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

Optimization of cadmium biosorption by Shewanella putrefaciens using a Box-Behnken design

Microbial adsorption of heavy metals has been attracted more interest in the recent years. However, there are very few studies in investigating the biosorption of heavy metals by Shewanella putrefaciens, which is a metal reducing bacterium. Firstly, the effects of contact time, pH value, temperature, biomass dosage and initial cadmium concentration on the cadmium adsorption by Shewanella putrefaciens were studied by single factor experiments. Then, the response surface methodology (RSM) based on Box-Behnken design was used to optimize the cadmium adsorption by Shewanella putrefaciens. The results showed that the empirical model was suitable for experimental data, and the maximum cadmium removal efficiency by Shewanella putrefaciens was 86.54% under the optimum conditions of contact time 4.0 days, pH value 5, initial cadmium concentration of 20 mg/L, which was further verified by experiments. In addition, scanning electron microscope - Energy Dispersive Spectrometer (SEM-EDS) analysis showed that the bacteria were seriously deformed, and a "bamboo" shape was observed on the surface which consisted of cadmium according to the EDS analysis. Fourier transform infrared spectroscopy (FT-IR) analysis was used to evaluate the possible functional groups involving in interaction between cells and metal ions. The results showed that the distribution of cadmium on the cell surface was related to the carboxyl, amide, hydroxyl and phosphoric acid groups of Shewanella putrefaciens. These studies can provide a comprehensive understanding of the process and mechanism of microbial removal of heavy metals, and theoretical support for the follow-up practice of using biological adsorbents to remediate heavy metal contaminated soil.

Overview of bioremediation with technology assessment and emphasis on fungal bioremediation of oil contaminated soils

Environmental contamination is a problem that requires sustainable solutions. Bioremediation technologies have been developed in the last decades and are increasingly used to mitigate environmental accidents and systematic contaminations. A review of bioremediation technologies, based on published article and patent documents, is presented for different types of contaminated matrices, bioremediation agents and contaminants. The worldwide database of the European Patent Office was searched using radicals of keyword as well as the International Patent Classification (IPC) to identify patents in our areas of concern. Technological domains, annual filing volume, legal status, assignee countries and development collaborations are presented and examples are discussed. The total number of patents is compared with the total number of articles. A SWOT analysis for bioremediation technologies is presented. The technologies for water (53%), soils (36%), and sludges (11%) are growing yearly at nearly constant rates. The bioremediation agents are predominantly bacteria (57%), enzymes (19%), fungi (13%), algae (6%), plants (4%) and protozoa. The major contaminants are oils (38%), followed by metals (21%), organic waste (21%), polymers (10%), food (5%), cellulose (5%) and biodiesel. Most of the patents are generally originated from China and United States of America. The soils bioremediation technology of oil is centered on bacteria usage (about two thirds of the articles and patents), being fungi a technology with critical mass and high growth potential. A recent trend in oil bioremediation of soils is the combination of bioremediation agents (fungi and bacteria) in the same process, thus making the process more robust to environment changes.

Occurrence and diversity of cultivable autochthonous microscopic fungi in substrates of old environmental loads from mining activities in Slovakia

Distribution and biodiversity of soil microscopic fungi in 5 areas of old environmental loads in Slovakia were studied in relation to very low amount of organic matter (%TOC from 0.2 to 3.54) and to the pH gradient from ultra-acidic (< 3.5) to very strongly alkaline (> 9.0). All soil samples were affected by several hundred years of mining activities and contained heavy metals and other toxic elements: arsenic, cadmium, copper, zinc, antimony, lead. Concentrations of toxicants highly exceeded their limited values. Fifty-three genera and 112 species of microscopic fungi were identified. Among them, Zygomycota occurred very rarely (8 genera and 12 species), except of samples with the highest content of TOC (2.01-3.54% - samples 2 and 6), regardless their pH. Though, on the other hand, from some similar samples (3, 5 and 9), incl. those with relatively high TOC (0.14-2.62%), the lower fungi were not recovered. Forty one genera and 95 species of Ascomycota represented the most abundant fungal phylum in all investigated samples. Among them, Penicillium chrysogenum var. chrysogenum, Aspergillus niger and Neosartorya fischeri were isolated the most often. Phytopathogenic moulds of Bionectria ochroleuca, Lewia infectoria, Phoma macrostoma and Phlebia acerina were also occurred frequently. The highest biodiversity of microfungal community was recorded in the extreme acidic environment, followed by the neutral, ultra-acidic and the very strong acidic ones. There was no similarity in microfungal spectrum found in the samples studied. Except of the ultra acidic and extreme acidic samples (1-2) as well as the ultra acidic and strong acidic ones (1-4) with the most rich mycobiota, that may indicate a certain similarity degree.

Effect of daily exposure to Pb-contaminated water on Salvinia biloba physiology and phytoremediation performance

Lead (Pb) removal from water column was evaluated in batch experiments using naturally occurring Salvinia biloba Raddi (S. biloba) specimens collected from Middle Paraná River and exposed every 24 h to a fresh discharge of water contaminated with 2.65 ± 0.07, 12.62 ± 0.02 or 30.57 ± 0.01 mg L^-1 Pb, during 10 consecutive days. S. biloba demonstrated a great ability for metal concentration-dependent Pb removal under these stressful conditions. Additionally, Pb toxicity in plants was assessed by the quantification of physiological parameters in root-like modified fronds (named "roots"), and its aerial leaf-like fronds (named "leaves") of submerged S. biloba. Photosynthetic (carotenoids, chlorophyll a, b, and total) and antioxidant pigments (anthocyanins and flavonoids), soluble carbohydrate content, and membrane stability index of both roots and leaves were affected as the metal concentration increased. In general, root deterioration was more pronounced than that in leaves, suggesting a greater implication of the former organs in Pb removal by S. biloba. All of these deleterious effects were well correlated with qualitative changes observed at plant phenotype during the assay. In conclusion, S. biloba may be considered as a water fern useful in phytoremediation strategies towards management of residual water bodies contaminated with Pb. In addition, these macrophytes could also be valuable for water biomonitoring contributing to improve risk assessments related to metal presence in wastewaters.

Embedding live bacteria in porous hydrogel/ceramic nanocomposites for bioprocessing applications

In this work, we present a biocompatible one-pot processing route for ceramic/hydrogel nanocomposites in which we embed live bacteria. In our approach, we fabricate a highly stable alginate hydrogel with minimal shrinkage, highly increased structural and mechanical stability, as well as excellent biocompatibility. The hydrogel was produced by ionotropic gelation and reinforced with alumina nanoparticles to form a porous 3D network. In these composite gels, the bacteria Escherichia coli and Bacillus subtilis were embedded. The immobilized bacteria showed high viability and similar metabolic activity as non-embedded cells. Even after repeated glucose consumption cycles, the material maintained high structural stability with stable metabolic activity of the immobilized bacteria. Storing the bionanocomposite for up to 60 days resulted in only minor loss of activity. Accordingly, this approach shows great potential for producing macroscopic bioactive materials for biotechnological processes.

Laboratory and field microcosms as useful experimental systems to study the bioaugmentation treatment of tannery effluents

Tannery effluents require effective treatment prior to their final disposal, and the use of native bacterial consortia could be an appropriate strategy for this purpose. In the present work, consortium SFC 500-1 was found to be highly tolerant to different metals, metalloids and aromatic compounds like phenols. It was also able to remove the black dye commonly used in tanneries. Moreover, it promoted a significant reduction in chemical oxygen demand and exhibited high capability for the simultaneous removal of Cr(VI) and phenol. However, the effectiveness of the remediation processes markedly varied from one experimental system (Erlenmeyer flasks) to another (field microcosm system), highlighting the importance of moving from a small-scale study system to one involving more realistic environmental scenarios. In addition, we found a decrease in the toxicity of the effluent treated with consortium SFC 500-1. Taken together, our results indicate that this consortium possesses great potential for the treatment of tannery effluents. We conclude that for the development of a bioremediation strategy, it is necessary to develop experiments at a larger scale under conditions similar to those of the original system, in order to complete the scenario first created by in vitro approaches.

Bioremediation of Waste Water to Remove Heavy Metals Using the Spent Mushroom Substrate of Agaricus bisporus

The presence of heavy metals in waste water brings serious environmental pollution that threatens human health and the ecosystem. Bioremediation of heavy metals has received considerable and growing interest over the years. Thus, this paper presents the use of the Spent Mushroom Substrate (SMS) of Agaricus bisporus cultivation as a bioremediating agent to remove heavy metals that are present in industrial waters. These metals include chromium, lead, iron, cobalt, nickel, manganese, zinc, copper and aluminium. In particular, this study analyses the performance of SMS bioreactors with different groups of heavy metals at various concentrations. Between 80% and 98% of all contaminants that were analysed can be removed with 5 kg of SMS at hydraulic retention times of 10 and 100 days. The best removal efficiencies and longevities were achieved when removing iron (III), nickel and cobalt from contaminated water at a pH of 2.5. These results suggest that SMS can successfully treat waste water that has been contaminated with heavy metals.

In vitro evaluation of mercury (Hg2+) effects on biofilm formation by clinical and environmental isolates of Klebsiella pneumoniae

The increase in urbanization and industrialization has contributed to the contamination of different environments by means of xenobiotic compounds, such as heavy metals, causing changes in microbial communities. Among these metals, the Mercury (Hg²⁺) is one the most prevalent toxic metals for the environment The present study aimed to evaluate the effect of mercury on the formation of biofilm by environmental (collected from urban stream water) and clinical isolates of Klebsiella pneumoniae. In addition, antibiotic resistance, virulence factors, and genetic diversity were investigated. Taxonomic identity of eight isolates (one reference, two clinical, and five environmental isolates) was performed by MALDI-TOF-MS, while the antibiotic susceptibility profile was assessed by the disc diffusion method. The ability to form biofilms was evaluated by culture on Congo red agar and by crystal violet staining. Biofilm structure was analyzed by scanning electron microscopy. The hydrophobicity profile and the presence of the virulence genes cps, fimH, and mrkD was investigated. The presence of merA and its relationship with antimicrobial resistance were also assessed. The identity of all isolates was confirmed by MALDI-TOF-MS, and different profiles of resistance to mercury and antibiotics as well as of biofilm formation were identified for the clinical and environmental isolates. All isolates were hydrophilic and positive for the virulence genes cps, fimH, and mrkD; only the clinical isolate K36-A2 was positive for merA. The diversity of the isolates was confirmed by ERIC-PCR, which revealed high heterogeneity among the isolates. In conclusion, the data demonstrate that the investigated isolates present different responses to exposure to Hg²⁺ and correspond to distinct populations of K. pneumoniae disseminated in the investigated environment. The data obtained in this work will aid in understanding the mechanisms of survival of this pathogen under adverse conditions.

Bioaccumulation of Cadmium and Lead in the Muscle Tissue of <i>Mullus barbatus</i> in Skikda and Jijel Bays Eastern Algeria

The bays of Skikda and Jijel present an ecosystem of great biological diversity and a significant economic interest (fishing and trading ports, industrial zones and tourism). They are threatened by the inputs of industrial effluents that are loaded with different substances, especially heavy metals. These pollutants have the distinction of being toxic and non-biodegradable, they accumulate in the different levels of the food chain which represents a danger for human health. The present work aimed to evaluate the impact of metal pollution in both bays via the study of the bioaccumulation of heavy metals namely, cadmium (Cd) and lead (Pb) in red mulletMullusbarbatusLinnaeus, 1758. Forty two (42) fish samples were obtained from 4 sites, 2 from bays of Skikda and 2 from bays of Jijel. After preparation, lyophilisation and mineralisation, samples were analysed by Atomic Absorption Spectroscopy (AAS) for detection of Cd and Pb concentrations in µg/g of dry weights. Lead has reached the values of 141.666±5.238 and 89±3.464 µg/g in Skikda sites and 20±1.527 and 10±0.577 µg/g in Jijel sites, while Cd has reached 0.76±0.023 and 0.3±0.011 µg/g in Skikda, with lowest values in Jijel 0.116±0.008 and 0.1±0.005 µg/g. The highest levels were recorded in the areas that are subject to anthropogenic pollution, namely the port areas and the oil industry (Skikda bays). While the low concentrations were found in less polluted areas like Jijel bays. The results obtained in this study are alarming and reflect significant level of pollution especially in the bays of Skikda. Preventive and remedial measures and awareness raising are needed.

Heavy metal mediated phytotoxic impact on winter wheat: oxidative stress and microbial management of toxicity by Bacillus subtilis BM2

Heavy metals are toxic environmental contaminants, which severely affect microbial composition and functions and, concurrently, crop production. Due to these issues, the present study focussed on the selection of metal tolerant microbes endowed with metal detoxification abilities and their role in the management and remediation of metal contaminated soils. The metal tolerant bacterium BM2, identified as Bacillus subtilis by 16SrRNA gene sequencing, survived well under metal pressure and tolerated 1600 and 2000 μg mL-1 of Ni and Pb, respectively. The inhibitory impact of metals on wheat increased consistently with a progressive increase in metal concentration. Deposition of Ni and Pb within root and leaf and oxidative stress were validated by SEM, EDX and CLSM. The overall growth parameters of wheat grown under metal stress were improved following B. subtilis BM2 colonization. As an example, B. subtilis with 195 mg Pb kg-1 enhanced the length and dry biomass of shoots by 14% and 23%, respectively, over the control. Also, strain BM2 improved the grain yield significantly by 49% at 870 mg Ni kg-1 and by 50% at 585 mg Pb kg-1 compared to uninoculated plants. Moreover, B. subtilis BM2 relieved the metal stress on wheat and caused a significant drop in proline and malondialdehyde content and the activities of antioxidant enzymes, like catalase (CAT), superoxide dismutase (SOD) and glutathione reductase (GR). This study, therefore, provided solutions to the metal toxicity problems faced by winter wheat and clearly suggests that the metal detoxification potential of B. subtilis BM2 could be greatly useful in the management of metal polluted soils.

Bioinoculants for Bioremediation Applications and Disease Resistance: Innovative Perspectives

Soil microbial species that act as PGPR or bioinoculants have the capability of improving plant health and promoting its growth. They facilitate plants for uptake nutrients from their surroundings. They provide resistivity to pathogenic pests and also play many roles in the bioremediation process. Bioremediation is the biological approach for the elimination of toxic contaminants by the approach of beneficial microbes. By the consortium of beneficial microbes and plant, a large number of heavy metal and organic contaminants can be controlled. With this advancement of bioremediation, microbial species that act as bioinoculants also help in the enhancement of induced systemic resistance (ISR) and their consortium triggers it by controlling SA, JA, ET and hormonal signaling pathways. Here, this review discusses the progress made on these areas and how the beneficial microbes that act as bioinoculants towards triggering bioremediation and ISR mechanism.

An insight into Cadmium poisoning and its removal from aqueous sources by Graphene Adsorbents

Graphene alone, in modified form or its composites had find their explicit position in the field of adsorption technology and hence assist in detection and removal of heavy metals like Cd (permissible limit 0.1 mg/L), which can cause various physiological problems if entered in variety of biota. Attributed to their unique physiognomies graphene-based adsorbent had classed themselves superior as compared to other carbonaceous adsorbent like CNT's or activated carbon, etc. This assessment summarizes the validity of graphene and its composite as a superior adsorbent for decontamination of Cd from aqueous environment; in addition, this evaluation also pronounces the toxicity profile of trace graphene and necessity of regeneration of the adsorbent.

Functional and structural biomarkers to monitor heavy metal pollution of one of the most contaminated freshwater sites in Southern Europe

This study evaluated the biological effects of highly polluted freshwater environment (Regi Lagni channels, S Italy) on the aquatic moss Leptodictyum riparium, exposed in bags at three sites representative of different environmental conditions and characterized by different heavy metal burdens. Bioaccumulation, ultrastructural alterations, Reactive Oxygen Species (ROS) production, antioxidant enzymes activity and DNA damage were assessed. To better evaluate the biological response of the moss species to heavy metals, the same biological parameters were assessed also in L. riparium samples cultured in vitro using metal mixtures at the same concentrations as measured at the 3 field exposure sites. Heavy metals were accumulated into the moss tissues causing severe ultra-structural damages at higher concentration case studies, and the ROS production as well as the activity of the enzyme followed a concentration-dependent increase. However, the DNA damage trend suggested a threshold effect that changed between field and in vitro experiment. The enrichment factor suggests that the concentration in the most polluted site is close to the upper limit of L. riparium to accumulate metals. Overall, combining measures of the morpho-functional traits at different level contribute to improving the knowledge about the tolerance of L. riparium to heavy metal stress, suggesting that this moss could be suitable for biomonitoring activity in field conditions.

Inoculation of soil with cadmium-resistant bacterium Delftia sp. B9 reduces cadmium accumulation in rice (Oryza sativa L.) grains

Bioremediation of heavy metal polluted soil using metal-resistant bacteria has received increasing attentions. In the present study, we isolated a heavy metal-resistant bacterial strain from a Cd-contaminated soil, and conducted pot experiments to evaluate the effect of bacterial inoculation in soil on soil Cd speciation, rice grain biomass and Cd accumulation. We find that the isolated bacterial strain is a Gram-negative bacterium, and named as Delftia sp. B9 based on the 16S rDNA gene sequence analysis. TEM-EDS manifests that Cd can be bioaccumulated inside cell, resulting in intracellular dissolution. The Cd contents of rice grain in the two rice cultivars (early and late rice) are all below the standard limit for Food Safety of People's Republic of China (0.2 mg/kg) after the treatment of both living and non-living cells. Non-living cells are more applicable than the use of living cells for the short time bioremediation. The average content of soil exchangeable fraction of Cd decreases whereas the residual fraction increases with bacterial inoculation. All our results suggest Delftia sp. B9 is able to the stabilization of Cd in soil and reduce Cd accumulation in rice grain, therefore, this strain is potentially suitable for the bioremediation of Cd-contaminated paddy soils.

Biosorption and Bioaccumulation Abilities of Actinomycetes/Streptomycetes Isolated from Metal Contaminated Sites

Heavy metal pollution is of great concern. Due to expansion of industrial activities, a large amount of metal is released into the environment, disturbing its fragile balance. Conventional methods of remediation of heavy metal-polluted soil and water are expensive and inefficient. Therefore, new techniques are needed to provide environmentally friendly and highly selective remediation. Streptomycetes, with their unique growth characteristics, ability to form spores and mycelia, and relatively rapid colonization of substrates, act as suitable agents for bioremediation of metals and organic compounds in polluted soil and water. A variety of mechanisms could be involved in reduction of metals in the environment, e.g., sorption to exopolymers, precipitation, biosorption and bioaccumulation. Studies performed on biosorption and bioaccumulation potential of streptomycetes could be used as a basis for further development in this field. Streptomycetes are of interest because of their ability to survive in environments contaminated by metals through the production of a wide range of metal ion chelators, such as siderophores, which provide protection from the negative effects of heavy metals or specific uptake for specialized metabolic processes. Many strains also have the equally important characteristic of resistance to high concentrations of heavy metals.

Chromium uptake by lettuce as affected by the application of organic matter and Cr(VI)-irrigation water: Implications to the land use and water management

Toxic chromium [(Cr(VI)] in food chain has created an alarming situation for human life and ecosystems. The present study through a greenhouse pot experiment aims to (a) investigate the ability of organic matter in reducing Cr uptake by lettuce (Lactuca sativa L.) from a sandy loam soil irrigating with Cr(VI)-water, (b) to provide a way for the restriction of Cr transfer from contaminated soils and irrigation water to plants/crops and (c) to contribute to the better management of soil (land) and water use, without reduction of the agricultural production. Since soil and groundwater contamination by Cr is a potential risk in a worldwide scale, due to industrial activities and/or natural processes, organic carbon may play a key role in the mobility of added Cr(VI) to soil via irrigation water, in a significant way. The cultivation of lettuce, using organic matter in the form of leonardite (10 and 30 wt%) and Cr(VI)-irrigation water (100, 200 and 300 mgL-1), showed that the uptake of Cr in both shoots and roots increased with increasing concentration of Cr in the irrigation water. The highest Cr values in shoots (average = 10 mg/kg) and in roots (average = 28 mg/kg) were recorded in those plants cultivated in soil after the addition of Cr(VI)- water without organic matter, whereas the lowest Cr values in shoots (average = 0.44 mg/kg) and in roots (average = 0.7 mg/kg) were recorded in those plants cultivated in soil with addition of 30 wt% organic matter. The used leonardite as organic matter that is an oxidized form of lignite, due to its high content of humic acid is considered to be a useful organic fertilizer that provides possibilities for combining food production with soil protection. Therefore, the application of the natural organic material leonardite, as a land management technique, seems to be a cost-effective method consistent to related protocols for the protection of the soil quality.

Brown marine macroalgae as natural cation exchangers for toxic metal removal from industrial wastewaters: A review

The discharge of inadequately treated or untreated industrial wastewaters has greatly contributed to the release of contaminants into the environment, including toxic metals. Toxic metals are persistent and bioaccumulative, being their removal from wastewaters prior to release into water bodies of great concern. Literature reports the use of brown marine macroalgae for toxic metals removal from aqueous solutions as an economic and eco-friendly technique, even when applied to diluted solutions. Minor attention has been given to the application of this technique in the treatment of real wastewaters, which present a complex composition that can compromise the biosorption performance. Therefore, the main goal of this comprehensive review is to critically outline studies that: (i) applied brown marine macroalgae as natural cation exchanger for toxic metals removal from real and complex matrices; (ii) optimised the biosorption process in a fixed-bed column, which was further scaled-up to pilot plants. An overview of toxic metals sources, chemistry and toxicity, which are relevant aspects to understand and develop treatment techniques, is initially presented. The problem of water resources pollution by toxic metals and more specifically the participation of metal finishing industries in the environmental contamination are issues also covered. The current and potential decontamination methods are presented including a discussion of their advantages and drawbacks. The literature on biosorption was reviewed in detail, considering especially the ion exchange properties of cell wall constituents, such as alginate and fucoidan, and their role in metal sequestration. Besides that, a detailed description of biosorption process design, especially in continuous mode, and the application of mechanistic models is addressed.

Toxicity and Bioremediation of Heavy Metals Contaminated Ecosystem from Tannery Wastewater: A Review

The discharge of untreated tannery wastewater containing biotoxic substances of heavy metals in the ecosystem is one of the most important environmental and health challenges in our society. Hence, there is a growing need for the development of novel, efficient, eco-friendly, and cost-effective approach for the remediation of inorganic metals (Cr, Hg, Cd, and Pb) released into the environment and to safeguard the ecosystem. In this regard, recent advances in microbes-base heavy metal have propelled bioremediation as a prospective alternative to conventional techniques. Heavy metals are nonbiodegradable and could be toxic to microbes. Several microorganisms have evolved to develop detoxification mechanisms to counter the toxic effects of these inorganic metals. This present review offers a critical evaluation of bioremediation capacity of microorganisms, especially in the context of environmental protection. Furthermore, this article discussed the biosorption capacity with respect to the use of bacteria, fungi, biofilm, algae, genetically engineered microbes, and immobilized microbial cell for the removal of heavy metals. The use of biofilm has showed synergetic effects with many fold increase in the removal of heavy metals as sustainable environmental technology in the near future.

Immobilization of cadmium by immobilized Alishewanella sp. WH16-1 with alginate-lotus seed pods in pot experiments of Cd-contaminated paddy soil

This study prepared immobilized Alishewanella sp. WH16-1 using alginate and lotus seed pods as a matrix and investigated the effects of its immobilization on Cd²⁺ in a culture solution and in soil. Compared with the free WH16-1 strain, the immobilized WH16-1 strain possessed greater stability for long-term use and storage and higher removal ability for Cd²⁺ in the culture solution. A model of Cd²⁺ removal by the immobilized WH16-1 strain was proposed. The immobilized WH16-1 strain was incubated in the pot experiments of Cd-contaminated paddy soil for 120 days, and the pot experiments of Cd-contaminated paddy soil without the immobilized WH16-1 strain were used as a control. Compared with the control, the exchangeable and carbonate-bound Cd in the paddy soil incubated with the immobilized WH16-1 strain significantly decreased by 33.6% (P < 0.05) and 17.36%, respectively, and the Cd concentrations in the rice significantly decreased by 78.31% (P < 0.05). The results indicate that alginate-lotus seed pods can be used as excellent cost-effective cell carriers for the immobilization of Alishewanella sp. WH16-1 and that the immobilized WH16-1 strain may be applicable for the biological stabilization of Cd in Cd-contaminated paddy soil.

Characterization and mechanism of copper biosorption by a highly copper-resistant fungal strain isolated from copper-polluted acidic orchard soil

In this paper, a highly copper-resistant fungal strain NT-1 was characterized by morphological, physiological, biochemical, and molecular biological techniques. Physiological response to Cu(II) stress, effects of environmental factors on Cu(II) biosorption, as well as mechanisms of Cu(II) biosorption by strain NT-1 were also investigated in this study. The results showed that NT-1 belonged to the genus Gibberella, which exhibited high tolerance to both acidic conditions and Cu(II) contamination in the environment. High concentrations of copper stress inhibited the growth of NT-1 to various degrees, leading to the decreases in mycelial biomass and colony diameter, as well as changes in morphology. Under optimal conditions (initial copper concentration: 200 mg L^-1, temperature 28 °C, pH 5.0, and inoculum dose 10%), the maximum copper removal percentage from solution through culture of strain NT-1 within 5 days reached up to 45.5%. The biosorption of Cu(II) by NT-1 conformed to quasi-second-order kinetics and Langmuir isothermal adsorption model and was confirmed to be a monolayer adsorption process dominated by surface adsorption. The binding of NT-1 to Cu(II) was mainly achieved by forming polydentate complexes with carboxylate and amide group through covalent interactions and forming Cu-nitrogen-containing heterocyclic complexes via Cu(II)-π interaction. The results of this study provide a new fungal resource and key parameters influencing growth and copper removal capacity of the strain for developing an effective bioremediation strategy for copper-contaminated acidic orchard soils.

Mechanistic studies on the biosorption of Pb(II) by Pseudomonas aeruginosa

The biosorption of Pb(II) ions from aqueous solution has been studied using both the intact and thermolyzed cells of Pseudomonas aeruginosa. Further, the role of the major cell wall components, namely DNA, protein, polysaccharide, and lipid, in Pb(II) binding has been assessed using an enzymatic treatment method. The Pb(II) bioremediation capability of P. aeruginosa cells has been investigated by varying the parameters of pH, time of interaction, amount of biomass, and concentration of Pb(II). The complete bioremoval of Pb(II) using intact cells has been achieved for an initial Pb(II) concentration of 12.4 mg L^-1 at pH 6.2 and temperature 29 ± 1 °C. The biosorption isotherm follows Langmuirian behavior with a Gibbs free energy of -30.7 kJ mol^-1, indicative of chemisorption. The biosorption kinetics is consistent with a pseudo-second-order model. The possible Pb(II) binding mechanisms of P. aeruginosa cells are discussed based on characterization using zeta potential measurements, Fourier transform infra-red spectroscopy, and energy dispersive X-ray spectroscopy. The results confirm that among the major cell wall components studied, polysaccharide shows the highest contribution towards Pb(II) binding, followed by DNA, lipid, and protein. Similar studies using thermolyzed cells show higher Pb(II) uptake compared to the intact cells both before and after enzymatic treatment.

Nickel exposure reduces enterobactin production in Escherichia coli

Escherichia coli is a well‐studied bacterium that can be found in many niches, such as industrial wastewater, where the concentration of nickel can rise to low‐millimolar levels. Recent studies show that nickel exposure can repress pyochelin or induce pyoverdine siderophore production in Pseudomonas aueroginosa. Understanding the molecular cross‐talk between siderophore production, metal homeostasis, and metal toxicity in microorganisms is critical for designing bioremediation strategies for metal‐contaminated sites. Here, we show that high‐nickel exposure prolongs lag phase duration as a result of low‐intracellular iron levels in E. coli. Although E. coli cells respond to low‐intracellular iron during nickel stress by maintaining high expression of iron uptake systems such as fepA, the demand for iron is not met due to a lack of siderophores in the extracellular medium during nickel stress. Taken together, these results indicate that nickel inhibits iron accumulation in E. coli by reducing the presence of enterobactin in the extracellular medium.

Genome-Guided Characterization of Ochrobactrum sp. POC9 Enhancing Sewage Sludge Utilization-Biotechnological Potential and Biosafety Considerations

Sewage sludge is an abundant source of microorganisms that are metabolically active against numerous contaminants, and thus possibly useful in environmental biotechnologies. However, amongst the sewage sludge isolates, pathogenic bacteria can potentially be found, and such isolates should therefore be carefully tested before their application. A novel bacterial strain, Ochrobactrum sp. POC9, was isolated from a sewage sludge sample collected from a wastewater treatment plant. The strain exhibited lipolytic, proteolytic, cellulolytic, and amylolytic activities, which supports its application in biodegradation of complex organic compounds. We demonstrated that bioaugmentation with this strain substantially improved the overall biogas production and methane content during anaerobic digestion of sewage sludge. The POC9 genome content analysis provided a deeper insight into the biotechnological potential of this bacterium and revealed that it is a metalotolerant and a biofilm-producing strain capable of utilizing various toxic compounds. The strain is resistant to rifampicin, chloramphenicol and β-lactams. The corresponding antibiotic resistance genes (including bla_OCH and cmlA/floR) were identified in the POC9 genome. Nevertheless, as only few genes in the POC9 genome might be linked to pathogenicity, and none of those genes is a critical virulence factor found in severe pathogens, the strain appears safe for application in environmental biotechnologies.

Tea saponins: effective natural surfactants beneficial for soil remediation, from preparation to application

Tea saponins, found in Camellia plants, are natural non-ionic surfactants that offer obvious beneficial effects in soil remediation. Most tea saponins are extracted from the Camellia oleifera seed meal, with the leaves and flowers of Camellia sinensis as potential sources. Water extraction and ultrasound-assisted water extraction combined with acetone precipitation are recommended for the industrial extraction and purification of tea saponins, considering multiple factors. The detailed physical, chemical and biochemical properties of tea saponins need to be clarified, especially whether tea saponins with slightly different structures from distinct sources have different soil remediation properties. Applied in leaching remediation, phytoremediation and microbial remediation, tea saponins desorb heavy metals from contaminated soil as well as enhancing their bioavailability. Tea saponins improve the accumulation of pollutants by hyperaccumulators as well as the degradation of organic pollutants by microorganisms. Currently the mechanisms of tea saponins are not clear, although they are proven to be effective natural surfactants for the remediation of contaminated soils. This review enriches our understanding of tea saponins from various aspects and encourages further studies of industrial extraction and purification, and the field remediation mechanisms of tea saponins, making better use of Camellia plants and contributing to environmental protection.

Biological approaches to tackle heavy metal pollution: A survey of literature

Pollution by heavy metals has been identified as a global threat since the inception of industrial revolution. Heavy metal contamination induces serious health and environmental hazards due to its toxic nature. Remediation of heavy metals by conventional methods is uneconomical and generates a large quantity of secondary wastes. On the other hand, biological agents such as plants, microorganisms etc. offer easy and eco-friendly ways for metal removal; hence, considered as efficient and alternative tools for metal removal. Bioremediation involves adsorption, reduction or removal of contaminants from the environment through biological resources (both microorganisms and plants). The heavy metal remediation properties of microorganisms stem from their self defense mechanisms such as enzyme secretion, cellular morphological changes etc. These defence mechanisms comprise the active involvement of microbial enzymes such as oxidoreductases, oxygenases etc, which influence the rates of bioremediation. Further, immobilization techniques are improving the practice at industrial scales. This article summarizes the various strategies inherent in the biological sorption and remediation of heavy metals.

Microbe and plant assisted-remediation of organic xenobiotics and its enhancement by genetically modified organisms and recombinant technology: A review

Environmental problems such as the deterioration of groundwater quality, soil degradation and various threats to human, animal and ecosystem health are closely related to the presence of high concentrations of organic xenobiotics in the environment. Employing appropriate technologies to remediate contaminated soils is crucial due to the site-specificity of most remediation methods. The limitations of conventional remediation technologies include poor environmental compatibility, high cost of implementation and poor public acceptability. This raises the call to employ biological methods for remediation. Bioremediation and microbe-assisted bioremediation (phytoremediation) offer many ecological and cost-associated benefits. The overall efficiency and performance of bio- and phytoremediation approaches can be enhanced by genetically modified microbes and plants. Moreover, phytoremediation can also be stimulated by suitable plant-microbe partnerships, i.e. plant-endophytic or plant-rhizospheric associations. Synergistic interactions between recombinant bacteria and genetically modified plants can further enhance the restoration of environments impacted by organic pollutants. Nevertheless, releasing genetically modified microbes and plants into the environment does pose potential risks. These can be minimized by adopting environmental biotechnological techniques and guidelines provided by environmental protection agencies and other regulatory frameworks. The current contribution provides a comprehensive overview on enhanced bioremediation and phytoremediation approaches using transgenic plants and microbes. It also sheds light on the mitigation of associated environmental risks.

Metal-Adapted Bacteria Isolated From Wastewaters Produce Biofilms by Expressing Proteinaceous Curli Fimbriae and Cellulose Nanofibers

Bacterial biofilm plays a pivotal role in bioremediation of heavy metals from wastewaters. In this study, we isolated and identified different biofilm producing bacteria from wastewaters. We also characterized the biofilm matrix [i.e., extracellular polymeric substances (EPS)] produced by different bacteria. Out of 40 isolates from different wastewaters, only 11 (27.5%) isolates (static condition at 28°C) and 9 (22.5%) isolates (agitate and static conditions at 28 and 37°C) produced air–liquid (AL) and solid–air–liquid (SAL) biofilms, respectively, only on salt-optimized broth plus 2% glycerol (SOBG) but not in other media tested. Biomass biofilms and bacteria coupled with AL biofilms were significantly (P ≤ 0.001) varied in these isolates. Escherichia coli (isolate ENSD101 and ENST501), Enterobacter asburiae (ENSD102), Enterobacter ludwigii (ENSH201), Pseudomonas fluorescens (ENSH202 and ENSG304), uncultured Vitreoscilla sp. (ENSG301 and ENSG305), Acinetobacter lwoffii (ENSG302), Klebsiella pneumoniae (ENSG303), and Bacillus thuringiensis (ENSW401) were identified based on 16S rRNA gene sequencing. Scanning electron microscope (SEM) images revealed that biofilm matrix produced by E. asburiae ENSD102, uncultured Vitreoscilla sp. ENSG301, A. lwoffii ENSG302, and K. pneumoniae ENSG303 are highly fibrous, compact, and nicely interlinked as compared to the biofilm developed by E. ludwigii ENSH201 and B. thuringiensis ENSW401. X-ray diffraction (XRD) results indicated that biofilm matrix produced by E. asburiae ENSD102, uncultured Vitreoscilla sp. ENSG301, and A. lwoffii ENSG302 are non-crystalline amorphous nature. Fourier transform infrared (FTIR) spectroscopy showed that proteins and polysaccharides are the main components of the biofilms. Congo red binding results suggested that all these bacteria produced proteinaceous curli fimbriae and cellulose-rich polysaccharide. Production of cellulose was also confirmed by Calcofluor binding- and spectrophotometric assays. E. asburiae ENSD102, Vitreoscilla sp. ENSG301, and A. lwoffii ENSG302 were tested for their abilities to form the biofilms exposure to 0 to 2000 mg/L of copper sulfate (for Cu), zinc sulfate (for Zn), lead nitrate (for Pb), nickel chloride (for Ni), and potassium dichromate (for Cr), several concentrations of these metals activated the biofilm formation. The polysaccharides is known to sequester the heavy metals thus, these bacteria might be applied to remove the heavy metals from wastewater.

Microbial Diversity of Mer Operon Genes and Their Potential Rules in Mercury Bioremediation and Resistance

Background:

Mercury is a toxic metal that is present in small amounts in the environment, but its level is rising steadily, due to different human activities, such as industrialization. It can reach humans through the food chain, amalgam fillings, and other sources, causing different neurological disorders, memory loss, vision impairment, and may even lead to death; making its detoxification an urgent task.

Methods:

Various physical and chemical mercury remediation techniques are available, which generally aim at: (i) reducing its mobility or solubility; (ii) causing its vaporization or condensation; (iii) its separation from contaminated soils. Biological remediation techniques, commonly known as bioremediation, are also another possible alternative, which is considered as cheaper than the conventional means and can be accomplished using either (i) organisms harboring themeroperon genes (merB,merA,merR,merP,merT,merD,merF,merC,merE,merHandmerG), or (ii) plants expressing metal-binding proteins. Recently, differentmerdeterminants have been genetically engineered into several organisms, including bacteria and plants, to aid in detoxification of both ionic and organic forms of mercury.

Results:

Bacteria that are resistant to mercury compounds have at least a mercuric reductase enzyme (MerA) that reduces Hg+2to volatile Hg0, a membrane-bound protein (MerT) for Hg+2uptake and an additional enzyme, MerB, that degrades organomercurials by protonolysis. Presence of bothmerA andmerB genes confer broad-spectrum mercury resistance. However,merA alone confers narrow spectrum inorganic mercury resistance.

Conclusion:

To conclude, this review discusses the importance of mercury-resistance genes in mercury bioremediation. Functional analysis ofmeroperon genes and the recent advances in genetic engineering techniques could provide the most environmental friendly, safe, effective and fantastic solution to overcome mercuric toxicity.

Isolation and identification of the native population bacteria for bioremediation of high levels of arsenic from water resources

Health of millions of people is threatened by the risk of drinking arsenic-contaminated water worldwide. Arsenic naturally conflicts with the concept of life, but recent studies showed that some microorganisms use toxic minerals as the source of energy. Hence, the researchers should consider the development of cost-effective and highly productive procedures to remove arsenic. The current study was conducted on a native bacterial population of Seyed-Jalaleddin Spring Kurdistan, Iran. Accordingly, the arsenic amount in water samples was measured >500 μg/L by the two field and in vitro methods. Water samples were transferred to laboratory and cultured on chemically defined medium (CDM) with arsenic salts. A total of 14 native arsenic-resistant bacterial strains were isolated and after providing pure culture and performing biochemical tests, the isolates were identified using polymerase chain reaction (PCR) and 16s rRNA genomic sequencing. The potential of bacterial strains for the biotransformation of arsenic was assessed by the qualitative assessment of AgNO₃ method and efficiency of arsenic speciation was determined for the first time by silver diethyldithiocarbamate (SDDC) method with an error of less than 5%. Among the isolated strains, only strain As-11 and strain As-12 showed arsenic transformation characteristics and were registered in NCBI database by the access numbers KY119262 and KY119261, respectively. Results of the current study indicated that strain As-11 had the potential of biotransformation of As(V) to As(III) and vice versa with the efficiency of 78% and 48%, respectively. On the other hand, strain As-12 had the potential for biotransformation of As(V) to As(III) and vice versa with the efficiency of 28% and 45%, respectively.

Adsorption of cadmium and lead onto live and dead cell mass of Pseudomonas aeruginosa: A dataset

In this research heavy metals, Cd and Pb, adsorption efficiency was evaluated in aqueous solutions using live and dead biomass of Pseudomonas aeruginosa bacteria. The various important parameters including; pH, temperature, Cd and Pb concentrations, contact time, live and dead cell mass were examined. First, the resistant P. aeruginosa to heavy metals identified and isolated from contaminated soil. Then, the Minimum Inhibitory Concentration (MIC) of Cd and Pb was determined for P. aeruginosa. The highest adsorption efficiency for Cd and Pb were 87% and 98.5%, under dead cell mass of 125 mg, pH 7, temperature 35 °C and contact time 90 min, respectively. The results of this study showed that P. aeruginosa have a high ability to adsorption of Cd and Pb in aqueous solutions.

Biogeochemical cycling of metals impacting by microbial mobilization and immobilization

Microbial mobilization and immobilization processes can affect the bioavailability and mobility of metals thereby influencing their toxicity and can therefore be utilized to treat solid and liquid wastes contaminated by metals. However, the microbial mobilization and immobilization of metals depends on the microbial metabolism, the environment conditions. In this review, mobilization and immobilization of metals are discussed with regard to the presence and function of involved microorganisms and in relation to applications such as bioleaching. Furthermore, the biosorption process is evaluated as a possible approach for microbial immobilization of metal on the basis of four mechanisms: (1) physical adsorption, (2) ion exchange, (3) complexation, and (4) microprecipitation. In addition, sulfide precipitation by sulfate reducing bacteria was included as an example of an application of microbial immobilization. Based on the evaluation and recommendations in this paper, bioremediation strategies for metals can be improved thus increasing the opportunity for field applications.

Bioremediation of cadmium- and zinc-contaminated soil using Rhodobacter sphaeroides

Bioremediation using microorganisms is a promising technique to remediate soil contaminated with heavy metals. In this study, Rhodobacter sphaeroides was used to bioremediate soils contaminated with cadmium (Cd) and zinc (Zn). The study found that the treatment reduced the overall bioavailable fractions (e.g., exchangeable and carbonate bound phases) of Cd and Zn. More stable fractions (e.g., Fe-Mn oxide, organic bound, and residual phases (only for Zn)) increased after bioremediation. A wheat seedling experiment revealed that the phytoavailability of Cd was reduced after bioremediation using R. sphaeroides. After bioremediation, the exchangeable phases of Cd and Zn in soil were reduced by as much as 30.7% and 100.0%, respectively; the Cd levels in wheat leaf and root were reduced by as much as 62.3% and 47.2%, respectively. However, when the soils were contaminated with very high levels of Cd and Zn (Cd 54.97-65.33 mg kg^-1; Zn 813.4-964.8 mg kg^-1), bioremediation effects were not clear. The study also found that R. sphaeroides bioremediation in soil can enhance the Zn/Cd ratio in the harvested wheat leaf and root overall. This indicates potentially favorable application in agronomic practice and biofortification. Although remediation efficiency in highly contaminated soil was not significant, R. sphaeroides may be potentially and practically applied to the bioremediation of soils co-contaminated by Cd and Zn.

Biotechnology in the management and resource recovery from metal bearing solid wastes: Recent advances

Solid metalliferous wastes (sludges, dusts, residues, slags, red mud and tailing wastes) originating from ferrous and non-ferrous metallurgical industries are a serious environmental threat, when waste management practices are not properly followed. Metalliferous wastes generated by metallurgical industries are promising resources for biotechnological extraction of metals. These wastes still contain significant amounts of valuable non-ferrous metals, sometimes precious metals and also rare earth elements. Elemental composition and mineralogy of the metallurgical wastes is dependent on the nature of mining site and composition of primary ores mined. Most of the metalliferous wastes are oxidized in nature and contain less/no reduced sulfidic minerals (which can be quite well processed by biohydrometallurgy). However, application of biohydrometallurgy is more challenging while extracting metals from metallurgical wastes that contain oxide minerals. In this review, origin, elemental composition and mineralogy of the metallurgical solid wastes are presented. Various bio-hydrometallurgical processes that can be considered for the extraction of non-ferrous metals from metal bearing solid wastes are reviewed.