Reduction of hexavalent chromium by Exiguobacterium mexicanum isolated from chromite mines soil

Functional genomic analysis of Bacillus cereus BC4 strain for chromium remediation in contaminated soil

Soil provides a habitat for microorganisms that can mitigate metal contamination. This study presents Bacillus cereus BC4 strain, which shows significant potential for metal pollution remediation. This bacterium achieved a 98.6 % reduction in Chromium (VI) concentrations from 300 mg/L to negligible levels under specific conditions (pH 8, 37 °C, and 120 rpm agitation) in LB medium. The complete genome of Bacillus cereus BC4 was sequenced using Oxford Nanopore Technology, revealing a circular chromosome and a plasmid with a total of 5537,675 base pairs and a G + C content of 35.44 %. Fourteen genes critical for Cr metabolism were identified. qRT-PCR demonstrated that under low Cr(VI) stress, two genes, chrA and nitR1, were up-regulated, indicating their role in Cr resistance. The genome revealed gene clusters essential for resilience against various metals, including chromium, arsenic, copper, manganese, and cadmium, as well as for synthesizing secondary metabolites crucial for survival and adaptation. Additionally, genes associated with biopolymer synthesis were identified, emphasizing the organism's diverse genetic capabilities. This genomic study led to the submission of the complete genome to GenBank (CP101135), enhancing the understanding and potential of Bacillus cereus BC4 in chromium remediation and environmental restoration.

Functional profiling of the rhizospheric Exiguobacterium sp. for dimethoate degradation, PGPR activity, biofilm development, and ecotoxicological risk

This study introduces an indigenous bacterial strain, Exiguobacterium sp. (L.O), isolated from sugarcane fields in Sevur, Tamil Nadu, which has adapted to prolonged exposure to dimethoate. The strain demonstrated the capability to utilize 150 ppm of dimethoate as its sole carbon source, achieving a remarkable degradation rate of 95.87% within 5 days in mineral salt media. Gas chromatography-mass spectrometry (GC-MS) analyses identified the presence of intermediate by-products formed during degradation, like methyl diethanol amine and aspartyl glycine ethyl ester. Notably, phosphorothioic O, O, S-acid, an expected end product in the degradation of dimethoate, was also identified, further confirming the strain's effective metabolic breakdown of the pesticide. Further degradation study and analysis of changes in functional group was performed by FTIR, and a hypothetical degradation pathway was elucidated showing the course of dimethoate metabolism by the strain. Exiguobacterium sp. (L.O) also displayed significant plant growth-promoting traits, including the production of HCN, IAA, and ammonia and the formation of biofilms, which enhance its utility in agricultural applications. The ecotoxicity study revealed the degradation by-products exhibited reduced toxicity compared to the parent compound dimethoate, highlighting the strain's potential not only for bioremediation but also for supporting sustainable agricultural practices. This research presents a novel application of Exiguobacterium sp. (L.O), integrating the bioremediation of the organophosphate pesticide dimethoate with agricultural enhancement. This approach is critical for addressing the challenges associated with pesticide pollution in agricultural practices. This study is likely the first to demonstrate the application of this strain in the degradation of dimethoate, as suggested by an extensive review of the literature.

A kinetic modelling approach to explore mechanism of Cr(VI) detoxification by a novel strain Pseudochrobactrum saccharolyticum NBRI-CRB 13 using response surface methodology

A novel Pseudochrobactrum saccharolyticum strain NBRI-CRB 13, isolated from tannery sludge, was studied to grow up to 500 mgL-1 of Cr(VI) and showed Cr(VI) detoxification by reducing > 90% of Cr(VI) at different concentrations 25, 50 and 100 mgL-1. Kinetic studies showed that first-order models were fitted (R2 = 0.998) to the time-dependent Cr(VI) reduction with degradation rate constant (k) (1.03-0.429 h-1). Cr(VI) detoxification was primarily related to the extracellular fraction of microbial cells, which showed a maximum extracellular reductase enzyme activity led to 94.6% reduction of Cr(VI). Moreover, the strain showed maximum extracellular polymeric substances (EPS) production at 100 mgL-1 Cr(VI), which is presumably the reason for Cr(VI) removal as EPS serves as the metal binding site for Cr(VI) ions. Further, an optimization study using Box-Behnken design was conducted considering parameters viz., pH, temperature, and initial concentration of Cr(VI). The maximum percent reduction of Cr(VI) was obtained at pH 6.5, temperature 30 °C with 62.5 mgL-1Cr(VI) concentration. Further, the Cr(VI) reduction and adsorption ability of strain P. saccharolyticum NBRI-CRB13 were confirmed by SEM-EDS, FTIR, and XRD analyses. FTIR analysis confirmed the presence of functional groups (-OH, -COOH, -PO4) on bacterial cell walls, which were more likely to interact with positively charged chromium ions. The study elucidated the reduction of Cr(VI) by the novel bacterium within 24 h using the response surface methodology approach and advocated its application in real-time situations.

Cr(VI) removal performance from wastewater by microflora isolated from tannery effluents in a semi-arid environment: a SEM, EDX, FTIR and zeta potential study

Hexavalent chromium removal from the environment remains a crucial worldwide challenge. To address this issue, microbiological approaches are amongst the straightforward strategies that rely mainly on the bacteria's and fungi's survival mechanisms upon exposure to toxic metals, such as reduction, efflux system, uptake, and biosorption. In this work, scanning electron microscopy, energy-dispersive X-ray spectrophotometry, Fourier transform infrared spectroscopy, and zeta potential measurements were used to investigate the ability of chromium adsorption by Bacillus licheniformis, Bacillus megaterium, Byssochlamys sp., and Candida maltosa strains isolated from tannery wastewater. Scanning electron microscopy combined with energy dispersive X-ray spectroscopy revealed alterations in the cells treated with hexavalent chromium. When exposed to 50 mg/L Cr6+, Bacillus licheniformis and Candida maltosa cells become rough, extracellular secretions are reduced in Bacillus megaterium, and Byssochlamys sp. cells are tightly bound and exhibit the greatest Cr weight percentage. In-depth analysis of Fourier transform infrared spectra of control and Cr-treated cells unveiled Cr-microbial interactions involving proteins, lipids, amino acids, and carbohydrates. These findings were supported by zeta potential measurements highlighting significant variations in charge after treatment with Cr(VI) with an adsorption limit of 100 mg/L Cr6+ for all the strains. Byssochlamys sp. showed the best performance in Cr adsorption, making it the most promising candidate for treating Cr-laden wastewater.

Growth-dependent cr(VI) reduction by Alteromonas sp. ORB2 under haloalkaline conditions: toxicity, removal mechanism and effect of heavy metals

Bacterial reduction of hexavalent chromium (VI) to chromium (III) is a sustainable bioremediation approach. However, the Cr(VI) containing wastewaters are often characterized with complex conditions such as high salt, alkaline pH and heavy metals which severely impact the growth and Cr(VI) reduction potential of microorganisms. This study investigated Cr(VI) reduction under complex haloalkaline conditions by an Alteromonas sp. ORB2 isolated from aerobic granular sludge cultivated from the seawater-microbiome. Optimum growth of Alteromonas sp. ORB2 was observed under haloalkaline conditions at 3.5-9.5% NaCl and pH 7-11. The bacterial growth in normal culture conditions (3.5% NaCl; pH 7.6) was not inhibited by 100 mg/l Cr(VI)/ As(V)/ Pb(II), 50 mg/l Cu(II) or 5 mg/l Cd(II). Near complete reduction of 100 mg/l Cr(VI) was achieved within 24 h at 3.5-7.5% NaCl and pH 8-11. Cr(VI) reduction by Alteromonas sp. ORB2 was not inhibited by 100 mg/L As(V), 100 mg/L Pb(II), 50 mg/L Cu(II) or 5 mg/L Cd(II). The bacterial cells grew in the medium with 100 mg/l Cr(VI) contained lower esterase activity and higher reactive oxygen species levels indicating toxicity and oxidative stress. In-spite of toxicity, the cells grew and reduced 100 mg/l Cr(VI) completely within 24 h. Cr(VI) removal from the medium was driven by bacterial reduction to Cr(III) which remained in the complex medium. Cr(VI) reduction was strongly linked to aerobic growth of Alteromonas sp. The Cr(VI) reductase activity of cytosolic protein fraction was pronounced by supplementing with NADPH in vitro assays. This study demonstrated a growth-dependent aerobic Cr(VI) reduction by Alteromonas sp. ORB2 under complex haloalkaline conditions akin to wastewaters.

Community assembly and microbial interactions in an alkaline vanadium tailing pond

Intensive development of vanadium-titanium mines leads to an increasing discharge of vanadium (V) into the environment, imposing potential risks to both environmental system and public health. Microorganisms play a key role in the biogeochemical cycling of V, influencing its transformation and distribution. In addition, the characterization of microbial community patterns serves to assess potential threats imposed by elevated V exposure. However, the impact of V on microbial community remains largely unknown in alkaline V tailing areas with a substantial amounts of V accumulation and nutrient-poor conditions. This study aims to explore the characteristics of microbial community in a wet tailing pond nearby a large-scale V mine. The results reveal V contamination in both water (0.60 mg/L) and sediment tailings (340 mg/kg) in the tailing pond. Microbial community diversity shows distinctive pattern between environmental metrices. Genera with the functional potential of metal reduction\resistance, nitrogen metabolism, and carbon fixation have been identified. In this alkaline V tailing pond, V and pH are major drivers to induce community variation, particularly for functional bacteria. Stochastic processes primarily govern the assemblies of microbial community in the water samples, while deterministic process regulate the community assemblies of sediment tailings. Moreover, the co-occurrence network pattern unveils strong selective pattern for sediment tailings communities, where genera form a complex network structure exhibiting strong competition for limited resource. These findings reveal the patterns of microbial adaptions in wet vanadium tailing ponds, providing insightful guidelines to mitigate the negative impact of V tailing and develop sustainable management for mine-waste reservoir.

A sustainable solution for alleviating hexavalent chromium from water streams using Lactococcus lactis AM99 as a novel Cr(VI)-reducing bacterium

Chromium, extensively used in various industries, poses significant challenges due to its environmental impact. The threat of Cr(VI) causes critical concerns in aquatic ecosystems as a consequence of the fluidity of water. The conventional approach for the treatment of effluents containing Cr(VI) is reducing Cr(VI) to low-noxious Cr(III). This research is related to a Gram positive bacterium newly isolated from tannery effluent under aerobic conditions. To characterize functional groups on the isolate, Fourier transform infrared spectroscopy was utilized. The effect of different factors on Cr(VI) bioreduction was investigated, including temperature, initial Cr(VI) concentration, acetate concentration, and Tween 80 surfactant. Under optimal conditions (37 °C and 0.90 g/L sodium acetate), the bioreduction rate of the isolate, identified as Lactococcus lactis AM99, achieved 88.0 % at 300 mg/L Cr(VI) during 72 h (p < 0.05). It was observed that Cr(VI) bioreduction was enhanced by the acetate in both the quantity and intensity, while Tween 80 had no impact on the reaction. The strain AM99 exhibited remarkable characteristics, notably a marginal decrease in growth at elevated concentrations of hexavalent chromium and an exceptional potential to reduce Cr(VI) even at very low biomass levels, surpassing any prior findings in the associated research. Furthermore, The isolate could tolerate 1400 mg/L Cr(VI) in a solid medium. These distinctive features make the isolate a promising and well-suited candidate for remediating Cr(VI)-polluted environments. Additionally, the impact of biogenic extracellular polymer produced by the strain AM99 on reduction was examined at different temperatures.

Screening of cadmium-chromium-tolerant strains and synergistic remediation of heavy metal-contaminated soil using king grass combined with highly efficient microbial strains

The present study involved the isolation of two cadmium (Cd) and chromium (Cr) resistant strains, identified as Staphylococcus cohnii L1-N1 and Bacillus cereus CKN12, from heavy metal contaminated soils. S. cohnii L1-N1 exhibited a reduction of 24.4 % in Cr6+ and an adsorption rate of 6.43 % for Cd over a period of 5 days. These results were achieved under optimal conditions of pH (7.0), temperature (35 °C), shaking speed (200 rpm), and inoculum volume (8 %). B. cereus strain CKN12 exhibited complete reduction of Cr6+ within a span of 48 h, while it demonstrated a 57.3 % adsorption capacity for Cd over a period of 120 h. These results were achieved under conditions of optimal pH (8.0), temperature (40 °C), shaking speed (150 rpm), and inoculum volume (2-3 %). Additionally, microcharacterization and ICP-MS analysis revealed that Cr and Cd were accumulated on the cell surface, whereas Cr6+ was mainly reduced extracellularly. Subsequently, a series of pot experiments were conducted to provide evidence that the inclusion of S. cohnii L1-N1 or B. cereus CKN12 into the system resulted in a notable enhancement in both the plant height and biomass of king grass. In particular, it was observed that the presence of S. cohnii L1-N1 or B. cereus CKN12 in king grass led to a significant reduction in the levels of Cd and Cr in the soils (36.0 % and 27.8 %, or 72.9 % and 47.4 %, respectively). Thus, the results of this study indicate that the combined use of two bacterial strains can effectively aid in the remediation of tropical soils contaminated with moderate to light levels of Cd and Cr.

Bacteria-driven copper redox reaction coupled electron transfer from Cr(VI) to Cr(III): A new and alternate mechanism of Cr(VI) bioreduction

Cr(VI) released into the environment inevitably co-exists with other contaminants, such as heavy metal ions, thus altering the performance of bacteria for Cr(VI) reduction; however, the mechanism underlying Cr(VI)-reducing bacterial response to heavy metal ions remains elusive. Herein, we investigate the toxic effects of Cu(II) and Cr(VI) on Cr(VI)-reducing bacterium Pannonibacter phragmitetus D-6 (hereafter D-6), which changes the primary metabolic pattern of Cr(VI). At Cu(II) concentrations of 10-100 mg/L, the efficiency of Cr(VI) reduction increases significantly. The co-exposure of Cr(VI) and Cu(II) induces D-6 to preferentially respond to Cu(II) through electrostatic forces, which is then reduced to Cu(I) outside and inside the bacterial cells. The original pathways for Cr(VI) reduction are weakened via downregulating genes related to Cr(VI) transport and reduction. A new mechanism involving Cu(II)-mediated electron transfer from D-6 to Cr(VI) is elucidated. Specially, Cu(II) accumulates around the cells as an electron shuttle and promotes Cr(VI) reduction. Genes encoding cytochromes involved in electron transfer are significantly up-regulated, thus promoting Cu(II) reduction. The Cu(II)/Cu(I) redox cycle ensures the continuous bioremoval of Cr(VI) in a cycle test. This study reveals an overlooked mechanism for Cr(VI) reduction, which provides theoretical guidance for designing practical microbial process to remediate Cr(VI) contamination.

Investigation on Cr(VI)-bioreduction mechanism and reduction products by a novel Microbacterium sp. strain NEAU-W11

Cr(VI) widely exists in the environment and has highly toxic, carcinogenic and mutagenic effects on all organisms. Physical/chemical methods to remove chromium pollution are economically expensive and have disadvantages like high reagent consumption, energy requirements and so on, while bioremediation is an eco-friendly, simple and cost-effective way. In this study, a novel Cr(VI)-reducing strain, Microbacterium sp. NEAU-W11, was reported, and its reduction mechanism was investigated. Microbacterium sp. NEAU-W11 could effectively degrade Cr(VI) under the conditions of pH 7-10, 15-35 °C, and the coexistence of metal pollutants such as Pb and Ni, etc. In addition, both Fe3+ and Cu2+ could improve the reducing ability of strain NEAU-W11, and glucose and lactose as electron donors also had promoting effect. Heat treatment of resting cells confirmed that chromium removal was not biological sorption but biological reduction. The active reductase of strain NEAU-W11 to chromium(VI) mainly existed in the cell cytoplasm, which is the first report in the genus Microbacterium. Micro-characterization of strain NEAU-W11 and the reduction products identified the reduction products as Cr(III)-ligand complexes bound to extracellular polymeric substances (EPS). Collectively, this study systematically investigated the degradation mechanism of Microbacterium sp. NEAU-W11 and the distribution of degradation product Cr(III), providing a new reduction mechanism for the genus Microbacterium, providing a new perspective for a comprehensive understanding of the degradation and transport of chromium by bacteria, and providing theoretical reference for the migration of metal ions in environmental governance.

Facilitating New Chromium Reducing Microbes to Enhance Hexavalent Chromium Reduction by In Situ Sonoporation-Mediated Gene Transfer in Soils

Chromium (Cr) is a heavy metal with a high toxicity and pathogenicity. Microbial reduction is an effective strategy to remove Cr(VI) at contaminated sites but suffers from the low populations and activities of Cr-reducing microorganisms in soils. This study proposed an in situ sonoporation-mediated gene transfer approach, which improved soil Cr(VI) reduction performance by delivering exogenous Cr-transporter chrA genes and Cr-reducing yieF genes into soil microorganisms with the aid of ultrasound. Besides the increasing populations of Cr-resistant bacteria and elevated copy numbers of chrA and yieF genes after sonoporation-mediated gene transfer, three new Cr-reducing strains were isolated, among which Comamonas aquatica was confirmed to obtain Cr-resistant capability. In addition, sonoporation-mediated gene transfer was the main driving force significantly shaping soil microbial communities owing to the predominance of Cr-resistant microbes. This study pioneered and evidenced that in situ soil sonoporation-mediated gene transfer could effectively deliver functional genes into soil indigenous microbes to facilitate microbial functions for enhanced bioremediation, e.g., Cr-reduction in this study, showing its feasibility as a chemically green and sustainable remediation strategy for heavy metal contaminated sites.

Symbiotic hemolymph bacteria reduce hexavalent chromium to protect the host from chromium toxicity in Procambarus clarkii

Hexavalent chromium (Cr(VI)) is a cytotoxic heavy metal pollutant that adversely affects all life forms. Interestingly, the crustacean Procambarus clarkii exhibits a relatively high tolerance to heavy metals. The underlying mechanisms remain unclear. In this study, we investigated the role of symbiotic bacteria in P. clarkii in alleviating Cr(VI)-induced damage and explored their potential mechanisms of action. Through transcriptomic analysis, we observed that Cr(VI) activated P. clarkii's antimicrobial immune responses and altered the bacterial composition in the hemolymph. After antibiotic treatment to reduce bacterial populations, Cr(VI)-induced intestinal and liver damage worsened, and crayfish exhibited lower levels of GSH/CAT/SOD activity. The Exiguobacterium, the symbiotic bacteria in the hemolymph of P. clarkii, were proved to be primary contributor to Cr(VI) tolerance. Further investigation suggested that it resists Cr(VI) through the activation of the ABC transporter system and the reduction of Cr(VI) via the reductase gene nfsA. To validate the role of Exiguobacterium in Cr(VI) tolerance, crayfish treated with antibiotics then supplemented with Exiguobacterium H6 and recombinant E. coli (with the nfsA gene), reduced Cr(VI)-induced ovarian damage. Overall, this study revealed that the symbiotic bacteria Exiguobacterium can absorb and reduce hexavalent chromium, mitigating Cr(VI)-induced damage in P. clarkii. These findings provide new insights into hexavalent chromium tolerance mechanisms in crustaceans.

Cr(VI) reduction by Agrobacterium sp. Cr-1 and Lysinibacillus sp. Cr-2, novel Cr(VI)-reducing strains isolated from chromium plant soil

The bioremediation of Cr(VI)-contaminated soil is a promising strategy; however, the performance of Cr(VI)-reducing bacteria is limited by the toxicity of Cr(VI). In this study, two novel Cr(VI)-reducing bacteria were isolated from a Cr salt plant and identified as Agrobacterium sp. and Lysinibacillus sp. The Cr(VI) reduction conditions of the two strains were optimized. At a Cr(VI) concentration of 500 mg/L, Agrobacterium sp. Cr-1 reduced Cr(VI) with a removal rate of 96.91%, while that for Lysinibacillus sp. Cr-2 was 92.82%. First-order reaction kinetic equations simulated the positive relationship between time and Cr(VI) concentration during Cr(VI) reduction in these two strains. Agrobacterium sp. Cr-1 was further studied, and the effects of different cell components on Cr(VI) reduction were detected. The extracellular extracts of Agrobacterium sp. Cr-1 played a major role in Cr(VI) reduction, followed by intracellular extracts and cell membranes. The scanning electron microscope-energy dispersive spectrometer (SEM-EDS) images show that the precipitation was Cr. The high Cr(VI) reducing ability of Agrobacterium sp. Cr-1 suggests that this strain is promising for the remediation of Cr(VI)-contaminated sites.

Anoxygenic phototrophic purple non-sulfur bacteria: tool for bioremediation of hazardous environmental pollutants

The extraordinary metabolic flexibility of anoxygenic phototrophic purple non-sulfur bacteria (PNSB) has been exploited in the development of various biotechnological applications, such as wastewater treatment, biohydrogen production, improvement of soil fertility and plant growth, and recovery of high-value compounds. These versatile microorganisms can also be employed for the efficient bioremediation of hazardous inorganic and organic pollutants from contaminated environments. Certain members of PNSB, especially strains of Rhodobacter sphaeroides and Rhodopseudomonas palustris, exhibit efficient remediation of several toxic and carcinogenic heavy metals and metalloids, such as arsenic, cadmium, chromium, and lead. PNSB are also known to utilize diverse biomass-derived lignocellulosic organic compounds and xenobiotics. Although biodegradation of some substituted aromatic compounds by PNSB has been established, available information on the involvement of PNSB in the biodegradation of toxic organic pollutants is limited. In this review, we present advancements in the field of PNSB-based bioremediation of heavy metals and organic pollutants. Furthermore, we highlight that the potential role of PNSB as a promising bioremediation tool remains largely unexplored. Thus, this review emphasizes the necessity of investing extensive research efforts in the development of PNSB-based bioremediation technology.

Influencing factors and mechanism of Cr(VI) reduction by facultative anaerobic Exiguobacterium sp. PY14

Microbial reduction is an effective way to deal with hexavalent chromium [Cr(VI)] contamination in the environment, which can significantly mitigate the biotoxicity and migration of this pollutant. The present study investigated the influence of environmental factors on aqueous Cr(VI) removal by a newly isolated facultative anaerobic bacterium, Exiguobacterium sp. PY14, and revealed the reduction mechanism. This strain with a minimum inhibitory concentration of 400 mg/L showed the strongest Cr(VI) removal capacity at pH 8.0 because of its basophilic nature, which was obviously depressed by increasing the Cr(VI) initial concentration under both aerobic and anaerobic conditions. In contrast, the removal rate constant for 50 mg/L of Cr(VI) under anaerobic conditions (1.82 × 10-2 h-1) was 3.3 times that under aerobic conditions. The co-existence of Fe(III) and Cu(II) significantly promoted the removal of Cr(VI), while Ag(I), Pb(II), Zn(II), and Cd(II) inhibited it. Electron-shuttling organics such as riboflavin, humic acid, and anthraquinone-2,6-disulfonate promoted the Cr(VI) removal to varying degrees, and the enhancement was more significant under anaerobic conditions. The removal of aqueous Cr(VI) by strain PY14 was demonstrated to be due to cytoplasmic rather than extracellular reduction by analyzing the contributions of different cell components, and the end products existed in the aqueous solution in the form of organo-Cr(III) complexes. Several possible genes involved in Cr(VI) metabolism, including chrR and chrA that encode well-known Chr family proteins responsible for chromate reduction and transport, respectively, were identified in the genome of PY14, which further clarified the Cr(VI) reduction pathway of this strain. The research progress in the influence of crucial environmental factors and biological reduction mechanisms will help promote the potential application of Exiguobacterium sp. PY14 with high adaptability to environmental stress in Cr(VI) removal in the actual environment.

Hexavalent chromium detoxification by haloalkaliphilic Nesterenkonia sp strain NRC-Y immobilized in different matrices.. [DOI: 10.21203/rs.3.rs-2870155/v1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

In order to develop a bioprocess for Cr(VI) detoxification in industrial effluent, a previously isolated potent Cr(VI) reducing haloalkaliphilic Nesterenkonia sp strain NRC-Y was immobilized in nine different matrices including natural polymer, modified natural, synthetic and mixtures of natural and synthetic polymers and investigated for Cr(VI) detoxification. Among the tested carriers and immobilization approaches, Nesterenkonia sp NRC-Y cells encapsulated in amidated pectin beads showed highest Cr(VI) reduction efficiency (58.4% of initial Cr(VI) concentration 150 mg/L after 20 h), followed by alginate, alginate-PVA, alginate-PVA-chitosan, PVA-PVP, and PVA respectively. Therefore, amidated pectin was selected for further investigation and immobilization of both whole cells and chromate reductase. Operational stability study revealed that immobilized whole cell was more efficient and stable than immobilized chromate reductase and the free cells retaining about 60%, 27.0% and 11.5% of its initial activity after four successive batches, respectively. The temperature and pH optima for the immobilized cells were 35⸰C and 7.0, respectively. The pH and thermal stability of Nesterenkonia sp NRC-Y cells were significantly enhanced upon immobilization in amidated pectin beads. The developed immobilized biocatalyst was applied for Cr(VI) reduction in industrial effluent samples, and was able to completely reduce Cr(VI) within 4 and 8 h for effluents for initial Cr(VI) concentrations of 10 and 30 mg/L, respectively. To the best of our knowledge, this is the first report about of Cr(VI) detoxification by immobilized Nesterenkonia sp NRC-Y in amidated pectin beads. The developed immobilized biocatalyst is promising and has the potential for large-scale Cr(VI) detoxification application.

Mechanism of Cr(VI) reduction by an indigenous Rhizobium pusense CR02 isolated from chromite mining quarry water (CMQW) at Sukinda Valley, India

Toxicological assessment of CMQW generated due to chromite mining activities at Sukinda Valley has revealed high chromium contamination along with Zn and Fe. The present study focused on the mechanism of chromate reduction by an indigenous multi-metal tolerant bacterium, Rhizobium pusense CR02, isolated from CMQW. The isolated strain has shown resistance up to 520 mg/L of Cr(VI) with an IC50 value of 385.4 mg/L. The highest reduction rate 8.6 × 10^-2/h was recorded with 20 mg/L of initial concentration of Cr(VI). Extracellular (3.06 ± 0.012 U/mL), intracellular (3.60 ± 0.13 U/mL), and membrane (1.89 ± 0.01 U/mL) associated chromate reductases were found to be involved for reduction. The extracellular polymeric substances (EPS) produced by the isolate also enhanced reduction activity of 46.32 ± 1.69 mg/L after 72 h with an initial concentration of 50 mg/L. FTIR analysis revealed the involvement of functional groups -OH, -CO, and -NH for Cr(VI) biosorption whereas P=O, -CO-NH- and -COOH interacted with Cr(III). Zeta potential with less negative surface charge favored reduction of Cr(VI). Treatment of CMQW by bacterial isolate detoxified Cr(VI) minimizing chromosomal aberrations in root cells of Allium cepa L., suggesting the role of Rhizobium pusense CR02 as a promising bio-agent for Cr(VI) detoxification.

Bioremediation of hexavalent chromium by transformation of Escherichia coli DH5α with chromate reductase (ChrR) genes of Pseudomonas putida isolated from tannery effluent

AIMS

Hexavalent chromium Cr(VI), a toxic heavy metal, is a serious pollutant of tannery effluent, and its accumulation in soil and water causes severe environmental concerns of increasing public health issues. The present study focus on the isolation and identification of chromium-reducing bacteria collected from the tannery industry in Dindigul, Tamil Nadu. Chromium-reducing bacteria Pseudomonas putida were identified by 16S rRNA sequencing followed by BLAST search. The plasmid with Cr(VI) reductase gene was isolated from Pseudomonas putida and transferred to E. coli DH5α for further studies.

METHODS AND RESULTS

The bacterial cultures were kept under controlled conditions for 72 h to observe the growth rates and bacterial resistance to chromium. When strains wild type and transformant E. coli DH5α were grown in chromium supplemented media revealed significant growth, but strains cured type Pseudomonas putida and E. coli DH5α were minimum growth. The Cr(VI) reduction employed by transformant E. coli DH5α and wild Pseudomonas putida was 42.52 ± 1.48% and 44.46 ± 0.55%, respectively. The culture supernatant of the wild Pseudomonas putida and transformant E. coli DH5α showed an increased reduction of Cr(VI) compared to cell extract supernatant and cell debris due to the extracellular activity of chromium reductase has been responsible for Cr(VI) reduction. Besides, the chromium reductase gene was confirmed in the isolated Pseudomonas putida and transformant E. coli DH5α.

CONCLUSIONS

Transformant bacteria could employ an alternative method for heavy metal detoxification in contaminated environments like tannery effluent and mining processes.

Bioreduction and bioremoval of hexavalent chromium by genetically engineered strains (Escherichia coli MT2A and Escherichia coli MT3)

The number of studies on the removal of hazardous metals from water using genetic engineering technologies is growing. A high rate of metal ion removal from the environment is ensured, particularly through the expression of cysteine and thiol-rich proteins such as metallothioneins in bacterial cells. In this study, we used recombinant strains created by cloning the human metallothioneins MT2A and MT3 into Escherichia coli Jm109 to assess the removal and reduction of hexavalent chromium (Cr(VI)) from aqueous solutions. MT2A was the most effective strain in both Cr(VI) removal (89% in 25 mg/L Cr(VI)) and Cr(VI) reduction (76% in 25 mg/L Cr(VI)). The amount of Cr adsorbed per dry cell by the MT2A strain was 22 mg/g. The biosorption of total Cr was consistent with the Langmuir isotherm model. Scanning electron microscope (SEM) images revealed that the morphological structures of Cr(VI)-treated cells were significantly damaged when compared to control cells. Scanning transmission electron microscope (STEM) images showed black spots in the cytoplasm of cells treated with Cr(VI). Shifts in the Fourier transform infrared spectroscopy analysis (FTIR) spectra of the cells treated with Cr(VI) showed that the groups interacting with Cr were hydroxyl, amine, amide I, amide II, phosphoryl and carbonyl. When all of the experimental data was combined, it was determined that both MT2A and MT3 were effective in removing Cr(VI) from aqueous solutions, but MT2A was more effective, indicating that MT2A may be employed as a biotechnological tool.

Biotic and Abiotic Biostimulation for the Reduction of Hexavalent Chromium in Contaminated Aquifers

Hexavalent chromium is a carcinogenic heavy metal that needs to be removed effectively from polluted aquifers in order to protect public health and the environment. This work aims to evaluate the reduction of Cr(VI) to Cr(III) in a contaminated aquifer through the stimulation of indigenous microbial communities with the addition of reductive agents. Soil-column experiments were conducted in the absence of oxygen and at hexavalent chromium (Cr(VI)) groundwater concentrations in the 1000–2000 μg/L range. Two carbon sources (molasses and EVO) and one iron electron donor (FeSO4·7H2O) were used as ways to stimulate the metabolism and proliferation of Cr(VI) reducing bacteria in-situ. The obtained results indicate that microbial anaerobic respiration and electron transfer can be fundamental to alleviate polluted groundwater from hazardous Cr(VI). The addition of organic electron donors increased significantly Cr(VI) reduction rates in comparison to natural soil attenuation rates. Furthermore, a combination of organic carbon and iron electron donors led to a longer life span of the remediation process and thus increased total Cr(VI) removal. This is the first study to investigate biotic and abiotic Cr(VI) removal by conducting experiments with natural soil and by applying biostimulation to modify the natural existing microbial communities.

Efficient visible-light-driven reduction of hexavalent chromium catalyzed by conjugated organic species modified hourglass-type phosphomolybdate hybrids

Four conjugated-organic-species modified hourglass-type phosphomolybdate hybrids with a 0-D + 1-D → 3-D supramolecular structure exhibited favorable photocatalytic activity and stability towards Cr(vi) reduction.

Bioreduction of hexavalent chromium by a novel haloalkaliphilic Salipaludibacillus agaradhaerens strain NRC-R isolated from hypersaline soda lakes

A novel Cr(VI)-resistant haloalkaliphilic bacterial strain NRC-R, identified as Salipaludibacillus agaradhaerens, was isolated from hypersaline soda lakes and characterized for its Cr(VI) bioreduction efficiency. Strain NRC-R grew well and effectively reduced Cr(VI) under a wide range of sodium chloride, pH, shaking velocity and temperature, showing maximum Cr(VI) reduction at 8% NaCl, pH 10, 150 rpm and 35 °C, respectively. Strain NRC-R was able to grow and reduce Cr(VI) effectively in the presence of different heavy metals and oxyanions (Pb²⁺, Zn²⁺, Co²⁺, Mn²⁺, Ni²⁺, Mo²⁺, HPO₄ ^-, NO₃ ^-, SO₄ ^2- and HCO₃ ^-). Furthermore, Fe³⁺ and Cu²⁺ significantly enhanced the Cr(VI) removal by about 1.5 fold. Strain NRC-R could reduce Cr(VI) using a variety of electron donors, exhibiting a maximum reduction in the presence of NADH and fructose. The bioremoval of Cr(VI) using strain NRC-R was due to direct enzymatic reduction and the chromate reductase activity was mainly detected in the bacterial cell membrane. Under the optimized conditions, strain NRC-R showed a remarkable Cr(VI) bioreduction with highest reduction rate of 240 uM/h. Cr(VI) concentrations of up to 3 mM (888.5 mg/L) and 4 mM (1177 mg/L) were completely reduced within 16 h and 32 h, respectively. TEM and SEM-EDX analyses confirmed the biosorption of chromium species into the cells. To the best of our knowledge, this is the first report about Cr(VI) reduction by S. agaradhaerens. In conclusion, S. agaradhaerens NRC-R was a highly efficient Cr(VI) reducing haloalkaliphilic bacterium that has a significant potential in the bioremediation of Cr(VI)-contaminated environments.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-03082-2.