Role of soil rhizobacteria in phytoremediation of heavy metal contaminated soils

Integrating waste valorization and symbiotic microorganisms for sustainable bioremediation of metal(loid)-polluted soils

Remediation strategies for metal(loid)-polluted soils vary among the wide range of approaches, including physical, chemical, and biological remediation, or combinations of these. In this study, we assessed the effectiveness of a set of soil remediation treatments based on the combined application of inorganic (marble sludge) and organic amendments (vermicompost, and dry olive residue [DOR] biotransformed by the saprobic fungi Coriolopsis rigida and Coprinellus radians) and inoculation with arbuscular mycorrhizal fungi (AMFs) (Rhizophagus irregularis and Rhizoglomus custos). The treatments were applied under greenhouse conditions to soil residually polluted by potentially toxic elements (PTEs) (Pb, As, Zn, Cu, Cd, and Sb), and wheat was grown in the amended soils to test the effectiveness of the treatments in reducing soil toxicity and improving soil conditions and plant performance. Therefore, we evaluated the influence of the treatments on the main soil properties and microbial activities, as well as on PTE availability and bioaccumulation in wheat plants. Overall, the results showed a positive influence of all treatments on the main soil properties. Treatments consisting of a combination of marble and organic amendments, especially biotransformed DOR amendments, showed the greatest effectiveness in improving the soil biological status, promoting plant growth and survival, and reducing PTE availability and plant uptake. Furthermore, AMF inoculation further enhanced the efficacy of DOR amendments by promoting the immobilization of PTEs in soil and stimulating the phytostabilization mechanisms induced by AMFs, thus playing an important bioprotective role in plants. Therefore, our results highlight that biotransformed DOR may represent an efficient product for use as a soil organic amendment when remediating metal(loid)-polluted soils, and that its application in combination with AMFs may represent a promising sustainable bioremediation strategy for recovering soil functions and reducing toxicity in polluted areas.

Unveiling the potential of A. fabrum and γ-aminobutyric acid for mitigation of nickel toxicity in fenugreek

Nickel (Ni) is a heavy metal that adversely affects the growth of different crops by inducing oxidative stress and nutrient imbalance. The role of rhizobacteria (RB) is vital to resolve this issue. They can promote root growth and facilitate the uptake of water and nutrients, resulting in better crop growth. On the other hand, γ-aminobutyric acid (GABA) can maintain the osmotic balance and scavenge the reactive oxygen species under stress conditions. However, the combined effect of GABA and RB has not been thoroughly explored to alleviate Ni toxicity, especially in fenugreek plants. Therefore, in the current pot study, four treatments, i.e., control, A. fabrum (RB), 0.40 mM GABA, and 0.40 mM GABA + RB, were applied under 0Ni and 80 mg Ni/kg soil (80Ni) stress. Results showed that RB + 0.40 mM GABA caused significant improvements in shoot length (~ 13%), shoot fresh weight (~ 47%), shoot dry weight (~ 47%), root length (~ 13%), root fresh weight (~ 60%), and root dry weight (~ 15%) over control under 80 Ni toxicity. A significant enhancement in total chlorophyll (~ 14%), photosynthetic rate (~ 17%), stomatal CO2 concentration (~ 19%), leaves and roots N (~ 10 and ~ 37%), P (~ 18 and ~ 7%) and K (~ 11 and ~ 30%) concentrations, while a decrease in Ni (~ 83 and ~ 49%) concentration also confirmed the effectiveness of RB + 0.40 mM GABA than control under 80Ni. In conclusion, fabrum + 0.40 mM GABA can potentially alleviate the Ni toxicity in fenugreek plants. The implications of these findings extend to agricultural practices, environmental remediation efforts, nutritional security, and ecological impact. Further research is recommended to elucidate the underlying mechanisms, assess long-term effects, and determine the practical feasibility of using A. fabrum + 0.40GABA to improve growth in different crops under Ni toxicity.

Lead and copper removal from sterile dumps by phytoremediation with Robinia pseudoacacia

In Romania, huge quantities of gangue material from the mining activity practiced in the past were improperly stored and led to the pollution of the environment. Thus, this work is framed to manage the sterile dump of the "Radeș" mine (Alba, Romania) through a 12-week phytoremediation process. The efficient use of Robinia pseudoacacia was studied through the implementation, at the laboratory level, of a phytoremediation experiment based on various variants prepared by mixtures of gangue material, uncontaminated soil, and dehydrated sludge. The prepared variants, all planted with R. pseudoacacia, were watered with tap water, potassium monobasic phosphate, and enzyme solution. The bioconcentration and translocation factors for lead showed values ˂ 1, which indicates a potential presence of an exclusion system for Pb or a reduced Pb bioavailability since the R. pseudoacacia accumulates high concentrations of metals absorbed on and inside the roots. For copper, both factors had values > 1 indicating the suitability of R. pseudoacacia to readily translocate copper into the epigean organs. In the investigated experimental conditions, the highest efficiency in the removal of copper (93.0%) and lead (66.4%) by plants was obtained when gangue material was not mixed with other materials and wetted with enzymatic solution.

Role of Plant-Growth-Promoting Rhizobacteria in Plant Machinery for Soil Heavy Metal Detoxification

Heavy metals migrate easily and are difficult to degrade in the soil environment, which causes serious harm to the ecological environment and human health. Thus, soil heavy metal pollution has become one of the main environmental issues of global concern. Plant-growth-promoting rhizobacteria (PGPR) is a kind of microorganism that grows around the rhizosphere and can promote plant growth and increase crop yield. PGPR can change the bioavailability of heavy metals in the rhizosphere microenvironment, increase heavy metal uptake by phytoremediation plants, and enhance the phytoremediation efficiency of heavy-metal-contaminated soils. In recent years, the number of studies on the phytoremediation efficiency of heavy-metal-contaminated soil enhanced by PGPR has increased rapidly. This paper systematically reviews the mechanisms of PGPR that promote plant growth (including nitrogen fixation, phosphorus solubilization, potassium solubilization, iron solubilization, and plant hormone secretion) and the mechanisms of PGPR that enhance plant-heavy metal interactions (including chelation, the induction of systemic resistance, and the improvement of bioavailability). Future research on PGPR should address the challenges in heavy metal removal by PGPR-assisted phytoremediation.

Exogenous Ca/Mg quotient reduces the inhibitory effects of PEG induced osmotic stress on Avena sativa L

Abstract Drought is one of the most damaging abiotic stress that hinder plant growth and development. The present study aimed to determine the effects of various Ca/Mg quotients under polyethylene glycol (PEG)–induced osmotic stress on growth, uptake and translocation of Ca and Mg in Avena sativa (L). Plants were grown in nutrient solution supplemented with three different Ca/Mg molar quotients (0.18, 2, and 4). After 30 days plants were exposed to two different PEG (Polyethylene glycol) concentrations (0.6 MPa & 0.2 MPa) for 8 days, and solutions were renewed after 4 days. A solution containing Ca and Mg nutrients has mitigated the negative impact caused via osmotic stress on relative growth rate (RGR), absolute growth rate (AGR), crop growth rate (CGR), leaf area ratio (LAR), Leaf index ratio (LAI), root-shoot ratio (RSR), water use efficiency (WUE) and net assimilation rate (NAR). In addition, it adversely affected germination parameters, including final emergence percentage (FEP), mean germination time (MGT), Timson germination Index (TGI), germination rate index (GRI) and percent field capacity (%FC), of oat (Avena sativa L.). Mg and Ca in shoot and root and Ca translocation factor decreased with increasing Ca in solution, while Mg translocation factor increased with increasing Ca in nutrient solution. In this work, the combined effects of various Ca/Mg quotients and osmotic stress produced by polyethylene glycol (PEG) in different concentrations (0.6 MPa, 0.2 MPa) on the growth and element uptake of Avena sativa L. are examined. As a result, the Ca/Mg Quotient may naturally combat the moderate drought stress experienced by field crops.

Rhizospheric bacteria: the key to sustainable heavy metal detoxification strategies

The increasing rate of industrialization, anthropogenic, and geological activities have expedited the release of heavy metals (HMs) at higher concentration in environment. HM contamination resulting due to its persistent nature, injudicious use poses a potential threat by causing metal toxicities in humans and animals as well as severe damage to aquatic organisms. Bioremediation is an emerging and reliable solution for mitigation of these contaminants using rhizospheric microorganisms in an environmentally safe manner. The strategies are based on exploiting microbial metabolism and various approaches developed by plant growth promoting bacteria (PGPB) to minimize the toxicity concentration of HM at optimum levels for the environmental clean-up. Rhizospheric bacteria are employed for significant growth of plants in soil contaminated with HM. Exploitation of bacteria possessing plant-beneficial traits as well as metal detoxifying property is an economical and promising approach for bioremediation of HM. Microbial cells exhibit different mechanisms of HM resistance such as active transport, extra cellular barrier, extracellular and intracellular sequestration, and reduction of HM. Tolerance of HM in microorganisms may be chromosomal or plasmid originated. Proteins such as MerT and MerA of mer operon and czcCBA, ArsR, ArsA, ArsD, ArsB, and ArsC genes are responsible for metal detoxification in bacterial cell. This review gives insights about the potential of rhizospheric bacteria in HM removal from various polluted areas. In addition, it also gives deep insights about different mechanism of action expressed by microorganisms for HM detoxification. The dual-purpose use of biological agent as plant growth enhancement and remediation of HM contaminated site is the most significant future prospect of this article.

Soil Microbial Community Composition and Tolerance to Contaminants in an Urban Brownfield Site

Brownfields are unused sites that contain hazardous substances due to previous commercial or industrial use. The sites are inhospitable for many organisms, but some fungi and microbes can tolerate and thrive in the nutrient-depleted and contaminated soils. However, few studies have characterized the impacts of long-term contamination on soil microbiome composition and diversity at brownfields. This study focuses on an urban brownfield-a former rail yard in Los Angeles that is contaminated with heavy metals, volatile organic compounds, and petroleum-derived pollutants. We anticipate that heavy metals and organic pollutants will shape soil microbiome diversity and that several candidate fungi and bacteria will be tolerant to the contaminants. We sequence three gene markers (16S ribosomal RNA, 18S ribosomal RNA, and the fungal internal transcribed spacer (FITS)) in 55 soil samples collected at five depths to (1) profile the composition of the soil microbiome across depths; (2) determine the extent to which hazardous chemicals predict microbiome variation; and (3) identify microbial taxonomic groups that may metabolize these contaminants. Detected contaminants in the samples included heavy metals, petroleum hydrocarbons, polycyclic aromatic hydrocarbons, and volatile organic compounds. Bacterial, eukaryotic, and fungal communities all varied with depth and with concentrations of arsenic, chromium, cobalt, and lead. 18S rRNA microbiome richness and fungal richness were positively correlated with lead and cobalt levels, respectively. Furthermore, bacterial Paenibacillus and Iamia, eukaryotic Actinochloris, and fungal Alternaria were enriched in contaminated soils compared to uncontaminated soils and represent taxa of interest for future bioremediation research. Based on our results, we recommend incorporating DNA-based multi-marker microbial community profiling at multiple sites and depths in brownfield site assessment standard methods and restoration.

The Effect of Cadmium Tolerant Plant Growth Promoting Rhizobacteria on Plant Growth Promotion and Phytoremediation: A Review

Cadmium (Cd) is a heavy metal of considerable toxicity with destructive impacts on plants, microbes and environments. Its toxicity is due to mishandling and manual hazards in plants and is primarily observed within the soil to cause decline of plants and microbial activity inside the rhizosphere. Cadmium accumulation in crops and the probability of Cd entering the food chain are grave for public health in the worldwide. Cadmium toxicity leads to depletion in seed germination, initial seedling growth, plant biomass, chlorosis, necrosis, hindrance of photosynthetic machinery and other physiological and biological activities in plants. Cadmium triggers the reactive oxygen species (ROS) that influences gene mutation and DNA damage that affects the cell cycle and cell division. Cd toxicity altered the levels of phenolic compounds, carbohydrates, glycine betaine, proline and organic acids in crops. Under stress conditions, the plant growth promoting rhizobacteria (PGPR) have various properties such as enzymatic activities, plant growth hormones production, phosphate solubilization, siderophores production and chelating agents that help the plants tolerate against Cd stress and also increase phenolic compound levels and osmolytes. Hence, this review highlights the crucial role of cadmium tolerant PGPR for crop production, declining metal phytoavailability and enhancing morphological and physiological boundaries of plants under stress conditions. It could be an environment friendly and cost effective technology under sustainable crop production.

Bio-efficacy of Geobacillus thermodenitrificans PS41 against larvicidal, fungicidal, and plant growth-promoting activities

The microbial interactions with plant hosts were known to establish plant growth and beneficial productivity. Some bacteria, fungi, actinomycetes, yeast, and algae have proven as potential effective microbes in agricultural field. In this study, the insecticidal effect of Geobacillus thermodenitrificans PS41 secondary metabolites was tested against third instar larvae of Spodoptera litura, with mortality rate 60.26 ± 1.5% which might influence the agropest management. The test bacterial metabolites were subjected to GC-MS analysis. Totally, 17 different compounds were identified from the ethyl acetate extract metabolites of PS41 strain. The highest peak was obtained with behenic alcohol compound followed by 1-octadecene and penta erythrityl tetrachloride. The Geobacillus thermodenitrificans PS41 secondary metabolites showed potential antifungal activity against plant pathogenic fungi. The highest inhibit was attained against Cladosporium sp., (25 mm) followed by Rhizoctonia solani and Alternaria brassicola (23 mm). However, no toxic effect was exerted upon earthworm (Perionyx excavatus) when treated with PS41 bacterial metabolites. The potential PS41 strain was also found supporting the plant growth. The potential bacterial strain PS41 did not show antagonistic activity against soil bacteria such as Rhizobium sp., Azotobacter sp., Azospirullum brasilense, Pseudomonas fluorescens and Bacillus megaterium. The potential test organism, Geobacillus thermodenitrificans PS41, possessing biopesticide and biofertilizer properties can be a suitable ecofriendly organic applicant in agricultural field for enhancing crop production.

Plant growth promoting rhizobacteria modulates the antioxidant defense and the expression of stress-responsive genes providing Pb accumulation and tolerance of grass pea

To ensure the success of phytoremediation, it is important to consider the appropriate combination of plants and microorganisms. This study was conducted to get a better insight into the underlying molecular and biochemical mechanism of grass pea (Lathyrus sativus L.) induced by plant growth promoting rhizobacteria (PGPR), when exposed for 3, 6, 9, and 14 days to 1 mM Pb in a hydroponic system. The significant positive effect of bacterial inoculation was reproduced in various parameters. Results indicated that inoculation of PGPR significantly increased the accumulation of Pb by 20%, 66%, 43%, and 36% in roots and by 46%, 55%, 37%, and 46% in shoots, respectively after 3, 6, 9, and 14 days of metal exposure compared to the uninoculated plants. The metal accumulation in grass pea plants triggered a significant elevation in the synthesis of non-protein thiols (NPT), particularly in inoculated plant leaves where it was about 3 and 2-fold higher than the uninoculated set on the 6th and the 9th day. Nevertheless, Pb treatment significantly increased oxidative stress and membrane damage in leaves with the highest hydrogen peroxide (H2O2) production and tissue malondialdehyde (MDA) concentration recorded in uninoculated plants. Furthermore, the PGPR inoculation alleviated the oxidative stress, improved significantly plant tolerance, and modulated the activities of antioxidant enzymes (SOD, CAT, APX, GR, DHAR, and MDHAR). Similarly, the expression patterns of LsPCS, LsGCN, LsCNGC, LsGR, and LsGST through qRT-PCR demonstrated that bacterial inoculation significantly induced gene expression levels in leaves 6 days after Pb treatment, indicating that PGPR act as regulators of stress-responsive genes. The findings suggest the key role of PGPR (R. leguminosarum (M5) + Pseudomonas fluorescens (K23) + Luteibacter sp. + Variovorax sp.) in enhancing Pb accumulation, reducing metal toxicity, strengthening of the antioxidant system, and conferring higher Pb tolerance to grass pea plants. Hence, the association Lathyrus sativus-PGPR is an effective tool to achieve the goal of remediation of Pb contaminated sites.

Signaling and Detoxification Strategies in Plant-Microbes Symbiosis under Heavy Metal Stress: A Mechanistic Understanding

Plants typically interact with a variety of microorganisms, including bacteria, mycorrhizal fungi, and other organisms, in their above- and below-ground parts. In the biosphere, the interactions of plants with diverse microbes enable them to acquire a wide range of symbiotic advantages, resulting in enhanced plant growth and development and stress tolerance to toxic metals (TMs). Recent studies have shown that certain microorganisms can reduce the accumulation of TMs in plants through various mechanisms and can reduce the bioavailability of TMs in soil. However, relevant progress is lacking in summarization. This review mechanistically summarizes the common mediating pathways, detoxification strategies, and homeostatic mechanisms based on the research progress of the joint prevention and control of TMs by arbuscular mycorrhizal fungi (AMF)-plant and Rhizobium-plant interactions. Given the importance of tripartite mutualism in the plant-microbe system, it is necessary to further explore key signaling molecules to understand the role of plant-microbe mutualism in improving plant tolerance under heavy metal stress in the contaminated soil environments. It is hoped that our findings will be useful in studying plant stress tolerance under a broad range of environmental conditions and will help in developing new technologies for ensuring crop health and performance in future.

Application of microbe-impregnated tannery solid waste biochar in soil enhances growth performance of sunflower

Synergistic effect of biochar and microbes in soil enhances performance of plants. Hazardous tannery solid waste can be reduced by one-third in volume by conversion to biochar. A greenhouse trial was set up with soil having different doses of metal resistant microbe-impregnated biochar (MIBC) prepared from tannery solid waste. Consortia of autochthonous strains of Trichoderma and Bacillus were inoculated on BC and the behavior and fate of metals were evaluated for their bioavailability to sunflower. Sunflower was grown in pots for 80 days having six different amendments of tannery solid waste biochar (0-10% w/w) with and without Trichoderma and Bacillus consortia and its morphological and biochemical attributes as well as metal uptake were observed. The results illustrated that application of BC at 2% rate without inoculation increased the shoot length and dry biomass by 19.8% and 77.4%, respectively, while plant growth and performance were reduced at higher amendments of BC. However, application of MIBC with Trichoderma or/and Bacillus consortium significantly improved the plant attributes at all levels of amendment. The results indicated that MIBC having Trichoderma and Bacillus consortia at 10% rate increased shoot length and dry biomass by 65.3% and 516% compared to control without BC. Application of BC without inoculation reduced the uptake of Cu, Fe, and Ni and increased the mobilization of all other metals for uptake in sunflower. Mobilization and uptake of Cd, Cr, Cu, Ni, Pb, and Zn decreased with MIBC having Trichoderma and Bacillus consortia whereas that of Fe and Mg were noted. A considerable decrease in proline and total phenolic content was demonstrated by MIBC-grown sunflower. The data of metal fractionation in BC also supported the above findings. Therefore, MIBC can be used as a promising option for enhancing growth performance and ensuring the physiological safety of sunflower as an energy crop.

Helping plants to deal with heavy metal stress: the role of nanotechnology and plant growth promoting rhizobacteria in the process of phytoremediation

Heavy metals (HMs) are not destroyable or degradable and persist in the environment for a long duration. Thus, eliminating and counteracting the HMs pollution of the soil environment is an urgent task to develop a safe and sustainable environment. Plants are in close contact with the soil and can play an important role in soil clean-up, and the process is known as phytoremediation. However, under HM contaminated conditions, plants suffer from several complications, like nutrient and mineral deficiencies, alteration of various physiological and biological processes, which reduces the plant's growth rate. On the other hand, the bioavailability of HMs is another factor for reduced phytoremediation, as most of the HMs are not bioavailable to plants for efficient phytoremediation. The altered plant growth and reduced bioavailability of HMs could be overcome and enhance the phytoremediation efficiency by incorporating either nanotechnology, i.e., nanoparticles (NPs) or plant growth promoting rhizobacteria (PGPR) along with phytoremediation. Single incorporation of NPs and PGPR might improve the growth rate in plants by enhancing nutrient availability and uptake and also by regulating plant growth regulators under HM contaminated conditions. However, there are certain limitations, like a high dose of NPs that might have toxic effects on plants. Thus, the combination of two techniques such as PGPR and NPs-based remediation can conquer the limitations of individual techniques and consequently enhance phytoremediation efficiency. Considering the negative impacts of HMs on the environment and living organisms, this review is aimed at highlighting the concept of phytoremediation, the single or combined integration of NPs and PGPR to help plants deal with HMs and their basic mechanisms involved in the process of phytoremediation. Additionally, the complications of using NPs and PGPR in the phytoremediation process are discussed to determine future research questions and this will assist to stimulate further research in this field and increase its effectiveness in practical application.

Azospirillum spp. from Plant Growth-Promoting Bacteria to Their Use in Bioremediation

Xenobiotic contamination, a worldwide environmental concern, poses risks for humans, animals, microbe health, and agriculture. Hydrocarbons and heavy metals top the list of toxins that represent a risk to nature. This review deals with the study of Azospirillum sp., widely reported as plant growth-promoting bacteria in various cultures. However, its adaptation properties in adverse environments make it a good candidate for studying remediation processes in environments polluted with hydrocarbons and heavy metals. This review includes studies that address its properties as a plant growth promoter, its genomics, and that evaluate its potential use in the remediation of hydrocarbons and heavy metals.

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.

Impact of Soil Inoculation with Bacillus amyloliquefaciens FZB42 on the Phytoaccumulation of Germanium, Rare Earth Elements, and Potentially Toxic Elements

Bioaugmentation promises benefits for agricultural production as well as for remediation and phytomining approaches. Thus, this study investigated the effect of soil inoculation with the commercially available product RhizoVital^®42, which contains Bacillus amyloliquefaciens FZB42, on nutrient uptake and plant biomass production as well as on the phytoaccumulation of potentially toxic elements, germanium, and rare earth elements (REEs). Zea mays and Fagopyrum esculentum were selected as model plants, and after harvest, the element uptake was compared between plants grown on inoculated versus reference soil. The results indicate an enrichment of B. amyloliquefaciens in inoculated soils as well as no significant impact on the inherent bacterial community composition. For F. esculentum, inoculation increased the accumulation of most nutrients and As, Cu, Pb, Co, and REEs (significant for Ca, Cu, and Co with 40%, 2042%, and 383%, respectively), while it slightly decreased the uptake of Ge, Cr, and Fe. For Z. mays, soil inoculation decreased the accumulation of Cr, Pb, Co, Ge, and REEs (significant for Co with 57%) but showed an insignificant increased uptake of Cu, As, and nutrient elements. Summarily, the results suggest that bioaugmentation with B. amyloliquefaciens is safe and has the potential to enhance/reduce the phytoaccumulation of some elements and the effects of inoculation are plant specific.

Analysis of soil bacterial communities and physicochemical properties associated with Fusarium wilt disease of banana in Malaysia

Fusarium wilt (FW) caused by Fusarium oxysporum f. sp. cubense Tropical Race 4 (TR4) is a soil-borne disease that infects bananas, causing severe economic losses worldwide. To reveal the relationship between bacterial populations and FW, the bacterial communities of healthy and TR4-infected rhizosphere and bulk soils were compared using 16S rRNA gene sequencing. Soil physicochemical properties associated with FW were also analyzed. We found the community structure of bacteria in the healthy and TR4 infected rhizosphere was significantly different compared to bulk soil within the same farm. The rhizosphere soils of infected plants exhibited higher richness and diversity than healthy plant with significant abundance of Proteobacteria. In the healthy rhizosphere soil, beneficial bacteria such as Burkholderia and Streptomyces spp. were more abundant. Compared to the infected rhizosphere soil, healthy rhizosphere soil was associated with RNA metabolism and transporters pathways and a high level of magnesium and cation exchange capacity. Overall, we reported changes in the key taxa of rhizospheric bacterial communities and soil physicochemical properties of healthy and FW-infected plants, suggesting their potential role as indicators for plant health.

Prospect and Challenges for Sustainable Management of Climate Change-Associated Stresses to Soil and Plant Health by Beneficial Rhizobacteria

Climate change imposes biotic and abiotic stresses on soil and plant health all across the planet. Beneficial rhizobacterial genera, such as Bacillus, Pseudomonas, Paraburkholderia, Rhizobium, Serratia, and others, are gaining popularity due to their ability to provide simultaneous nutrition and protection of plants in adverse climatic conditions. Plant growth-promoting rhizobacteria are known to boost soil and plant health through a variety of direct and indirect mechanisms. However, various issues limit the wider commercialization of bacterial biostimulants, such as variable performance in different environmental conditions, poor shelf-life, application challenges, and our poor understanding on complex mechanisms of their interactions with plants and environment. This study focused on detecting the most recent findings on the improvement of plant and soil health under a stressful environment by the application of beneficial rhizobacteria. For a critical and systematic review story, we conducted a non-exhaustive but rigorous literature survey to assemble the most relevant literature (sorting of a total of 236 out of 300 articles produced from the search). In addition, a critical discussion deciphering the major challenges for the commercialization of these bioagents as biofertilizer, biostimulants, and biopesticides was undertaken to unlock the prospective research avenues and wider application of these natural resources. The advancement of biotechnological tools may help to enhance the sustainable use of bacterial biostimulants in agriculture. The perspective of biostimulants is also systematically evaluated for a better understanding of the molecular crosstalk between plants and beneficial bacteria in the changing climate towards sustainable soil and plant health.

Assessment of the Streptomyces-plant system to mitigate the impact of Cr(VI) and lindane in experimental soils

Phytoremediation techniques have been proposed as ecological methods to clean up contaminated sites. This study is aimed to evaluate the effect of the Streptomyces sp. Waksman & Henrici and Zea mays L. plant system on the dissipation of Cr(VI) and/or lindane from a co-contaminated soil, being 2 mg kg^-1 of lindane and 150 mg kg^-1 of chromium used. Lindane dissipation was improved in the presence of plant-microorganism association; however, Cr(VI) removal was higher when plants or the microorganism were separately. In co-contaminated systems, chromium content in plant tissues was lower than metal content in plants grown only with Cr(VI), suggesting that lindane could interfere with metal accumulation in the plant. The high malondialdehyde (MDA) concentration detected in non-inoculated plants grown with chromium could be consequence of high metal concentration in plant tissues. Interestingly, plants inoculated with Streptomyces sp. Z38 growing with Cr(VI) showed decrease in MDA concentration, indicating that the bacterium could activate defense mechanisms in the plant. Also, inoculated plants showed the highest value of superoxide dismutase activity. Lettuce plants used as bioindicators grew better in biologically treated soils compared with lettuce grown on non-treated soil. The results presented in this work provide the basis that will allow the optimization of future trials on a larger scale.

Rhizosphere Bacteria in Plant Growth Promotion, Biocontrol, and Bioremediation of Contaminated Sites: A Comprehensive Review of Effects and Mechanisms

Agriculture in the 21st century is facing multiple challenges, such as those related to soil fertility, climatic fluctuations, environmental degradation, urbanization, and the increase in food demand for the increasing world population. In the meanwhile, the scientific community is facing key challenges in increasing crop production from the existing land base. In this regard, traditional farming has witnessed enhanced per acre crop yields due to irregular and injudicious use of agrochemicals, including pesticides and synthetic fertilizers, but at a substantial environmental cost. Another major concern in modern agriculture is that crop pests are developing pesticide resistance. Therefore, the future of sustainable crop production requires the use of alternative strategies that can enhance crop yields in an environmentally sound manner. The application of rhizobacteria, specifically, plant growth-promoting rhizobacteria (PGPR), as an alternative to chemical pesticides has gained much attention from the scientific community. These rhizobacteria harbor a number of mechanisms through which they promote plant growth, control plant pests, and induce resistance to various abiotic stresses. This review presents a comprehensive overview of the mechanisms of rhizobacteria involved in plant growth promotion, biocontrol of pests, and bioremediation of contaminated soils. It also focuses on the effects of PGPR inoculation on plant growth survival under environmental stress. Furthermore, the pros and cons of rhizobacterial application along with future directions for the sustainable use of rhizobacteria in agriculture are discussed in depth.

Nickel stress-tolerance in plant-bacterial associations

Nickel (Ni) is an essential element for plant growth and is a constituent of several metalloenzymes, such as urease, Ni-Fe hydrogenase, Ni-superoxide dismutase. However, in high concentrations, Ni is toxic and hazardous to plants, humans and animals. High levels of Ni inhibit plant germination, reduce chlorophyll content, and cause osmotic imbalance and oxidative stress. Sustainable plant-bacterial native associations are formed under Ni-stress, such as Ni hyperaccumulator plants and rhizobacteria showed tolerance to high levels of Ni. Both partners (plants and bacteria) are capable to reduce the Ni toxicity and developed different mechanisms and strategies which they manifest in plant-bacterial associations. In addition to physical barriers, such as plants cell walls, thick cuticles and trichomes, which reduce the elevated levels of Ni entrance, plants are mitigating the Ni toxicity using their own antioxidant defense mechanisms including enzymes and other antioxidants. Bacteria in its turn effectively protect plants from Ni stress and can be used in phytoremediation. PGPR (plant growth promotion rhizobacteria) possess various mechanisms of biological protection of plants at both whole population and single cell levels. In this review, we highlighted the current understanding of the bacterial induced protective mechanisms in plant-bacterial associations under Ni stress.

Microbe-assisted phytoremediation of environmental pollutants and energy recycling in sustainable agriculture

The perception of phytoremediation is efficiently utilized as an eco-friendly practice of green plants combating and cleaning up the stressed environment without harming it. The industrial revolution was followed by the green revolution which fulfilled the food demands of the growing population caused an increase in yield per unit area in crop production, but it also increased the use of synthetic fertilizers in agriculture. Globally, the intensive use of inorganic fertilizers in agriculture has led to serious health problems and irreversible environmental damage. Biofertilizers improve the growth of the plant and can be applied as an alternative to chemical/synthetic fertilizers. Cyanobacteria, bacteria, and fungi are known as some of the principal microbe groups used to produce biofertilizers that form symbiotic associations with plants. Microorganisms perform a key role in phosphate solubilization and mobilization, nitrogen fixation, nutrient management, biotic elicitors and probiotics, and pollution management (biodegradation agents), specifically bacteria which also help in atmospheric nitrogen fixation and are thus available for the growth of the plant. Management or biodegradation of hazardous chemical residues and heavy metals produced by a huge number of large-scale industries should be given primary importance to be transformed by various bacterial strains, fungi, algae. Currently, modern omics technologies such as metagenomic, transcriptomic, and proteomic are being used to develop strategies for studying the ecology of microorganisms, as well as their use in environmental monitoring and bioremediation. This review briefly discusses some of the major groups of microorganisms that can perform different functions responsible for plant health, crop production, phytoremediation and also focus on the omics techniques reportedly used in environmental monitoring to tackle the pollution load.

Combined application of two Bacillus species enhance phytoremediation potential of Brassica napus in an industrial metal-contaminated soil

This study aimed to assess the impact of individual as well as combined application of Lysinibacillus macroides and Bacillus safensis in phytoremediation potential of Brassica napus grown in soil contaminated by industrial effluents. In response to five metals; copper, chromium, nickel, lead, and cadmium, results revealed that germination percentage, fresh and dry weights, and photosynthetic pigments of B. napus decreased under contaminated soil. On the other hand, electrolyte leakage due to cellular injury, metabolites (proline and glycine betaine), antioxidant enzymes (superoxide dismutase, catalase, peroxidase, ascorbate peroxidase), accumulation of hydrogen peroxide and metals in plant's roots, shoots and leaves increased. Inoculation significantly reduced these effects as proved by the enhancement of germination percentage, fresh and dry biomass, and photosynthetic pigments. Simultaneously, the antioxidant enzymes, metabolites contents (proline and glycine betaine) and metal concentrations in plant's roots, shoots and leaves decreased. Combined application of both Bacilli strains was found more effective as compared to individual inoculation. It was concluded that metal resistant Bacillus species in combination had growth effects on B. napus and enhanced its phytoremediation efficiency in contaminated soil.Novelty statementBrassica napus; a hyper-accumulator of metals, loses phytoremediation potential with the passage of growth. Two Bacillus species (Lysinibacillus macroides and Bacillus safensis) having known bioremediation abilities were employed individually as well as in combination under metals contaminated soil to increase phytoremediation efficiency of B. napus. The metals containing soil used is a unique aspect in this study because selected soil, contaminated by industrial effluents, has not been evaluated or reported earlier. Combined application of Bacilli improved phytoremediation potential of B. napus more as compared to application of individual Bacillus strain which is yet another unique aspect of this investigation.

Unraveling the Underlying Heavy Metal Detoxification Mechanisms of Bacillus Species

The rise of anthropogenic activities has resulted in the increasing release of various contaminants into the environment, jeopardizing fragile ecosystems in the process. Heavy metals are one of the major pollutants that contribute to the escalating problem of environmental pollution, being primarily introduced in sensitive ecological habitats through industrial effluents, wastewater, as well as sewage of various industries. Where heavy metals like zinc, copper, manganese, and nickel serve key roles in regulating different biological processes in living systems, many heavy metals can be toxic even at low concentrations, such as mercury, arsenic, cadmium, chromium, and lead, and can accumulate in intricate food chains resulting in health concerns. Over the years, many physical and chemical methods of heavy metal removal have essentially been investigated, but their disadvantages like the generation of chemical waste, complex downstream processing, and the uneconomical cost of both methods, have rendered them inefficient,. Since then, microbial bioremediation, particularly the use of bacteria, has gained attention due to the feasibility and efficiency of using them in removing heavy metals from contaminated environments. Bacteria have several methods of processing heavy metals through general resistance mechanisms, biosorption, adsorption, and efflux mechanisms. Bacillus spp. are model Gram-positive bacteria that have been studied extensively for their biosorption abilities and molecular mechanisms that enable their survival as well as their ability to remove and detoxify heavy metals. This review aims to highlight the molecular methods of Bacillus spp. in removing various heavy metals ions from contaminated environments.

Insights into the Interactions among Roots, Rhizosphere, and Rhizobacteria for Improving Plant Growth and Tolerance to Abiotic Stresses: A Review

Abiotic stresses, such as drought, salinity, heavy metals, variations in temperature, and ultraviolet (UV) radiation, are antagonistic to plant growth and development, resulting in an overall decrease in plant yield. These stresses have direct effects on the rhizosphere, thus severely affect the root growth, and thereby affecting the overall plant growth, health, and productivity. However, the growth-promoting rhizobacteria that colonize the rhizosphere/endorhizosphere protect the roots from the adverse effects of abiotic stress and facilitate plant growth by various direct and indirect mechanisms. In the rhizosphere, plants are constantly interacting with thousands of these microorganisms, yet it is not very clear when and how these complex root, rhizosphere, and rhizobacteria interactions occur under abiotic stresses. Therefore, the present review attempts to focus on root–rhizosphere and rhizobacterial interactions under stresses, how roots respond to these interactions, and the role of rhizobacteria under these stresses. Further, the review focuses on the underlying mechanisms employed by rhizobacteria for improving root architecture and plant tolerance to abiotic stresses.

Exploring the global research trends in biofertilizers: a bibliometric approach

This research article attempts a bibliometric analysis of global research on biofertilizers carried out from 2000 to 2019. The main purpose of this analysis is in technology foresight; to understand where the research interest lies within the domain of biofertilizer and also to identify the major research networks. The analysis is based on 344 research articles identified using the ISI Web of Science tool, which is processed further using VOSviewer. The results demonstrated that there is an increase in the number of articles, particularly from countries like Brazil, India China, the USA, and Iran. The research focus has been on the assessment of nitrogen fixation capacity of biofertilizers, and the yield improvement due to biofertilizers, and the economics of biofertilizer application. Our findings can act as a useful reference for the researchers, and provide insights for directing future research on biofertilizers.

Rhizospheric Bacillus amyloliquefaciens Protects Capsicum annuum cv. Geumsugangsan From Multiple Abiotic Stresses via Multifarious Plant Growth-Promoting Attributes

Plant growth-promoting rhizobacteria (PGPR) are beneficial microorganisms that can be utilized to improve plant responses against biotic and abiotic stresses. In this study, we investigated whether PGPR (Bacillus amyloliquefaciens) isolated from the endorhizosphere of Sasamorpha borealis have the potential to sustain pepper growth under drought, salinity, and heavy metal stresses. The bacterial strain was determined based on 16S rDNA and gyrB gene sequencing and characterized based on the following biochemical traits: nitrogen fixation; 1-aminocyclopropane-1-carboxylate deaminase activity; indole acetic acid production; inorganic phosphate, potassium, zinc, and silicon solubilization; and siderophore production. Various abiotic stresses were applied to 28-day-old pepper seedlings, and the influence of the PGPR strain on pepper seedling growth under these stress conditions was evaluated. The application of PGPR improved survival of the inoculated pepper plants under stress conditions, which was reflected by higher seedling growth rate and improved physiochemical traits. The PGPR-treated plants maintained high chlorophyll, salicylic acid, sugar, amino acid, and proline contents and showed low lipid metabolism, abscisic acid, protein, hydrogen peroxide contents, and antioxidant activities under stress conditions. Gene expression studies confirmed our physiological and biochemical findings. PGPR inoculation led to enhanced expression of XTH genes and reduced expression of WRKY2, BI-1, PTI1, and binding immunoglobulin protein (BiP) genes. We conclude that the PGPR strain described in this study has great potential for use in the phytoremediation of heavy metals and for enhancing pepper plant productivity under stress conditions, particularly those involving salinity and drought.

16S rRNA gene profiling of rhizospheric microbial community of Eichhornia crassipes

The rhizosphere of a plant is an important interface for the plant-microbe interaction that plays a significant role in the uptake and removal of heavy metal from contaminated sites. Eichhornia crassipes is a free-floating macrophyte and a well-known metal hyperaccumulator. It is a promising plant, which harbors a diverse microbial community in its rhizosphere. Therefore it is hypothesized that it can be a good habitat for microorganisms that supports plant growth and increases its phytoremediation potential. The rhizospheric DNA was extracted from the procured plant samples. The library was prepared and sequenced using the Illumina platform. 16S rRNA data from the Next Generation Sequencing (NGS) platform was analyzed using the QIIME software package. Alpha diversity was estimated from statistical indices i.e. Shannon index, Chao1 index, and observed species. The rarefaction plots, rank abundance curve, krona graph, and heat map were generated to study the rhizospheric community in detail. Metagenome consisted of 225,408 flash reads, 185,008 non-chimeric sequences with 17,578 Operational Taxonomic Units (OTU's), and 4622 OTU's without singletons. The data of present study are available at NCBI Bioproject (PRJNA631882). The taxonomic analysis of OTU's showed that the sequences belonged to major Phyla revealing the dominance of Proteobacteria, Bacteroidetes, Cyanobacteria, and Verrucomicrobia. The most abundant Genera in the sampled rhizosphere recorded were Thiothrix and Flavobacterium.

Evaluation potential of PGPR to protect tomato against Fusarium wilt and promote plant growth

Soilborne fungal diseases are most common among vegetable crops and have major implications for crop yield and productivity. Eco-friendly sustainable agriculture practices that can overcome biotic and abiotic stresses are of prime importance. In this study, we evaluated the ability of plant growth-promoting rhizobacterium (PGPR) Bacillus aryabhattai strain SRB02 to control the effects of tomato wilt disease caused by Fusarium oxysporum f. sp. lycopersici (strain KACC40032) and promote plant growth. In vitro bioassays showed significant inhibition of fungal growth by SRB02. Inoculation of susceptible and tolerant tomato cultivars in the presence of SRB02 showed significant protection of the cultivar that was susceptible to infection and promotion of plant growth and biomass production in both of the cultivars. Further analysis of SRB02-treated plants revealed a significantly higher production of amino acids following infection by F. oxysporum. Analysis of plant defense hormones after inoculation by the pathogen revealed a significantly higher accumulation of salicylic acid (SA), with a concomitant reduction in jasmonic acid (JA). These results indicate that B. aryabhattai strain SRB02 reduces the effects of Fusarium wilt disease in tomato by modulating endogenous phytohormones and amino acid levels.

Phytoextraction of Lead Using a Hedge Plant [Alternanthera bettzickiana (Regel) G. Nicholson]: Physiological and Biochemical Alterations through Bioresource Management

Phytoremediation is a cost-effective and environmentally friendly approach that can be used for the remediation of metals in polluted soil. This study used a hedge plant–calico (Alternanthera bettzickiana (Regel) G. Nicholson) to determine the role of citric acid in lead (Pb) phytoremediation by exposing it to different concentrations of Pb (0, 200, 500, and 1000 mg kg−1) as well as in a combination with citric acid concentration (0, 250, 500 µM). The analysis of variance was applied on results for significant effects of the independent variables on the dependent variables using SPSS (ver10). According to the results, maximum Pb concentration was measured in the upper parts of the plant. An increase in dry weight biomass, plant growth parameters, and photosynthetic contents was observed with the increase of Pb application (200 mg kg−1) in soil while a reduced growth was experienced at higher Pb concentration (1000 mg kg−1). The antioxidant enzymatic activities like superoxide dismutase (SOD) and peroxidase (POD) were enhanced under lower Pb concentration (200, 500 mg kg−1), whereas the reduction occurred at greater metal concentration Pb (1000 mg kg−1). There was a usual reduction in electrolyte leakage (EL) at lower Pb concentration (200, 500 mg kg−1), whereas EL increased at maximum Pb concentration (1000 mg kg−1). We concluded that this hedge plant, A. Bettzickiana, has the greater ability to remediate polluted soils aided with citric acid application.

Perspectives of future water sources in Qatar by phytoremediation: biodiversity at ponds and modern approach

Anthropogenic and industrial wastewater (IWW) could be an additional future source of water to support the needs of the people of the State of Qatar. New lagoons have been built using modern technologies to optimize water use and waste recycling, as well as increasing the green spaces around the country. To achieve successful development of these new lagoons, lessons should be learned from the old ponds by examining their biodiversity, ecology, and the roles played by aquatic plants and algae to remediate wastewaters at these ponds. The perspectives of using IWW (from oil and gas activities), that is currently pumped deep into the ground are presented. Instead of causing great damage to groundwater, IWW can be stored in artificial ponds prepared for ridding it of all impurities and pollutants of various types, organic and inorganic, thereby making it serviceable for various human uses. Phycoremediation, bioremediation, and phytoremediation methods adopted by algae, bacteria and aquatic native plants are discussed, and special attention should be paid to those that proved successful in removing heavy metals and degrading organic compounds. At least three native plants namely: Amaranthus viridis, Phragmites australis, and Typha domingensis should be paid special attention, since these plants are efficient in remediation of arsenic and mercury; elements found abundantly in wastewater of gas activities. Some promising modern and innovative experiences and biotechnologies to develop efficient transgenic plants and microorganisms in removing and degrading pollutants are discussed, as an important strategy to keep the ecosystem clean and safe. Novelty statementIndustrial wastewater (IWW) could be an alternative source of water at the Arabian Gulf region. Currently, IWW is pumped deep into the ground causing a great damage to groundwater; little information about this issue has been reported. Such IWW can be stored in artificial ponds designed for ridding them of all impurities of various types; various remediation methods can be used. Modern biotechnology to develop transgenic plants and microorganisms to enhance these remediation methods can be adopted.

Analysis of the long-term effectiveness of biochar immobilization remediation on heavy metal contaminated soil and the potential environmental factors weakening the remediation effect: A review

Currently, the research and application of biochar in the remediation of heavy metal contaminated soil has become a hotspot, especially regarding the remediation of agricultural land. Biochar has been proved to be effective in reducing the content of available heavy metals in the soil as well as the heavy metals in plants. However, the long-term effectiveness of biochar immobilization has not been widely studied. In this review, retrospective search was carried out on the published literature results concerning remediation effects of biochar on different areas of heavy metal contaminated soil in the recent years, its application in field remediation (several years), and some potential abiotic and biotic factors that may weaken the immobilization effects of biochar. This results indicate that: (1) biochar is widely used in the remediation of heavy metal contaminated soil in different areas and has excellent immobilization effect. (2) Most of the research demonstrate that the immobilization effect of biochar is effective for 2-3 years or according to few results even for 5 years. However, there have been various reports claiming that the immobilization effect of biochar decreases with time. (3) Abiotic factors such as acid rain, flooded environment, changes in soil condition (pH, redox and dissolved organic matter) and changes in biochar (Cl^- and alkali leaching) can significantly weaken the immobilization effect of biochar. (4) Biotic factors such as plant roots, earthworms and soil microorganisms can also significantly reduce the immobilization effect of biochar. Therefore, field experiments having longer time span with biochar need to be further carried out, and the developmental research of modified biochar with a more stable immobilization effect also needs further attention.

PGPR-mediated induction of systemic resistance and physiochemical alterations in plants against the pathogens: Current perspectives

Plant growth-promoting rhizobacteria (PGPR) are diverse groups of plant-associated microorganisms, which can reduce the severity or incidence of disease during antagonism among bacteria and soil-borne pathogens, as well as by influencing a systemic resistance to elicit defense response in host plants. An amalgamation of various strains of PGPR has improved the efficacy by enhancing the systemic resistance opposed to various pathogens affecting the crop. Many PGPR used with seed treatment causes structural improvement of the cell wall and physiological/biochemical changes leading to the synthesis of proteins, peptides, and chemicals occupied in plant defense mechanisms. The major determinants of PGPR-mediated induced systemic resistance (ISR) are lipopolysaccharides, lipopeptides, siderophores, pyocyanin, antibiotics 2,4-diacetylphoroglucinol, the volatile 2,3-butanediol, N-alkylated benzylamine, and iron-regulated compounds. Many PGPR inoculants have been commercialized and these inoculants consequently aid in the improvement of crop growth yield and provide effective reinforcement to the crop from disease, whereas other inoculants are used as biofertilizers for native as well as crops growing at diverse extreme habitat and exhibit multifunctional plant growth-promoting attributes. A number of applications of PGPR formulation are needed to maintain the resistance levels in crop plants. Several microarray-based studies have been done to identify the genes, which are associated with PGPR-induced systemic resistance. Identification of these genes associated with ISR-mediating disease suppression and biochemical changes in the crop plant is one of the essential steps in understanding the disease resistance mechanisms in crops. Therefore, in this review, we discuss the PGPR-mediated innovative methods, focusing on the mode of action of compounds authorized that may be significant in the development contributing to enhance plant growth, disease resistance, and serve as an efficient bioinoculants for sustainable agriculture. The review also highlights current research progress in this field with a special emphasis on challenges, limitations, and their environmental and economic advantages.

Azotobacter: A potential bio-fertilizer for soil and plant health management

Stressor (biotic as well as abiotic) generally hijack the plant growth and yield characters in hostile environment leading to poor germination of the plants and yield. Among the plant growth promoting rhizobacteria, Azotobacter spp. (Gram-negative prokaryote) are considered to improve the plant health. Various mechanisms are implicated behind improved plant health in Azotobacter spp. inoculated plants. For example, acceleration of phytohormone like Indole-3-Acetic Acid production, obviation of various stressors, nitrogen fixation, pesticides and oil globules degradation, heavy metals metabolization, etc. are the key characteristics of Azotobacter spp. action. In addition, application of this bacteria has also become helpful in the reclamation of soil suggesting to be a putative agent which can be used in the transformation of virgin land to fertile one. Application of pesticides of chemical origin are being put on suspension mode as the related awareness program is still on. As far as the limitations of this microbe is concerned, commercial level formulations availability is still a great menace. Present review has been aimed to appraise the researchers pertaining to utility of Azotobacter spp. in the amelioration of plant health in sustainable agroecosystem. The article has been written with the target to gather maximum information into single pot so that it could reach to the dedicated researchers.

Evaluation of compost quality and bioprotection potential against Fusarium wilt of date palm

The Fusarium wilt of date palm caused by Fusarium oxysporum f.sp. albedinis (Foa) is the most damaging disease in Morocco. It has destroyed more than 12 million trees in the country. Until now, there is no efficient approach to manage this disease. However, biocontrol of soil-borne phytopathogens with compost is currently an established horticultural approach. This study aimed to evaluate the effect of compost extracts in the biocontrol of Foa and their mechanism of action and to investigate parameters involved in compost maturity. Maturity (phytotoxicity test), sanitary quality (pathogen indicators) and in vitro suppressive effect of four composts were investigated. Date palm by-products compost produced with forced aeration composting system is not phytotoxic presenting a germination index of 83.78%. In contrast, two commercial composts were phytotoxic. The maturity was negatively correlated with salinity, C/N ratio and total soluble phenols. Concentrations of pathogen indicators were below suggested limits in all composts. The biocontrol test showed that the date palm wastes compost was more effective; from the sixth day of incubation, the unsterilized extract of this compost at 10% showed the highest antagonistic effect against the pathogen with an inhibition rate of 100%. The inhibition rate using microfiltrated extract didn't exceed 30%. All autoclaved extracts lost their antagonistic effect against the indicator fungus. Thus, the suppressive effect is mainly due to the indigenous microorganisms. Findings indicate the relevance of maturity in the evaluation of compost quality and the efficiency of date palm wastes compost in the suppression of Foa.

Statistical modeling-approach for optimization of Cu2+ biosorption by Azotobacter nigricans NEWG-1; characterization and application of immobilized cells for metal removal

Heavy metals are environmental pollutants affect the integrity and distribution of living organisms in the ecosystem and also humans across the food chain. The study targeted the removal of copper (Cu²⁺) from aqueous solutions, depending on the biosorption process. The bacterial candidate was identified using 16S rRNA sequencing and phylogenetic analysis, in addition to morphological and cultural properties as Azotobacter nigricans NEWG-1. The Box-Behnken design was applied to optimize copper removal by Azotobacter nigricans NEWG-1 and to study possible interactive effects between incubation periods, pH and initial CuSO₄ concentration. The data obtained showed that the maximum copper removal percentage of 80.56% was reached at run no. 12, under the conditions of 200 mg/L CuSO₄, 4 days’ incubation period, pH, 8.5. Whereas, the lowest Cu²⁺ removal (12.12%) was obtained at run no.1. Cells of Azotobacter nigricans NEWG-1 before and after copper biosorption were analyzed using FTIR, EDS and SEM. FTIR analysis indicates that several functional groups have participated in the biosorption of metal ions including hydroxyl, methylene, carbonyl, carboxylate groups. Moreover, the immobilized bacterial cells in sodium alginate-beads removed 82.35 ± 2.81% of copper from the aqueous solution, containing an initial concentration of 200 mg/L after 6 h. Azotobacter nigricans NEWG-1 proved to be an efficient biosorbent in the elimination of copper ions from environmental effluents, with advantages of feasibility, reliability and eco-friendly.

Phytoremediation potential of Arundo donax (Giant Reed) in contaminated soil by heavy metals

Soil pollution from heavy metals poses a serious risk for environment and public health. Phytoremediation is an eco-friendly and cheaper alternative compared to chemical-physical techniques. We carried out in vitro tests where three microorganisms Trichoderma harzianum, Saccharomyces cerevisiae and Wickerhamomyces anomalus were exposed to eight different heavy metals (one metal at a time) in order to evaluate resistance, growth and bioaccumulation capability for each metal (Ni, Cd, Cu, V, Zn, As, Pb, Hg). Taking into account the natural characteristics of T. harzianum, (resistance to environmental stress, resistance to pathogenic fungi, ability to establish symbiotic relationships with superior green plants) and the good bioaccumulation capacity for V, As, Cd, Hg, Pb shown after in vitro tests, it was chosen as a microorganism to be used in greenhouse tests. Controlled exposure tests were performed in greenhouse, where Arundo donax and mycorrhized Arundo donax with T. harzianum were exposed for 7 months at two different doses (L1 and L2) of a heavy metal mix, so as to assess whether the symbiotic association could improve the bioaccumulation capability of the superior green plant A. donax. Heavy metals were determined with ICP-MS. The average bioaccumulation percentage values of A. donax for L1 and L2 were, respectively: Ni (31%, 26%); Cd (35%, 50%); Cu (30%, 35%); As (19%, 27%); Pb (18%, 14%); Hg (42%, 45%); V (39%, 26%); Zn (23%, 9%). The average bioaccumulation percentage values of mycorrhized A. donax with T. harzianum for L1 and L2 were, respectively: Ni (27%, 38%); Cd (44%, 42%); Cu (36%, 29%); As (17%, 23%); Pb (37%, 54%); Hg (44%, 60%); V (16%, 20%); Zn (14%, 7%). A. donax showed the highest BAF (bioaccumulation factor) for Cd (0.50), Cu (0.35), As (0.27) and Hg (0.45) after exposure to L2; mycorrhized A. donax with T. harzianum showed the highest BAF for Ni (0.38), Cd (0.42), Pb (0.54) and Hg (0.60) after exposure to L2. A. donax showed the highest TF (translocation factor) values for Cd (0.28) and Hg (0.26) after exposition at L1 and L2 respectively; A. donax mycorrhized with T. harzianum showed the highest TF values for Cd (0.70), As (0.56), V (0.24), Pb (0.18) after exposition at L2, and Zn (0.30) after exposition at L1. Our study showed a good growth capability in contaminated soils and a good bioaccumulation capability of heavy metals, both for A. donax and mycorrhized A. donax with T. harzianum. Furthermore, for three metals (Ni, Pb and Hg) the bioaccumulation capability was improved by the symbiosis of T. harzianum with A. donax. So, these results proved the suitability both for A. donax and mycorrhized A. donax with T. harzianum for phytoremediation processes.

Bioremediation Methods for the Recovery of Lead-Contaminated Soils: A Review

Currently, the pollution of soils by heavy metals is a problem of paramount relevance and requires the development of proper remediation techniques. In particular, lead is a frequently detected soil contaminant that poses adverse effects to the environment and human health. In this review, we provide an overview of the bioremediation treatments promoted by plants (phytoremediation), fungi, or bacteria that could be applied to areas polluted by lead. These restoration processes have the advantage of being environmentally friendly and cost-effective solutions that exploit plants to immobilize and extract contaminants from soil and water, and fungi and bacteria to degrade them. Phytoremediation is an extensively studied and mature practice, with many in-the-field applications where numerous plant species have been employed. In contrast, bioremediation processes promoted by fungi and bacteria are very promising but, up to now, studies have been mostly performed at a laboratory scale with only a few implementations in real-world situations; therefore, further research is needed.

Tolerance of Impatiens balsamina L., and Crotalaria retusa L. to grow on soil contaminated by used lubricating oil: A comparative study

Screening of plant species with an ability to grow on contaminated soil is the most critical step in the planning of a phytoremediation program. While flourishing growth of Impatiens balsamina L. and Crotalaria retusa L. has been observed in areas adjacent to automobile service stations in Sri Lanka, no systematic study of their tolerance to used lubricating oil (ULO) contaminated soil has been carried out. Therefore, the aim of the present study was to investigate the comparative responses of I. balsamina L. and C. retusa L. to soil contaminated with ULO. Both species exhibited 100% seed germination in soils treated with 1%-5% w/w ULO. After 120 h exposure, root lengths and biomass of germinated seedlings of both species were significantly (p < 0.05) reduced in all treatments above 3% w/w ULO. The measured growth parameters of plants following 90 d exposure to 0.5-3% w/w ULO, indicated significant (p < 0.05) negative effects on I. balsamina and C. retusa at >1% w/w and >2% w/w ULO, respectively. There were no significant effects on chlorophyll content or root anatomy of either species under any treatments. Therefore, we concluded that I. balsamina can tolerate up to 1% of ULO and C. retusa up to 2% w/w ULO without displaying any negative effects. Comparatively higher biodegradation of ULO in the rhizosphere, root nodule formation, increases in root length and root hair density are all possible strategies for the exhibited higher tolerance of C. retusa. Therefore, the overall results indicate that C. retusa has the greater potential to be used in phytoremediation of ULO contaminated soils. The findings of the present study will be beneficial in planning phytoremediation program for ULO contaminated soil.

Funneliformis mosseae Enhances Root Development and Pb Phytostabilization in Robinia pseudoacacia in Pb-Contaminated Soil

It is possible that arbuscular mycorrhizal fungi play a pivotal role in root development and Pb phytostabilization in plants grown in Pb-contaminated soil. In this study, a pot experiment was conducted over 4 months to evaluate the effects of Funneliformis mosseae strain BGCXJ01A on root characteristics of black locust (Robinia pseudoacacia L.) seedlings in Pb-contaminated soil. Four Pb treatments (0, 90, 900, and 3,000 mg kg^–1) were applied to soil in the presence and absence of F. mosseae. Inoculation with F. mosseae prominently improved root length, surface area, volume, and tip number in the plants across all Pb treatments. The F. mosseae inoculation also increased root diameter and fork number, especially under high Pb treatments. The presence of F. mosseae significantly increased the root activity and root tolerance index. However, there was little difference in specific root length between inoculated and non-inoculated plants. The biomass of roots, stems, and leaves all increased following inoculation with F. mosseae. Inoculated plants had greater accumulation and translocation capacities for Pb in the roots and stems, but lower capacities were found in the leaves when compared with those in non-inoculated plants. These results highlight that F. mosseae can alleviate the toxic effects of Pb on root development and can immobilize Pb in the roots and stems of R. pseudoacacia grown in Pb-contaminated soil. This study provides a model system for phytoremediation of Pb-contaminated soil via reciprocal symbiosis between arbuscular mycorrhizal fungi and woody legumes.

Magnesium oxide nanoparticles and thidiazuron enhance lead phytoaccumulation and antioxidative response in Raphanus sativus L

An in vitro study was conducted to evaluate the effects of thidiazuron (TDZ) growth regulator and magnesium oxide (MgO) nanoparticles on radish (Raphanus sativus L.) under lead (Pb) stress. Effects of TDZ and MgO on seed germination, growth, biomass, total phenolics and flavonoids, antioxidant potential, and Pb phytoaccumulation in different plant parts were assessed. Nanoparticles of MgO were synthesized with leaf extract of Sageretia thea (Osbeck) plant. Thidiazuron and MgO nanoparticles were added to growth media in individual and in combinations. Lead (50 mg L^-1) was added to growth media. Thidiazuron and MgO nanoparticles increased plant growth, phenolic and flavonoid contents, free radical scavenging activity, and lead phytoaccumulation. The increase was highly significant in TDZ and MgO nanoparticle combination treatments (T5, T6). Treatment (T6) showed a sixfold increase in Pb accumulation (1721.73 ± 17.4 μg g^-1 dry biomass) as compared to control (274.29 ± 4.23 μg^-1g^-1). Total phenolic and dry biomass showed significantly positive correlation in leaves (R² = 0.73), stem (R² = 0.58), and roots (R² = 0.72). The correlation of Pb accumulation and phenolic contents was significantly positive in root (R² = 0.80), stem (R² = 0.92), and leaves (R² = 0.69). Flavonoid showed a positive correlation with dry biomass and Pb accumulation. Antioxidant activity was highly increased in leaves followed by stem and root. Findings show that TDZ in combination with MgO nanoparticles can play a significant role in secondary metabolite production and Pb phytoaccumulation.