Microbial Cr Research Articles

Comparison of hexavalent chromium and cadmium removal from wastewater in constructed rapid infiltration system with layered double hydroxides (LDHs) modified maifanite substrates: Physical, chemical and microbial mechanism

In order to protect the ecological environment on which human beings depend, it is of great practical significance to prevent and control the heavy metal pollution in surface water. Cr(VI) in anionic form and Cd(II) in cationic form were selected to conduct the heavy metal purification experiments. Cr(VI) and Cd(II) removal by constructed rapid infiltration systems (CRIS) contain Zn-CO3-LDHs (ZnAl-CO3-LDHs and ZnFe-CO3-LDHs) coated maifanite substrates were compared. After ZnAl-CO3-LDHs coated, the Cr(VI) and Cd(II) removal amount increased by 87.53 % and 19.00 %, respectively. It could be inferred from XRD, FTIR, and Zeta potential results that the Cr(VI) physicochemical removal mechanism of maifanite changed to anion exchange, chemical bonding and electrostatic adsorption from electrostatic adsorption and chemical bonding after Zn-CO3-LDHs coated. By contrast, isomorphous replacement, electrostatic adsorption and chemical bonding of Cd(II) induced by Zn-CO3-LDHs replaced the electrostatic adsorption by natural maifanite. Additionally, the effective Cr(VI)/Cd(II) accumulation on Zn-CO3-LDHs modified maifanite reduced the microbial quantity, but increased EPS secretion and changed the community structure, which achieved the microbial Cr(VI)/Cd(II) removal through extracellular complexation and intracellular accumulation. This research result may provide essential reference for exploring the feasibility of heavy metal removal of CRIS with LDHs modified substrate.

Microbial communities outperform extracellular polymeric substances in the reduction of hexavalent chromium by anaerobic granular sludge

The reduction of hexavalent chromium [Cr(VI)] by microbiota agents is a potential method for ecological remediation in heavily contaminated soil. This study investigated the contribution of microbial community and extracellular polymeric substances (EPS) to reduce Cr(VI) using lab-cultured anaerobic granular sludge (AGS). The AGS demonstrated an impressive reduction efficiency of 99.3% for Cr(VI) in soil, operating within an initial Cr(VI) concentration range of 500–3000 mg/kg. The distribution of typical electron acceptors, e.g., SO42-, NO3-, and Fe3+, had a negligible influence on microbial Cr(VI) reduction. The contribution of EPS on Cr(VI) reduction accounted for only 2.1% of the total Cr(VI) reduction, implying that Cr(VI) reduction driven by AGS was primarily caused by the direct effect of microbiota. High-throughput sequencing analysis confirmed that an increasing trend in the relative abundance of Cr reduction bacteria (Sphingopyxis: 0.40–5.54%, Thermonas: 0.41–5.70%, Edaphobaculum: 0.35–6.64%, etc.), and the stability (0.716–4.114) of microbial co-occurrence networks was significantly promoted by the end of the experiment. Materials characterization indicated that successful reduction of Cr(VI) to trivalent chromium [Cr(III)] and formed in Cr(OH)3, and functional groups, O-H, N-H, -CH, and -CH2, of AGS likely played a vital role in Cr(VI) reduction. Overall, these findings offer insights into the remediation technique of microbial Cr(VI) reduction in highly-contaminated soil based on the interaction network stability of the microbial community in AGS.

The interaction of ZnO nanoparticles, Cr(VI), and microorganisms triggers a novel ROS scavenging strategy to inhibit microbial Cr(VI) reduction.

Cr(VI) contaminated water usually contains other contaminants like engineered nanomaterials (ENMs). During the process of microbial treatment, the inevitable interaction of Cr(VI), ENMs, and microorganisms probably determines the efficiency of Cr(VI) biotransformation, however, the corresponding information remains elusive. This study investigated the interaction of ZnO nanoparticles (NPs), Cr(VI), and Pannonibacter phragmitetus BB (hereafter BB), which changed the process of microbial Cr(VI) reduction. ZnO NPs inhibited Cr(VI) reduction, but had no effect on bacterial viability. In particular, Cr(VI) induced BB to produce organic acids and to drive Zn2+ dissolution from ZnO NPs inside and outside of cells. The dissolved Zn2+ not only promoted Cr(VI) reduction to Cr(V)/Cr(IV) by strengthening sugar metabolism and inducing increase in NAD(P)H production, but also hindered Cr(V)/Cr(IV) transformation to Cr(III) through down-regulating Cr(VI) reductase genes. A novel bacterial driven ROS scavenging mechanism leading to the inhibition of Cr(VI) reduction was elucidated. Specifically, the accumulated Cr(VI) and Cr(V)/Cr(IV) formed a redox dynamic equilibrium, which triggered the disproportionation of superoxide radicals mimicking superoxide dismutase through the flip-flop of Cr(VI) and Cr(V)/Cr(IV) in bacterial cells. This study provided a realistic insight into design the applicability of biological remediation technology for Cr(VI) contaminant and evaluating environmental risks of ENMs.

External sodium acetate improved Cr(VI) stabilization in a Cr-spiked soil during chemical-microbial reduction processes: Insights into Cr(VI) reduction performance, microbial community and metabolic functions

Interest combined chemical and microbial reduction for Cr(VI) remediation in contaminated sites has greatly increased. However, the effect of external carbon sources on Cr(VI) reduction during chemical-microbial reduction processes has not been studied. Therefore, in this study, the role of external sodium acetate (SA) in improving Cr(VI) reduction and stabilization in a representative Cr(VI)-spiked soils was systemically investigated. The results of batch experiments suggested that the soil Cr(VI) content declined from 1000 mg/kg to 2.6–5.1 mg/kg at 1–5 g C/kg SA supplemented within 15 days of reaction. The external addition of SA resulted in a significant increase in the relative abundances of Cr(VI)-reducing microorganisms, such as Tissierella, Proteiniclasticum and Proteiniclasticum. The relative abundance of Tissierella increased from 9.1% to 29.8% with the SA treatment at 5 g C/kg soil, which was the main contributors to microbial Cr(VI) reduction. Redundancy analysis indicated that pH and SA were the predominant factors affecting the microbial community in the SA treatments at 2 g C/kg soil and 5 g C/kg soil. Functional prediction suggested that the addition of SA had a positive effect on the metabolism of key substances involved in Cr(VI) microbial reduction. This work provides new insightful guidance on Cr(VI) remediation in contaminated soils.

Chromium (VI) reduction by two-chamber bioelectrochemical system with electrically conductive wall

Hexavalent chromium (Cr(VI)) poses a threat to public health and the environment owing to its high toxicity. Various microbial Cr(VI) reduction methods have been proposed to treat contaminated water; however, they are not effective at high concentrations of Cr(VI), which inhibit microbial growth and viability. To overcome the limitations of conventional microbial methods, we set up a novel bioelectrochemical reactor in which microbial cells were separated from Cr(VI) by an electroconductive barrier consisting of a vertically aligned carbon nanotube (VACNT) composite to remove toxic heavy metal ions. This study demonstrated that chromium compounds could be biologically and sustainably treated with the VACNT composite without growth inhibition, using electron transfer from an electron shuttle secreted by Pseudomonas aeruginosa, even at the microbial inhibitory concentration of Cr(VI). The content of Cr(VI) was measured using the diphenylcarbazide method, and Cr(III), the reduction product, was identified using X-ray photoelectron spectroscopy. Pyocyanin, a phenazine secreted by P. aeruginosa, was identified as an electron mediator in Cr(VI) reduction, using aeration and phz mutant assay. The bioelectrochemical reactor used in this study exhibited a Cr(VI) removal rate of 99.6% at 100 mg/L, which is the highest reduction rate achieved using P. aeruginosa. The microbial Cr(VI) reduction system developed in this study allows microbial Cr(VI) reduction to overcome the hurdles of biological treatment of toxic materials and has advanced the understanding of microbial electron transfer.

Ethane-driven chromate and nitrate bioreductions in a membrane biofilm reactor

Chromium (Cr(VI)) and co-contaminating nitrate (NO3–) pose major threats to human health. Hydrogen- and methane-driven Cr(VI) and NO3– bioreductions have been documented, but unknown is whether or not non-methane short-chain gaseous alkanes (SCGAs), such as ethane (C2H6), also can drive Cr(VI) and NO3– reductions. This study demonstrated simultaneous Cr(VI) and NO3– bioreductions driven by C2H6 oxidation in a membrane biofilm reactor (MBfR) that delivered C2H6 directly to a biofilm by its diffusion through non-porous membranes. Scanning electron microscopy and X-ray photoelectron spectroscopy showed that the biofilm was mainly composed of cocci and bacillus and that a Cr(III) precipitate was the major product of microbial Cr(VI) reduction. Microbial community analysis showed that the C2H6-oxidizing bacterium Mycobacterium oxidized C2H6 and delivered organic intermediates to Cr(VI)-reducing bacteria TM7a, Sediminibacterium, and Deinococcus. Because the first step of ethane catabolism is a monooxygenation, the influent had to have dissolved oxygen, and the dominant respiratory process was O2 reduction. By providing insights into how Cr(VI) and NO3– bioreductions can be driven by C2H6 oxidation, these finding support the application of SCGAs for ex-situ bioremediation of waters contaminated with Cr(VI) and NO3–.

Microbial Cr(vi) Reduction by Indigenous Activated Sludge Bacilli and Pure Culture of Escherichia Coli Atcc 33456

Microbial Cr(vi) Reduction by Indigenous Activated Sludge Bacilli and Pure Culture of Escherichia Coli Atcc 33456

Immobilizing chromate reductase NfoR on magnetic biochar reduced Cr(VI) in copper-containing wastewater

Heavy metals (e.g. copper) in smelting wastewater inhibited microbial Cr(VI) reduction, previous report found that a novel chromate reductase NfoR exhibited Cu(II)-enhanced activity and broad-spectrum metal tolerance. In this study, magnetic biochar (MB) from pine sawdust was fabricated to immobilize NfoR via covalent binding. MB-NfoR showed a 2.6- and 2.1-folds higher activity in reducing Cr(VI) to Cr(III) compared to free NfoR and MB, respectively. XPS and EPR analysis showed that NfoR catalysis and persistent free radicals (PFRs) are responsible for Cr(VI) reduction mechanism of MB-NfoR. The addition of Cu(II) increased the MB-NfoR activity 2.3-fold compared to that of without Cu(II). The maximum Cr(VI) removal by MB-NfoR in the pure Cr(VI) solution was 98% at 45 °C and pH 6.0, and the MB-NfoR retained 68.3% of its initial activity even after five consecutive cycles. Application of MB-NfoR in smelting wastewater can simultaneously remove 94% Cr(VI) and 52.1% Cu(II). Furthermore, treated smelting wastewater by MB-NfoR showed no phytotoxicity and enriched numerous oligotrophic bacteria . This study provides a novel ‘one-stop’ strategy for decontamination of effluents combined with heavy metals, utilizing synergies offered by constituents in such waste streams. • Cu(II)-enhanced NfoR was covalently immobilized on magnetic biochar (MB). • NfoR- and PFRs-mediated reduction are the Cr(VI) removal mechanism of MB-NfoR. • MB-NfoR removes 94% Cr(VI) and 52.1% Cu(II) from smelting wastewater.

Reductive Cr(VI) Removal under Different Reducing and Electron Donor Conditions—A Soil Microcosm Study

Increased groundwater and soil contamination by hexavalent chromium have led to the employment of a variety of detoxification methods. Biological remediation of Cr(VI) polluted aquifers is an eco-friendly method that can be performed in situ by stimulating the indigenous microbial population with organic and inorganic electron donors. In order to study the effect of different redox conditions on microbial remediated Cr(VI) reduction to Cr(III), microcosm experiments were conducted under anaerobic, anoxic, and sulfate-reducing conditions and at hexavalent chromium groundwater concentrations in the 0–3000 μg/L range, with groundwater and soil collected from an industrial area (Inofyta region). As electron donors, molasses, emulsified vegetable oil (EVO), and FeSO4 were employed. To quantitatively describe the degradation kinetics of Cr(VI), pseudo-first-order kinetics were adopted. The results indicate that an anaerobic system dosed with simple or complex external organic carbon sources can lead to practically complete Cr(VI) reduction to Cr(III), while the addition of Fe2+ can further increase Cr(VI) removal rate significantly. Furthermore, Cr(VI) microbial reduction is possible in the presence of NO3− at rates comparable to anaerobic Cr(VI) microbial reduction, while high sulfate concentrations have a negative effect on Cr(VI) bioreduction rates in comparison to lower sulfate concentrations.

Hexavalent chromium reduction and bioremediation potential of Fusarium proliferatum S4 isolated from chromium-contaminated soil.

Microbial remediation, utilizing reduction of Cr(VI) to Cr(III), is considered a promising method for lowering toxic environmental chromium levels. In this study, a Cr(VI)-resistant fungal strain, Fusarium proliferatum S4 (F. proliferatum), was isolated from seriously chromium-polluted soil at Haibei Chemical Plant, China. This strain for treatment chromium-containing solution resulted in 100.00%, 93%, and 74% removal at initial concentrations of 10, 30, and 50mg L-1 Cr(VI), respectively, after 12days of treatment in a batch mode. Contributions of different cell fractions to Cr(VI) removal were explored. The Cr(VI) removal capacity of various cell components from strong to weak was as follows: cytoplasm, cell secretions, and cell debris. Observations obtained by scanning electron microscopy and transmission electron microscopy with energy dispersive X-ray spectroscopy revealed that not only the cell surfaces but also the intracellular contents were involved Cr through adsorption, reduction, or accumulation. Fourier transform infrared spectra indicated that a large number of functional groups (amino, carbonyl, carboxyl, and phosphate groups) participated in chromium binding on the cell surface. X-ray photoelectron spectroscopy confirmed the presence of Cr on the cell surface only as Cr(III). The results have important implications for an in-depth understanding of microbial chromate reduction by F. proliferatum. This study provides an insight into the microbial Cr(VI) bioreduction efficiency, and mechanisms in the chromium-contaminated environment.

Implantation of Bacillus pseudomycoides Chromate Transporter Increases Chromate Tolerance in Bacillus subtilis.

Chromium of anthropogenic origin contaminates the environment worldwide. The toxicity of chromium, a group I human carcinogen, is greatest when it is in a hexavalent oxidation state, Cr(VI). Cr(VI) is actively transported into the cell, triggering oxidative damage intracellularly. Due to the abundance of unspecific intracellular reductants, any microbial species is capable of bio-transformation of toxic Cr(VI) to innocuous Cr(III), however, this process is often lethal. Only some bacterial species are capable of sustaining the vegetative growth in the presence of a high concentration of Cr(VI) and thus operate as self-sustainable bioremediation agents. One of the successful microbial Cr(VI) detoxification strategies is the activation of chromate efflux pumps. This work describes transplantation of the chromate efflux pump from the potentially pathogenic but highly Cr resistant Bacillus pseudomycoides environmental strain into non-pathogenic but only transiently Cr tolerant Bacillus subtilis strain. In our study, we compared the two Bacillus spp. strains harboring evolutionarily diverged chromate efflux proteins. We have found that individual cells of the Cr-resistant B. pseudomycoides environmental strain accumulate less Cr than the cells of B. subtilis strain. Further, we found that survival of the B. subtilis strain during the Cr stress can be increased by the introduction of the chromate transporter from the Cr resistant environmental strain into its genome. Additionally, the expression of B. pseudomycoides chromate transporter ChrA in B. subtilis seems to be activated by the presence of chromate, hinting at versatility of Cr-efflux proteins. This study outlines the future direction for increasing the Cr-tolerance of non-pathogenic species and safe bioremediation using soil bacteria.

Effect of TOC Concentration of Humic Substances as an Electron Shuttle on Redox Functional Groups Stimulating Microbial Cr(VI) Reduction.

Humic substances as an electron shuttle play an essential role in the biogeochemistry processes. However, the influence of total organic carbon (TOC) concentrations of humic substances on microbial Cr(VI) reduction remains unclear. In this study, the rates and extents of Cr(VI) reduction by Shewanella oneidensis MR-1 in the presence of Leonardite humic acids (LHA) and Pahokee peat humic acids (PPHA) with different TOC concentrations were evaluated. We found that the enhanced reduction in Cr(VI) was associated with TOC concentrations of 2.5–50 mg C/L of HA samples. The result shows that HA as an electron shuttle impacted both rates and extents of microbial Cr (VI) reduction, which delivered differently in terms of low TOC concentration range of 2.5 to 15 mg C/L and high concentration range of 15–50 mg C/L. The rates of Cr(VI) reduction significantly enhanced in the low TOC concentration range of HA compared to a high concentration range. The highest acceleration rate of Cr(VI) reduction was achieved at 15 mg C/L of HA. The quinone-like fluorophore was responsible for the main redox-active functional groups of HA by the three-dimensional excitation-emission spectroscopy. The fluorescence intensity of quinone-like fluorophore of HA in the low TOC concentration range was positively correlated with its acceleration coefficient, corresponding to the highest microbial Cr(VI) reduction rate obtained in 15 mg C/L of HA. These findings highlighted the effect of the TOC concentration of HA on microbial Cr(VI) reduction processes. It emphasized that the low TOC concentration of HA contributed to the high rates of Cr(VI) reduction, which is critical for better understanding the fate of Cr(VI) and evaluating the effectiveness of Cr(VI) restoration strategies in the future.

Size-dependent visible-light-enhanced Cr(VI) bioreduction by hematite nanoparticles

Light irradiation would affect the electron transfer between dissimilatory metal-reducing bacteria (DMRB) and semiconducting minerals, which may impose a great influence on the biogeochemistry cycle of heavy metals. However, the size effect of semiconducting minerals on the its electron transfer with DMRB and microbial Cr(VI) reduction under visible light irradiation is little known. Herein, the Cr(VI) reduction by Shewanella oneidensis MR-1 (MR-1) was investigated in the presence of hematite nanoparticles with average diameters of 10 nm and 50 nm in dark and under visible light irradiation. It is found that hematite nanoparticles adhered onto MR-1 cells to form the composites, leading to the decrease in surface sites and Zeta potential. Hematite mediated-Cr(VI) bioreduction rate under visible light irradiation was 0.342 h−1, which is 3.4 folds enhancement compared with that in dark and 4.4 folds compared with the MR-1 alone under visible light irradiation. Decreasing nanoparticle size of hematite from 50 nm to 10 nm promoted the Cr(VI) reduction under visible light irradiation but impeded it in dark. It was deduced that the bioelectrons from MR-1 could promote the separation of photoelectron-hole pairs of light-irradiated hematite, which consequently enhanced the Cr(VI) bioreduction by MR-1-hematite composites. Moreover, mutant strains experiments demonstrated the vital role of c-cytochrome for the conducting network actively established by MR-1 with hematite nanoparticles. Those findings expand the understanding of the electron transfer pathway for enhancing Cr(VI) reduction by hematite-MR-1 composites, and the impact of particle size on the interaction between semiconducting mineral and electroactive bacteria under light irradiation.

Hexavalent Chromium Removal and Prokaryotic Community Analysis in Glass Column Reactor Packed with Aspen Wood as Solid Organic Substrate.

Microbial hexavalent chromium (Cr(VI)) reduction is a promising method for Cr(VI)-laden wastewater treatment. However, the soluble organic substrate required for heterotrophic microbial Cr(VI) reduction necessitates constant supervision, and an excessive supply of soluble organic substrate can result in deterioration of the quality of the effluent. In this study, we evaluated aspen wood, a low-cost lignocellulose biomass, as a solid organic substrate for heterotrophic Cr(VI) reduction. A laboratory-scale aspen wood-packed glass column reactor inoculated with activated sludge was operated for 148days for evaluation. Following reactor operation, an effective average dissolved Cr(VI) removal rate of 0.75mg L-1h-1 was confirmed under an average dissolved Cr(VI) loading rate of 0.90mg L-1h-1. Subsequently, 16S ribosomal ribonucleic acid gene amplicon sequencing analysis revealed that the dominant prokaryotic operational taxonomic units detected in the reactor were associated with prokaryotic lineages with the capacity for lignocellulose biodegradation, Cr compound resistance, and Cr(VI) reduction. Proteobacteria and Chloroflexi were two major prokaryotic phyla in the reactor. Our data indicate that aspen wood is an effective solid organic substrate for the development of simplified, effective, and low-cost microbial Cr(VI)-removing reactors.

Transcriptome analysis provides new insights into the tolerance and reduction of Lysinibacillus fusiformis 15-4 to hexavalent chromium.

Microbial bioremediation of Cr(VI)-contaminated environments has drawn extensive concern. However, the molecular processes underlying the microbial Cr(VI) tolerance and reduction remain unclear. We isolated a Cr(VI)-reducing Lysinibacillus fusiformis strain 15-4 from soil on the Qinghai-Tibet Plateau. When grown in 1mM and 2mM Cr(VI)-containing medium, strain 15-4 could reduce 100% and 93.7% of Cr(VI) to Cr(III) after 36h and 60h of incubation, respectively. To know the molecular processes in response to Cr(VI), transcriptome sequencing was carried out using RNA-Seq technology. The results annotated a total of 3913 expressed genes in the strain. One thousand ninety-eight genes (28.1%) were significantly (fold change ≥ 2, false discovery rate ≤ 0.05) expressed in response to Cr(VI), of which 605 (55.1%) were upregulated and 493 (44.9%) were downregulated. The enrichment analysis showed that a total of 630 differentially expressed genes (DEGs) were enriched to 122 KEGG pathways, of which 8 pathways were significantly (p < 0.05) enriched in Cr(VI)-treated sample, including ATP-binding cassette (ABC) transporters (97 DEGs), ribosome (40), sulfur metabolism (16), aminoacyl-tRNA biosynthesis (19), porphyrin metabolism (20), quorum sensing (44), oxidative phosphorylation (17), and histidine metabolism (10), suggesting that these pathways play key roles to cope with Cr(VI) in the strain. The highly upregulated DEGs consisted of 29 oxidoreductase, 18 dehydrogenase, 14 cell redox homeostasis and stress response protein, and 10 DNA damage and repair protein genes. However, seven Na+:H+ antiporter complex-coding DEGs and most of transcriptional regulator-coding DEGs were significantly downregulated in the Cr-treated sample. Many of FMN/NAD(P)H-dependent reductase-encoding genes were greatly induced by Cr, suggesting the involvement of these genes in Cr(VI) reduction in strain 15-4. Sulfur and iron ions as well as the thiol-disulfide exchange reactions might play synergistic roles in Cr reduction.Key points• Lysinibacillus fusiformis 15-4 was able to tolerate and reduce Cr(VI) to Cr(III).• Transcriptome analysis revealed that 1098 DEGs and 8 key KEGG pathways significantly responded to Cr(VI).• Sulfur metabolism, protein biosynthesis, and porphyrin metabolism were the key pathways associated with the survival of strain 15-4 in response to Cr(VI).

Chemical-Assisted Microbially Mediated Chromium (Cr) (VI) Reduction Under the Influence of Various Electron Donors, Redox Mediators, and Other Additives: An Outlook on Enhanced Cr(VI) Removal.

Chromium (Cr) (VI) is a well-known toxin to all types of biological organisms. Over the past few decades, many investigators have employed numerous bioprocesses to neutralize the toxic effects of Cr(VI). One of the main process for its treatment is bioreduction into Cr(III). Key to this process is the ability of microbial enzymes, which facilitate the transfer of electrons into the high valence state of the metal that acts as an electron acceptor. Many underlying previous efforts have stressed on the use of different external organic and inorganic substances as electron donors to promote Cr(VI) reduction process by different microorganisms. The use of various redox mediators enabled electron transport facility for extracellular Cr(VI) reduction and accelerated the reaction. Also, many chemicals have employed diverse roles to improve the Cr(VI) reduction process in different microorganisms. The application of aforementioned materials at the contaminated systems has offered a variety of influence on Cr(VI) bioremediation by altering microbial community structures and functions and redox environment. The collective insights suggest that the knowledge of appropriate implementation of suitable nutrients can strongly inspire the Cr(VI) reduction rate and efficiency. However, a comprehensive information on such substances and their roles and biochemical pathways in different microorganisms remains elusive. In this regard, our review sheds light on the contributions of various chemicals as electron donors, redox mediators, cofactors, etc., on microbial Cr(VI) reduction for enhanced treatment practices.

Cu(II) nonspecifically binding chromate reductase NfoR promotes Cr(VI) reduction.

Cu(II)-enhanced microbial Cr(VI) reduction is common in the environment, yet its mechanism is unknown. The specific activity of chromate reductase, NfoR, from Staphylococcus aureus sp. LZ-01 was augmented 1.5-fold by Cu(II). Isothermal titration calorimetry and spectral data show that Cu(II) binds to NfoR nonspecifically. Further, Cu(II) stimulates the nitrobenzene reduction of NfoR, indicating that Cu(II) promotes electron transfer. The crystal structure of NfoR in complex with CuSO4 (1.46 Å) was determined. The overall structure of NfoR-Cu(II) complex is a dimer that covalently binds with FMN and Cu(II)-binding pocket is located at the interface of the NfoR dimer. Structural superposition revealed that NfoR resembles the structure of class II chromate reductase. Site-directed mutagenesis revealed that Leu46 and Phe123 were involved in NADH binding, whereas Trp70 and Ser45 were the key residues for nitrobenzene binding. Furthermore, His100 and Asp171 were preferential affinity sites for Cu(II) and that Cys163 is an active site for FMN binding. Attenuation reductase activity in C163S can be partially restored to 54% wild type by increasing Cu(II) concentration. Partial restoration indicates dual-channel electron transfer of NfoR via Cu(II) and FMN. We propose a catalytic mechanism for Cu(II)-enhanced NfoR activity in which Cu(I) is formed transiently. Together, the current results provide an insight on Cu (II)-induced enhancement and benefit of Cr(VI) bioremediation.

Microbial electrolysis cell produced biogas as sustainable electron donor for microbial chromate reduction

Microbial electrolysis cell (MEC) is an attractive technology to produce biogas containing hydrogen (H2) and methane (CH4). Both gaseous products are effective electron donors for chromate [Cr(VI)] bioreduction when either is applied individually. However, microbial Cr(VI) reduction driven by H2 and CH4 mixture remains poorly understood. This study investigates Cr(VI) biotransformation supported by fluctuant MEC-produced biogas. Synergy between H2 and CH4 promoted Cr(VI) detoxification. Maximum Cr(VI) removal rate reached 1.81 ± 0.17 mg/L·d with 200.5 ± 13.6 μmol H2 and 203.3 ± 9.0 μmol CH4 produced by MEC at 0.6 V in a 4-d cycle. Biogas yield and H2/CH4 mole ratio were mainly regulated by Geobacter and Methanobacterium. The process in which Cr(VI) reduced to insoluble Cr(III) was proposed in two major pathways. A single autohydrogenotrophic genus (e.g., Hydrogenophaga, Thiobacillus) oxidized H2 independently coupled with Cr(VI) reduction. Cr(VI) reduction was also achieved through CH4-metabolizing microorganisms (e.g., Methanobacterium, Methanosaeta), which convert CH4 into organic metabolites to donate electrons for heterotrophic Cr(VI) reducers (e.g., Geobacter, Anaerolineaceae). Genes relevant to Cr(VI) reduction (yieF) and CH4 oxidation (mcrA) increased significantly. Electrons were effectively transferred through extracellular cytochrome c and intracellular nicotinamide adenine dinucleotide. This study offers an innovative route to sustainable bioremediation of Cr(VI)-polluted groundwater and promising direct utilization of mixed biogas from MEC.

Successive use of microorganisms to remove chromium from wastewater.

Heavy metal pollution is a direct consequence of the extensive utilization of heavy metals in various industrial processes. The persistence and nondegradability of heavy metals cause them to bioaccumulate in nature, and when they come in direct contact with the pristine environment, they not only contaminate it severely but also pose dire consequences to the health of all living forms on earth, including humans. Chromium (Cr) is one of the heavy metals which has been extensively used in various industrial processes such as mining, alloy manufacturing, tanning of hides and skins, pigment production, etc. However, it is regarded as a priority pollutant due to its highly toxic, teratogenic, mutagenic, and carcinogenic nature, and the U.S. Environmental Protection Agency (EPA) also categorized it into group "A" human carcinogen. In contrast to water-soluble hexavalent chromium (Cr6+), its reduced form, trivalent chromium (Cr3+), is relatively benign and readily precipitated at environmental pH. Thus, bioremediation of Cr6+ through microorganisms including bacteria, yeast, and algae provides a promising approach to decontaminate a metal-polluted environment. This review describes an overview of the microbial reduction of Cr6+, resistance mechanism, and the antioxidant profiling exhibited by these microorganisms when exposed to Cr6+. It also describes the pilot-scale study of the successive use of bacterial, fungal, and algal strains and the subsequent use of microbially purified water for the cultivation of plant growth. Multiple metal-resistant microorganisms are a good bioresource for green chemistry to eradicate environmental Cr6+. KEY POINTS: • Hexavalent chromium (Cr6+) is highly toxic for living organisms including humans. • Microbial Cr resistance is mediated at the genetic, proteomic, and molecular levels. • Successive use of microorganisms is the best strategy to exterminate Cr6+from the environment.

The mechanism of nitrate-Cr(VI) reduction mediated by microbial under different initial pHs

To date, comparatively little research is known about the role of pH conditions in bioremediation of Cr(VI) contaminated aquifers. This study explored microbial Cr(VI) reduction and denitrification under different initial pHs. The underlying mechanism was also investigated. When testing 50 mg/L–N nitrate and 10 mg/L Cr(VI), complete contaminants removal was observed at initial pH 10.0 and 11.0, and only 10 %–30 % of removal achieved under other conditions, which might be ascribe to the significant up-regulation of functional genes narG (8.31 and 10.46 folds) and azoR (24.90 and 15.96 folds) at initial pH 10.0 and 11.0. Metagenomic sequencing showed that alkali tolerant bacteria played major roles in the NO3−-Cr(VI) reduction (i.e. Pannonibacter increased by 13.08 % and 25.24 % at initial pH 10.0 and 11.0), and metabolic pathways of Degradation and Energy were found of increased abundant. Furthermore, a significative study suggested that potential interspecies cooperation existed at initial pH 11.0 to facilitating the simultaneous removal of contaminants, and Pannonibacter indicus might be an important participant in the degradation of contaminants. The results of this study will fully understand the metabolic patterns of bacteria under alkaline conditions, expand the range of available functional bacteria, and enhance the practical aspects of co-contaminants remediation.