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.     

Optimization of Chromium(VI) Detoxification by Pseudomonas aeruginosa and Its Application for Treatment of Industrial Waste and Contaminated Soil

The reduction of Cr(VI) to Cr(III) is a potential detoxification process. In this study, seven Pseudomonas spp. were isolated and screened for chromium reduction. Isolate P4 was able to grow in the presence of 8000 μM chromium, in spite of the fact that the isolate was not previously exposed to any metal stress. Isolate P4 was identified as Pseudomonas aeruginosa strain SRD chr3 by 16S rDNA sequence analysis. Shake flask study showed 78% reduction of 1000 μM Cr(VI) after 6 h of incubation. The optimum pH for chromium reduction by the isolate was between 6 and 8. Isolate Pseudomonas aeruginosa gave 50–80% Cr(VI) reduction even in the presence of 100 mM of Cu, Mn, Ni, and Zn and 300–800 mM NaCl in 24 h, compared with the absence of any of these metals. In a 5-L reactor, the isolate showed 84.85% reduction of Cr(VI) even at the 70th cycle, with a hydraulic retention time of 24 h from the effluent of a hard chrome plating (electroplating) industry, which contained 2100 mg L^?1 hexavalent chromium. The chromate-amended soil inoculated with the isolate showed 2800 μM chromium removal from 4000 μM Cr(VI) kg^?1 of soil, which corresponds to 70% removal. The isolate had the ability to degrade stimulated waste containing 10,000 μM chromium.     

Trivalent chromium pretreatment alleviates the toxicity of oxidative damage in wheat plants exposed to hexavalent chromium

Treating plants with abiotic or biotic factors can lead to the establishment of a unique primed state of defense. Primed plants display enhanced defense reactions upon further challenge with environmental stressors. Here, we report that trivalent chromium (Cr(III)) pretreatment can alleviate hexavalent chromium (Cr(VI)) toxicity in 2-week-old wheat plants. The data indicate that Cr(III)-pretreated wheat displayed longer survival times and less heavy metal toxicity symptoms under Cr(VI) exposure than the control. To investigate the possible mechanism from an antioxidant defense perspective, we determined the H₂O₂ and lipid peroxide content (TBARS), the activities of antioxidant enzymes (SOD, CAT, APX and GR) and the antioxidant metabolite content (ascorbate and glutathione content, AsA/DHA and GSH/GSSG ratios) in pretreated wheat roots. The results showed that 0.5 μM Cr(III) pretreatment can alleviate oxidative damage, such as H₂O₂ and TBARS accumulation, in root tissues compared to the control during the first 3 days of Cr(VI) exposure. Furthermore, we determined that this pretreatment can significantly increase the antioxidant enzyme activities and total ascorbate and glutathione contents compared to the control treatment. In addition, redox homeostasis declined slightly in pretreated wheat compared to the control in the presence of Cr(VI). We discuss a possible mechanism for Cr(III)-mediated protection of wheat.     

Mechanical properties of Callitriche cophocarpa leaves under Cr(VI)/Cr(III) influence

Submersed Callitriche cophocarpa is an outstanding Cr phytoremediator in water systems. The mineral elements in waters can penetrate the submersed plant surface. This has led us to the hypothesis that the absorbed Cr can alter the mechanical properties of leaves. These properties were measured by applying atomic force microscopy. C. cophocarpa shoots were immersed in 100 µM (5.2 mg/l) Cr solution for 7 days. Cr was applied independently at two distinct oxidation states as Cr(VI) and Cr(III), known from different physicochemical properties and toxic effects. The contents of elements which were proportional to the fluorescence signal in individual leaves were evaluated using micro-X-ray fluorescence spectroscopy. The results obtained showed that the leaf epidermis significantly changes its elastic properties upon incubation with Cr-supplemented solution. When compared to the control, a drop in the leaf’s stiffness observed for Cr(III) was ca. 42 %. In the case of Cr(VI)-treated leaves, the stiffness raised to ca. 17 %. The changes in elasticity were significantly correlated with the contents of Ca (Pearson’s coefficient r = 0.87, p < 0.017). The results led us to ascertain that it is Cr(III) but not Cr(VI) that significantly influences Ca removal from leaves thus decreasing the stiffness of the leaf’s epidermis.     

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.     

Use of synchrotron- and plasma-based spectroscopic techniques to determine the uptake and biotransformation of chromium(III) and chromium(VI) by Parkinsonia aculeata

In this study, a combination of inductively coupled plasma optical emission spectroscopy and X-ray absorption spectroscopy (XAS) was used to study the uptake and speciation of chromium in Parkinsonia aculeata, commonly known as Mexican Palo Verde. Plants were treated for 14 days in a modified Hoagland solution containing chromium(III) or chromium(VI) at several concentrations. The results showed that plants treated with 70 mg Cr(III) L(-1) and 30 mg Cr(VI) L(-1) had similar Cr concentrations in leaves (～200 mg kg(-1) dry weight, DW). The results also showed that neither Cr(III) nor Cr(VI) affected the uptake of phosphorus and sulfur. However, the concentration of calcium in the stems of plants treated with Cr(VI) at 40 mg L(-1) (about 6000 mg Ca kg(-1) DW) was significantly higher compared to the Ca concentration (about 3000 mg kg(-1) DW) found in the stems of plants treated with 150 mg Cr(III) L(-1). However, no differences were observed in potassium and magnesium concentrations. The iron concentration (about 1000 mg kg(-1) DW) in roots treated with 40 mg Cr(VI) L(-1) was similar to the iron concentration found in the roots of plants treated with 110 mg Cr(III) L(-1). The XAS data showed that Cr(VI) was reduced to Cr(III) in/on the plant roots and transported as Cr(III) to the stems and leaves. The XAS studies also showed that Cr(III) within plants was present as an octahedral complex.     

Effects of dichromate on growth and root system architecture of Arabidopsis thaliana seedlings

The hexavalent form of chromium [Cr(VI)] is toxic for most organisms; however, very little information is available regarding the effects of this metal on plant morphogenesis. In this work, we investigated the effects of Cr(VI) on the growth and development of Arabidopsis thaliana, a species widely used as a model for studying the diverse physiological and cellular processes in plants. Elongation of root hairs and biomass production were stimulated by relatively low concentrations (100 μM) of Cr(VI) as potassium dichromate. Concentrations of Cr(VI) greater than 200 μM were toxic to plants as revealed both by arrested growth of roots and shoots and the development of chlorosis in leaves. At 200 μM the primary root growth was totally inhibited but the plants continued their growth manifesting different alterations in root development. These alterations correlated with changes in mitotic activity and in cellular expansion. The analyses of A. thaliana transgenic plants that express the auxin-inducible marker DR5:uidA, and the response of the auxin-resistant mutants axr2 and aux1–7 to dichromate suggest that auxins do not participate as mediators in the cellular and physiological responses to this metal. The primary root growth inhibition by 200 μM dichromate was alleviated by more than 70% by increasing the sulfate, phosphate or nitrate concentration in the media, which suggests a relation of dichromate with these mineral nutrients.     

Chromium (VI) induced changes in growth and root plasma membrane redox activities in pea plants

The effect of chromium (Cr) on growth as well as root plasma membrane redox reactions and superoxide radical production was studied in pea (Pisum sativum L. cv. Azad) plants exposed for 7 days to 20 and 200 μM Cr (VI), respectively, supplied as potassium dichromate. The growth of pea plants declined significantly at 200 μM Cr, as indicated by reduced leaf area and biomass. Relative to the control plants (no Cr exposure), the Cr content of roots increased significantly, both at 20 and 200 μM Cr. Following exposure to 200 μM Cr, there was a significant increase in root lipid peroxidation and hydrogen peroxide (H₂O₂) content, while both the Fv/Fm ratio and chlorophyll content were reduced. Exposure to Cr increased NADPH-dependent superoxide production in pea root plasma membrane vesicles, with the effect being more significant at 200 μM Cr than at 20 μM Cr. Treatment with Cr rapidly increased the activities of NADPH oxidase: relative to the controls, plants exposed to 20 μM Cr showed approximately a 67% increase in activity while there was a threefold increase in those plants exposed to 200 μM Cr. NADH-ferricyanide oxido-reductase activity was found to be inhibited by 16 and 51% at 20 and 200 μM Cr, respectively. The results of this study suggest that exposure to excess Cr damages pea root plasma membrane structure and function, resulting in decreased photosynthesis and poor plant growth.     

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).     

Transformation from Organo-Cr (III) to Trivalent Chromium Mineral (Guyanaite/Grimaldiite) and its Environmental Implication

Remediation of heavy-metal contamination by biomineralization has become an environmentally very important issue in the last two decades. Here we describe the transformation of amorphous organo-Cr(III) to chromium hydroxide oxide (guyanaite/grimaldiite) by hydrothermal treatment (HTT). First, glycine-Cr(III) was synthesized to serve as a simple model for exploring the conditions favoring HTT. Cell-bound Cr(III) was obtained by the reduction of hexavalent chromium [Cr(VI)] to trivalent chromium [Cr(III)] by Bacillus cereus. Then the reduced Cr(III) was chelated by ligands at the cell surface, forming cell-bound Cr(III). Subsequently, HTT was applied to treat cell-bound Cr(III) at different temperatures and for different lengths of time. The results showed that, by this treatment at 200°C for 7 days or at 250°C for 1 day, glycine-Cr(III) was converted to trivalent chromium mineral (guyanaite/grimaldiite), having the form of nanosheets with a length of 10～20 nm and a width of 3～5 nm under the described conditions. Cell-bound Cr(III) could also be converted to guyanaite/grimaldiite at 250°C for 9 days if it was bound by an organic compound more complex than glycine. Our finding showed that organo-Cr(III) could be transformed into minerals by an appropriate hydrothermal process, which is applicable to bioremediation of heavy-metal pollution. Our findings also suggest that organo-Cr(III) may play an important role in the biogeochemistry of chromium.     

Bioreduction of hexavalent chromium by <Emphasis Type="Italic">Pseudomonas stutzeri</Emphasis> L1 and <Emphasis Type="Italic">Acinetobacter baumannii</Emphasis> L2

Bioreduction of the very toxic hexavalent chromium ion [Cr(VI)] to the non-toxic trivalent chromium ion [Cr(III)] is a key remediation process in chromium-contaminated sites. In this study, we investigated the bioreduction of Cr(VI) by Pseudomonas stutzeri L1 and Acinetobacter baumannii L2. The optimum pH (5–10), temperature (27, 37 and 60 °C) and initial chromium Cr(VI) concentration (100–1000 mg L^?1) for Cr(VI) reduction by strains L1 and L2 were determined using the diphenylcarbazide method. In the presence of L1 and L2, the bioreduction rate of Cr(VI) was 40–97 and 84–99%, respectively. The bioreduction of Cr(VI) by L2 was higher, reaching up to 84%—than that by L1. The results showed that strain L2 was able to survive even if exposed to 1000 mg L^?1 of Cr(VI) and that this tolerance to the effects of Cr(VI) was linked to the activity of soluble enzyme fractions. Overall, A. baumannii L2 would appear to be a potent Cr(VI)-tolerant candidate for the bioremediation of chromium (VI)-contaminated wastewater effluent.     

Kocuria Flava Induced Growth and Chromium Accumulation in Cicer Arietinum L

In the present investigation a chromate tolerant rhizobacterium Kocuria flava was isolated and inoculated to the Cicer arietinum L to evaluate its effects on growth and chromium accumulation upon exposure of different concentration of chromium (1–10 μg ml^?1) as Cr (VI) for 24 d. K. flava inoculated plant of C. arietinum demonstrated luxuriant growth as compared to non inoculated plant at respective concentration of Cr (VI). K. flava found to ameliorate chromium induced phytotoxicity in terms of chlorophylls, carotenoid and protein contents and thus helps the plant in acquiring higher biomass with high chromium concentration. After 24 d, maximum concentration of chromium recorded in root of C. arietinum (4892.39 μg g^?1dw) inoculated with K. flava as compared to non inoculated plant (1762.22 μg g^?1dw) upon exposure of 5 μg ml^?1Cr (VI). Therefore, application of C. arietinum in association with K. flava could be more efficient in decontamination of chromium polluted site. Moreover, K. flava may be used as a bioresource for developing microbes assisted phytoremediation system due to its compatibility.     

Isolation of indigenous Staphylococcus sciuri from chromium-contaminated paddy field and its application for reduction of Cr(VI) in rice plants cultivated in pots

Accumulation of Cr(VI) in rice seeds cultivated in Cr-contaminated soil of the Sundarbans (India) is an environmental problem. Cr(VI) concentration in this soil was 6.2 ± 0.3 mg/kg, whereas total chromium was 32.04 ± 1.60 mg/kg. A Cr(VI)-removing bacterium isolated from Cr-contaminated paddy field soil of Sundarbans was identified as Staphylococcus sciuri. Enrichment culture of S. sciuri was applied to pot cultivation of rice in Cr-contaminated soil. After 8 weeks, 71 ± 3% Cr(VI) (final concentration 2.15 ± 0.01 mg/kg) and 65 ± 2% total Cr removal (end concentration 11.3 ± 0.5 mg/kg) were attained in bacterium-treated soils. Growth parameters indicated healthy development of plants cultivated in bacterium-treated soils that was not observed in control plants. Total Cr removal attained in rice seeds of plants cultivated in bacterium-treated soils compared with control rice seeds was 78 ± 4%. Total Cr concentration in test seeds was 0.72 ± 0.05 mg/kg (World Health Organization [WHO] permissible limit: 1.30 mg/kg), whereas the same in control seeds was 3.27 ± 0.16 mg/kg. Cr(VI) reduction achieved in rice seeds cultivated in bacterium-treated soil compared with control rice seeds was 95 ± 5%. Cr(VI) concentration in rice seeds cultivated in treated soil was 0.050 ± 0.003 mg/kg, whereas the same in untreated control was 0.93 ± 0.05 mg/kg. Successful paddy field soil bioremediation by any Staphylococcus species was demonstrated for the first time.     

Kinetic Study and Mathematical Modeling of Chromium(VI) Reduction and Microorganism Growth Under Mixed Culture

The kinetics of chromium(VI) reduction by Pseudomonas aeruginosa (P. aeruginosa) and Escherichia coli (E. coli) was studied under both pure and mixed cultures. Initially, the study of kinetics was performed in pure culture. It was observed that the growth of the two bacteria was both inhibited in the presence of chromium(VI). The maximum specific growth rate (μ _m ) of P. aeruginosa decreased from 2.3942 h^?1 (without Cr(VI)) to 1.8551 h^?1 (with Cr(VI)). Under the mixed culture, the growth of E. coli was inhibited by P. aeruginosa. The maximum specific growth rate (μ _m ) of E. coli decreased form 0.871 h^?1 (in pure culture) to 0.153 h^?1 (in mixed culture). When the concentration of each bacterium was 4.5 × 10⁸ cells ml^?1, the half-velocity reduction rate constant (K _C) and the maximum specific reduction rate constant (v _max) of chromium(VI) were 80.05 mg chromium(VI) l^?1 and 3.674 mg chromium(VI) cells^?1 h^?1, respectively. The results showed that the simulation appeared in good agreement with the experimental data, supporting the series of mathematical models represented the bacteria growth and chromium(VI) reduction in both pure and mixed cultures usefully.     

Chromium tolerance,oxidative stress response,morphological characteristics,and FTIR studies of phytopathogenic fungus <Emphasis Type="Italic">Sclerotium rolfsii</Emphasis>

Sclerotium rolfsii is one of the most destructive fungal plant pathogens that can infect over 500 plants and can adapt to diverse environmental conditions. The present research work was carried out to evaluate the impact of both hexa- and trivalent chromium (Cr) on growth, morphology, enzymatic characteristics, and metal accumulation in S. rolfsii under laboratory conditions. Experiments were performed in both malt extract broth and agar growth medium amended with six different concentrations (10, 20, 40, 60, 80, and 100 ppm) of each Cr(III) and Cr(VI) ions inoculated with fungus and incubated for 6–7 days at 25 ± 3 °C. In broth medium, the total protein content was declined and activities of antioxidant enzymes were increased with an increase in metal concentrations. Lower concentrations (10 ppm) of the metal ions stimulated the growth of fungus and higher concentrations (60–100) inhibited it. The Fourier transform infrared spectroscopy (FTIR) assessment showed hydroxyl, carboxyl, and amine groups as major metal binding sites. In agar medium, tolerance index was decreased up to 0.56 at 10–80 ppm of Cr(III) and up to 0.62 at 10–60 ppm of Cr(VI). Considerable modifications were observed in hyphal and sclerotial morphology with an increase in concentration of metal ions. The current study concluded that interference of Cr with growth and physiological process of S. rolfsii could affect its infection level on its host plant. This study provides important information regarding cultivation of susceptible plant varieties in Cr-polluted soil as evidenced by pathogen growth up to 50 ppm of Cr(III) and Cr(VI) ions.     

Speciation of chromium in chromium yeast

High-performance liquid chromatography was used to separate Cr(III) and Cr(VI) in samples with detection by inductively coupled plasma mass spectrometry(ICP-MS). The separation was achieved on a weak anion exchange column. The mobile phase was pH 7.0 ammonium nitrate solution. The redox reaction between Cr(III) and Cr(VI) was avoided during separation and determination. This separation method could be used to separate the samples with large concentration differences between Cr(III) and Cr(VI). The alkaline digestion was used to extract chromium in solid sample, which had no effect on the retention time and the peak area of the Cr(VI). However, the conversion of Cr(VI) from Cr(III) was observed during alkaline digestion, which displayed positive relation with the ratio of Cr(III) and Cr(VI) in samples. Both Cr(III) and Cr(VI) contents of chromium yeasts cultured in media with different chromium additions were determined. The spike recoveries of Cr(VI) for chromium yeasts were in the range of 95–108 %.     

Healthy Response from Chromium Survived Pteridophytic Plant-Ampelopteris prolifera with the Interaction of Mycorrhizal Fungus-Glomus deserticola

Interaction between arbuscular mycorrhizal fungus Glomus deserticola and pteridophytic member Ampelopteris prolifera was found abundant on entire growth level based on elemental composition and gaseous exchange as a potential remediation system for phytoextraction of chromium. Inoculated A. prolifera (AM) and non-inoculated A. prolifera (Non-AM) were supplied with two Cr species: 12 mmol of trivalent cation (Cr⁺³) [Cr(III)] and 0.1 mmol of divalent dichromate anion (Cr₂O₇ ^?2) [Cr(VI)]. Both Cr species were found to be depressed in overall growth and inefficient stomatal conductance (g_s) and net photosynthesis (NP). Mycorrhizal association was found to be natural scavenger of Cr toxicity as indicated by greater growth in plants exposed to Cr species, and increased gas exchange of Cr(III) treated plants. Though, chromium reduction resulted lower level of nitrogen (N), phosphorus (P), and potassium (K) but interestingly elevated the level of aluminum (Al), iron (Fe), and zinc (Zn) uptake in many folds which is the significance of sustainable growth of plant.     

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.     

Single and Combined Effects of Exposure Concentration and Duration on Biological Responses of Ceratophyllum demersum L. Exposed to Cr Species

This study aimed to demonstrate the ways in which two chromium species, Cr (III) and Cr (VI), can affect various physiological and biochemical parameters in the plant Ceratophyllum demersum L., and to evaluate the single and combined impact of exposure concentration and duration. C. demersum was exposed to Cr (III) and Cr (VI) at a variety of concentrations (1, 2, 5, and 10 mM) and for differing durations (1, 2, 4, and 7 days), after which Cr accumulation, relative growth rate (RGR), malondialdehyde (MDA) content, electrical conductivity (EC), photosynthetic pigmentation, proline content and antioxidant enzyme activities were examined. The single and combined effects of exposure duration and Cr concentration on each parameter were determined using a two-way analysis of variance. For both the Cr (III) and Cr (VI) applications, it was observed that concentration had a significant effect on all parameters assessed. However, duration had no statistically significant effect on proline content in the Cr (III) application, or on MDA and protein content in the Cr (VI) application. It was determined that concentration exerted greater effects than duration for both Cr species studied. In addition, the results indicated that duration and concentration had a synergistic effect on variations of RGR, EC, protein content, and antioxidant enzyme activities in both the Cr (III) and Cr (VI) applications. These results may be useful when planning further phytoremediation and plant biotechnology studies.     

Bioreduction of Hexavalent Chromium from Soil Column Leachate by Pseudomonas stutzeri

Bioreduction of Cr(VI) to less toxic Cr(III) by chromate-reducing bacteria has offered an ecological and economical option for chromate detoxification. The present study reports isolation of chromate-resistant bacterial strain Cr8 from chromium slag, identified as Pseudomonas stutzeri, based on 16S rRNA gene sequencing and their potential use in Cr(VI) reduction. The reduced product associated with bacterial cell was characterized by scanning electron microscopy–energy-dispersive x-ray spectroscopy (SEM-EDS) and x-ray diffraction (XRD) analyses. At initial concentrations of 100 and 200 mg L^?1 Cr(VI), P. stutzeri Cr8 reduced Cr(VI) completely within 24 h, whereas it reduced almost 1000 mg L^?1 Cr(VI) at the end of 120 h. Further, soil column leaching experiments were performed and found that bacterial cells reduced Cr(VI) leachate at faster rate that almost disappeared at the end of 168 h. The leachate precipitates also revealed efficient chromate bioreduction. The remediation process utilizing P. stutzeri could be considered as a viable alternative to reduce Cr(VI) contamination, especially emanating from the overburden dumps of chromite ores and mine drainage.