both Cr(VI) and total chromium (TCr) in the effluent significantly decreased after acclimation. In the acclimated activated sludge, the chromium removal efficiency was 100% Cr(VI) and 98.56% TCr at influent Cr(VI) levels of 20 mg/day, 100% Cr(VI) and 98.92% TCr at influent Cr(VI) levels of 40 mg/day, and 98.64% Cr(VI) and 97.16% TCr at influent Cr(VI) levels of 60 mg/day. The corresponding effluent Cr(VI) and TCr concentrations were 0 and 0.012 mg/l, 0 and 0.018 mg/l, and 0.034 mg/l and 0.071 mg/l, respectively. When the influent glucose increased from 1125 to 1500 mg/l at influent Cr(VI) dosage of 60 mg/day, the Cr(VI) and TCr removal efficiency with the acclimated activated sludge improved from 98.64% and 97.16% to 100% and 98.48%, respectively, and the chromium concentration in the effluent decreased from 0.034 mg/l of Cr(VI) and 0.071 mg/l of TCr to 0 (Cr(VI)) and 0.038 mg/l (TCr). The effluent COD and turbidity was around 40 mg/l and 0, respectively, after the activated sludge was acclimated. Further studies showed that after the activated sludge was acclimated, its specific dehydrogenases activity (SDA) and protein contents increased. The SDA and protein increased respectively 15% and 10% when influent Cr(VI) increased from 20 to 60 mg/day.     

Mycorrhizal fungi enhance accumulation and tolerance of chromium in sunflower (Helianthus annuus)

Chromium (Cr) is a heavy metal risk to human health, and a contaminant found in agricultural soils and industrial sites. Phytoremediation, which relies on phytoextraction of Cr with biological organisms, is an important alternative to costly physical and chemical methods of treating contaminated sites. The ability of the arbuscular mycorrhizal fungus (AM),Glomus intraradices, to enhance Cr uptake and plant tolerance was tested on the growth and gas exchange of sunflower (Helianthus annuus L.). Mycorrhizal-colonized (AM) and non-inoculated (Non-AM) sunflower plants were subjected to two Cr species [trivalent cation (Cr³⁺) Cr(III) , and divalent dichromate anion (Cr₂O₇⁻) Cr(VI) ]. Both Cr species depressed plant growth, decreased net photosynthesis (A) and increased the vapor pressure difference; however, Cr(VI) was more toxic. Chromium accumulation was greatest in roots, intermediate in stems and leaves, and lowest in flowers. Greater Cr accumulation occurred with Cr(VI) than Cr(III). AM enhanced the ability of sunflower plants to tolerate and hyperaccumulate Cr. At higher Cr levels greater mycorrhizal dependency occurred, as indicated by proportionally greater growth, higherA and reduced visual symptoms of stress, compared to Non-AM plants. AM plants had greater Cr-accumulating ability than Non-AM plants at the highest concentrations of Cr(III) and Cr(VI), as indicated by the greater Cr phytoextraction coefficient. Mycorrhizal colonization (arbuscule, vesicle, and hyphae formation) was more adversely affected by Cr(VI) than Cr(III), however high levels of colonization still occurred at even the most toxic levels. Arbuscules, which play an important role in mineral ion exchange in root cortical cells, had the greatest sensitivity to Cr toxicity. Higher levels of both Cr species reduced leaf tissue phosphorus (P). While tissue P was higher in AM plants at the highest Cr(III) level, tissue P did not account for mycorrhizal benefits observed with Cr(VI) plants.     

Uptake of chromium(III) and chromium(VI) compounds in the yeast cell structure

The study presented in this article investigated the influence of different Cr(III) and Cr(VI) compounds in the cultivation medium on the uptake and localization of chromium in the cell structure of the yeast Candida intermedia. The morphology of the yeast cell surface was observed by the scanning electron microscopy. Results demonstrated that the growth inhibitory concentration of Cr(III) in the cultivation medium induced changes in the yeast cell shape and affected the budding pattern, while inhibitory concentration of Cr(VI) did not cause any visible effects on morphological properties of the yeast cells. The amount of total accumulated chromium in yeast cells and the distribution of chromium between the yeast cell walls and spheroplasts were determined by atomic absorption spectroscopy. No significant differences were found neither in total chromium accumulation nor in the distribution of chromium in yeast cell walls and spheroplasts between the two of Cr(VI) compounds. Conversely, substantial differences between Cr(III) compounds were demonstrated in the total uptake as well as the localization of chromium in yeast cells.     

Cometabolism of Cr(VI) by Shewanella oneidensis MR-1 produces cell-associated reduced chromium and inhibits growth

Microbial reduction is a promising strategy for chromium remediation, but the effects of competing electron acceptors are still poorly understood. We investigated chromate (Cr(VI)) reduction in batch cultures of Shewanella oneidensis MR-1 under aerobic and denitrifying conditions and in the absence of an additional electron acceptor. Growth and Cr(VI) removal patterns suggested a cometabolic reduction; in the absence of nitrate or oxygen, MR-1 reduced Cr(VI), but without any increase in viable cell counts and rates gradually decreased when cells were respiked. Only a small fraction (1.6%) of the electrons from lactate were transferred to Cr(VI). The 48-h transformation capacity (Tc) was 0.78 mg (15 micromoles) Cr(VI) reduced. [mg protein](-1) for high levels of Cr(VI) added as a single spike. For low levels of Cr(VI) added sequentially, Tc increased to 3.33 mg (64 micromoles) Cr(VI) reduced. [mg protein](-1), indicating that it is limited by toxicity at higher concentrations. During denitrification and aerobic growth, MR-1 reduced Cr(VI), with much faster rates under denitrifying conditions. Cr(VI) had no effect on nitrate reduction at 6 microM, was strongly inhibitory at 45 microM, and stopped nitrate reduction above 200 microM. Cr(VI) had no effect on aerobic growth at 60 microM, but severely inhibited growth above 150 microM. A factor that likely plays a role in Cr(VI) toxicity is intracellular reduced chromium. Transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) of denitrifying cells exposed to Cr(VI) showed reduced chromium precipitates both extracellularly on the cell surface and, for the first time, as electron-dense round globules inside cells.