Bioremediation of Heavy Metals from Soil and Aquatic Environment: An Overview of Principles and Criteria of Fundamental Processes

Toxic metals in the regulation of epithelial-mesenchymal plasticity: demons or angels?

Epithelial cells can trans-differentiate into motile mesenchymal cells through a dynamic process known as epithelial-mesenchymal transition (EMT). EMT is crucial in embryonic development and wound healing but also contributes to human diseases such as organ fibrosis and cancer progression. Heavy metals are environmental pollutants that can affect human health in various ways, including causing cancers. The cytotoxicity and carcinogenicity of heavy metals are complex, and studies have demonstrated that some of these metals can affect the progress of EMT. Here, we focus on reviewing the roles of six environmentally common toxic metals concerning EMT: arsenic (AS), cadmium (Cd), cobalt (Co), chromium (Cr), nickel (Ni), and copper (Cu). Noteworthily, the effects of these elements on EMT may vary according to the form, dose, and exposure time; the dual role of heavy metals (e.g., AS, Cd, and Cu) on EMT is also observed, in which, sometimes they can promote while sometimes inhibit the EMT process. Given the vast number of toxicologically relevant metals that exist in nature, we believe a comprehensive understanding of their effects on EMT is required to dictate in what circumstances these metals act more likely as demons or angels.

Novel strategies and advancement in reducing heavy metals from the contaminated environment

The most contemporary ecological issues are the dumping of unprocessed factories' effluent. As a result, there is an increasing demand for creative, practical, environmentally acceptable, and inexpensive methodologies to remediate inorganic metals (Hg, Cr, Pb, and Cd) liquidated into the atmosphere, protecting ecosystems. Latest innovations in biological metals have driven natural treatment as a viable substitute for traditional approaches in this area. To eliminate pesticide remains from soil/water sites, technologies such as oxidation, burning, adsorption, and microbial degradation have been established. Bioremediation is a more cost-effective and ecologically responsible means of removing heavy metals than conventional alternatives. As a result, microorganisms have emerged as a necessary component of methyl breakdown and detoxification via metabolic reactions and hereditary characteristics. The utmost operative variant for confiscating substantial metals commencing contaminated soil was A. niger, which had a maximum bioaccumulation efficiency of 98% (Cd) and 43% (Cr). Biosensor bacteria are both environmentally sustainable and cost-effective. As a result, microbes have a range of metal absorption processes that allow them to have higher metal biosorption capabilities. Additionally, the biosorption potential of bacterium, fungus, biofilm, and algae, inherently handled microorganisms that immobilized microbial cells for the elimination of heavy metals, was reviewed in this study. Furthermore, we discuss some of the challenges and opportunities associated with producing effective heavy metal removal techniques, such as those that employ different types of nanoparticles.

Thermally enhanced bioremediation: A review of the fundamentals and applications in soil and groundwater remediation

Thermally enhanced bioremediation (TEB), a new concept proposed in recent years, explores the combination of thermal treatment and bioremediation to address the challenges of the low efficiency and long duration of bioremediation. This study presented a comprehensive review regarding the fundamentals of TEB and its applications in soil and groundwater remediation. The temperature effects on the bioremediation of contaminants were systematically reviewed. The thermal effects on the physical, chemical and biological characteristics of soil, and the corresponding changes of contaminants bioavailability and microbial metabolic activities were summarized. Specifically, the increase in temperature within a suitable range can proliferate enzymes enrichment, extracellular polysaccharides and biosurfactants production, and further enhancing bioremediation. Furthermore, a systematic evaluation of TEB applications by utilizing traditional in situ heating technologies, as well as renewable energy (e.g., stored aquifer thermal energy and solar energy), was provided. Additionally, TEB has been applied as a biological polishing technology post thermal treatment, which can be a cost-effective method to address the contaminants rebounds in groundwater remediation. However, there are still various challenges to be addressed in TEB, and future research perspectives to further improve the basic understanding and applications of TEB for the remediation of contaminated soil and groundwater are presented.

Potential probiotic strains with heavy metals and mycotoxins bioremoval capacity for application in foodstuffs

Heavy metals and mycotoxins in foodstuffs are one of the major concerns of our world nowadays. Food decontamination with the help of microbial biomass is a cheap, easy, efficient, and green method known as bioremoval. Probiotics are able to reduce the availability of heavy metals and toxins in food products. The purpose of this review is to summarize the probiotics and potential probiotics' interesting role in food bio-decontamination. After a brief glance at the definition of potential probiotic strains with bioremoval ability, LABs (lactic acid bacteria) are described as they are the most important groups of probiotics. After that, the role of the main probiotic and potential probiotic strains (Bacillus, Lactobacillus, Lactococcus, Enterococcus, Bifidobacterium, Pediococcus, Propionibacterium, Streptococcus, and Saccharomyces cerevisiae) for heavy metals and mycotoxins bioremoval are described. Additionally, the bioremoval mechanism and the effect of some factors in bioremoval efficiency are explained. Finally, the investigations about probiotic and contaminant stability are mentioned. It is worth mentioning that this review article can be exerted in different food and beverage industries to eliminate the heavy metals and mycotoxins in foodstuffs.

Effects of Tetracycline and Copper on Water Spinach Growth and Soil Bacterial Community

The effects of tetracycline (TC) and copper (Cu) on the growth of water spinach and the bacterial community structure in soil were examined in this study. The results revealed that a single Cu treatment decreased water spinach development more severely than TC, and that the toxic effects of TC and Cu on water spinach were synergistic at low doses and antagonistic at high concentrations. The single Cu treatment had the largest influence on the activities of three antioxidant enzymes (Superoxide Dismutase (SOD), Peroxidase (POD), Catalase (CAT)) and the content of Malondialdehyde (MDA) in water spinach leaves, followed by the TC and Cu composed treatment, with the single TC treatment having the least effect. The results of 16Sr RNA sequence analysis showed that the richness and diversity of soil bacterial communities were reduced by either a single TC or Cu treatment. Cu had a greater effect on the composition of the microbial community at genus level than TC. In conclusion, Cu had a greater influence on the growth of water spinach and soil microbial community composition than TC. TC and Cu exhibited synergistic effects at low concentrations and antagonistic effects at high concentrations on relevant indicators when Cu concentration was fixed.

Biogenic Silver Nanoparticles as a Stress Alleviator in Plants: A Mechanistic Overview

Currently, the growth and yield of crops are restrained due to an increase in the occurrence of ecological stresses globally. Biogenic generation of nanomaterials is an important step in the development of environmentally friendly procedures in the nanotechnology field. Silver-based nanomaterials are significant because of their physical, chemical, and biological features along with their plentiful applications. In addition to useful microbes, the green synthesized Ag nanomaterials are considered to be an ecologically friendly and environmentally biocompatible method for the enhancement of crop yield by easing stresses. In the recent decade, due to regular droughts, infrequent precipitation, salinity, and increased temperature, the climate alternation has changed certain ecological systems. As a result of these environmental changes, crop yield has decreased worldwide. The role of biogenic Ag nanomaterials in enhancing methylglyoxal detoxification, antioxidant defense mechanisms, and generating tolerance to stresses-induced ROS injury has been methodically explained in plants over the past ten years. However, certain studies regarding stress tolerance and metal-based nanomaterials have been directed, but the particulars of silver nanomaterials arbitrated stresses tolerance have not been well-reviewed. Henceforth, there is a need to have a good understanding of plant responses during stressful conditions and to practice the combined literature to enhance tolerance for crops by utilization of Ag nanoparticles. This review article illustrates the mechanistic approach that biogenic Ag nanomaterials in plants adopt to alleviate stresses. Moreover, we have appraised the most significant activities by exogenous use of Ag nanomaterials for improving plant tolerance to salt, low and high temperature, and drought stresses.

Toxic and Trace Elements in Seaweeds from a North Atlantic Ocean Region (Tenerife, Canary Islands)

Canary Islands is a North Atlantic Ocean archipelago in the Macaronesian region that stand out for its great algae diversity and its climatic conditions. However, even in this low industrialised area, human activities tend to increase the marine pollution. Asparagopsis spp. and Liagora spp. algae are red algae frequent in the Canary Islands’ coasts. Therefore, they could be used as bio-indicators of marine pollution for trace elements. A total of 30 samples of both algae’s species from Tenerife’s southern coast, specifically in Playa Grande, Porís de Abona, in Arico (Tenerife, Spain) were used to determine trace element content (Mn, B, Ba, Cu, Cd, Co, Fe, Li, Mo, Ni, Pb, Sr, V, Zn, Al, Cr) through inductively coupled plasma atomic emission spectroscopy (ICP—OES). Highest Fe concentrations were found in Liagora spp. concentrations (1190 ± 1545 mg/kg dw) and Al (288 ± 157 mg/kg dw) was more significant in Asparagopsis spp. High concentrations of B were also registered in both species 80.2 ± 34.2 mg/kg dw and 77.9 ± 34.2 mg/kg dw, respectively. The recorded concentrations show a high contamination scenario in the collected area. Porís is known by its marine diversity and by its higher pollution levels, compared with other locations of Tenerife, due to the currents present on the Canary Island and its singular north orientation, actions must be taken to reduce pollution.

Phytoremediation of pollutants from wastewater: A concise review

As there is a global water crisis facing the whole world, it is important to find alternative solutions to treat wastewater for reuse. Hence, plants have an effective role in removing pollutants from wastewater, which has been emphasized in this review article. Biological treatment of wastewater can be considered an eco-friendly and cost-effective process that depends on in the future. Living organisms, including plants, can remediate pollutants in wastewater, especially in agricultural fields, such as dyes, heavy metals, hydrocarbons, pharmaceuticals, and pesticides. This review discusses the different activities of plants in pollutant elimination from wastewater and sheds light on the utilization of plants in this scope. This review focuses on the remediation of the most common contaminants present in wastewater, which are difficult to the removal with microorganisms, such as bacteria, fungi, and algae. Moreover, it covers the major role of plants in wastewater treatment and the potential of phytoremediation as a possible solution for the global water crisis.

Microbial Interventions in Bioremediation of Heavy Metal Contaminants in Agroecosystem

Soil naturally comprises heavy metals but due to the rapid industrialization and anthropogenic events such as uncontrolled use of agrochemicals their concentration is heightened up to a large extent across the world. Heavy metals are non-biodegradable and persistent in nature thereby disrupting the environment and causing huge health threats to humans. Exploiting microorganisms for the removal of heavy metal is a promising approach to combat these adverse consequences. The microbial remediation is very crucial to prevent the leaching of heavy metal or mobilization into the ecosystem, as well as to make heavy metal extraction simpler. In this scenario, technological breakthroughs in microbes-based heavy metals have pushed bioremediation as a promising alternative to standard approaches. So, to counteract the deleterious effects of these toxic metals, some microorganisms have evolved different mechanisms of detoxification. This review aims to scrutinize the routes that are responsible for the heavy metal(loid)s contamination of agricultural land, provides a vital assessment of microorganism bioremediation capability. We have summarized various processes of heavy metal bioremediation, such as biosorption, bioleaching, biomineralization, biotransformation, and intracellular accumulation, as well as the use of genetically modified microbes and immobilized microbial cells for heavy metal removal.

Preservation and Recovery of Metal-Tolerant Fungi from Industrial Soil and Their Application to Improve Germination and Growth of Wheat

Heavy metals contaminate soil and adversely affect plant growth. These soils contain different fungi and bacteria which exhibit metal tolerance and work as bioremediation agents to detoxify polluted soils. In the present study, polluted soil samples were collected to estimate the contamination of copper (Cu) and cadmium (Cd). From this contaminated soil, metal tolerant fungi were isolated and characterized. Copper and cadmium were found in a range of 190.2–300.4 mg/kg and 46.8–56.1 mg/kg, respectively. For the isolation of metal tolerant fungi, soil dilutions were made in water and inoculated on potato dextrose agar (PDA) media. Fungal growth was observed on PDA and successive screening resulted in the isolation of four multi-metal tolerant fungal species, including Penicillim oxalicum, Fusarium solani, Aspergillus niger and Trichoderma harzianum. Sequencing of 18S rRNA genes of isolated fungi also efficiently identified them. To reveal minimum inhibitory concentrations (MIC), these fungi were exposed to increasing concentrations of cadmium and copper chlorides (100 to 1000 ppm) and a variable MIC range of 400 ppm to 1000 ppm was estimated. Based on tolerance index analysis, F. solani was found to be resistant at 1 mM copper, while P. oxalicum was the most tolerant species under cadmium stress. F. solani and P. oxalicum demonstrated the highest biosorption capacity of Cu and Cd, respectively. Both metals negatively affected wheat seedlings in a pot experiment, while the treatment of both F. solani and P. oxalicum positively influenced the germination and growth of wheat. Based on these observations, it could be inferred that F. solani and P. oxalicum can be used for the myco-remediation of Cu and Cd, respectively.

A comprehensive study on aquatic chemistry, health risk and remediation techniques of cadmium in groundwater

Cadmium (Cd), a non-essential trace element, it's intrusion in groundwater has ubiquitous implications on the environment and human health. This review is an approach to comprehensively emphasize on i) chemistry and occurrence of Cd in groundwater and its concomitant response on human health ii) sustainable Cd remediation techniques, iii) and associated costs. Current study is depending on meta-analysis of Cd contaminations in groundwater and discusses its distributions around the globe. Literature review primarily comprises from the last three decades online electronic published database, which mainly includes i) research literatures, ii) government reports. On the basis of meta-data, it was concluded that Cd mobility depends on multiple factors: such as pH, redox state, and ionic strength, dissolved organic (DOC) and inorganic carbon (DIC). A substantially high Cd concentration has been reported in Lagos, Nigeria (0.130 mg/L). In India, groundwater is continuing to be contaminated by Cd in the proximity of industrial, agricultural areas, high concentrations (>8.20 mg/L) were reported in Tamil Nadu and Maharashtra. Depending on chemical behavior and ionic radius cadmium disseminate into the food chain and ultimately cause health hazard that can be measured by various index-based assessment tools. Instead of chemical adsorbents, nanoparticles, phytoextraction, and bioremediation techniques can be very useful in the remediation and management of Cd polluted groundwater at a low-cost. For Cd pollution, the development of a comprehensive framework that links the hydro-geological, bio-geochemical processes to public health is important and need to be further studied.

Microbial bioremediation strategies with wastewater treatment potentialities - A review

The demand for innovative waste treatment techniques has arisen because of the establishment and operation of rigorous waste discharge guidelines into the environment. Due to the rapid increase in the human population, wastewater treatment is a procedure of increasing significance. As a result, wastewater treatment systems are intended to sustain high activities and densities of such microorganisms which meet the different purification requirements. The waste produced by the pharmaceutical industry, if not adequately treated, has harmful repercussions for the environment as well as public health. Bioremediation is an innovative and optimistic technology that can be used to remove and reduce heavy metals from polluted water and contaminated soil. Because of cost-effectiveness and environmental compatibility, bioremediation using microorganisms has an excellent potential for future development. A diverse range of microorganisms, including algae, fungi, yeasts, and bacteria, can function as biologically active methylators, capable of modifying toxic species. Microorganisms play a crucial role in heavy metal bioremediation. Nanotechnology may minimize industry expenses by producing environmentally friendly nanomaterials to alleviate these contaminants. The use of microorganisms in nanoparticle synthesis gives green biotechnology a positive impetus to cost reduction and sustainable production as a developing nanotechnology sector.

Indigenous microbial populations of abandoned mining sites and their role in natural attenuation

Environmental contamination by toxic effluents discharged by anthropogenic activities including the mining industries has increased extensively in the recent past. Microbial communities and their biofilms inhabiting these extreme habitats have developed different adaptive strategies in metabolizing and transforming the persistent pollutants. They also play a crucial role in natural attenuation of these abandoned mining sites and act as a major driver of many biogeochemical processes, which helps in ecological rehabilitation and is a viable approach for restoration of wide stretches of land. In this review, the types of mine wastes including the overburden and mine drainage and the types of microbial communities thriving in such environments were probed in detail. The types of biofilms formed along with their possible role in metal bioremediation were also reviewed. This review also provides an overview of the shift in microbial communities in natural reclamation process and also provides an insight into the restoration of the enzyme activities of the soils which may help in further revegetation of abundant mining areas in a sustainable manner. Moreover, the role of indigenous microbiota in bioremediation of heavy metals and their plant growth-promoting activity weres discussed to assess their role in phytoremedial processes.

Biogeochemical Redox Processes Controlling the Element Cycling: Insights from Karst-Type Bauxite, Greece

The occurrence of very-high-grade bauxite ores of karst-type deposits resulting from Fe-leaching is of particular importance, because they are widespread in the Mediterranean metallogenetic province and result in the natural beneficiation of the ore quality. The present study focuses on mineral transformations and variations of major and trace elements, including platinum-group elements (PGE) and mineral chemistry along a bauxite profile from the Parnassos-Ghiona deposit, Greece underlying a fault. The most salient feature of the multicolor ores (grey–whitish, yellowish, deep red, deep grey to brown-red color, from top to bottom) is their association with fossilized and present-day microorganisms, which, by their reducing and/or oxidizing activity, catalyze redox reactions and provide nucleation sites for the precipitation of secondary minerals. Texture relationships between mineral and variations in the mineral chemistry, suggesting the sequence in their formation, indicate a multistage evolution. The recorded compositional variations show that the Al enrichment is accompanied by increase in the TOC, As, Pd and U, and chondrite-normalized REE patterns exhibit a similar trend and positive Ce anomalies. A lower (Pt + Pd) content and higher Pd/Pt ratio, ranging from 1.0 to 5.5 in the bauxite profile compared to those in Fe-Ni laterite deposits with the Pd/Pt ratio ranging from 0.1 to 0.68, reflect the higher solubility and mobility of Pd compared to that of Pt, and differences in their origin and genesis. A positive correlation between Pd and As and the elevated As content (up to 960 mg/kg) in multicolor ores compared to brown-red samples (average 10 mg/kg As) confirms their mobilization and redeposition.

Nanobioremediation: A sustainable approach for the removal of toxic pollutants from the environment

In recent years, the proportion of organic and inorganic contaminants has increased rapidly due to growing human interference and represents a threat to ecosystems. The removal of these toxic pollutants from the environment is a difficult task. Physical, chemical and biological methods are implemented for the degradation of toxic pollutants from the environment. Among existing technologies, bioremediation in combination with nanotechnology is the most promising and cost-effective method for the removal of pollutants. Numerous studies have shown that exceptional characteristics of nanomaterials such as improved catalysis and adsorption properties as well as high reactivity have been subjects of great interest. There is an emerging trend of employing bacterial, fungal and algal cultures and their components, extracts or biomolecules as catalysts for the sustainable production of nanomaterials. They can serve as facilitators in the bioremediation of toxic compounds by immobilizing or inducing the synthesis of remediating microbial enzymes. Understanding the association between microorganisms, contaminants and nanoparticles (NPs) is of crucial importance. In this review, we focus on the removal of toxic pollutants using the cumulative effects of nanoparticles with microbial technology and their applications in different domains. Besides, we discuss how this novel nanobioremediation technique is significant and contributes towards sustainability.

Fungal-bacterial biofilm mediated heavy metal rhizo-remediation

Heavy metal pollution due to excessive use of chemical fertilizers (CF) causes major damage to the environment. Microbial biofilms, closely associated with the rhizosphere can remediate heavy metal-contaminated soil by reducing plant toxicity. Thus, this study was undertaken to examine the remedial effects of microbial biofilms against contaminated heavy metals. Fungi and bacteria isolated from soil were screened for their tolerance against Cd²⁺, Pb²⁺, and Zn²⁺. Three bacterial and two fungal isolates were selected upon the tolerance index (TI) percentage. Fungal-bacterial biofilms (FBBs) were developed with the most tolerant isolates and were further screened for their bioremediation capabilities against heavy metals. The best biofilm was evaluated for its rhizoremediation capability with different CF combinations using a pot experiment conducted under greenhouse conditions with potatoes. Significantly (P < 0.05), the highest metal removal percentage was observed in Trichoderma harzianum and Bacillus subtilis biofilm under in situ conditions. When compared to the 100% recommended CF, the biofilm with 50% of the recommended CF (50CB) significantly (P < 0.05) reduced soil available Pb²⁺ by 77%, Cd²⁺ by 78% and Zn²⁺ by 62%. In comparison to initial soil, it was 73%, 76%, and 57% lower of Pb²⁺, Cd²⁺, and Zn²⁺, respectively. In addition, 50CB treatment significantly (P < 0.05) reduced the metal penetration into the tuber tissues in comparison with 100 C. Thus, the function of the developed FBB with T. harzianum-B. subtilis can be used as a potential solution to remediate soil polluted with Pb²⁺ Cd²⁺ and Zn²⁺ metal contaminants.

Vinegar residue biochar: A possible conditioner for the safe remediation of alkaline Pb-contaminated soil

A better understanding how to modulate alkaline soil-plant systems with lead (Pb) toxicity with by vinegar residue biochar is important for the remediation of Pb-contaminated soil. Leaching column and pot experiments were conducted to investigate the effect of vinegar residue biochar on Pb speciation, soil properties, and plant growth under Pb stress. The results indicate that biochar could effectively decrease the exchangeable and carbonated-bound Pb but increase the Fe-Mn oxide and residue fractions in the soil with Pb at 500 mg kg^-1. Biochar did not effectively immobilize Pb in the soil with Pb at 1000 mg kg^-1. After leaching, biochar evidently increased the organic carbon and dissolved organic carbon content of the soil, but slightly affected the pH, cation exchange capacity and carbonate content. The biochar addition at 0.5% had no significant effect on soil aggregates, and biochar at 2.0% and 5.0% significantly decreased soil aggregate stability. The dry weight and soluble protein content of pak choi (Brassica chinensis L.) increased with biochar treatment. Lead assimilation by plants was inhibited by the decreased availability of Pb in biochar-treated soils. Soil enzymes activities also significantly increased, then facilitated biochemical reactions in the soil environment. The applied biochar has shown an important role in mitigating Pb toxicity by increasing the soil organic carbon, dissolved organic carbon content, enzyme activities, and plant growth. The low dose biochar (0.5-2.0%) are recommended as references for subsequent experiments, especially in alkaline loam soil.

Insight Into Microbes and Plants Ability for Bioremediation of Heavy Metals

Contamination of ground and surface water, soil, and air by harmful and carcinogenic chemicals is one of the most prevalent problems in the modern industrialized world. Heavy metal toxicity has demonstrated to be paramount hazardous and there are various risks associated with it. In addition, these heavy metals have adverse effects on human health and plant physiology. The field of bioremediation has undergone an impactful revolution in recent years due to an exponential increase in various issues related to soil and water pollution. Bioremediation is an advanced and efficient technology, which involves the use of biological means such as microorganisms and plants to degrade heavy metal contaminants. Among the millions of microbes present in the ecosystem, the highest metal adsorption ability is possessed by species belonging to genus Penicillium, Streptomyces, Bacillus, Rhizopus, Chlorella, Ascophyllum, Sargassum, and Aspergillus. Among different plant species, Allium, Eucalyptus, Helianthus, and Hibiscus are the main heavy metal absorbers. The present review concentrates on the research in the bioremediation of important heavy metals through the use of plants and microbes.

Significance of Shewanella Species for the Phytoavailability and Toxicity of Arsenic-A Review

Simple Summary

The availability of some toxic heavy metals, such as arsenic (As), is related to increased human and natural activities. This type of metal availability in the environment is associated with various health and environmental issues. Such problems may arise due to direct contact with or consumption of plant products containing this metal in some of their parts. A microbial approach that employs a group of bacteria (Shewanella species) is proposed to reduce the negative consequences of the availability of this metal (As) in the environment. This innovative strategy can reduce As mobility, its spread, and uptake by plants in the environment. The benefits of this approach include its low cost and the possibility of not exposing other components of the environment to unfavourable consequences.

Abstract

The distribution of arsenic continues due to natural and anthropogenic activities, with varying degrees of impact on plants, animals, and the entire ecosystem. Interactions between iron (Fe) oxides, bacteria, and arsenic are significantly linked to changes in the mobility, toxicity, and availability of arsenic species in aquatic and terrestrial habitats. As a result of these changes, toxic As species become available, posing a range of threats to the entire ecosystem. This review elaborates on arsenic toxicity, the mechanisms of its bioavailability, and selected remediation strategies. The article further describes how the detoxification and methylation mechanisms used by Shewanella species could serve as a potential tool for decreasing phytoavailable As and lessening its contamination in the environment. If taken into account, this approach will provide a globally sustainable and cost-effective strategy for As remediation and more information to the literature on the unique role of this bacterial species in As remediation as opposed to conventional perception of its role as a mobiliser of As.

Cadmium transfer between maize and soybean plants via common mycorrhizal networks

More than 80% terrestrial plants establish mutualistic symbiosis with soil-borne arbuscular mycorrhizal fungi (AMF). These fungi not only significantly improve plant nutrient acquisition and stress resistance, but also mitigate heavy metal phytotoxicity, Furthermore, the extraradical mycorrhizal mycelia can form common mycorrhizal networks (CMNs) that link roots of multiple plants in a community. Here we show that the networks mediate migration of heavy metal cadmium (Cd) from maize (Zea mays L.) to soybean (Glycine max (Linn.) Merr.) plants. CMNs between maize and soybean plants were established after inoculation of maize plants with AMF Funneliformis mosseae. Application of CdCl2 in maize plants led to 64.4% increase in the shoots and 48.2% increase in the roots in Cd content in CMNs-connected soybean plants compared to the control without Cd treatment in maize. Meanwhile, although the CMNs-connected soybean plants did not directly receive Cd supply, they upregulated transcriptional levels of Cd transport-related genes HATPase and RSTK 2.13- and 5.96-fold, respectively, induced activities of POD by 44.8% in the leaves, and increased MDA by 146.2% in the roots. Furthermore, Cd addition inhibited maize growth but mycorrhizal colonization improved plant performance in presence of Cd stress. This finding demonstrates that mycorrhizal networks mediate the transfer of Cd between plants of different species, suggesting a potential to use CMNs as a conduit to transfer toxic heavy metals from main food crops to heavy metal hyperaccumulators via intercropping.

Bioelectrochemical systems-based metal recovery: Resource, conservation and recycling of metallic industrial effluents

Metals represent a large proportion of industrial effluents, which due to their high hazardous nature and toxicity are responsible to create environmental pollution that can pose significant threat to the global flora and fauna. Strict ecological rules compromise sustainable recovery of metals from industrial effluents by replacing unsustainable and energy-consuming physical and chemical techniques. Innovative technologies based on the bioelectrochemical systems (BES) are a rapidly developing research field with proven encouraging outcomes for many industrial commodities, considering the worthy options for recovering metals from industrial effluents. BES technology platform has redox capabilities with small energy-intensive processes. The positive stigma of BES in metals recovery is addressed in this review by demonstrating the significance of BES over the current physical and chemical techniques. The mechanisms of action of BES towards metal recovery have been postulated with the schematic representation. Operational limitations in BES-based metal recovery such as biocathode and metal toxicity are deeply discussed based on the available literature results. Eventually, a progressive inspection towards a BES-based metal recovery platform with possibilities of integration with other modern technologies is foreseen to meet the real-time challenges of viable industrial commercialization.

Utilization of Chlorella pyrenoidosa for Remediation of Common Effluent Treatment Plant Wastewater in Coupling with Co-relational Study: An Experimental Approach

Earlier investigations on biological methods of wastewater treatment have revealed that algal based wastewater treatment could be a green, cost effective and efficient approach for the removal of heavy metals. So, this study aimed to assess the potential of microalga Chlorella pyrenoidosa for remediation of heavy metals (Cr, Cu, Pb, Zn, Cd, Mn, and Ni) from varying concentration (25%, 50%, 75 and 100%) of wastewater collected from Common Effluent Treatment Plant. Heavy metals such as Cr, Cu, Pb, Zn, Cd, Mn, and Ni have been removed significantly from the wastewater, with percentage removal ranging from 73%, 60%, 75%, 66%, 87%, 83%, and 74% with 50% test solution, 57%, 59%, 70%, 56%, 72%, 66%, and 62% with 75% test solution, and 47%, 55%, 56%, 71%, 61%, 77%, and 72% with 100% test solution respectively. Studies on biochemical assay (protein, carbohydrate, and pigment) of Chlorella pyrenoidosa were also an important part of the present investigation to understand the interaction of heavy metals with algal biochemical compounds using Pearson correlation co-efficient. Biomass grown in CETP wastewater can be used for synthesis of various fruitful value-added end products like bio-diesel, pharmaceutical products, cosmetic products, bio-adsorbent etc.

Microbial response and adaption to thallium contamination in soil profiles

Thallium (Tl) is a trace metal with high toxicity. Comprehensive investigation of spatial distribution of Tl and microorganism is still limited in soils from mining area. In this study, 16S rRNA sequencing and network analysis were used for deciphering the co-occurrence patterns of bacterial communities in two different types of soil profiles around a typical Tl-bearing pyrite mine. The results showed that geochemical parameters (such as pH, S, Tl, Fe and TOM) were the driving forces for shaping the vertical distribution of microbial community. According to network analysis, a wide diversity of microbial modules were present in both soil profiles and affected by depth, significantly associated with variations in Tl geochemical fractionation. Phylogenetic information further unveiled that the microbial modules were mainly dominated by Fe reducing bacteria (FeRB), Fe oxidizing bacteria (FeOB), S oxidizing bacteria and Mn reducing bacteria. The results of metagenome indicated that Fe, Mn and S cycle in soil are closely involved in the biogeochemical cycle of Tl. The findings of co-occurrence patterns in the bacterial network and correlation between microorganisms and different geochemical fractions of Tl may benefit the strategy of bioremediation of Tl-contaminated soils with indigenous microbes.

Biosynthesis of Silver Nanoparticles by Marine Actinobacterium Nocardiopsis dassonvillei and Exploring Their Therapeutic Potentials

Nanoparticles have recently emerged as a popular research topic. Because of their potential applications in therapeutic applications, biosynthesized silver nanoparticles (Bio-AgNPs) have gained much attention in recent years. Cell-free extracts (CFE) from a marine culture of actinobacteria and silver nitrate were used to reduce Ag+ ions and create Bio-AgNPs. Nocardiopsis dasonvillei KY772427, a new silver-tolerant actinomycete strain, was isolated from marine water and used to synthesize AgNPs. In order to characterize Bio-AgNPs, UV-Vis spectral analysis, Fourier transform infrared (FTIR), transmission electron microscopy (TEM), and dynamic light scattering spectroscopy (DLS) were all utilized. Using UV-Vis spectroscopy, a peak in the surface plasmon resonance (SPR) spectrum at 430 nm revealed the presence of Bio-AgNPs. The TEM revealed spherical AgNPs with a diameter of 29.28 nm. DLS determined that Bio-AgNPs have a diameter of 56.1 nm and a negative surface charge (-1.46 mV). The minimum inhibitory concentration (MIC) of Bio-AgNPs was determined against microbial strains. Using resazurin-based microtiter dilution, the synergistic effect of Bio-AgNPs with antimicrobials was investigated. Pseudomonas aeruginosa had the lowest MIC of Bio-AgNPs (4 μg/ml). Surprisingly, the combination of antimicrobials and Bio-AgNPs had a significant synergistic effect on the tested strains. The insecticidal activity of Bio-AgNPs (200 μg/ml) against Macrosiphum rosae was found to be maximal after 36 h. Additionally, Bio-AgNPs demonstrated significant scavenging activity against 2,2'-diphenyl-1-picrylhydrazyl (DPPH) and hydroxyl (OH - ) radicals, with IC 50 values of 4.08 and 8.9 g/ml, respectively. In vitro studies using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay revealed a concentration-dependent decrease in cell viability when CaCo2 cells were exposed to Bio-AgNPs. With the decrease in cell viability, lactate dehydrogenase leakage (LDH) increased. The findings of this study open up a new avenue for the use of marine Nocardiopsis dasonvillei to produce Bio-AgNPs, which have significant antimicrobial, antioxidant, insecticidal, and anticancer potential.

Mucilaginibacter sp. Strain Metal(loid) and Antibiotic Resistance Isolated from Estuarine Soil Contaminated Mine Tailing from the Fundão Dam

In 2015 a mine dam with Mn-Fe-rich tailings collapsed releasing million tons of sediments over an estuary, in the Southwest of Brazil. The tailings have a high concentration of metals that contaminated soil until the present day. The high contaminant concentrations possibly caused a selection for microorganisms able to strive in such harsh conditions. Here, we isolated metal(loid) and anti-biotic resistance bacteria from the contaminated estuarine soil. After 16S rDNA sequencing to identify the strains, we selected the Mucilaginibacter sp. strain for a whole-genome sequence due to the bioprospective potential of the genus and the high resistance profile. We obtained a complete genome and a genome-guided characterization. Our finding suggests that the 21p strain is possibly a new species of the genus. The species presented genes for resistance for metals (i.e., As, Zn, Co, Cd, and Mn) beyond resistance and cross-resistance for antibiotics (i.e., quinolone, aminoglycoside, β-lactamase, sulphonamide, tetracycline). The Mucilaginibacter sp. 21p description as new species should be further explored, as their extracellular polymeric substances and the potential of this strain as bioremediation and as a growth promoter in high met-al(loid) contaminated soil.

An overview and assessment of the existing technological options for management and resource recovery from beach wrack and dredged sediments: An environmental and economic perspective

The present work discusses the problems and management options of beach wrack and dredged sediments. Beach wrack and dredged sediments near the shores have affected the coastal ecosystem, badly. The piles of beach wrack residues might be a significant emitter of greenhouse gases (GHGs) and dredged sediment is a substantial source of heavy metals and other pollutants. The recovery of valuable resources such as metals and nutrients from these so-called "wastes" is a sustainable strategy to enhance the resilience of the coastal ecosystem and management. The beach wrack meadows can be a potential source for green energy production. Even the demand for biodegradable polymers can be supplied by utilizing the waste beach wracks. The residues of beach wrack species like Posidonia oceanica, Zostera marina, Ulva spc. and Enhalus acorodies can be very beneficial species in terms of economic growth. Red algae have been the most favored and efficient candidate for methane yield. In case of dredged sediment, dewatering of sediment is an essential step for successful resource extraction. Although, extraction methods are almost similar to that applied for soil treatment, which includes pretreatment, physical partitioning, washing, thermal treatment, biological extraction, and immobilization. The fractionation study can be a beneficial tool for determining the metal species present in the sediment. Immobilization techniques are successful but continuous monitoring is required. The vitrification technique is highly effective but very expensive. Thermal treatment is useful for volatile metals such as mercury (Hg), but costs are high. Biological extractions are comparatively cheap but time-consuming. Henceforth, very few extraction methods are available for sediment and required further advancement in this field.

Hydroxyapatite-based adsorbents: Applications in sequestering heavy metals and dyes

Hydroxyapatite (HAp) is a calcium phosphate material that was used primarily in bone regeneration and repair as a result of its chemical similarity with bone. However, HAp has emerged as a very promising adsorbent for sequestering contaminants like heavy metals, dyes, hydrocarbons as well as other emerging pollutants from wastewater as a result of its versatility and encouraging adsorptive properties. Contaminants like heavy metals and dyes have been a major source of environmental concern. Research studies involving the use of HAp as adsorbents for the adsorptive treatment of heavy metal- and dye-contaminated wastewater have become increasingly popular due to its eco-friendliness, easy synthesis, unique adsorption properties etc. Various methods are available for the synthesis of HAp and its composites with some of these methods used in combination with other methods to obtain more efficient HAp-based adsorbents. In this work, the adsorptive removal of heavy metals and dyes by HAp and its composites was extensively reviewed as well as the parametric effects of process factors like contact time, solution pH, temperature, solute concentration etc on the adsorption process. Kinetic, thermodynamic, and isotherm models for elucidating the adsorption process were also considered. Generally, from the works reviewed, HAp-based adsorbents were found to be very effective for sequestering heavy metals and dyes from solution and thus presents a low-cost option for adsorptive wastewater treatment.

Wheat Microbiome: Structure, Dynamics, and Role in Improving Performance Under Stress Environments

Wheat is an important cereal crop species consumed globally. The growing global population demands a rapid and sustainable growth of agricultural systems. The development of genetically efficient wheat varieties has solved the global demand for wheat to a greater extent. The use of chemical substances for pathogen control and chemical fertilizers for enhanced agronomic traits also proved advantageous but at the cost of environmental health. An efficient alternative environment-friendly strategy would be the use of beneficial microorganisms growing on plants, which have the potential of controlling plant pathogens as well as enhancing the host plant's water and mineral availability and absorption along with conferring tolerance to different stresses. Therefore, a thorough understanding of plant-microbe interaction, identification of beneficial microbes and their roles, and finally harnessing their beneficial functions to enhance sustainable agriculture without altering the environmental quality is appealing. The wheat microbiome shows prominent variations with the developmental stage, tissue type, environmental conditions, genotype, and age of the plant. A diverse array of bacterial and fungal classes, genera, and species was found to be associated with stems, leaves, roots, seeds, spikes, and rhizospheres, etc., which play a beneficial role in wheat. Harnessing the beneficial aspect of these microbes is a promising method for enhancing the performance of wheat under different environmental stresses. This review focuses on the microbiomes associated with wheat, their spatio-temporal dynamics, and their involvement in mitigating biotic and abiotic stresses.

Cadmium Uptake and Growth Responses of Seven Urban Flowering Plants: Hyperaccumulator or Bioindicator?

The application of flowering plants is the basis of urban forest construction. A newly-found flowering hyperaccumulator is crucial for remediating urban contaminated soil sustainably by cadmium (Cd). This study evaluated growth responses, Cd uptake and bioaccumulation characteristics of seven urban flowering plants. Based on growth responses of these plants, Calendula officinalis L. showed high tolerance to at least 100 mg kg−1 Cd, in terms of significant increase in biomass and with no obvious changes in height. After 60 d exposure to 100 mg kg−1 Cd, the accumulated Cd in shoots of the plant reached 279.51 ± 13.67 μg g−1 DW, which is above the critical value defined for a hyperaccumulator (100 μg g−1 DW for Cd). Meanwhile, the plant could accumulate Cd to as much as 926.68 ± 29.11 μg g−1 DW in root and 1206.19 ± 23.06 μg g−1 DW in plant, and had higher Cd uptake and bioaccumulation values. According to these traits, it is shown that Calendula officinalis L. can become a potential Cd-hyperaccumulator for phytoremediation. By contrast, Dianthus caryophyllus L. is very sensitive to Cd stress in terms of significantly decreased biomass, height and Cd uptake, indicating the plant is considered as a Cd-bioindicator.

Isolation and characterization of a highly effective bacterium Bacillus cereus b-525k for hexavalent chromium detoxification

The chromate resistant Gram-positive Bacillus cereus strain b-525k was isolated from tannery effluents, demonstrating optimal propagation at 37 °C and pH 8. The minimum inhibitory concentration (MIC) test showed that B. cereus b-525k can tolerate up to 32 mM Cr⁶⁺, and also exhibit the ability to resist other toxic metal ions including Pb²⁺ (23 mM), As³⁺ (21 mM), Zn²⁺ (17 mM), Cd²⁺ (5 mM), Cu²⁺ (2 mM), and Ni²⁺ (3 mM) with the resistance order as Cr ⁶⁺ > Pb²⁺ > As³⁺ >Zn²⁺ >Cd²⁺ >Ni²⁺ >Cu²⁺. B. cereus b-525k showed maximum biosorption efficiency (q) of 51 mM Cr⁶⁺/g after 6 days. Chromate stress elicited pronounced production of antioxidant enzymes such as catalase (CAT) 191%, glutathione transferase (GST) 192%, superoxide dismutase (SOD) 161%, peroxidase (POX) 199%, and ascorbate peroxidase (APOX) (154%). Within B. cereus b-525k, the influence of Cr⁶⁺ stress (2 mM) did stimulate rise in levels of GSH (907%) and non-protein thiols (541%) was measured as compared to the control (without any Cr⁶⁺ stress) which markedly nullifies Cr⁶⁺ generated oxidative stress. The pilot scale experiments utilizing original tannery effluent showed that B. cereus b-525k could remove 99% Cr⁶⁺ in 6 days, thus, it could be a potential candidate to reclaim the chromate contaminated sites.

Arsenic biotransformation and mobilization: the role of bacterial strains and other environmental variables

Over several decades, arsenic (As) toxicity in the biosphere has affected different flora, fauna, and other environmental components. The majority of these problems are linked with As mobilization due to bacterial dissolution of As-bearing minerals and its transformation in other reservoirs such as soil, sediments, and ground water. Understanding the process, mechanism, and various bacterial species involved in these processes under the influence of some ecological variables greatly contributes to a better understanding of the fate and implications of As mobilization into the environments. This article summarizes the process, role, and various types of bacterial species involved in the transformation and mobilization of As. Furthermore, insight into how Fe(II) oxidation and resistance mechanisms such as methylation and detoxification against the toxic effect of As(III) was highlighted as a potential immobilization and remediation strategy in As-contaminated sites. Furthermore, the significance and comparative advantages of some useful analytical tools used in the evaluation, speciation, and analysis of As are discussed and how their in situ and ex situ applications support assessing As contamination in both laboratory and field settings. Nevertheless, additional research involving advanced molecular techniques is required to elaborate on the contribution of these bacterial consortia as a potential agronomic tool for reducing As availability, particularly in natural circumstances. Graphical abstract. Courtesy of conceptual model: Aminu Darma.

Microbial recovery of critical metals from secondary sources

The continuous development of technologies involving critical metals, both in Europe and over the world, and geopolitical challenges in areas rich in critical metal sources, imposed increased research efforts to recover them from secondary sources, by eco-efficient processes. Yet, microbes-metal interactions are not sufficiently exploited to recover metals from secondary sources, although they are already used in ore extraction. This review examines and compare strategies and processes involving microorganisms for critical metals recovery, since conventional physico-chemical methods are energy-intensive and often polluting. Two groups of microbial assisted recovery processes are discussed: metal mobilization from metal bearing waste, and selective metal separation from leaching solutions by immobilization on microbial biomass. Because most of the identified microbial technologies are developed on laboratory scale, the increase of biorecovery efficiency is compulsory for enhancing scaling-up potential. Future developments focused on novel microorganisms and high-performance strategies for critical metal recovery by microbial processes are considered.

Effects of heavy metals on bacterial community structures in two lead-zinc tailings situated in northwestern China

We evaluated the variations of bacterial communities in six heavy metal contaminated soils sampled from Yanzi Bian (YZB) and Shanping Cun (SPC) tailings located in northwestern China. Statistical analysis showed that both the heavy metals and soil chemical properties could affect the structure and diversity of the bacterial communities in the tailing soils. Cd, Cu, Zn, Cr, Pb, pH, SOM (soil organic matters), TP (total phosphorus) and TN (total nitrogen) were the main driving factors of the bacterial community variations. As a consequence, the relative abundances of certain bacterial phyla including Proteobacteria, Chloroflexi, Firmicutes, Nitrospirota and Bacteroidota were significantly increased in the tailing soils. Further, we found that the abundance increasement of these phyla were mainly contributed by certain species, such as s__unclassified_g__Thiobacillus (Proteobacteria), s__unclassified_g__Sulfobacillus (Firmicutes) and Leptospirillum ferriphilum (Nitrospirota). Thus, these species were considered to be strongly heavy metal tolerant. Together, our findings will provide a useful insight for further bioremediations of these contaminated areas.

Recent Developments in Microbe-Plant-Based Bioremediation for Tackling Heavy Metal-Polluted Soils

Soil contamination with heavy metals (HMs) is a serious concern for the developing world due to its non-biodegradability and significant potential to damage the ecosystem and associated services. Rapid industrialization and activities such as mining, manufacturing, and construction are generating a huge quantity of toxic waste which causes environmental hazards. There are various traditional physicochemical techniques such as electro-remediation, immobilization, stabilization, and chemical reduction to clean the contaminants from the soil. However, these methods require high energy, trained manpower, and hazardous chemicals make these techniques costly and non-environment friendly. Bioremediation, which includes microorganism-based, plant-based, microorganism-plant associated, and other innovative methods, is employed to restore the contaminated soils. This review covers some new aspects and dimensions of bioremediation of heavy metal-polluted soils. The bioremediation potential of bacteria and fungi individually and in association with plants has been reviewed and critically examined. It is reported that microbes such as Pseudomonas spp., Bacillus spp., and Aspergillus spp., have high metal tolerance, and bioremediation potential up to 98% both individually and when associated with plants such as Trifolium repens, Helianthus annuus, and Vallisneria denseserrulata. The mechanism of microbe's detoxification of metals depends upon various aspects which include the internal structure, cell surface properties of microorganisms, and the surrounding environmental conditions have been covered. Further, factors affecting the bioremediation efficiency and their possible solution, along with challenges and future prospects, are also discussed.

Metal nanoparticles and its application on phenolic and heavy metal pollutants

The perpetual exposure of several manmade materials and their activities such as urbanization, industrialization, transportation, mining, construction, petroleum refining, manufacturing, preservatives, disinfectants etc., release various pollutants like organic, inorganic, and heavy metals which pollute the air, water, and soil. This poses various environmental issues which are relevant to the ecosystem and human wellbeing that intensify the implementation of new expedient treatment technologies. Likewise, phenolic and heavy metal pollutants find their way into the environment. These phenolic and heavy metals are toxic to the liver, heart and carcinogenic. Therefore, the removal of these kinds of pollutants from the environment is a highly challenging issue. As conventional treatment technologies have consequent drawbacks, new interests have been developed to remediate and remove pollutants from the ecosystem using metal nanoparticles (MPNs). To date, many researchers all over the world have been investigating novel approaches to enhance various remediation application technologies. One such approach that the researchers are constantly showing interest in is the use of nanomaterials with potential applications towards the environment. In this regard, MPNs like Copper (Cu), Nickel (Ni), Palladium (Pd), Gold (Au), Silver (Ag), Platinum (Pt), Titanium (Ti), and other nano metals are serving as a suitable agent to eliminate emerging contaminants in various fields, particularly in the removal of phenolic and heavy metal pollutants. This chapter discusses the mechanism and application of various MPNs in eliminating various phenolic and heavy metal pollutants from the environment.

Ligand-protected nanoclusters and their role in agriculture, sensing and allied applications

Nano biotechnology, when coupled with green chemistry, can revolutionize human life because of the vast opportunities and benefits it can offer to the quality of human life. Luminescent metal nanoclusters (NCs) have recently developed as a potential research area with applications in different areas like medical, imaging, sensing etc. Recently these new candidates have proved to be beneficial in the food supply chain enabling controlled release of nutrients, pesticides and as nanosensors for the detection of contaminants and play roles in healthy food storage and maintaining food quality. An assortment of nanomaterials has been employed for these applications and reviews have been published on the use of nanotechnology in agriculture. Ligand-protected metal nanoclusters are a distinctive class of small organic-inorganic nanostructures that garnered immense research interest in recent years owing to their stability at specific "magic size" compositions along with tunable properties that make them promising candidates for a wide range of nanotechnology-based applications. This review tries to consolidate the recent developments in the area of ligand-protected nanoclusters in connection with the detection of pesticides, food contaminants, heavy metal ions and plant growth monitoring for healthy agricultural practices. Its antimicrobial activity to manage the microbial contamination is highlighted. The review also throws light on the various perspectives by which food production and allied areas will be transformed in future.

Evaluation of Cadmium Bioaccumulation-Related Physiological Effects in Salvinia biloba: An Insight towards Its Use as Pollutant Bioindicator in Water Reservoirs

Free-living macrophytes play an important role in the health of aquatic ecosystems. Therefore, the use of aquatic plants as metal biomonitors may be a suitable tool for the management of freshwater reservoirs. Hence, in this study, we assessed the effects of cadmium (Cd) in Salvinia biloba specimens collected from the Middle Paraná River during a 10-day experiment employing artificially contaminated water (100 μM Cd). S. biloba demonstrated a great ability for Cd bioaccumulation in both the root-like modified fronds (named "roots") and the aerial leaf-like fronds (named "leaves") of the plants. Additionally, Cd toxicity was determined by the quantification of photosynthetic pigments (chlorophylls a and b, and carotenoids), flavonoids, and soluble carbohydrate contents in S. biloba over time (1, 3, 5, 7, and 10 days). In general, deterioration was more pronounced in leaves than in roots, suggesting a greater implication of the former in long-term Cd sequestration in S. biloba. Deleterious effects in the appraised parameters were well correlated with the total amount of Cd accumulated in the leaves, and with the qualitative changes observed in the plants' phenotype during the 10-day metal exposure assay. The flavonoids and carotenoids in leaves were highly affected by low Cd levels followed by root carbohydrates. In contrast, chlorophylls and root flavonoids were the least impacted physiological parameters. Therefore, our results demonstrate that S. biloba displays dissimilar organ-linked physiological responses to counteract Cd phytotoxicity and that these responses are also time-dependent. Though further research is needed, our work suggests that easy-handled physiological data obtained from autochthonous free-floating S. biloba specimens may be used as a valuable tool for metal-polluted water biomonitoring.

Ultrastructural evidences for chromium(III) immobilization by Escherichia coli K-12 depending on metal concentration and exposure time

Microorganisms can mediate in heavy metal sequestration through several cellular strategies and pathways. This offers an efficient way to remediate heavy metal polluted environments. This paper describes the ability of Escherichia coli K-12 to capture chromium(III) (Cr(III)) and the ultrastructural effects of this metal on cells, as well as the cellular metal localization and the possible sequestration strategy uses for it. The study was mainly performed by using several electron microscopy techniques and is based on the chromium trivalent concentration and the related exposure time. Transmission electron microscopy (TEM) assay was performed along with field emission scanning electron microscopy (FESEM) for morphological responses. Furthermore, TEM was coupled with an energy dispersive X-ray (TEM-EDX) and TEM with selected area electron diffraction (TEM-SAED) to conduct analytical assays. The exposed cultures to 10 and 12 mM Cr(III) at 12 h and to 5, 7, 10, 12, 13, and 15 mM of Cr(III) at 24 h indicated the presence of multiple electrodense granules that were significantly enriched in chromium and phosphorus content via EDX analysis. Moreover, these granules were observed to be attached to external membrane and/or surrounding cells in the respective ultrathin sections analyzed under TEM. According to these results, E. coli K-12 possesses the ability to immobilize Cr(III) in external polyphosphate granules through a strategy of accumulation, where cell response to Cr(III) toxicity seems to have a dose-dependent and time-dependent relation, thereby offering significant potential for bioremediation in Cr(III)-contaminated areas.

Accumulation and Transfer of Cadmium Isotope (112Cd) by Sonchus asper Intercropped with Vicia faba and Implications for Phytoremediation

Accumulation and transfer of Cd by Sonchus asper intercropped with a legume crop, Vicia faba were determined via ¹¹²Cd as a tracer to assess the potential of Sonchus asper as a hyperaccumulator. In this study, Sonchus asper and Vicia faba were planted side by side in soils amended with arbuscular mycorrhizae, and ¹¹²Cd was applied to either Sonchus asper or Vicia faba planted soils while the neighboring plant received ¹¹²Cd across a nylon net impermeable to the roots. Mean concentration of ¹¹²Cd in the shoot of Sonchus asper was 7.0 times higher than Vicia faba. The translocation factor of ¹¹²Cd in Sonchus asper were 39 and 400 times higher than in Vicia faba under the ¹¹²Cd direct and indirect treatments, respectively. The results suggested that the intercropping of Sonchus asper as a hyperaccumulator with Vicia faba, might be a feasible approach for phytoremediation of Cd contaminated soil.

ED-XRF: a promising method for accurate and rapid quantification of metals in a bacterial matrix

ABSTRACTThe remediation of metal-polluted water using bacterial biofilms is a promising technology. In order to help its development, the present study aims to evaluate the feasibility to utilize XRF spectrometry for accurate and rapid measurement of metal concentrations in bacterial biofilms used in treatment plants. For that purpose, an ED-XRF spectrometer was used to measure Cd, Cu, Fe, Mn, Ni and Zn concentrations within a matrix of marine bacteria Pseudomonas fluorescens BA3SM1 and its metabolites. Contaminated and control cultures of the strain BA3SM1 were dried and crushed, then analysed by ED-XRF. The LOD value of the analysed metals was between 2.08 and 10.5 µg g^-1. Metal concentrations were also measured by ICP-AES or ICP-MS to support ED-XRF results. The two techniques showed a good linear correlation with a slope of at least 0.949 and R² of at least 0.985. These results confirm the possibility to measure metal contents by ED-XRF in bacterial matrices.

Vertical distribution of the toxic metal(loid)s chemical fraction and microbial community in waste heap at a nonferrous metal mining site

Over the past few decades, nonferrous mining has produced numerous waste rock and part of the waste that has not been properly treated was generally dumped at roadsides and hill slopes. However, the vertical distributions of toxic metal(loid)s and composition of microbial communities in waste heap and the under-laid pristine soil are rarely studied. In this work, the fraction-related distributions of toxic metal(loid)s were investigated at a waste heap profile and the indigenous microbial assemblages were also analyzed by Illumina sequencing of 16 s rRNA genes. Results showed that compared to the under-laid pristine soil, content of toxic metal(loid)s, especially Cd, As and Pb, in waste rock layer were higher. Most of As in subsoil existed as non-specifically sorbed and specifically-sorbed fractions, which could be ascribed to the migration from the upper layer. The mobility was significantly correlated with Eh, EC, clay content, CEC and the total content of metal(loid)s. Phyla Proteobacteria, Acidobacteria and Firmicutes dominated the microbial communities. The microbial community compositions at the genus level were similar, but their relative abundances were mainly influenced by pH, CEC, Eh, SOM, and bioavailability content of toxic metal(loid)s. Besides, microbial functions of elements (S, Fe, Mn and As) oxidation/reduction and metabolites (siderophore, biosurfactant, organic acid, phosphatase and urease) potentially were used for pollutants bioremediation.

Bioaccumulation of Fluoride in Plants and Its Microbially Assisted Remediation: A Review of Biological Processes and Technological Performance

Fluoride is widely found in soil–water systems due to anthropogenic and geogenic activities that affect millions worldwide. Fluoride ingestion results in chronic and acute toxicity, including skeletal and dental fluorosis, neurological damage, and bone softening in humans. Therefore, this review paper summarizes biological processes for fluoride remediation, i.e., bioaccumulation in plants and microbially assisted systems. Bioremediation approaches for fluoride removal have recently gained prominence in removing fluoride ions. Plants are vulnerable to fluoride accumulation in soil, and their growth and development can be negatively affected, even with low fluoride content in the soil. The microbial bioremediation processes involve bioaccumulation, biotransformation, and biosorption. Bacterial, fungal, and algal biomass are ecologically efficient bioremediators. Most bioremediation techniques are laboratory-scale based on contaminated solutions; however, treatment of fluoride-contaminated wastewater at an industrial scale is yet to be investigated. Therefore, this review recommends the practical applicability and sustainability of microbial bioremediation of fluoride in different environments.

Surface properties and heavy metals chelation of lipopeptides biosurfactants produced from date flour by Bacillus subtilis ZNI5: optimized production for application in bioremediation

The present study summarizes the valorization of date flour by the production of lipopeptide biosurfactant (BioS) by Bacillus subtilis ZNI5 (MW091416). A Taguchi design permitted the formulation of a medium composed only of 6% date flour and 0.5% yeast extract within 2 days of incubation at 150 rpm with a maximal surface tension (ST) reduction of about 27.8 mN/m. The characterization of the lipopeptide shows a CMC value of about 400 mg/L with a minimal ST of 30 mN/m and an ability to disperse oil to about 80 mm at 800 mg/L. Having reduced phytotoxicity, the ZNI5 BioS and ZNI5 strain were assayed for Copper and Cobalt chelation and biosorption. The improvement of the germination index of radish seeds irrigated by the treated contaminated water showed the great potential application of ZNI5 lipopeptide in the bioremediation of heavy metals.

Heavy metal fixation of lead-contaminated soil using Morchella mycelium

With the exploitation of lead-zinc deposits, lead content around mining areas has seriously exceeded the recommended level. The most challenging problem is how to reduce lead contamination in soil efficiently. In this study, we developed a method to remediate lead-contaminated soil by adding Morchella mycelium. First, we compared the repair effects of mycelium and hyperaccumulator by conducting pot experiments. Then, we investigated the mechanism through which mycelium repairs lead-contaminated soil by conducting simulation experiments. Results showed that using mycelium was a more efficient way to repair soil than using hyperaccumulator. Compared with the untreated group, mycelium reduced the lead content of crops by 34.83 % and raised dry biomass by 134.05 % when lead addition was 800 mg/kg. After mycelium fixation, soil catalase, urease, cellulase, and sucrase activities were significantly enhanced, and the bioavailability of lead decreased significantly. The lead solution exposure simulation test showed that Morchella mycelium immobilized lead due to its extracellular secretions. That is, mycelium secreted metabolites and lead to form salt crystals, reducing bioavailable lead content. In addition, Morchella mycelium restoration may effectively improve soil fertility and increase crop yields. Thus, mycelium may be used successfully in alternative green repair methods for environmental heavy metal remediation.

Hemarthria compressa—Aspergillus niger—Trichoderma pseudokoningii Mediated Trilateral Perspective for Bioremediation and Detoxification of Industrial Paper Sludge

The present study was carried out to evaluate the effects of different fungal species on the metal uptake from paper sludge by Hemarthria compressa L. Paper sludge (PS) in various concentrations (0, 30% and 60%) were used with four treatments of two fungal species (F0 = Control, F1 = Aspergillus niger, F2 = Trichoderma pseudokoningii, F3 = A. niger + T. pseudokoningii). Paper sludge as rooting medium steadily influenced growth and physio-biochemical attributes of H. compressa in F0. Results revealed discrete variations in growth attributes with different PS concentrations and each fungal treatment. The maximum damages in ionic homeostasis (Na+, K+, Ca2+, Mg2+) due to PS toxicity were evident in parallel with declined chlorophyll concentration. The highest growth, total chlorophyll and biomass of the plants were observed when PS was treated with combined fungal strains (F3). Translocation factor decreased with F3 that is indicative of changes in Cd, Pb, and Cu movement to shoot from root. The plants with combined fungal treatment also showed greater catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD) activity. Ions, such as Ca, Mg, and Na, also increased gradually with combined fungal treatment. The correlation matrix displayed a close association among diverse fungal and PS levels with multiple plant attributes. PCA-Biplot confirmed the outcome of correlational analyses among different characters of H. compressa. The combined fungal mediate the extraction of heavy metals from the paper sludge by H. compressa recommends the high accumulation of heavy metal (HM) and possible reutilization of metal free sludge as a fertilizer, for application in cultivated fields. Furthermore, the effects of fungal species on HM attenuation in PS are linear and can be used in different habitats. Advancement in such research work will also be helpful in understanding the mechanisms and enhancing the ability of other native microbes to remediate metals.

Metal accumulations in aquatic organisms and health risks in an acid mine-affected site in South China

Metal contamination from base metal sulphide mines is a major environmental challenge that poses many ecological and health risks. We examined the metal concentrations in the Dabaoshan mine in South China in water, sediments, and aquatic organisms and their specific characteristics (i.e. size, body tissue, species, and habitat) along the Hengshi and Wengjiang River courses to assess acid mine drainage remediation efforts. Metal concentrations of arsenic, cadmium, chromium, copper, lead, nickel, thallium, and zinc were examined in tissues (i.e. gills, intestines, and muscles) of 17 freshwater species of fish, shrimps, and crabs. Metals in tissues followed the trend: intestines > gills > muscles; nearly all intestine samples exceeded the safe limits of metals analysed in this study. There is a positive correlation between distance from the mine and metal concentrations related to the flow of surface water and the habitat of aquatic organisms. The concentrations of arsenic, copper, and zinc were the highest in aquatic organisms, and the distribution was influenced by physical (distance from mine, currents, and seasonality), chemical (pH and competing ions), and biological (species, habitat, and predator-prey relation) factors. Large demersal fish and benthic fauna had higher concentrations of metals. Bioaccumulation and biomagnification of metals, as well as the high metal pollution index and target hazard quotient (arsenic, cadmium, copper, lead, thallium, and zinc), occurred in bottom feeders (C. aumtus, X. argentea) and fish belonging to higher trophic levels (P. fulvidraco, O. mossambicus). Lead and cadmium indicated the highest level of biomagnification from prey to predator. Health risks exist from the dietary intake of common aquatic species such as tilapia and carp besides crustaceans due to high arsenic, cadmium, lead, and thallium levels. Further reduction of metals is necessary to improve the effects of acid mine drainage in the catchment.

Bioremediation of heavy metals from industrial effluents by endophytes and their metabolic activity: Recent advances

Worldwide, heavy metals pollution is mostly caused by rapid population growth and industrial development which is accumulated in food webs causing a serious public health risk. Endophytic microorganisms have a variety of mechanisms for metal sequestration having metal biosorption capacities.Endophytic organisms like bacteria and fungi provide beneficial qualities that help plants to improve their health, reduce stress, and detoxify metals. Endophytes have a higher proclivity for improving metal and mineral solubility by cells that secrete low-molecular-weight organic acids and metal-specific ligands like siderophores, which change the pH of the soil and improve binding activity. Protein-related approaches like chromatin immunoprecipitation sequencing (ChIP-Seq) and modified enzyme-linked immunosorbent assay (ELISA test) can represent endophytic bacterial community and DNA-protein interactions during metal reduction. This review explored the role of endophytes in bioremediation approaches that can help in analyzing the potential and prospects in response to industrial effluents' detoxification.