Mechanisms for metal removal established via electron microscopy and spectroscopy: a case study on metal tolerant fungi Penicillium simplicissimum

"Efficient novel fungal-enriched biochar formulation for hexavalent chromium bioremediation"

Chromium (Cr), a key element in industrial processes such as leather tanning, poses severe environmental hazards, particularly its hexavalent form, Cr(VI), which is highly toxic and prevalent in tannery effluents/sludge. The persistence and toxicity of Cr(VI) necessitate the development of effective remediation strategies to mitigate its environmental impact. This study investigated the potential of Trichoderma yunnanense (NBRICRF_97) and its combination with 0.5% sugarcane bagasse biochar (SBC) for the reduction of Cr(VI). The results demonstrated that T. yunnanense alone achieved a 91.04% reduction of 50 mg L-1 Cr(VI) within 72 h. Combined with 0.5% SBC, the reduction efficiency increased to 99.65% within 48 h. However, the efficiency decreased at higher concentrations (200 mg L-1). The combination also improved fungal growth and increased extracellular ChrR enzyme activity (13.07 U mg-1 protein compared to the control). Total glutathione activity was boosted by 161.07% at 100 mg L-1 Cr(VI). Antioxidant enzymes (SOD, POD, CAT) and proline mitigated oxidative stress and FTIR analysis revealed changes in fungal cell wall functional groups (-OH and -NH) upon Cr(VI) exposure. SEM-EDX confirmed chromium deposition on fungal surfaces. These results underscore the Cr(VI) detoxification capabilities of T. yunnanense and the synergistic benefits of SBC, suggesting a promising bioremediation strategy for Cr(VI)-contaminated environments. The integration of T. yunnanense with SBC offers a sustainable and cost-effective approach for the bioremediation of Cr(VI)-contaminated sites, with potential for implementation in large-scale environmental cleanup efforts.

Exploring the biosorption of nickel and lead by Fusarium sp. biomass: kinetic, isotherm, and thermodynamic assessment

Fungal biomass is as a cost-effective and sustainable biosorbent utilized in both active and inactive forms. This study investigated the efficacy of inactivated and dried biomass of Fusarium sp. in adsorbing Ni2+ and Pb2+ from aqueous solutions. The strain underwent sequential cultivation and was recovered by filtration. Then, the biomass was dried in an oven at 80 ± 2 °C and sieved using a 0.1-cm mesh. The biosorbent was thoroughly characterized, including BET surface area analysis, morphology examination (SEM), chemical composition (XRF and FT-IR), thermal behavior (TGA), and surface charge determination (pH-PZC and zeta potential). The biosorption mechanism was elucidated by fitting equilibrium models of kinetics, isotherm, and thermodynamic to the data. The biosorbent exhibited a neutral charge, a rough surface, a relatively modest surface area, appropriate functional groups for adsorption, and thermal stability above 200 °C. Optimal biosorption was achieved at 25 ± 2 °C, using 0.05 g of adsorbent per 50 mL of metallic ion solution at initial concentrations ranging from 0.5 to 2.0 mg L-1 and at pH 4.5 for Pb2+ and Ni2+. Biosorption equilibrium was achieved after 240 min for Ni2+ and 1440 min for Pb2+. The process was spontaneous, mainly through chemisorption, in monolayer for Ni2+ and multilayer for Pb2+, with efficiencies of over 85% for both metallic ion removal. These findings underscore the potential of inactive and dry Fusarium sp. biomass (IDFB) as a promising material for the biosorption of Ni2+ and Pb2+.

Pathway to industrial application of heterotrophic organisms in critical metals recycling from e-waste

The transition to renewable energies and electric vehicles has triggered an unprecedented demand for metals. Sustainable development of these technologies relies on effectively managing the lifecycle of critical raw materials, including their responsible sourcing, efficient use, and recycling. Metal recycling from electronic waste (e-waste) is of paramount importance owing to ore-exceeding amounts of critical elements and high toxicity of heavy metals and organic pollutants in e-waste to the natural ecosystem and human body. Heterotrophic microbes secrete numerous metal-binding biomolecules such as organic acids, amino acids, cyanide, siderophores, peptides, and biosurfactants which can be utilized for eco-friendly and profitable metal recycling. In this review paper, we presented a critical review of heterotrophic organisms in biomining, and current barriers hampering the industrial application of organic acid bioleaching and biocyanide leaching. We also discussed how these challenges can be surmounted with simple methods (e.g., culture media optimization, separation of microbial growth and metal extraction process) and state-of-the-art biological approaches (e.g., artificial microbial community, synthetic biology, metabolic engineering, advanced fermentation strategies, and biofilm engineering). Lastly, we showcased emerging technologies (e.g., artificially synthesized peptides, siderophores, and biosurfactants) derived from heterotrophs with the potential for inexpensive, low-impact, selective and advanced metal recovery from bioleaching solutions.

Inhibitory effects of Pseudomonas sp. W112 on cadmium accumulation in wheat grains: Reduced the bioavailability in soil and enhanced the interception by plant organs

Microorganisms play an important role in heavy metal bioremediation and soil fertility. The effects of soil inoculation with Pseudomonas sp. W112 on Cd accumulation in wheat were investigated by analyzing the transport, subcellular distribution and speciation of Cd in the soil and plants. Pseudomonas sp. W112 application significantly decreased Cd content in the roots, internode and grains by 10.2%, 29.5% and 33.0%, respectively, and decreased Cd transfer from the basal nodes to internodes by 63.5%. Treatment with strain W112 decreased the inorganic and water-soluble Cd content in the roots and increased the proportion of residual Cd in both the roots and basal nodes. At the subcellular level, the Cd content in the root cell wall and basal node cytosol increased by 19.6% and 61.8%, respectively, indicating that strain W112 improved the ability of the root cell wall and basal node cytosol to fix Cd. In the rhizosphere soil, strain W112 effectively colonized and significantly decreased the exchangeable Cd, carbonate-bound Cd and iron-manganese oxide-bound Cd content by 43.5%, 27.3% and 17.6%, respectively, while it increased the proportion of residual Cd by up to 65.2%. Moreover, a 3.1% and 23.5% increase in the pH and inorganic nitrogen content in the rhizosphere soil, respectively, was recorded. Similarly, soil bacterial community sequencing revealed that inoculating with strain W112 increased the abundance of Pseudomonas, Thauera and Azoarcus, which are associated with inorganic nitrogen metabolism, and decreased that of Acidobacteria, which is indicative of soil alkalinization. Hence, root application of Pseudomonas sp. W112 improved soil nitrogen availability and inhibited Cd accumulation in the wheat grains in a two-stage process: by reducing the Cd availability in the rhizosphere soil and by improving Cd interception and fixation in the wheat roots and basal nodes. Pseudomonas sp. W112 may be a suitable bioremediation agent for restoring Cd-contaminated wheat fields.

Unveiling the performance of a novel alkalizing bacterium Enterobacter sp. LYX-2 in immobilization of available Cd

A novel alkalizing strain Enterobacter sp. LYX-2 that could resist 400 mg/L Cd was isolated from Cd-contaminated soil, which immobilized 96.05% Cd2+ from medium. Cd distribution analysis demonstrated that more than half of the Cd2+ was converted into extracellular precipitated Cd through mobilization of the alkali-producing mechanism by the strain LYX-2, achieving the high immobilization efficiency of Cd2+. Biosorption experiments revealed that strain LYX-2 had superior biosorption capacity of 48.28 mg/g for Cd. Pot experiments with Brassica rapa L. were performed with and without strain LYX-2. Compared to control, 15.92% bioavailable Cd was converted to non-bioavailable Cd and Cd content in aboveground vegetables was decreased by 37.10% with addition of strain LYX-2. Available Cd was mainly immobilized through extracellular precipitation, cell-surface biosorption and intracellular accumulation of strain LYX-2, which was investigated through Cd distribution, Scanning Electron Microscope and Energy-Dispersive X-ray Spectroscopy (SEM-EDS), Fourier Transform Infrared Spectroscopy (FT-IR), X-ray Photoelectron Spectroscopy (XPS) and Transmission Electron Microscopy (TEM) analysis. In addition, the application of strain LYX-2 significantly promoted the growth of vegetables about 2.4-fold. Above results indicated that highly Cd-resistant alkalizing strain LYX-2, as a novel microbial passivator, had excellent ability and reuse value to achieve the remediation of Cd-contaminated soil coupled with safe production of vegetables simultaneously.

Docking assisted mechanistic elucidation of bio conversion of hexavalent chromium by Serratia marcescens AJRR-22 that is effective yet long term sustainable in bio-geosphere

Environmental accumulation of hexavalent chromium [Cr(VI)] in the food chain can induce detrimental effects on plants and animals, which calls for effective remediation strategies using biological entities. The bacterium isolated from an iron mine in Odisha, India, is identified asSerratia marcescensAJRR-22. This multi-metal tolerant strain is capable of bio-converting up to 350 mg/L Cr(VI) within 72 h of incubation. Observable electron dense precipitates in transmission electron microscopic images, data patterns in fluorescence microscopy and flow cytometry clearly reveal the chromate reduction ability of the strain. The molecular study is depicted by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopic analyses. Furthermore, a simulation study to estimate the interactions of chromium bound flavin reductasewith predicted docked complexes suggests significant negative Gibbs free energy and a low inhibition constant (Ki), signifying strong spontaneous binding of Cr(VI) to the enzyme, which makes the strain an efficient candidate for chromium bioremediation.

Tailored bacteria tackling with environmental mercury: Inspired by natural mercuric detoxification operons

Mercury (Hg) and its inorganic and organic compounds significantly threaten the ecosystem and human health. However, the natural and anthropogenic Hg environmental inputs exceed 5000 metric tons annually. Hg is usually discharged in elemental or ionic forms, accumulating in surface water and sediments where Hg-methylating microbes-mediated biotransformation occurs. Microbial genetic factors such as the mer operon play a significant role in the complex Hg biogeochemical cycle. Previous reviews summarize the fate of environmental Hg, its biogeochemistry, and the mechanism of bacterial Hg resistance. This review mainly focuses on the mer operon and its components in detecting, absorbing, bioaccumulating, and detoxifying environmental Hg. Four components of the mer operon, including the MerR regulator, divergent mer promoter, and detoxification factors MerA and MerB, are rare bio-parts for assembling synthetic bacteria, which tackle pollutant Hg. Bacteria are designed to integrate synthetic biology, protein engineering, and metabolic engineering. In summary, this review highlights that designed bacteria based on the mer operon can potentially sense and bioremediate pollutant Hg in a green and low-cost manner.

Microbial Removal of Heavy Metals from Contaminated Environments Using Metal-Resistant Indigenous Strains

Contamination of soil with heavy metals has become a matter of global importance due to its impact on agriculture, environmental integrity, and therefore human health and safety. Several microbial strains isolated from soil contaminated by long-term chemical and petrochemical activities were found to manifest various levels of tolerance to Cr, Pb, and Zn, out of which Bacillus marisflavi and Trichoderma longibrachiatum exhibited above-moderate tolerance. The concentrations of target heavy metals before and after bioremediation were determined using electrochemical screen-printed electrodes (SPE) modified with different nanomaterials. The morpho-structural SEM/EDX analyses confirmed the presence of metal ions on the surface of the cell, with metal uptake being mediated by biosorption with hydroxyl, carboxyl, and amino groups as per FTIR observations. T. longibrachiatum was observed to pose a higher bioremediation potential compared to B. marisflavi, removing 87% of Cr and 67% of Zn, respectively. Conversely, B. marisflavi removed 86% of Pb from the solution, compared to 48% by T. longibrachiatum. Therefore, the fungal strain T. longibrachiatum could represent a viable option for Cr and Zn bioremediation strategies, whereas the bacterial strain B. marisflavi may be used in Pb bioremediation applications.

Bioremoval of heavy metals from aqueous solution using dead biomass of indigenous fungi derived from fertilizer industry effluents: isotherm models evaluation and batch optimization

The present work investigated the utilization of dead biomass of the highly multi-heavy metals tolerant indigenous fungal strain NRCA8 isolated from the mycobiome of fertilizer industry effluents that containing multiple heavy metal ions at high levels to remove Pb2+, Ni2+, Zn2+, and Mn2+ as multiple solutes from multi-metals aqueous solutions for the first time. Based on morphotype, lipotype and genotype characteristics, NRCA8 was identified as Cladosporium sp. NRCA8. The optimal conditions for the bioremoval procedure in the batch system were pH 5.5 for maximum removal (91.30%, 43.25%, and 41.50%) of Pb2+, Zn2+ and Mn2+ but pH 6.0 supported the maximum bioremoval and uptake of Ni2+ (51.60% and 2.42 mg/g) by NRCA8 dead biomass from the multi-metals aqueous solution, respectively. The 30 min run time supported the highest removal efficiency and uptake capacity of all heavy metals under study. Moreover, the equilibrium between the sorbent NRCA8 fungal biomass and sorbates Ni2+, Pb2+ and Zn2+ was attained after increasing the dead biomass dose to 5.0 g/L. Dead NRCA8 biomass was described by scanning electron microscopy, energy-dispersive X-ray spectroscopy and Fourier transform infrared spectrometer before and after biosorption of Pb2+, Ni2+, Zn2+ and Mn2+ under multiple metals system. The Langmuir, Freundlich and Dubinin-Kaganer-Radushkevich isotherms were applied to characterize the adsorption equilibrium between Pb2+, Ni2+, Mn2+ and Zn2+ and the adsorbent NRCA8. By comparing the obtained coefficient of regression (R2) by Freundlich (0.997, 0.723, 0.999, and 0.917), Langmiur (0.974, 0.999, 0.974, and 0.911) and Dubinin-Radushkevich (0.9995, 0.756, 0.9996 and 0.900) isotherms values for Pb2+, Zn2+, Ni2+ and Mn2+ adsorption, respectively, it was found that the isotherms are proper in their own merits in characterization the possible of NRCA8 for removal of Pb2+, Zn2+, Ni2+ and Mn2+. DKR isotherm is the best for Pb2+ and Ni2+ (0.9995 and 0.9996) while Langmiur isotherm giving a good fit to the Zn2+ sorption (0.9990) as well as Freundlich isotherm giving a good fit to the Mn2+ sorption (0.9170). The efficiencies of Cladosporium sp. NRCA8 dead biomass for bioremoval of heavy metals from real wastewater under the optimized conditions were Pb2+, Ag+, Mn2+, Zn2+ and Al3+ ˃ Ni2+ ˃ Cr6+ ˃ Co2+ ˃ Fe3+ ˃ Cu2+ ˃ Cd2+. Dead NRCA8 biomass showed efficient ability to adsorb and reduce harmful components in the industrial effluents to a level acceptable for discharge into the environment.

Multimetal bioremediation from aqueous solution using dead biomass of Mucor sp. NRCC6 derived from detergent manufacturing effluent

Among ten metal-tolerant fungal isolates obtained from the microbiomes of detergent industry effluent, Mucor sp. NRCC6 showed the highest tolerance and an adaptive behavior toward the heavy metals Ni2+, Pb2+, Mn2+, and Zn2+. It gave the highest growth rates 0.790 ± 0.59, 0.832 ± 0.32, 0.774 ± 0.40, and 0.741 ± 1.06 mm/h along with the lowest growth inhibition 9.19, 4.37, 11.04, and 14.83% in the presence of Pb2+, Zn2+, Ni2+, and Mn2+, respectively, at a concentration of 5.0 g/L. Then, Mucor sp. NRCC6 was selected as a biotrap for the removal of these heavy metals. The optimized operating conditions were detected to be pH 6.0 for Pb2+, Zn2+, and Mn2+ and pH 5.5 for Ni2+ at 30 °C; agitation speed 150 rpm; contact time 30 min for Mn2+ and Ni2+, 30-60 min for Pb2+, and 90-180 min for Zn2+; NRCC6 biomass dosage 5.0 g/L for Ni2+ and Pb2+ and 10.0 g/L for Mn2+ and Zn2+; and initial concentration 12 mg/L of each ion in the multimetal aqueous solutions. Under these optimized conditions, the adsorption capacity for Pb2+, Ni2+, Mn2+, and Zn2+ reached 98.75, 59.25, 58.33, and 50.83%. The Langmuir isotherm was the best for describing the adsorption of Zn2+ (0.970) and Mn2+ (0.977). The Freundlich isotherm significantly giving a good fit to the adsorption of Pb2+ (0.998) while the adsorption of Ni2+ onto NRCC6 biomass can follow DKR (0.998). Furthermore, the current study revealed that Mucor sp. NRCC6 fungus is a new efficient and eco-friendly method that revealed a maximum removal of 100% for Pb2+ and Zn2+ as well as 97.39, 88.70, 78.95, 74.0, 70.22, 68.57, and 60.0% for Ni2+, Mn2+, Cd2+, Cu2+, Fe3+, As2+, and Cr6+ from the industrial wastewater, respectively.

Roles and significance of chelating agents for potentially toxic elements (PTEs) phytoremediation in soil: A review

Phytoremediation is a biological remediation technique known for low-cost technology and environmentally friendly approach, which employs plants to extract, stabilise, and transform various compounds, such as potentially toxic elements (PTEs), in the soil or water. Recent developments in utilising chelating agents soil remediation have led to a renewed interest in chelate-induced phytoremediation. This review article summarises the roles of various chelating agents and the mechanisms of chelate-induced phytoremediation. This paper also discusses the recent findings on the impacts of chelating agents on PTEs uptake and plant growth and development in phytoremediation. It was found that the chelating agents have increased the rate of metal absorption and translocation up to 45% from roots to the aboveground plant parts during PTEs phytoremediation. Besides, it was also explored that the plants may experience some phytotoxicity after adding chelating agents to the soil. However, due to the leaching potential of synthetic chelating agents, the use of organic chelants have been explored to be used in PTEs phytoremediation. Finally, this paper also presents comprehensive insights on the significance of using chelating agents through SWOT analysis to discuss the advantages and limitations of chelate-induced phytoremediation.

Toxicity of Heavy Metals and Recent Advances in Their Removal: A Review

Natural and anthropogenic sources of metals in the ecosystem are perpetually increasing; consequently, heavy metal (HM) accumulation has become a major environmental concern. Human exposure to HMs has increased dramatically due to the industrial activities of the 20th century. Mercury, arsenic lead, chrome, and cadmium have been the most prevalent HMs that have caused human toxicity. Poisonings can be acute or chronic following exposure via water, air, or food. The bioaccumulation of these HMs results in a variety of toxic effects on various tissues and organs. Comparing the mechanisms of action reveals that these metals induce toxicity via similar pathways, including the production of reactive oxygen species, the inactivation of enzymes, and oxidative stress. The conventional techniques employed for the elimination of HMs are deemed inadequate when the HM concentration is less than 100 mg/L. In addition, these methods exhibit certain limitations, including the production of secondary pollutants, a high demand for energy and chemicals, and reduced cost-effectiveness. As a result, the employment of microbial bioremediation for the purpose of HM detoxification has emerged as a viable solution, given that microorganisms, including fungi and bacteria, exhibit superior biosorption and bio-accumulation capabilities. This review deals with HM uptake and toxicity mechanisms associated with HMs, and will increase our knowledge on their toxic effects on the body organs, leading to better management of metal poisoning. This review aims to enhance comprehension and offer sources for the judicious selection of microbial remediation technology for the detoxification of HMs. Microbial-based solutions that are sustainable could potentially offer crucial and cost-effective methods for reducing the toxicity of HMs.

Enhanced mercury phytoremediation by Pseudomonodictys pantanalensis sp. nov. A73 and Westerdykella aquatica P71

Mercury is a non-essential and toxic metal that induces toxicity in most organisms, but endophytic fungi can develop survival strategies to tolerate and respond to metal contaminants and other environmental stressors. The present study demonstrated the potential of mercury-resistant endophytic fungi in phytoremediation. We examined the functional traits involved in plant growth promotion, phytotoxicity mitigation, and mercury phytoremediation in seven fungi strains. The endophytic isolates synthesized the phytohormone indole-3-acetic acid, secreted siderophores, and solubilized phosphate in vitro. Inoculation of maize (Zea mays) plants with endophytes increased plant growth attributes by up to 76.25%. The endophytic fungi stimulated mercury uptake from the substrate and promoted its accumulation in plant tissues (t test, p < 0.05), preferentially in the roots, which thereby mitigated the impacts of metal phytotoxicity. Westerdykella aquatica P71 and the newly identified species Pseudomonodictys pantanalensis nov. A73 were the isolates that presented the best phytoremediation potential. Assembling and annotation of P. pantanalensis A73 and W. aquatica P71 genomes resulted in genome sizes of 45.7 and 31.8 Mb that encoded 17,774 and 11,240 protein-coding genes, respectively. Some clusters of genes detected were involved in the synthesis of secondary metabolites such as dimethylcoprogen (NRPS) and melanin (T1PKS), which are metal chelators with antioxidant activity; mercury resistance (merA and merR1); oxidative stress (PRX1 and TRX1); and plant growth promotion (trpS and iscU). Therefore, both fungi species are potential tools for the bioremediation of mercury-contaminated soils due to their ability to reduce phytotoxicity and assist phytoremediation.

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.

Screening of Native Trichoderma Species for Nickel and Copper Bioremediation Potential Determined by FTIR and XRF

Soil pollution with heavy metals is a serious threat to the environment. However, soils polluted with heavy metals are considered good sources of native metal-resistant Trichoderma strains. Trichoderma spp. are free-living fungi commonly isolated from different ecosystems, establishing endophytic associations with plants. They have important ecological and biotechnological roles due to their production of a wide range of secondary metabolites, thus regulating plant growth and development or inducing resistance to plant pathogens. In this work we used indigenous Trichoderma strains that were previously isolated from different soil types to determine their tolerance to increased copper and nickel concentrations as well as mechanisms of metal removal. The concentrations of bioavailable metal concentrations were determined after extraction with diethylene-triamine pentaacetate (DTPA)-extractable metals (Cd, Cr, Co, Cu, Pb, Mn, Ni, and Zn) from the soil samples by inductively coupled plasma-optical emission spectrometry (ICP-OES). Two indigenous T. harzianum strains were selected for copper tolerance, and three indigenous T. longibrachiatum strains were selected for nickel tolerance tests. Strains were isolated from the soils with the highest and among the lowest DTPA-extractable metal concentrations to determine whether the adaptation to different concentrations of metals affects the mechanisms of remediation. Mechanisms of metal removal were determined using Fourier-transform infrared spectroscopy (FTIR) and X-ray fluorescence spectroscopy (XRF), non-destructive methods characterized by high measurement speed with little or no need for sample preparation and very low costs. Increased DTPA-extractable metal content for nickel and copper was detected in the soil samples above the target value (TV), and for nickel above the soil remediation intervention values (SRIVs), for total metal concentrations which were previously determined. The SRIV is a threshold of metal concentrations indicating a serious soil contamination, thus confirming the need for soil remediation. The use of FTIR and XRF methods revealed that the presence of both biosorption and accumulation of metals in the Trichoderma cells, providing good bioremediation potential for Ni and Cu.

A Feasibility Study on the Recall of Metallophilic Fungi from Fe(III)-Contaminated Soil and Evaluating Their Mycoremediation Capacity: Experimental and Theoretical Study

Mycoremediation is one of the most attractive, eco-friendly, and sustainable methods to mitigate the toxic effects of heavy metals. This study aimed to determine the mycoremediation capacity of metallophilic fungi isolated from heavy-metal-contaminated soil containing a high Fe(III) concentration (118.40 mg/kg). Four common fungal strains were isolated, including Curvularia lunata, Fusarium equiseti, Penicillium pinophilum, and Trichoderma harzianum. These fungal strains were exposed to gradually increasing concentrations of Fe(III) of 100, 200, 300, 400, 500, 600, 700, 800, 900, and 1000 mg/L. Sophisticated techniques and tests were employed to investigate the mycoremediation capability, including tolerance index (TI), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and adsorption isotherm. Furthermore, the impacts of initial concentration, pH, and temperature on the Fe(III) removal (%) and uptake capacity (mg/g) of the studied samples were investigated. The results were validated by statistical analysis using one-way ANOVA. It was found that the Fe(III) uptake with different ratios triggered alterations in the Fe(III) tolerance (TI) morphological (SEM), chemical (FTIR), and adsorption capacity properties. The highest Fe(III) tolerance for all studied fungal strains was achieved at 100 mg/L. Moreover, the optimum conditions of Fe(III) removal (%) for all studied fungal strains were within pH 7 and 28 °C, with similar performance at the initial Fe(III) concentration ranging from 50-200 mg/L. At the same time, the maximum Fe(III) uptake was achieved at pH 7, 20 °C, and 200 mg/L. Compared to other strains, the Fe(III) tolerance of T. harzianum was rise in the Fe(III) concentration. The Fe(III) uptake reaction was corroborated by best fitting with the Langmuir model, achieving optimum adsorption capacities of 61.34, 62.90, 63.30, and 72.46 mg/g for C.lunata, F. equiseti, P. pinophilum, T. harzianum, respectively. It can be deduced that the addressed fungi species can be applied in mycoremediation according to the utilized Fe(III) concentrations with more superiority for live T. harzianum.

Simultaneous Heavy Metal-Polycyclic Aromatic Hydrocarbon Removal by Native Tunisian Fungal Species

Multi-contamination by organic pollutants and toxic metals is common in anthropogenic and industrial environments. In this study, the five fungal strains Chaetomium jodhpurense (MH667651.1), Chaetomium maderasense (MH665977.1), Paraconiothyrium variabile (MH667653.1), Emmia lacerata, and Phoma betae (MH667655.1), previously isolated in Tunisia, were investigated for the simultaneous removal and detoxification of phenanthrene (PHE) and benzo[a]anthracene (BAA), as well as heavy metals (HMs) (Cu, Zn, Pb and Ag) in Kirk's media. The removal was analysed using HPLC, ultra-high performance liquid chromatography (UHPLC) coupled to a QToF mass spectrometer, transmission electron microscopy, and toxicology was assessed using phytotoxicity (Lepidium sativum seeds) and Microtox® (Allivibrio fisherii) assays. The PHE and BAA degradation rates, in free HMs cultures, reached 78.8% and 70.7%, respectively. However, the addition of HMs considerably affected the BAA degradation rate. The highest degradation rates were associated with the significant production of manganese-peroxidase, lignin peroxidase, and unspecific peroxygenase. The Zn and Cu removal efficacy was considerably higher with live cells than dead cells. Transmission electron microscopy confirmed the involvement of both bioaccumulation and biosorption processes in fungal HM removal. The environmental toxicological assays proved that simultaneous PAH and HM removal was accompanied by detoxification. The metabolites produced during co-treatment were not toxic for plant tissues, and the acute toxicity was reduced. The obtained results indicate that the tested fungi can be applied in the remediation of sites simultaneously contaminated with PAHs and HMs.

The trafficking of HgII by alleviating its toxicity via Citrobacter sp. IITISM25 in batch and pilot-scale investigation

The study aims to see how effective the Citrobacter species strain is in removing Hg^II under stressful conditions. For this, a response surface methodology was chosen to optimized pH, temperature, and biomass for effective biotransformation of Hg^II. The optimized value for pH, temperature, and biomass were 6.5, 30 °C, and 2 mg/l with 89% Hg^II removal potential. TEM-EDX showed accumulated mercury onto the bacterial surface. Pot study was conducted to check the potentiality of this strain in alleviating the toxicity in Solanum lycopersicum L. under different concentrations of mercury. The enhancement in antioxidative enzymes, as well as mercury accumulation, was observed in test plants inoculated with IITISM25. Obtained result showed a greater accumulation of mercury in the root system than that of the shoot system due to poor translocation. Moreover, mercury reductase enzyme synthesis was also boosted by the addition of β-mercaptoethanol and L-cysteine. The optimized condition for maximum enzyme synthesis was at pH 7.5 and temperature 30 °C with Km = 48.07 μmol and Vmax = 9.75 μmol/min. Thus, we can say that Citrobacter species strain IITISM25 can be effectively applied in remediation of Hg^II stress condition as well as promotion of Solanum lycopersicum L growth under stress conditions as a promising host.

Cr(VI) Removal by Recombinant Escherichia coli Harboring the Main Functional Genes of Sporosarcina saromensis M52

Hexavalent chromium [Cr(VI)], a recognized heavy metal pollutant, has attracted much attention because of its negative impact on the ecological environment and human health. A chromium-resistant strain, Sporosarcina saromensis M52, was discovered, and the functional genes orf2987, orf3015, orf0415, and orf3237 were identified in the strain by genomics. With the advancement of DNA recombination and gene-splicing technology, genetic engineering technology was used to produce recombinant strains 2987, 3015, 0415, and 3237. The study revealed Cr(VI) tolerance in the order of M52 ≈ 2987 > 3015 ≈ 0415 > 3237 and reduction abilities in the order of M52 ≈ 2987 > 3015 > 0415 ≈ 3237. SEM-EDS, XRD, FT-IR and XPS were utilized to examine the surface structure of the recombinant strains and analyze the surface components and main functional groups. A comprehensive review of the recombinant strains’ capacity to tolerate and reduce Cr(VI) revealed that orf2987 and orf0415 were the main functional genes in Sporosarcina saromensis M52, which may play a key role in removing Cr(VI) and protecting the strain, respectively. The optimum pH for recombinant strains 2987 and 0415 was 7.5–8.5, and the optimum temperature was 37°C. Cu²⁺ had the greatest promotional effect when Cr(VI) was removed by them, while SDS had an inhibitory effect. This research provided the foundation for further study into the mechanism of Cr(VI) reduction in Sporosarcina saromensis M52, as well as a theoretical basis for the development of effective engineered strains to repair Cr(VI) contamination.

Mechanistic study for mutual interactions of Pb2+ and Trichoderma viride

Fungi play significant roles in the geochemical processes of heavy metals in the environment. However, the interaction between heavy metals and fungi, especially at the cellular level, is quite complicated and remains unknown. This study explored the mutual interaction mechanism between Pb²⁺ and Trichoderma viride by combining batch experiments, spectroscopy, and in vitro approaches. Batch experiments revealed that Pb²⁺ had toxic effect on T. viride, originally causing the biomass of T. viride decreased from 1.3 g in the control group to 0 g in the presence of 200 mg/L Pb²⁺. The difference in biomass further led to varied pH, even decreasing from 5.7 at the outset to 3.4 due to the acid-production properties of T. viride. Moreover, structural deformation and damage of T. viride mycelium appeared when exposed to Pb²⁺, and were more evident at a higher dose of Pb²⁺ exposure. The growth curve exhibited that T. viride gradually adapted to Pb²⁺ exposure, which related to Pb²⁺ exposure concentration. Further, intracellular and extracellular secretions of T. viride changed with varying exposure concentrations of Pb²⁺, indicating that T. viride adapted differently to different concentrations of Pb²⁺, and MT participated in the detoxification of T. viride. SEM-EDX showed that T. viride could bio-adsorb and bioaccumulate more Pb²⁺ when exposed to more Pb²⁺, which was closely related to the content of P. And carbonyl, phosphate, and amino groups of T. viride participated in the Pb²⁺ biosorption onto T. viride, as evidenced by FT-IR and XPS. Meanwhile, the biomineralization and reduction of Pb²⁺ by T. viride were observed by XRD and XPS, which might be a possible factor for Pb²⁺ biosorption and bioaccumulation. CLSM showed that the bio-adsorbed and bioaccumulated Pb²⁺ were mainly distributed in the membrane of T. viride mycelium.

The aluminum distribution and translocation in two citrus species differing in aluminum tolerance

BACKGROUND

Many citrus orchards of south China suffer from soil acidification, which induces aluminum (Al) toxicity. The Al-immobilization in vivo is crucial for Al detoxification. However, the distribution and translocation of excess Al in citrus species are not well understood.

RESULTS

The seedlings of 'Xuegan' [Citrus sinensis (L.) Osbeck] and 'Shatianyou' [Citrus grandis (L.) Osbeck], that differ in Al tolerance, were hydroponically treated with a nutrient solution (Control) or supplemented by 1.0 mM Al³⁺ (Al toxicity) for 21 days after three months of pre-culture. The Al distribution at the tissue level of citrus species followed the order: lateral roots > primary roots > leaves > stems. The concentration of Al extracted from the cell wall (CW) of lateral roots was found to be about 8 to 10 times higher than in the lateral roots under Al toxicity, suggesting that the CW was the primary Al-binding site at the subcellular level. Furthermore, the Al distribution in CW components of the lateral roots showed that pectin had the highest affinity for binding Al. The relative expression level of genes directly relevant to Al transport indicated a dominant role of Cs6g03670.1 and Cg1g021320.1 in the Al distribution of two citrus species. Compared to C. grandis, C. sinensis had a significantly higher Al concentration on the CW of lateral roots, whereas remarkably lower Al levels in the leaves and stems. Furthermore, Al translocation revealed by the absorption kinetics of the CW demonstrated that C. sinensis had a higher Al retention and stronger Al affinity on the root CW than C. grandis. According to the FTIR (Fourier transform infrared spectroscopy) analysis, the Al distribution and translocation might be affected by a modification in the structure and components of the citrus lateral root CW.

CONCLUSIONS

A higher Al-retention, mainly attributable to pectin of the root CW, and a lower Al translocation efficiency from roots to shoots contributed to a higher Al tolerance of C. sinensis than C. grandis. The aluminum distribution and translocation of two citrus species differing in aluminum tolerance were associated with the transcriptional regulation of genes related to Al transport and the structural modification of root CW.

Combined effects of Penicillium oxalicum and tricalcium phosphate on lead immobilization: Performance, mechanisms and stabilities

Phosphorus (P) containing minerals are identified as effective Pb stabilizers in soil, while their low solubility limit the Pb immobilization efficiency. In this work, the combination of phosphate solubilizing fungi (PSF) Penicillium oxalicum and tricalcium phosphate (TCP) was constructed and applied to improve Pb immobilization stabilities in medium and soils. P. oxalicum+ TCP could significantly improve Pb²⁺ removal to above 99% under different TCP/Pb²⁺ and pH values. TCP and P. oxalicum could remarkably immobilize Pb by ion exchange, and PbC₂O₄ precipitation or surface adsorption, respectively. While the enhanced Pb immobilization in P. oxalicum+ TCP was explained by stronger Pb²⁺ interaction with tryptophan protein-like substances in extracellular polymeric substance, and the formation of the most stable Pb-phosphate compound hydroxypyromorphite (Pb₅(PO₄)₃OH). Toxicity characteristic leaching procedure test showed that only 0.91% of Pb²⁺ was leachable in P. oxalicum+ TCP treatment, significantly lower than that in P. oxalicum (2.90%) and TCP (7.52%) treatments. In addition, the lowest soil exchangeable Pb fraction (37.1%) and the highest available soil P (88.0 mg/kg) were both found in P. oxalicum+ TCP treatment. By synergistically forming stable Pb-containing products, thus the combination of PSF and P minerals could significantly improve Pb²⁺ immobilization and stability in soils.

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.

Mechanism of Cr(VI) reduction by Lysinibacillus sp. HST-98, a newly isolated Cr (VI)-reducing strain

Facing the increasingly severe Cr(VI) pollution, bioreduction has proved to be an eco-friendly remediation method. An isolated strain identified as Lysinibacillus can relatively reduce Cr(VI) well. Even if the concentration of Cr(VI) increased to 250mg/L, the strain HST-98 could also grow and remove Cr(VI) well. After optimization of reaction conditions, the optimal pH, temperature, and electron donor are 8~9, 36°C, and sodium lactate, respectively. Coexisting metal ions such as Cu2+, Co2+, and Mn2+ are beneficial to reduce Cr(VI), while Zn2+, Ni2+, and Cd2+ are just the opposite. What is more, the mechanism of the reduction by the strain HST-98 is chiefly mediated by intracellular enzymes. After gene sequence homology blast and analysis, the genes and enzymes related to chromium metabolism in strain HST-98 have been annotated, which helps us to further understand the reduction mechanism of the strain HST-98. In general, Lysinibacillus sp. HST-98 is a potential candidate to repair the Cr(VI)-contaminated sites.

Innovative method for encapsulating highly pigmented biomass from Aspergillus nidulans mutant for copper ions removal and recovery

Biosorption has been considered a promising technology for the treatment of industrial effluents containing heavy metals. However, the development of a cost-effective technique for biomass immobilization is essential for successful application of biosorption in industrial processes. In this study, a new method of reversible encapsulation of the highly pigmented biomass from Aspergillus nidulans mutant using semipermeable cellulose membrane was developed and the efficiency of the encapsulated biosorbent in the removal and recovery of copper ions was evaluated. Data analysis showed that the pseudo-second-order model better described copper adsorption by encapsulated biosorbent and a good correlation (r2 > 0.96) to the Langmuir isotherm was obtained. The maximum biosorption capacities for the encapsulated biosorbents were higher (333.5 and 116.1 mg g-1 for EB10 and EB30, respectively) than that for free biomass (92.0 mg g-1). SEM-EDXS and FT-IR analysis revealed that several functional groups on fungal biomass were involved in copper adsorption through ion-exchange mechanism. Sorption/desorption experiments showed that the metal recovery efficiency by encapsulated biosorbent remained constant at approximately 70% during five biosorption/desorption cycles. Therefore, this study demonstrated that the new encapsulation method of the fungal biomass using a semipermeable cellulose membrane is efficient for heavy metal ion removal and recovery from aqueous solutions in multiple adsorption-desorption cycles. In addition, this reversible encapsulation method has great potential for application in the treatment of heavy metal contaminated industrial effluents due to its low cost, the possibility of recovering adsorbed ions and the reuse of biosorbent in consecutive biosorption/desorption cycles with high efficiency of metal removal and recovery.

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).

Sustainable removal of Ni(II) from waste water by freshly isolated fungal strains

The release of untreated wastewater containing biotoxic substances in the form of heavy metals is one of the most crucial environmental and health challenges faced by our community. The recent advances in microbes derived removal has propelled bioremediation as a better and effective alternative to conventional techniques. Present study investigates the detoxification mechanisms evolved by the nickel (Ni(II)) resistant fungal strains, isolated from the industrial drain sites. The molecular detailing of the isolated fungal isolates confirms their identity as Neurospora crassa and Aspergillus flavus. Laboratory-scale experiments have established influence of different ranges of dose, pH, time, and metal concentration on the removal and uptake trends. Further, the variations in the carbon and nitrogen sources and agitation conditions has revealed the best substratum for achieving optimum results for the industrial exploitation of these microbes. SEM micrographs and FTIR spectra elucidates the superficial alterations on the mycelium of the fungal isolates and the involvement of active functional groups in the bioremediation of Ni(II) respectively. Biosorption of Ni(II) on living biomass has followed the Langmuir adsorption model. The findings of the study have provided a promising insight in the simultaneous action of different mechanistic removal approaches to explore a large scale removal of Ni(II) from the waste generating industries.

Kinetics and equilibrium study for the biosorption of lanthanum by Penicillium simplicissimum INCQS 40,211

Lanthanum (La) is a light rare-earth element that plays an essential role in manufacturing technological products, clean technologies, medical products, electron cathodes, scintillators, fluorescent lamps, and fertilizers. This study is the first investigation of La³⁺ biosorption using inactive lyophilized biomass from Penicillium simplicissimum INCQS 40,211. The maximum sorption capacity (q_max) for P. simplicissimum was 7.81 mg g^-1. La ³⁺ biosorption followed the Freundlich model, where the biosorption system possibly multilayer coverage of P. simplicissimum by lanthanum ions. The kinetic data for the adsorption process obeyed a pseudo-second-order (R ² > 0.92), indicating chemical sorption. The results indicated that inactive lyophilized biomass from Penicillium simplicissimum INCQS 40211are an excellent candidate for removing light rare-earth elements from aquatic environments.

Restoration of heavy metal-contaminated soil and water through biosorbents: A review of current understanding and future challenges

Heavy metal pollution in soil and water is a potential threat to human health as it renders food quality substandard. Different biosorbents such as microbial and agricultural biomass have been exploited for heavy metal immobilization in soil and sorptive removal in waters. Biosorption is an effective and sustainable method for heavy metal removal in soil and water, but the inherent challenges are to find cheap, selective, robust, and cost-effective bioadsorbents. Microbial and agricultural biomass and their modified forms such as nanocomposites and carbonaceous materials (viz., biochar, nanobiochar, biocarbon), might be useful for sequestration of heavy metals in soil via adsorption, ion exchange, complexation, precipitation, and enzymatic transformation mechanisms. In this review, potential biosorbents and their metal removal capacity in soil and water are discussed. The microbial adsorbents and modified composites of agricultural biomasses show improved performance, stability, reusability, and effectively immobilize heavy metals from soil and water. In the future, researchers may consider the modified composites, encapsulated biosorbents for soil and water remediation.

Morphological changes and bioaccumulation in response to cadmium exposure in Morchella spongiola, a fungus with potential for detoxification

Morchella is a genus of edible fungi with strong resistance to Cd and the ability to accumulate it in the mycelium. However, the mechanisms conferring Cd resistance in Morchella are unknown. In the present study, morphological and physiological responses to Cd were evaluated in the mycelia of Morchella spongiola. Variations in hyphal micro-morphology including twisting, folding and kinking in mycelia exposed to different Cd concentrations (0.15, 0.9, 1.5, 2.4, 5.0 mg/L) were observed using scanning electron microscopy. Deposition of Cd precipitates on cell surfaces (at Cd concentrations > 2.4 mg/L) was shown by SEM-EDS. Transmission electron microscopy analysis of cells exposed to different concentrations of Cd revealed the loss of intracellular structures and the localization of Cd depositions inside/outside the cell. FTIR analysis showed that functional groups such as C=O, -OH, -NH and -CH could be responsible for Cd binding on the cell surface of M. spongiola. In addition, intracellular accumulation was observed in cultures at low Cd concentrations (< 0.9 mg/L), while extracellular adsorption occurred at higher concentrations. These results provide valuable information on the Cd tolerance mechanism in M. spongiola and constitute a robust foundation for further studies on fungal bioremediation strategies.

Insights into nanomycoremediation: Secretomics and mycogenic biopolymer nanocomposites for heavy metal detoxification

Our environment thrives on the subtle balance achieved by the forever cyclical nature of building and rebuilding life through natural processes. Fungi, being the evident armor of bioremediation, is the indispensable element of the soil food web, contribute to be the nature's most dynamic arsenal with non-specific enzymes like peroxidase (POX), glutathione peroxidase (GPx), catalase (CAT), superoxide dismutase (SOD), non-enzymatic compounds like thiol (-SH) groups and non-protein compounds such as glutathione (GSH) and metallothionein (MT). Recently, the area of nanomycoremediation has been gaining momentum as a powerful tool for environmental clean-up strategies with its ability to detoxify heavy metals with its unique characteristics to adapt mechanisms such as biosorption, bioconversion, and biodegradation to harmless end products. The insight into the elaborate secretomic processes provides us with huge opportunities for creating a magnificent living bioremediation apparatus. This review discusses the scope and recent advances in the lesser understood area, nanomycoremediation, the state-of-the-art, innovative, cost-effective and promising tool for detoxification of heavy metal pollutants and focuses on the metabolic capabilities and secretomics with nanobiotechnological interventions.

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.

Study on detoxification and removal mechanisms of hexavalent chromium by microorganisms

Extensive industrial activities have led to an increase of the content of chromium in the environment, which causes serious pollution to the surrounding water, soil and atmosphere. The enrichment of chromium in the environment through the food chain ultimately affects human health. Therefore, the remediation of chromium pollution is crucial to development of human society. A lot of scholars have paid attention to bioremediation technology owing to its environmentally friendly and low-cost. Previous reviews mostly involved pure culture of microorganisms and rarely discussed the optimization of bioreduction conditions. To make up for these shortcomings, we not only introduced in detail the conditions that affect microbial reduction but also innovatively introduced consortium which may be the cornerstone for future treatment of complex field environments. The aim of this study is to summary chromium toxicity, factors affecting microbial remediation, and methods for enhancing bioremediation. However, the actual application of bioremediation technology is still facing a major challenge. This study also put forward the current research problems and proposed future research directions, providing theoretical guidance and scientific basis for the application of bioremediation technology.

Tolerance of three fungal species to lithium and cobalt: Implications for bioleaching of spent rechargeable Li-ion batteries

AIMS

This paper aims to quantify the growth and organic acid production of Aspergillus niger, Penicillium chrysogenum and Penicillium simplicissimum when these fungi are exposed to varying levels of lithium (Li) and cobalt (Co). The study also tests whether pre-exposing the fungi to these metals enables the fungi to develop tolerance to Li or Co.

METHODS AND RESULTS

When cultures of A. niger, P. chrysogenum or P. simplicissimum were exposed to 250 mg l^-1 of Li or Co, biomass production and excretion of organic acids were significantly inhibited after 5 days of growth compared to cultures grown in the absence of these metals. Pre-exposing cultures of A. niger to 250 mg l^-1 of Li or Co for 20 days significantly increased biomass production when the fungus was subsequently sub-cultured into 250 or 500 mg l^-1 of Li or Co. However, pre-exposure of P. chrysogenum or P. simplicissimum to 250 mg l^-1 of Li or Co for 20 days did not increase biomass production.

CONCLUSIONS

Aspergillus niger, but not the Penicillium species, developed tolerance to Li and to Co during the 20-day pre-exposure period. Therefore, processes that utilize fungal bioleaching with A. niger to mobilize and recover valuable metals such as Li or Co should consider a pre-exposure step for fungi to improve their tolerance to metal toxicity.

SIGNIFICANCE AND IMPACT OF THE STUDY

Fungi may have the ability to extract valuable metals such as Li and Co from spent rechargeable batteries. However, the toxicity of the extracted metals can inhibit fungal growth and organic acid production. Pre-exposure to metals may alleviate toxicity for some fungal species. This knowledge can be used to improve the design of bioleaching protocols, increasing the potential for fungal bioleaching to become an economical and environmentally friendly method of recovering Li and Co from spent batteries.

Bioremediation mechanism and potential of copper by actively growing fungus Trichoderma lixii CR700 isolated from electroplating wastewater

Present study investigated the Cu²⁺ removal potential of Trichoderma lixii CR700, isolated from enormously heavy metal polluted electroplating wastewater. In the batch study, actively growing CR700 was able to remove 84.6% of Cu²⁺ at the concentration 10 mg/L of Cu²⁺ within 120 h after incubation and the accumulated and surface adsorbed amount of Cu was 0.51 and 0.47 mg/g of dry biomass respectively. T. lixii CR700 also showed efficient Cu²⁺ removal potential in the pH ranges from 5.0 to 8.0, in the presence of other co-occurring contaminant such as heavy metal, anions and metabolic inhibitor as well from real tannery wastewater. Alteration on cell surface of Cu²⁺ treated mycelia of T. lixii CR700 was analyzed using scanning electron microscope. Fourier transform infrared spectroscopic analysis was performed to identify the role of surface functional group in Cu²⁺ adsorption which revealed that COO^─ functional group lead Cu²⁺ adsorption onto the surface of T. lixii CR700. Thus, T. lixii CR700 uses simultaneous surface sorption and accumulation mechanism in Cu²⁺ removal and can be potentially applied for bioremediation of Cu²⁺ contaminated wastewater in ecofriendly, safe and sustainable way.

Biological-based methods for the removal of volatile organic compounds (VOCs) and heavy metals

The current scenario of increased population and industrial advancement leads to the spoliation of freshwater and tapper of the quality of water. These results decrease in freshwater bodies near all of the areas. Besides, organic and inorganic compounds discharged from different sources into the available natural water bodies are the cause of pollution. The occurrence of heavy metals in water and volatile organic compounds (VOCs) in the air is responsible for a vast range of negative impacts on the atmosphere and human health. Nonetheless, high uses of heavy metals for human purposes may alter the biochemical and geochemical equilibrium. The major air contaminants which are released into the surroundings known as VOCs are produced through different kinds of sources, such as petrochemical and pharmaceutical industries. VOCs are known to cause various health hazards. VOCs are a pivotal group of chemicals that evaporate readily at room temperature. To get over this problem, biofiltration technology has been evolved for the treatment of heavy metals using biological entities such as plants, algae, fungi, and bacteria. Biofiltration technology is a beneficial and sustainable method for the elimination of toxic pollutants from the aquatic environment. Various types of biological technologies ranging from biotrickling filters to biofilters have been developed and they are cost-effective, simple to fabricate, and easy to perform. A significant advantage of this process is the pollutant that is transformed into biodegradable trashes which can decompose within an average time period, thus yielding no secondary pollutants. The aim of this article is to scrutinize the role of biofiltration in the removal of heavy metals in wastewater and VOCs and also to analyze the recent bioremediation technologies and methods.

Lead removal from water by a newly isolated Geotrichum candidum LG-8 from Tibet kefir milk and its mechanism

In this study, a yeast-like fungal strain (LG-8), newly isolated from spontaneous Tibet kefir in China, was identified as Geotrichum candidum on the basis of its morphological characteristics and ITS5.8S gene sequence. Interestingly, the strain was able to remove more than 99% of Pb²⁺ ions in water at low concentrations and a maximum of 325.68 mg lead/g of dry biomass. The results of selective passivation experiments suggested that phosphate, amide and carboxyl groups on the cell wall contributed to lead removal. Scanning electron microscopy (SEM) photomicrographs revealed that large amounts of micro/nanoparticles formed on the cell wall, and energy dispersive X-ray spectroscopy (EDX) results further indicated the presence of lead along with phosphorus and chlorine in the particles. Furthermore, the results of Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) analyses revealed that the particles were composed of pyromorphite [Pb₅(PO4)₃Cl], a highly insoluble lead mineral. Importantly, this is the first time that the biomineralization of lead into pyromorphite has been observed as the major mechanism for lead removal by G. candidum LG-8, providing a new strategy to scavenge heavy metals from aquatic environment in an eco-friendly manner.

Decolorization and detoxification of triphenylmethane dyes by isolated endophytic fungus, Bjerkandera adusta SWUSI4 under non-nutritive conditions

Biodecolorization by microorganisms is a potential treatment technique because they seem to be environmentally safe. In the present study, the decolorization and detoxification of cotton blue, crystal violet, malachite green and methyl violet by endophytic fungi were investigated. Preliminary screening result indicated that SWUSI4, identified as Bjerkandera adusta, demonstrated the best decolorization for the four TPM dyes within 14 days. Furthermore, optimization result demonstrated the decolorization rate could reach above 90% at 24 h by live cells of isolate SWUSI4 when 4 g biomass was added into 100-mL dyes solution with the concentration 50 mg/L and shaking (150 rpm) conditions. Moreover, decolorization mechanism analysis shows that the decolorization was caused by the isolate SWUSI4 that mainly includes both absorption of biomass and/or degradation of enzymes. Biosorption of dyes was attributed to binding to hydroxyl, amino, phosphoryl alkane, and ester–lipids groups based on Fourier transform infrared (FTIR) analyses. The biodegradation potential of SWUSI4 was further suggested by the change of peaks in the ultraviolet–visible (UV–vis) spectra and detection of manganese peroxidase and lignin peroxidase activities. Finally, the phytotoxicity test confirmed that the toxicity of TPM dyes after treatment with SWUSI4 was significantly lower than that before treatment. These results indicate that an endophytic SWUSI4 could be used as a potential TPM dyes adsorption and degradation agent, thus facilitating the study of the plant–endophyte symbiosis in the bioremediation processes.

The bioremediation potentials and mercury(II)-resistant mechanisms of a novel fungus Penicillium spp. DC-F11 isolated from contaminated soil

Bioremediation of Hg-contaminated soil using microbe-based strategies is a promising and efficient method as it is inexpensive and not harmful to the environment. In this study, a novel Hg(II)-volatilizing fungus Penicillium spp., DC-F11 was isolated and showed bioremediation potential for reducing Hg(II) phytotoxicity, total Hg, and exchangeable Hg in Hg(II)-polluted soil. Subsequently, the mechanisms of Hg(II) volatilization and resistance involved were investigated using multiple complementary techniques. The fungal cells could detoxify Hg(II) by extracellular sequestration via adsorption and precipitation. Moreover, a comparative transcriptome analysis uncovered the primary intracellular adaptive responses of the DC-F11 to Hg(II) stress, including mer-mediated detoxification system, thiol compound metabolism, and oxidative stress defense and damage repair metabolism. These results showed that the resistance of DC-F11 to Hg(II) was generally a multisystem collaborative process. Here, we report, for the first time, that the mer-mediated detoxification system was responsible for Hg(II) volatilization in fungus. These findings provide a better understanding of the mechanisms involved in Hg(II) volatilization and resistance that occur in fungi and also provide a strong theoretical basis for the future application of fungi in the bioremediation of Hg-polluted environments.

Burkholderia sp. Y4 inhibits cadmium accumulation in rice by increasing essential nutrient uptake and preferentially absorbing cadmium

Microbial remediation of heavy metal-polluted soil is a commonly used method. Burkholderia sp. Y4, isolated from cadmium (Cd)-contaminated rice rhizosphere soil, was investigated for its direct and indirect effects on Cd accumulation in rice by SEM-EDS, FITR and sequencing analysis of the soil bacterial community. Burkholderia sp. Y4 inoculation reduced Cd accumulation in rice roots, rachises, and grains of the two rice varieties T705 and X24 and increased levels of essential elements, especially Fe and Mn, which competitively inhibited Cd transport through cationic channels. Living Burkholderia sp. Y4 cells, rather than non-living ones, could colonize the surface of rice roots and accumulated more Cd through direct biosorption associated with -CO and -NH/-CO bonds of amino acids and proteins. The results of soil microbial community showed that the colonization of externally added Burkholderia sp. Y4 could be maintained over some time to impact the total rhizospheric environment. Burkholderia sp. Y4 inoculation decreased the abundance of microbes involved in the iron cycle (Acidobacteria) as well as of those mediating the transformation of ammonium nitrogen to nitrate nitrogen (Nitrosomonadaceae and Nitrospira). So Burkholderia sp. Y4 inoculation may indirectly change the availability of micronutrients and Cd in rice rhizosphere soil through iron-nitrogen coupled cycles to increase essential nutrient uptake and inhibit Cd accumulation in rice by preferential Cd-biosorption. Therefore, Burkholderia sp. Y4 is potentially suitable for the bioremediation of Cd-contaminated paddy soil.

Biosorption of copper by immobilized biomass of Aspergillus australensis. Effect of metal on the viability, cellular components, polyhydroxyalkanoates production, and oxidative stress

Heavy metals are toxic especially when they are introduced into the environment due to anthropogenic activities such as metallurgy, mining, and tanning. Removing these pollutants has become a worldwide concern since they cannot be degraded into nontoxic forms causing extended effects in the ecosystems. The use of an Aspergillus australensis was evaluated in order to remove Cu²⁺ from simulated wastewater. The fungus was isolated from river sludges contaminated with heavy metals and was first evaluated for the determination of Cu²⁺ tolerance levels. Microscopic fluorescence analysis was carried out to determine the effect of Cu²⁺ presence on the viability, cellular components, polyhydroxyalkanoates production, and oxidative stress of the fungus, as a response to the stress caused by exposure to metal. In order to achieve copper removal, the A. australensis biomass was produced using batch cultures, and the mycelium was immobilized on a textile media in order to compare the copper-removal efficiency of live or dead biomass. The optimal values of pH and temperature for biomass production were established by using a surface response analysis. Live immobilized biomass was capable of removing Cu²⁺ from 1.54 ± 0.19 to 2.66 ± 0.26 mg of copper/ g of dry biomass, while values of 1.93 ± 0.03 to 2.36 ± 0.29 mg of copper/g of dry biomass were observed when dead biomass was used. As was expected, copper removal using biomass varied depending on the pH and temperature used.

A Multi-Scale Approach to Investigate Adhesion Properties of Pseudomonas aeruginosa PAO1 to Geotrichum candidum LG-8, a Potential Probiotic Yeast

This study investigated properties of Pseudomonas aeruginosa PAO1 adhesion to Geotrichum candidum LG-8 cells in variable pH and salt conditions. The primary mechanism was revealed by multi-scale microscopy technics. The adhesion of PAO1 to the living fungus occurred within 1 h and was limited at concentrations of bile salts higher than 0.5%. The adhesion efficiency gradually increased to 58.1% with the pH increasing from 2.0 to 7.0 and then decreased to 48.2% at pH 9.0. However, the dead LG-8 has an advantage over the living ones to adhere PAO1 in same pH and bile salt conditions. Optical microscopy showed that both unsterilized and sterilized G. candidum LG-8 cells removed approximately one hundred fold bacteria in 4 h. Laser scanning confocal microscopy (LSCM) analysis indicated that polysaccharides of the fungus contributed to adhesion. Scanning electron microscopy (SEM) analysis proved that syrup-like EPS (extracellular polymeric substances) of LG-8 coating PAO1 was in part a mechanism. Atomic force microscopy (AFM) showed roughness of the LG-8 surface changed in the adhesion process. Furthermore, a pedestal-like structure of bacteria was observed by transmission electron microscopy (TEM) analysis, indicating that the bacteria were also actively involved in the adhesion process. G. candidum LG-8 is a potential candidate for the control of P. aeruginosa PAO1 in the food industry and immunodeficiency patients.

The shuttling effects and associated mechanisms of different types of iron oxide nanoparticles for Cu(II) reduction by Geobacter sulfurreducens

Iron oxide nanoparticles (IONPs), commonly occurring in soils, aquifers and subsurface sediments, may serve as important electron shuttles for the biotransformation of coexisting toxic metals. Here, we explored the impact of different IONPs (low-crystallinity goethite and ferrihydrite, high-crystallinity magnetite and hematite) on the reduction of Cu(II) by Geobacter sulfurreducens and the associated electron shuttle mechanisms. All four IONPs tested can function as electron shuttles to enhance long distance electron transfer from bacteria to Cu(II). Upon IONPs addition, the rate of Cu(II) reduction increased from 14.9 to 65.0-83.8 % in solution after 7 days of incubation. Formation of both Cu(I) and Cu(0) on the iron oxide nanoparticles was revealed by the X-ray absorption near-edge spectroscopy. The IONPs can be utilized as conduits by bacteria to directly transfer electrons and they can also reversibly accept and donate electrons as batteries through a charging-discharging cycle to transfer electron. The latter mechanism (geo-battery) played an important role in all four types of IONPs while the former one (geo-conductor) can only be found in the magnetite and hematite treatments due to the higher crystallinity. Our results shed new light on the biogeochemically mediated electron flux in microbe-IONPs-metal networks under anaerobic iron-reduction conditions.

Synthesis of calcite-based bio-composite biochar for enhanced biosorption and detoxification of chromium Cr (VI) by Zhihengliuella sp. ISTPL4

The current study presents a comprehensive analysis of the potential of actinobacterium Zhihengliuella sp. ISTPL4 and different composite materials for the removal of hexavalent chromium [Cr (VI)]. Genome analysis of strain indicated the presence of several oxidoreductases which includes chromate reductase, nitrate reductase, thioredoxin, superoxide dismutase and hydrogenase are other major candidate genes. Catalytic calcite-based bio-composite material was absorbed on biochar had highest Cr removal efficiency. The main mechanism involved in Cr biosorption by this strain was explained by the Langmuir isotherm model; under equilibrium conditions the maximum adsorption was observed 49 ± 0.3 mgg^-1. Kinetic studies showed that biosorption of Cr (VI) by this strain was a rate-limiting step and followed a pseudo-second-order kinetics (R2 = 0.99). SEM analysis is in line with EDX result indicating highest Cr removal by calcined biochar. MTT assay shown that the bacteria successfully convert toxic Cr (VI) to comparatively less toxic form such as Cr (III).

Removal mechanism of Pb(II) by Penicillium polonicum: immobilization, adsorption, and bioaccumulation

Currently, lead (Pb) has become a severe environmental pollutant and fungi hold a promising potential for the remediation of Pb-containing wastewater. The present study showed that Penicillium polonicum was able to tolerate 4 mmol/L Pb(II), and remove 90.3% of them in 12 days through three mechanisms: extracellular immobilization, cell wall adsorption, and intracellular bioaccumulation. In this paper. the three mechanisms were studied by Raman, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). The results indicated that Pb(II) was immobilized as lead oxalate outside the fungal cell, bound with phosphate, nitro, halide, hydroxyl, amino, and carboxyl groups on the cell wall, precipitated as pyromorphite [Pb₅(PO₄)₃Cl] on the cell wall, and reduced to Pb(0) inside the cell. These combined results provide a basis for additionally understanding the mechanisms of Pb(II) removal by P. polonicum and developing remediation strategies using this fungus for lead-polluted water.

Efficacy of Gamma-Irradiated Macroporous Microbial Biomat for Lead Removal: A Proposed Application to Aquacultures

Lead mobilization in aquaculture and its generated health hazards prompted the use of a cheap and reusable method for its removal within a short duration. A 3D macroporous microbial biomat formed of Trichoderma viride immobilized on luffah was used for Pb removal. The biomat was used to remove 79% of initial 400 mg/kg Pb in 24 h that increased to 87% under optimized conditions of pH, temperature, and contact time. In order to reduce the time needed for Pb bioremoval to 1 h, pretreated biomats were used, resulting in an increase in removal from 58% to 96% upon exposure to gamma radiation (0.01 kilogray [kGy]). The irradiated biomat was studied in terms of morphology, elemental analysis of surface biosorbed Pb and surface functional groups using scanning electron microscopy (SEM), energy dispersive X-ray (EDX), and Fourier transform infrared (FTIR) spectroscopy. The results show a difference in the adsorption pattern. The biomat was reused efficiently for 3 consecutive cycles and was also used in fixed bed column showing 89% removal for downward flow and in real aquaculture samples. Pretreated microbial biomats are very suitable for use in fixed bed reactors or as a biofilter and can be tested in recirculating aquaculture systems (RASs), thereby contributing to water conservation and aquaculture sustainability. Integr Environ Assess Manag 2020;16:508-516. © 2020 SETAC.

Multiple heavy metal tolerance and removal by an earthworm gut fungus Trichoderma brevicompactum QYCD-6

Fungal bioremediation is a promising approach to remove heavy-metal from contaminated water. Present study examined the ability of an earthworm gut fungus Trichoderma brevicompactum QYCD-6 to tolerate and remove both individual and multi-metals. The minimum inhibitory concentration (MIC) of heavy metals [Cu(II), Cr(VI), Cd(II) and Zn(II)] against the fungus was ranged 150–200 mg L⁻¹ on composite medium, and MIC of Pb(II) was the highest with 1600 mg L⁻¹ on potato dextrose (PD) medium. The Pb(II) presented the highest metal removal rate (97.5%) which mostly dependent on bioaccumulation with 80.0%, and synchronized with max biomass (6.13 g L⁻¹) in PD medium. However, on the composite medium, the highest removal rate was observed for Cu(II) (64.5%). Cellular changes in fungus were reflected by TEM analysis. FTIR and solid-state NMR analyses indicated the involvement of different functional groups (amino, carbonyl, hydroxyl, et al.) in metallic biosorption. These results established that the earthworm-associated T. brevicompactum QYCD-6 was a promising fungus for the remediation of heavy-metal wastewater.