Mechanisms of interaction of chromium with Aspergillus niger var tubingensis strain Ed8

An Approach to Evaluate Pb Tolerance and Its Removal Mechanisms by Pleurotus opuntiae

Widespread lead (Pb) contamination prompts various environmental problems and accounts for about 1% of the global disease burden. Thus, it has necessitated the demand for eco-friendly clean-up approaches. Fungi provide a novel and highly promising approach for the remediation of Pb-containing wastewater. The current study examined the mycoremediation capability of a white rot fungus, P. opuntiae, that showed effective tolerance to increasing concentrations of Pb up to 200 mg L−1, evidenced by the Tolerance Index (TI) of 0.76. In an aqueous medium, the highest removal rate (99.08%) was recorded at 200 mg L−1 whereas intracellular bioaccumulation also contributed to the uptake of Pb in significant amounts with a maximum of 24.59 mg g−1. SEM was performed to characterize the mycelium, suggesting changes in the surface morphology after exposure to high Pb concentrations. LIBS indicated a gradual change in the intensity of some elements after exposure to Pb stress. FTIR spectra displayed many functional groups including amides, sulfhydryl, carboxyl, and hydroxyl groups on the cell walls that led to binding sites for Pb and indicated the involvement of these groups in biosorption. XRD analysis unveiled a mechanism of biotransformation by forming a mineral complex as PbS from Pb ion. Further, Pb fostered the level of proline and MDA at a maximum relative to the control, and their concentration reached 1.07 µmol g−1 and 8.77 nmol g−1, respectively. High Pb concentration results in oxidative damage by increasing the production of ROS. Therefore, the antioxidant enzyme system provides a central role in the elimination of active oxygen. The enzymes, namely SOD, POD, CAT, and GSH, served as most responsive to clear away ROS and lower the stress. The results of this study suggested that the presence of Pb caused no visible adverse symptoms in P. opuntiae. Moreover, biosorption and bioaccumulation are two essential approaches involved in Pb removal by P. opuntiae and are established as worthwhile agents for the remediation of Pb from the environment.

Exploring the Cr(VI) removal mechanism of Sporosarcina saromensis M52 from a genomic perspective

Serious hexavalent chromium [Cr(VI)] pollution has continuously threatened ecological security and public health. Microorganism-assisted remediation technology has strong potential in the treatment of environmental Cr(VI) pollution due to its advantages of high efficiency, low cost, and low secondary pollution. Sporosarcina saromensis M52, a strain with strong Cr(VI) removal ability, isolated from coastal intertidal zone was used in this study. Scanning electron microscopy coupled with energy dispersive X-ray analysis indicated M52 was relatively stable under Cr(VI) stress and trace amount of Cr deposited on the cell surface. X-ray photoelectron spectroscopy and X-ray diffraction analyses exhibited M52 could reduce Cr(VI) into Cr(III). Fourier transform infrared spectroscopy showed the bacterial surface was mainly consisted of polysaccharides, phosphate groups, carboxyl groups, amide II (NH/CN) groups, alkyl groups, and hydroxyl groups, while functional groups involving in Cr(VI) bio-reduction were not detected. According to these characterization analyses, the removal of Cr(VI) was primarily depended on bio-reduction, instead of bio-adsorption by M52. Genome analyses further indicated the probable mechanisms of bio-reduction, including the active efflux of Cr(VI) by chromate transporter ChrA, enzymatic redox reactions mediated by reductases, DNA-repaired proteases ability to minimize the ROS damage, and the formation of specific cell components to minimize the biofilm injuries caused by Cr(VI). These studies provided a theoretical basis which was useful for Cr(VI) remediation, especially in terms of increasing its effectiveness. THE MAIN FINDING OF THE WORK: M52 realized the bioremediation of Cr(VI) majorly through bio-reduction, including Cr(VI) efflux, chromate reduction, DNA repair, and the formation of specific cell components, instead of bio-adsorption.

Assessment of hexavalent chromium (VI) biosorption competence of indigenous Aspergillus tubingensis AF3 isolated from bauxite mine tailing

The intention of this research was to find the most eminent metal tolerant and absorbing autochthonous fungal species from the waste dump of a bauxite mine. Out of the 4 (BI-1, BI-II, BI-III, and BI-IV) predominant isolates, BI-II had an excellent metal tolerance potential against most of the metals in the subsequent order: Cr(VI) (1500), Cu(II) (600), Pb(II) (500), and Zn(II) (500-1500 μg mL^-1). BI-II had shown tolerance to Cr(VI) up to 1500 mg L^-1. The excellent metal tolerant isolate was characterized and identified as Aspergillus tubingensis AF3 through 18S rRNA sequencing method and submitted to GenBank and received an accession number (MN901243). A. tubingensis AF3 had the efficiency to absorb Cr(VI) and Cu(II) at <70 & 46.3% respectively under the standard growth conditions. Under the optimized conditions (25 °C, pH 7.0, 0.5% of dextrose, and 12 days of incubation), A. tubingensis AF3 absorbed 74.48% of Cr(VI) in 12 days (reduction occurred as 822.3, 719.13, 296.66, and 255.2 mg L^-1 of Cr(VI) on the 3^rd, the 6^th, the 9^th and the 12^th day, respectively). The adsorbed metal was sequestered in the mycelia of the fungus in a precipitated form; it was confirmed by Scanning Electron Microscope (SEM) and Energy Dispersive X-ray analysis (EDX) analyses. The possible biosorption mechanisms were analyzed by Fourier-Transform Infrared Spectroscopy (FTIR) analysis, the results showed the presence of N-H primary amines (1649.98 cm^-1) and Alkanes (914.30 cm^-1) in the cell wall of the fungus, while being treated with Cr(VI) they supported and enhanced the Cr(VI) absorption. The entire results concluded that the biomass of A. tubingensis AF3 had the potential to absorb a high concentration of Cr(VI).

A proteomic analysis of Pseudevernia furfuracea after exposure to Cr+6 by MALDI-TOF mass spectrometry

The problem of heavy metal pollution in nature has increased rapidly in recent years. Hexavalent chromium (Cr⁺⁶) is one of the most toxic heavy metals that cause environmental pollution. Although many studies in the literature that illuminate the stress response mechanisms of biological organisms such as bacteria, algae, and plants against heavy metals, there is limited information about revealing the protein level changes of lichen species in response to heavy metal stress. Here, we used a MALDI-TOF-based proteomic assay to determine protein level changes in Pseudevernia furfuracea after exposure to Cr⁺⁶ heavy metal stress at 6, 18 and 24 h. It was determined that expression levels of 26, 149 and 66 proteins changed in P. furfuracea. 6, 18 and 24 h after Cr⁺⁶ application compared to the control sample, respectively. We identified 9 common proteins expressed at three different time levels (6, 18, 24 h) and evaluated their protein-protein interaction profiles with the STRING tool. According to the results of the study, it was determined that the expression level of six proteins was up-regulated (OP4, KIP3, BNI5, VSP64, HSP 60, BCK1) and three proteins were down-regulated (MNS1, ABZ2, ATG4) from the expression level of nine proteins in total with Cr⁺⁶ exposure. It was determined that nine proteins were also found to be effective in biological processes such as stress signaling, transcription regulation and cellular detoxification metabolisms. To confirm the protein expression level, we analyzed the HSP60 protein by western blot assay. It has been shown that exposure to Cr⁺⁶ exposure in P. furfuracea caused an increase in HSP60 protein level compared to the control sample (non-exposed Cr⁺⁶). In this study, new knowledge are presented for the use of P. furfuracea as a biosorption agent in the removal of industrial wastes in biotechnological applications.

Mycoremediation of heavy metals: processes, mechanisms, and affecting factors

Industrial processes and mining of coal and metal ores are generating a number of threats by polluting natural water bodies. Contamination of heavy metals (HMs) in water and soil is the most serious problem caused by industrial and mining processes and other anthropogenic activities. The available literature suggests that existing conventional technologies are costly and generated hazardous waste that necessitates disposal. So, there is a need for cheap and green approaches for the treatment of such contaminated wastewater. Bioremediation is considered a sustainable way where fungi seem to be good bioremediation agents to treat HM-polluted wastewater. Fungi have high adsorption and accumulation capacity of HMs and can be potentially utilized. The most important biomechanisms which are involved in HM tolerance and removal by fungi are bioaccumulation, bioadsorption, biosynthesis, biomineralisation, bioreduction, bio-oxidation, extracellular precipitation, intracellular precipitation, surface sorption, etc. which vary from species to species. However, the time, pH, temperature, concentration of HMs, the dose of fungal biomass, and shaking rate are the most influencing factors that affect the bioremediation of HMs and vary with characteristics of the fungi and nature of the HMs. In this review, we have discussed the application of fungi, involved tolerance and removal strategies in fungi, and factors affecting the removal of HMs.

Trichoderma viride involvement in the sorption of Pb(II) on muscovite, biotite and phlogopite: Batch and spectroscopic studies

In this study, batch and spectroscopic approaches were used to explore the sorption of Pb(II) on micas (i.e., muscovite, biotite and phlogopite) in the presence of Trichoderma viride (T. viride). Batch sorption showed that ion exchange, outer-sphere complexes (OSCs) and inner-sphere complexes (ISCs) contributed to Pb(II) sorption on biotite and phlogopite in the pH range of 2.0-7.4, whereas the ISCs were predominant for Pb(II) sorption on muscovite. X-ray diffraction and Fourier transform infrared (FT-IR) analyses have confirmed the changes of structure and surface properties of micas after co-culturing with T. viride, which could improve the sorption capacity of micas to Pb(II). Scanning electron microscopy revealed the bio-mineralization of Pb(II) on T. viride and mica-T. viride composites forming lead phosphates. Furthermore, FT-IR analysis showed that the groups of Si-OH, Al-OH from micas, and carboxyl, phosphate and amino groups from T. viride were synergistically contributing to Pb(II) sorption on mica-T. viride composite. X-ray photoelectron spectroscopy further confirmed that both OSCs and ISCs formed for Pb(II) sorption on micas; however, in the case of mica-T. viride composites, the synergistic effects of T. viride and micas were contributing to Pb(II) sorption through forming the ISCs and biomineralization.

Multi-approach analysis to assess the chromium(III) immobilization by Ochrobactrum anthropi DE2010

Ochrobactrum anthropi DE2010 is a microorganism isolated from Ebro Delta microbial mats and able to resist high doses of chromium(III) due to its capacity to tolerate, absorb and accumulate this metal. The effect of this pollutant on O. anthropi DE2010 has been studied assessing changes in viability and biomass, sorption yields and removal efficiencies. Furthermore, and for the first time, its capacity for immobilizing Cr(III) from culture media was tested by a combination of High Angle Annular Dark Field (HAADF) Scanning Transmission Electron Microscopy (STEM) imaging coupled to Energy Dispersive X-ray spectroscopy (EDX). The results showed that O. anthropi DE2010 was grown optimally at 0-2 mM Cr(III). On the other hand, from 2 to 10 mM Cr(III) microbial plate counts, growth rates, cell viability, and biomass decreased while extracellular polymeric substances (EPS) production increases. Furthermore, this bacterium had a great ability to remove Cr(III) at 10 mM (q = 950.00 mg g^-1) immobilizing it mostly in bright polyphosphate inclusions and secondarily on the cellular surface at the EPS level. Based on these results, O. anthropi DE2010 could be considered as a potential agent for bioremediation in Cr(III) contaminated environments.

Hexavalent chromium stress response, reduction capability and bioremediation potential of Trichoderma sp. isolated from electroplating wastewater

In the present study we are investigating the Cr_(VI) reduction potential of a multi-metal tolerant fungus (isolate CR700); isolated from electroplating wastewater. Based on the ITS region sequencing, the isolate was identified as Trichoderma lixii isolate CR700 and able to tolerate As(2000 mg/L), Ni(1500 mg/L), Zn(1200 mg/L), Cu(1200 mg/L), Cr(1000 mg/L), and 100 mg/L of Pb and Cd evident from tolerance assay. Cr_(VI) reduction experiment was conducted in Erlenmeyer flasks containing different concentration of Cr_(VI) (0-200 mg/L) amended potato dextrose broth medium followed by inoculating with a disk (0.5 cm diameter) of 7 days grown isolate CR700, and achieved a maximum of 99.4% within 120 h at 50 mg/L of Cr_(VI). However, the accumulation of total Cr by isolate CR700 was 2.12 ± 0.15 mg/g of dried biomass at the same concentration after 144 h of exposure. Isolate CR700 showed the capability to reduce Cr_(VI) at different physicochemical stress conditions such as pH, temperature, heavy metals, metabolic inhibitor and also in tannery wastewater. Fungus exhibited multifarious morphological and biochemical response under the exposure of Cr_(VI); the scanning electron microscopic analysis revealed that Cr_(VI) treated mycelia of isolate CR700 comparatively irregular, aggregated and swelled than without treated mycelia which might be due to the tolerance mechanism and vacuolar compartmentation of chromium. Moreover, energy dispersive spectroscopy and x-ray photoelectron spectroscopic analysis exposed the Cr_(III) precipitation on the mycelia surface of isolate CR700 and Fourier-transform infrared spectroscopic analysis suggested the contribution of the protein associated functional group in the complexation of Cr_(VI). The phytotoxicity test of fungal treated 100 mg/L of Cr_(VI) supernatant on Vigna radiata and Cicer arietinum revealed the successful detoxification/remediation of Cr_(VI).

Hexavalent chromium reduction ability and bioremediation potential of Aspergillus flavus CR500 isolated from electroplating wastewater

Hexavalent chromium reduction by microbes can mitigate the chromium toxicity to the environment. In the present study Cr_[VI] tolerant fungal isolate (CR500) was isolated from electroplating wastewater, was able to tolerate 800 mg/L of Cr_[VI. Based on the ITS region sequencing, the isolate was identified as Aspergillus flavus CR500, showed multifarious biochemical (reactive oxygen species, antioxidants response and non-protein thiol) and morphological (protrusion less, constriction and swelling/outwards growth in mycelia) response under Cr_[VI] stress. Batch experiment was conducted at different Cr_[VI] concentration (0-200 mg/L) to optimize the Cr_[VI] reduction and removal ability of isolate CR500; results showed 89.1% reduction of Cr_[VI] to Cr_[III] within 24 h and 4.9 ± 0.12 mg of Cr per gram of dried biomass accumulation within 144 h at the concentration of 50 mg/L of Cr_[VI]. However, a maximum of 79.4% removal of Cr was recorded at 5 mg/L within 144 h. Fourier-transform infrared spectroscopy, energy dispersive x-ray spectroscopy and X-ray diffraction analysis revealed that chromium removal also happened via adsorption/precipitation on the mycelia surface. Fungus treated and without treated 100 mg/L of Cr_[VI] solution was subjected to phytotoxicity test using Vigna radiata seeds and result revealed that A. flavus CR500 successfully detoxified the Cr_[VI] via reduction and removal mechanisms. Isolate CR500 also exhibited efficient bioreduction potential at different temperature (20-40 °C), pH (5.0-9.0), heavy metals (As, Cd, Cu, Mn, Ni and Pb), metabolic inhibitors (phenol and EDTA) and in sterilized tannery effluent that make it a potential candidate for Cr_[VI] bioremediation.

Chromium tolerance and accumulation in Aspergillus flavus isolated from tannery effluent

Cr(VI) tolerance in Aspergillus flavus, strain SFL, isolated from tannery effluent was measured and compared with a reference strain of A. flavus, A1120. On solid medium, SFL had a high level of Cr(VI) tolerance (1,600 mg/L), which was 16 times that of A1120 and greater than most previously analyzed fungal strains. When in 100 mg/L of Cr(VI), SFL completely depleted Cr(VI) within 72 h while A1120 depleted 85% of Cr(VI). SFL was more effective in reducing extracellular Cr(VI) than A1120. While A1120 showed greater biosorption of Cr(VI) than SFL, intracellular accumulation was approximately 50% greater in SFL and was more energy-dependent than A1120. Cr(VI) modified the external surface of the hyphae. Cr speciation detected the presence of only Cr(III), corresponding to Cr(OH)₃ , which precipitated on the hyphal surface. Cr(VI) bound to the functional groups carboxyl, amine, and hydroxyl in both SFL and A1120. Transmission electron microscopy energy-dispersive X-ray detected Cr on the fungal wall and within membrane-bound organelles of the cytoplasm. In conclusion, the greater tolerance of SFL to Cr(VI) relative to A1120 is due to more effective energy-dependant uptake of Cr(VI) into the cell and increased capacity of SFL to store Cr in intracellular vacuoles compared with A1120.

Marine Fungi: Biotechnological Perspectives from Deep-Hypersaline Anoxic Basins

Deep-sea hypersaline anoxic basins (DHABs) are one of the most hostile environments on Earth. Even though DHABs have hypersaline conditions, anoxia and high hydrostatic pressure, they host incredible microbial biodiversity. Among eukaryotes inhabiting these systems, recent studies demonstrated that fungi are a quantitatively relevant component. Here, fungi can benefit from the accumulation of large amounts of organic material. Marine fungi are also known to produce bioactive molecules. In particular, halophilic and halotolerant fungi are a reservoir of enzymes and secondary metabolites with valuable applications in industrial, pharmaceutical, and environmental biotechnology. Here we report that among the fungal taxa identified from the Mediterranean and Red Sea DHABs, halotolerant halophilic species belonging to the genera Aspergillus and Penicillium can be used or screened for enzymes and bioactive molecules. Fungi living in DHABs can extend our knowledge about the limits of life, and the discovery of new species and molecules from these environments can have high biotechnological potential.

Biosorption of hexavalent chromium by Pleurotus ostreatus

The existing techniques for the removal of heavy metals are expensive and frequently inefficient. Thus the application of biosorbents has arisen as an alternative, this being emergent technology that must be studied and explored, with the aim of promoting better environmental and human life quality. The objective of this study was to verify the capacity of active and inactive Pleurotus ostreatus fungal biomass in removing Cr(VI) ions by biosorption from synthetic aqueous solutions of these ions at concentrations of 10, 25, 50, 75, 100, 125 and 150 mg L-1. When using active biomass, the kinetic studies showed that 100% of biosorption was reached from the 25 mg L-1Cr(VI) solution in 360 hours, equivalent to the removal of 169.84 mg g-1 of total Cr. On the other hand the inactive biomass reached 100% of its saturation capacity in 22 minutes for a concentration of 50 mg L-1 of Cr(VI), equivalent to the removal of 368.21 mg g-1 of total Cr. The kinetic study was shown to be highly effective, presenting an efficiencies of times 500 and 750 for the active and inactive P. ostreatus biomasses, respectively, when compared to the limit of 0.1 mg L-1 of Cr(VI) for industrial effluents described in CONAMA resolution n° 430/2011.

Bioremediation of hexavalent chromium by endophytic fungi; safe and improved production of Lactuca sativa L

One of the main problems of the industrialized world is the accumulation of chromium (Cr) in soil, which is a serious threat to the crops. Complete removal of Cr from the contaminated soils poses a great challenge. However, this issue can be minimised by using plant growth promoting microbes as a bioremediation tool. In the present study, healthy plants established near the University campus in Mardan were selected for the isolation of Cr resistant endophytes. From the designated plants, 114 species of endophytic fungi were isolated. Among the 114 isolated strains, 4 strains have induced resistance in L. sativa against Cr. The strains were identified as Aspergillus fumigatus, Rhizopus sp., Penicillium radicum and Fusarium proliferatum based on ITS region (18 S rDNA) homology. The isolates have removed Cr from soil and culture media as well as bio-transformed it from highly toxic hexavalent to least toxic trivalent form, thus helped the Cr stressed L. sativa to restore its normal growth. The Rhizopus Sp. CUC23 has mainly accrued Cr and detoxified intracellularly, whereas A. fumigatus ML43 and P. radicum PL17 has detoxified up to 95% of Cr extracellularly. From the results, it is concluded that the selected endophytic strains might be used as biofertilizer for healthy and safe crop production in Cr contaminated soils.

Bioremediation strategies for chromium removal: Current research, scale-up approach and future perspectives

Industrial applications and commercial processes release a lot of chromium into the environment (soil, surface water or atmosphere) and resulting in serious human diseases because of their toxicity. Biological Cr-removal offers an alternative to traditional physic-chemical methods. This is considered as a sustainable technology of lower impact on the environment. Resistant microorganisms (e.g. bacteria, fungi, and algae) have been most extensively studied from this characteristic. Several mechanisms were developed by microorganisms to deal with chromium toxicity. These tools include biotransformation (reduction or oxidation), bioaccumulation and/or biosorption, and are considered as an alternative to remove the heavy metal. The aim of this review is summarizes Cr(VI)-bioremediation technologies oriented on practical applications at larger scale technologies. In the same way, the most relevant results of several investigations focused on process feasibility and the robustness of different systems (reactors and pilot scale) designed for chromium-removal capacity are highlighted.

Heavy Metal Stress, Signaling, and Tolerance Due to Plant-Associated Microbes: An Overview

Several anthropogenic activities including mining, modern agricultural practices, and industrialization have long-term detrimental effect on our environment. All these factors lead to increase in heavy metal concentration in soil, water, and air. Soil contamination with heavy metals cause several environmental problems and imparts toxic effect on plant as well as animals. In response to these adverse conditions, plants evolve complex molecular and physiological mechanisms for better adaptability, tolerance, and survival. Nowadays conventional breeding and transgenic technology are being used for development of metal stress resistant varieties which, however, are time consuming and labor intensive. Interestingly the use of microbes as an alternate technology for improving metal tolerance of plants is gaining momentum recently. The use of these beneficial microorganisms is considered as one of the most promising methods for safe crop-management practices. Interaction of plants with soil microorganisms can play a vital role in acclimatizing plants to metalliferous environments, and can thus be explored to improve microbe-assisted metal tolerance. Plant-associated microbes decrease metal accumulation in plant tissues and also help to reduce metal bioavailability in soil through various mechanisms. Nowadays, a novel phytobacterial strategy, i.e., genetically transformed bacteria has been used to increase remediation of heavy metals and stress tolerance in plants. This review takes into account our current state of knowledge of the harmful effects of heavy metal stress, the signaling responses to metal stress, and the role of plant-associated microbes in metal stress tolerance. The review also highlights the challenges and opportunities in this continued area of research on plant-microbe-metal interaction.

Characterization of Penicillium oxalicum SL2 isolated from indoor air and its application to the removal of hexavalent chromium

Removal of toxic Cr(VI) by microbial reduction is a promising approach to reducing its ecotoxicological impact. To develop bioremediation technologies, many studies have evaluated the application of microorganisms isolated from Cr(VI)-contaminated sites. Nonetheless, little attention has been given to microbes from the environments without a history of Cr(VI) contamination. In this study, we aimed to characterize the Cr(VI) tolerance and removal abilities of a filamentous fungus strain, SL2, isolated from indoor air. Based on phenotypic characterization and rDNA sequence analysis, SL2 was identified as Penicillium oxalicum, a species that has not been extensively studied regarding Cr(VI) tolerance and reduction abilities. SL2 showed high tolerance to Cr(VI) on solid and in liquid media, facilitating its application to Cr(VI)-contaminated environments. Growth curves of SL2 in the presence of 0, 100, 400, or 1000 mg/L Cr(VI) were well simulated by the modified Gompertz model. The relative maximal colony diameter and maximal growth rate decreased as Cr(VI) concentration increased, while the lag time increased. SL2 manifested remarkable efficacy of removing Cr(VI). Mass balance analysis indicated that SL2 removed Cr(VI) by reduction, and incorporated 0.79 mg of Cr per gram of dry biomass. In electroplating wastewater, the initial rate of Cr(VI) removal was affected by the initial contaminant concentration. In conclusion, P. oxalicum SL2 represents a promising new candidate for Cr(VI) removal. Our results significantly expand the knowledge on potential application of this microorganism.

Detoxification of hexavalent chromate by growing Paecilomyces lilacinus XLA

In the study, the capability of Paecilomyces lilacinus XLA (CCTCC: M2012135) to reduce Cr⁶⁺ and its main antagonistic mechanisms to Cr⁶⁺ were experimentally evaluated. Activated growing fungus XLA efficiently reduced over 90% Cr⁶⁺ in the media with Cr⁶⁺ concentration below 100 mg L^-1 at pH 6 after 14 days. After 1-day exposure to 100 mg L^-1 Cr⁶⁺, nearly 50% of Cr⁶⁺ was reduced. Moreover, SO₄^2- stimulated Cr⁶⁺ reduction, whereas other interferential ions inhibited Cr⁶⁺ reduction. The interaction mechanisms between XLA and Cr⁶⁺ mainly involve biotransformation, biosorption, and bioaccumulation, as detected by electron microscopy and chemical methods. The lower concentrations of Cr⁶⁺ (5 and 50 mg L^-1) stimulated the activities of superoxide dismutase (SOD), catalase (CAT) and glutathione (GSH) level in XLA, respectively, but the higher concentration of Cr⁶⁺ (150 mg L^-1) decreased the enzymatic activities and GSH concentration. The results implied that SOD, CAT and GSH were defensive guards to the oxidant stress produced by Cr⁶⁺. All these extracellular/intracellular defense systems endowed XLA with the ability to resist and detoxify Cr⁶⁺ by transforming its valent species. The fungus XLA could efficiently reduce Cr⁶⁺ under different environmental conditions (pH, interferential ions, and concentration). Moreover, XLA could endure the high concentration of Cr⁶⁺ probably due to its high biotransformation capability of Cr⁶⁺ and intracellular antioxidant systems for the detoxification of ROS generated by external Cr⁶⁺. All these results suggested that the fungus XLA can be applied to remediation of Cr⁶⁺-contaminated environments.

Potential of Salvinia auriculata biomass as biosorbent of the Cr(III): directed chemical treatment, modeling and sorption mechanism study

In this work, the mechanism of the Cr(III) sorption by Salvinia auriculata biosorbent was studied in two stages. To understand the influence of the sorption parameters on the Cr(III) uptake, preliminary tests were performed. First, S. auriculata biomass was separately treated with base and acid solutions. Second, acid and base treatment of samples was performed based on the knowledge data base of our group. It was achieved a higher Cr(III) sorption capacity above 15 mg g-1 as associated to an increase of the micro-pores specific area and biosorbent volume. The obtained kinetic data of raw and treated biosorbents were well described by the intra-particle diffusion model. In this model, Cr(III) adsorption onto treated biomass is progressively improved with appearing of different mass transfer zones from out layer up to micro-porous layers. The equilibrium data of raw biomass were best described by the Langmuir isotherm, whereas the equilibrium data of the treated biomass were best fit by a combination of both Langmuir and Dubinin-Radushkevich isotherms. At low concentrations the adsorption most likely occurred on the outer monolayer, as proposed by the Langmuir model, followed by the adsorption on the micro-porous layers, as validated by the Dubinin-Radushkevich isotherm.

The Geomicrobiology of Chromium (VI) Pollution: Microbial Diversity and its Bioremediation Potential

The role and significance of microorganisms in environmental recycling activities marks geomicrobiology one of the essential branches within the environmental biotechnology field. Naturally occurring microbes also play geo-active roles in rocks, leading to biomineralization or biomobilization of minerals and metals. Heavy metals, such as chromium (Cr), are essential micronutrients at very low concentrations, but are very toxic at higher concentrations. Generally, heavy metals are leached to the environment through natural processes or anthropogenic activities such as industrial processes, leading to pollution with serious consequences. The presence of potentially toxic heavy metals, including Cr, in soils does not necessarily result in toxicity because not all forms of metals are toxic. Microbial interaction with Cr by different mechanisms leads to its oxidation or reduction, where its toxicity could be increased or decreased. Chromite contains both Cr(III) and Fe(II) and microbial utilization of Fe(II)- Fe(III) conversion or Cr (III) - Cr (VI) could lead to the break-down of this mineral. Therefore, the extraction of chromium from its mineral as Cr (III) form increases the possibility of its oxidation and conversion to the more toxic form (Cr (VI)), either biologically or geochemically. Cr (VI) is quite toxic to plants, animals and microbes, thus its levels in the environment need to be studied and controlled properly. Several bacterial and fungal isolates showed high tolerance and resistance to toxic Cr species and they also demonstrated transformation to less toxic form Cr (III), and precipitation. The current review highlights toxicity issues associated with Cr species and environmental friendly bioremediation mediated by microorganisms.

Microbial Diversity of Chromium-Contaminated Soils and Characterization of Six Chromium-Removing Bacteria

Three soil samples obtained from different sites adjacent to a chromium slag heap in a steel alloy factory were taken to examine the effect of chromium contamination on soil bacterial diversity as determined by construction of 16S rDNA clone libraries and sequencing of selected clones based on restriction fragment length polymorphism (RFLP) analysis. Results revealed that Betaproteobacteria, Gammaproteobacteria, Firmicutes, and Alphaproteobacteria occurred in all three soil samples, although the three samples differed in their total diversity. Sample 1 had the highest microbial diversity covering 12 different classes, while Sample 3 had the lowest microbial diversity. Strains of six different species were successfully isolated, one of which was identified as Zobellella denitrificans. To our knowledge, this is the first report of a strain belonging to the genus Zobellella able to resist and reduce chromium. Among all isolates studied, Bacillus odysseyi YH2 exhibited the highest Cr(VI)-reducing capability, with a total removal of 23.5 % of an initial Cr(VI) concentration of 350 mg L(-1).

Cr(VI) reduction by gluconolactone and hydrogen peroxide, the reaction products of fungal glucose oxidase: Cooperative interaction with organic acids in the biotransformation of Cr(VI)

The Cr(VI) reducing capability of growing cells of the environmental A. tubingensis Ed8 strain is remarkably efficient compared to reference strains A. niger FGSC322 and A. tubingensis NRRL593. Extracellular glucose oxidase (GOX) activity levels were clearly higher in colonies developed in solid medium and in concentrated extracts of the spent medium of liquid cultures of the Ed8 strain in comparison with the reference strains. In addition, concentrated extracts of the spent medium of A. tubingensis Ed8, but not those of the reference strains, exhibited the ability to reduce Cr(VI). In line with this observation, it was found that A. niger purified GOX is capable of mediating the conversion of Cr(VI) to Cr(III) in a reaction dependent on the presence of glucose that is stimulated by organic acids. Furthermore, it was found that a decrease in Cr(VI) may occur in the absence of the GOX enzyme, as long as the reaction products gluconolactone and hydrogen peroxide are present; this conversion of Cr(VI) is stimulated by organic acids in a reaction that generates hydroxyl radicals, which may involve the formation of an intermediate peroxichromate(V) complex. These findings indicated that fungal glucose oxidase acts an indirect chromate reductase through the formation of Cr(VI) reducing molecules, which interact cooperatively with other fungal metabolites in the biotransformation of Cr(VI).