Enhancing phytoremediation of chromium-stressed soils through plant-growth-promoting bacteria

Integrated removal of chromium, lead, and cadmium using nano-zero-valent iron-supported biochar: Mechanistic insights and eco-toxicity assessment

The contamination of water and soil by heavy metals (HMs) is a global issue that should be given much more concern. Modified nano-zero-valent iron (nZVI) composites offer an effective strategy for HMs remediation, but few studies have focused on removing coexisting HMs and the eco-toxicity of the composite. In this study, corn straw biochar-supported nZVI composites (nZVI-BC) were synthesized, characterized and used for the removal of Cr6 +, Pb2+, and Cd2+ in single and multi-system at different composites dosages, metal concentrations, and solution pH. This study indicated that the composites exhibited enhanced removal capacities for Cr6+, Pb2+, and Cd2+ (respectively 82.24, 737.2, and 545.28 mg g-1), which were considerably superior to those observed with the sole application of biochar (0.05, 89.88, and 108.49 mg g-1) and nZVI (39.8, 297.35, and 191.02 mg g-1). Results of the remediation application of the composites to multi-metal systems revealed that intricate interplay existed between coexisting HMs, which hindered the simultaneous removal effect. The coexistence of Cr6+ and Cd2+ decreased both removal efficiencies by 58.16 % and 14.06 % at high Cr6+ levels, respectively, while the coexistence of Cd2+ and Pb2+ resulted in a decrease in Cd2+ removal efficiency by 14.3 %. An in-depth characterization of the underlying adsorption mechanism was performed by using kinetic and isotherms models such as Pseudo-first-order, Pseudo-second-order, Langmuir and Freundlich, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) analysis. Each HM exhibited a distinct adsorption mechanism. The primary removal processes for Cr6+ and Pb2+ involved adsorption, reduction, and precipitation, whereas Cd2+ was mainly removed by adsorption and precipitation. Eco-toxicity experiments revealed that nZVI-BC enhanced pak choi (Brassica rapa L.) seeds germination (13.32, 17.22, and 23.33 %) and vigor indexes (1.22, 1.44, and 1.15) under Cr6+, Pb2+, and Cd2+ contamination, respectively. Nevertheless, an observed shift in toxicity occurred when the composites dosage for Cr6+, Pb2+, and Cd2+ exceeded 2, 4, and 4 g L-1, respectively, thereby instigating adverse effects on the early stages of plant growth. This work elucidates the removal mechanism and intricate reactions between co-existing HMs, highlighting the potential of nZVI-BC as a remediation strategy for HMs contamination.

A phytoremediation approach for the restoration of coal fly ash polluted sites: A review

Coal fly ash (CFA) is a predominant waste by-product of coal combustion which is disposed of in open ash dams that utilize large pieces of land. This waste material is classified as a hazardous substance in South Africa as well as in other countries due to its fine particles that are easily blown to the atmosphere and the unacceptable levels of heavy metals and persistent organic pollutants. Contaminants in CFA can pollute surface and ground water, agricultural sites, soil and therefore pose risks to the health of humans and the environment. More than 500 million tons of CFA is produced yearly and over 200 million tons remain unused globally. The production will continue due to high consumer energy demands, especially in countries with heavy reliance on coal for power generation. Despite a significant progress made on the application of phytoremediation approach for decontamination of polluted sites, there is very limited evidence for its potential in the rehabilitation of CFA dumps. Low organic carbon, microbial activities and availability of nutrients including nitrogen contribute to restricted plant growth in CFA, and therefore converting ash dumps to barren lands devoid of vegetation. Leguminous plant species can fix atmospheric nitrogen through symbiotic association with bacteria. Therefore, their intercropping mixture development can improve the chemistry of the substrate and facilitate nutrients availability to the companion plants. This approach can enhance the performance of phytoremediation and promote sustainable practices. The paper provides an overview of the ongoing burden of CFA disposal and discusses the ecological and economic benefits of using legumes, aromatic and bioenergy plants. We identify knowledge gaps to establishing vegetation in ash dumping sites, and provide insights to encourage continued research that will enhance the applicability of phytoremediation in restoration programs.

Unveiling the chromate stress response in the marine bacterium Bacillus enclensis AGM_Cr8: a multifaceted investigation

In this study, we introduce Bacillus enclensis AGM_Cr8, a gram-positive marine bacterium isolated from the chronically polluted Versova Creek in Mumbai, India. AGM_Cr8 exhibits robust tolerance to chromate stress, thriving in marine agar media containing up to 3200 mg/l of hexavalent chromium [Cr(VI)], with the Minimum Inhibitory Concentration (MIC) established at 5000 mg/l. Notably, AGM_Cr8 also displays tolerance to other heavy metals, including Lead [Pb (II)] (1200 mg/l), Arsenic [As (III)] (400 mg/l), Cadmium [Cd(II)] (100 mg/l), and Nickel [Ni(II)] (100 mg/l). Scanning Electron Microscopy (SEM) reveals the presence of Cr(VI) on the bacterial surface, accompanied by the secretion of extracellular polymeric substances (EPSs) facilitating Cr(VI) sequestration. This observation is validated through Energy Dispersive Spectroscopy (EDS). Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy-Energy Dispersive Spectroscopy (STEM-EDS) confirm internal bioaccumulation of Cr(VI). X-ray photoelectron spectroscopy (XPS) identifies distinct peaks around 579 and 576 eV, indicating the coexistence of Cr(VI) and Cr(III), implying a bioreduction mechanism. De novo genome sequencing identifies twenty-two chromate-responsive genes, including putative chromate transporters (srpC1 and srpC2), suggesting an efflux mechanism. Other identified genes encode NAD(P)H-dependent FMN-containing oxidoreductase, NADH quinone reductase, ornithine aminotransferase, transporter genes (natA, natB, ytrB), and genes related to DNA replication and repair (recF), DNA mismatch repair (mutH), and superoxide dismutase. We therefore, propose a chromate detoxification pathway that involves an interplay of chromate transporters, enzymatic reduction of Cr(VI) to Cr(III), DNA repair and role of antioxidants in response to chromate stress. We have highlighted the potential of AGM_Cr8 for bioremediation in chromium-contaminated environments, given its robust tolerance and elucidated molecular mechanisms for detoxification.

Remediation of heavy metals polluted soil environment: A critical review on biological approaches

Heavy metals (HMs) pollution is a globally emerging concern. It is difficult to cost-effectively combat such HMs polluted soil environments. The efficient remediation of HMs polluted soil is crucial to protect human health and ecological security that could be carried out by several methods. Amidst, biological remediation is the most affordable and ecological. This review focused on the principles, mechanisms, performances, and influential factors in bioremediation of HMs polluted soil. In microbial remediation, microbes can alter metallic compounds in soils. They transform these compounds into their metabolism through biosorption and bioprecipitation. The secreted microbial enzymes act as transformers and assist in HMs immobilization. The synergistic microbial effect can further improve HMs removal. In bioleaching, the microbial activity can simultaneously produce H2SO4 or organic acids and leach HMs. The production of acids and the metabolism of bacteria and fungi transform metallic compounds to soluble and extractable form. The key bioleaching mechanisms are acidolysis, complexolysis, redoxolysis and bioaccumulation. In phytoremediation, hyperaccumulator plants and their rhizospheric microbes absorb HMs by roots through absorption, cation exchange, filtration, and chemical changes. Then they exert different detoxification mechanisms. The detoxified HMs are then transferred and accumulated in their harvestable tissues. Plant growth-promoting bacteria can promote phytoremediation efficiency; however, use of chelants have adverse effects. There are some other biological methods for the remediation of HMs polluted soil environment that are not extensively practiced. Finally, the findings of this review will assist the practitioners and researchers to select the appropriate bioremediation approach for a specific soil environment.

Thermotolerant plant growth-promoting bacteria enhance growth and nutrient uptake of lettuce under heat stress conditions by altering stomatal movement and chlorophyll fluorescence

This study investigates the effects of selected PGPB on lettuce growth performance under heat-stress conditions. Bacterial plant growth-promoting potentials have been characterized and identified successfully in ongoing studies. Based on in vitro plant growth-promoting potential, the top five bacteria were ranked and identified as Acinetobacter sp. GRB12, Bacillus sp. GFB04, Klebsiella sp. LFB06, Klebsiella sp. GRB10, and Klebsiella sp. GRB04. They were mixed to inoculate on lettuce (Lactuca sativa L.) in temperature-controlled greenhouses. Another in-vivo chamber experiment was conducted by using Bacillus sp. GFB04 and Klebsiella sp. GFB10. Plant physiological traits (chlorophyll fluorescence and transpiration) and nutrient contents were measured at harvest, along with growth, development, and yield component analyses. Uninoculated plants under heat-stress condition showed poor growth performance. In contrast, plants with PGPB inoculation showed improved growth under heat-stress conditions, as the uptake of nutrients was facilitated by the symbionts. Inoculation also improved lettuce photosystem II efficiency and decreased total water use under heat stress. In conclusion, the current study suggests that PGPB inoculation successfully enhances lettuce heat-tolerance. PGPB application could potentially help improve sustainable production of lettuce with less fertilization under increasing temperatures.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-024-01470-5.

In situ mycoremediation of acid rain and heavy metals co-contaminated soil through microbial inoculation with Pleurotus ostreatus

The combined pollution of acid rain and heavy metals in soil is a pressing environmental problem, especially in the regions with large-scale heavy industrial production activities. Low remediation efficiency and weak long-lasting stability are major challenges when disposing the heavy metals contaminated soil in acid rain polluted sites. Herein, a specific microbe, strain CT13 was isolated and domesticated to exhibit high tolerance to both acid rain and cadmium (Cd). Then, an in situ mycoremediation method by adopting a bioaugmentation technology of strain CT13 inoculation with Pleurotus ostreatus was developed. The remediation performance was investigated in acidic conditions with Cd concentrations in soil ranging from 0 to 15 mg/kg. While most of the bacteria strains (e.g. strain CT6/13) significantly improved the dry weight of mushroom and Cd accumulation in neutral environment, the performance of strain CT6 was remarkably deteriorated in acid rain environment. In contrast, strain CT13 maintained its behavior in acidic conditions, displaying ∼30 % and 150 % enhancements (vs the neutral environment) in the dry weight of mushroom and Cd accumulation, respectively. In addition, inoculation of strain CT13 led to significant reductions in the content of superoxide dismutase, peroxidase and lipid peroxidation in the fruiting body of P. ostreatus, indicating an improvement in the mushroom's tolerance to both acid rain and heavy metals. The synergistic effect of strain CT13 and P. ostreatus realized the significant improvement in soil remediation efficiency and long-lasting stability in acidic conditions, providing valuable insights into the remediation of heavy metal contaminated soil in the regions affected by acid rain.

Bioremediation of hazardous heavy metals by marine microorganisms: a recent review

Heavy metals (HMs) like Zn, Cu, Pb, Ni, Cd, and Hg, among others, play a role in several environmental problems. The marine environment is polluted by several contaminants, such as HMs. A variety of physico-chemical methods usually available for sanitation HMs remediation suffer from either limitation. Bioremediation is a promising way of dealing with HMs pollution. Microbes have the ability with various potencies to resist HMs tension. The current review discusses the main sources and influences of HMs, the role of marine microorganisms in HMs bioremediation, as well as the microbial mechanisms for HMs detoxification and transformation. This review paper aims to provide an overview of the bioremediation technologies that are currently available for the removal of HMs ions from industrial and urban effluent by aquatic organisms such as bacteria, fungi, and microalgae, particularly those that are isolated from marine areas. The primary goals are to outline various studies and offer helpful information about the most important aspects of the bioelimination techniques. The biotreatment practices have been primarily divided into three techniques based on this topic. They are biosorption, bioaccumulation, bioleaching, and biotransformation. This article gives the brief view on the research studies about bioremediation of HMs using marine microorganisms. The current review also deals with the critical issues and recent studies based on the HMs biodetoxification using aquatic microorganisms.

Water Quality Degradation Due to Heavy Metal Contamination: Health Impacts and Eco-Friendly Approaches for Heavy Metal Remediation

Water quality depends on its physicochemical and biological parameters. Changes in parameters such as pH, temperature, and essential and non-essential trace metals in water can render it unfit for human use. Moreover, the characteristics of the local environment, geological processes, geochemistry, and hydrological properties of water sources also affect water quality. Generally, groundwater is utilized for drinking purposes all over the globe. The surface is also utilized for human use and industrial purposes. There are several natural and anthropogenic activities responsible for the heavy metal contamination of water. Industrial sources, including coal washery, steel industry, food processing industry, plastic processing, metallic work, leather tanning, etc., are responsible for heavy metal contamination in water. Domestic and agricultural waste is also responsible for hazardous metallic contamination in water. Contaminated water with heavy metal ions like Cr (VI), Cd (II), Pb (II), As (V and III), Hg (II), Ni (II), and Cu (II) is responsible for several health issues in humans, like liver failure, kidney damage, gastric and skin cancer, mental disorders and harmful effects on the reproductive system. Hence, the evaluation of heavy metal contamination in water and its removal is needed. There are several physicochemical methods that are available for the removal of heavy metals from water, but these methods are expensive and generate large amounts of secondary pollutants. Biological methods are considered cost-effective and eco-friendly methods for the remediation of metallic contaminants from water. In this review, we focused on water contamination with toxic heavy metals and their toxicity and eco-friendly bioremediation approaches.

Nano zero-valent iron enhances the absorption and transport of chromium in rice (Oryza sativa L.): Implication for Cr risks management in paddy fields

Chromium (Cr) accumulating in soil caused serious pollution to cultivated land. At present, nano zero-valent iron (nZVI) is considered to be a promising remediation material for Cr-contaminated soil. However, the nZVI impact on the behavior of Cr in the soil-rice system under high natural geological background value remains unknown. We studied the effects of nZVI on the migration and transformation of Cr in paddy soil-rice by pot experiment. Three different doses of nZVI (0, 0.001 % and 0.1 % (w/w)) treatments and one dose of 0.1 % (w/w) nZVI treatment without plant rice were set up. Under continuous flooding conditions, nZVI significantly increased rice biomass compared with the control. At the same time, nZVI significantly promoted the reduction of Fe in the soil, increased the concentration of oxalate Fe and bioavailable Cr, then facilitated the absorption of Cr in rice roots and the transportation to the aboveground part. In addition, the enrichment of Fe(III)-reducing bacteria and sulfate-reducing bacteria in soil provided electron donors for Cr oxidation, which helps to form bioavailable Cr that is easily absorbed by plants. The results of this study can provide scientific basis and technical support for the remediation of Cr -polluted paddy soil with high geological background.

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

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

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.

Mesorhizobium improves chickpea growth under chromium stress and alleviates chromium contamination of soil

Environmental pollution has become a transnational issue that impacts ecosystems, soil, water, and air and is directly related to human health and well-being. Chromium pollution decreases the development of plant and microbial populations. It warrants the need to remediate chromium-contaminated soil. Decontaminating chromium-stressed soils via phytoremediation is a cost-effective and environmentally benign method. Using multifunctional plant growth-promoting rhizobacteria (PGPR) lower chromium levels and facilitates chromium removal. PGPR work by altering root architecture, secreting chemicals that bind metals in the rhizosphere, and reducing phytotoxicity brought on by chromium. The present study aimed to investigate the chromium bioremediation capacity of metal-tolerant PGPR isolate while promoting the growth of chickpeas in the presence of varying levels of chromium (15.13, 30.26, and 60.52 mg/kg of chromium). The isolate, Mesorhizobium strain RC3, substantially reduced chromium content (60.52 mg/kg) in the soil. It enhanced the root length by 10.87%, the shoot length by 12.38%, the number of nodules by 6.64%, and nodule dry weight by 13.77% at 90 days. After 135 days of sowing, more improvement in the root length (18.05), shoot length (21.60%)the chlorophyll content (6.83%), leghaemoglobin content (9.47%), and the highest growth in the crop seed yield (27.45%) and crop protein content (16.83%)The isolate reduced chromium accumulation in roots, shoots, and grains chickpea. Due to chromium bioremediation and its plant growth-promoting and chromium-attenuating qualities, Mesorhizobium strain RC3 could be used as a green bioinoculant for plant growth promotion under chromium stress.

Analysis of the Physiological Response and Reactive Oxygen Species in Castor Oil Plant (Ricinus Communis) in the Phytoremediation Processes with Plant Growth Promoter Bacteria (PGPB)

In the phytoremediation processes of mine tailings with Ricinus communis inoculated with PGPB, it was found that the Serratia K120 bacterium favors the translocation of Al, As, Cu, Pb, Cr, Cd, and Mn to the aerial part of the plant, with a significant difference (p < 0.05) concerning for the control. The bioaccumulation factor (BF) was > 1 in Al with all the bacteria, Pb, Serratia K120, Fe, Pantoea 113, Cu, Pb, Cd, Mn in Serratia MC119 and Serratia K120, Fe and As in Serratia K120 and Pantoea 134, indicating that Ricinus communis inoculated with PGPB functions as a hyper accumulating plant. The PGPB help to reduce the stress in the plants generated by the heavy metals, decreasing the H2O2, and increasing the activity of the enzymes SOD, CAT, APX, POX, and GR, for which the bacteria Serratia K120 and Pantoea 113 can be used as bioinoculants to favor phytoremediation processes.

Hexavalent chromium-reducing plant growth-promoting rhizobacteria are utilized to bio-fortify trivalent chromium in fenugreek by promoting plant development and decreasing the toxicity of hexavalent chromium in the soil

BACKGROUND

Fenugreek is known to have good anti-diabetes properties. Moreover, several studies accounted that the trivalent form of chromium [Cr(III)] also have anti-diabetic properties. However, its hexavalent form i.e., Cr(VI) is known to be highly toxic and carcinogenic to living beings and retarded plant growth even if it is present in low concentration in soil. Many plant growth-promoting rhizobacteria (PGPR) are reported to have the potential to reduce the Cr(VI) into Cr(III) in soil. In view of the above, the present objective was designed to effectively utilize Cr(VI) reducing PGPRs for the growth and development of fenugreek plant in Cr(VI) amended soil, apart from reducing Cr(VI) in soil and fortification of Cr(III) in the aerial part of plants.

METHODS

The experiment was carried out to evaluate the effect of Cr(VI)-reducing PGPRs viz. Bacillus cereus (SUCR44); Microbacterium sp. (SUCR140); Bacillus thuringiensis (SUCR186) and B. subtilis (SUCR188) on growth, uptake and translocation of Cr as well as other physiological parameters in fenugreek grown under artificially Cr(VI) amended soil (100 mg kg^-1 of Cr(VI) in soil).

RESULTS

The aforementioned concentration of Cr(VI) in soil cause severe reduction in root length (41 %), plant height (43 %), dry root (38 %) and herb biomass (48 %), when compared with control negative (CN; uninoculated plant not grown in Cr(VI) contaminated soil). However, the presence of Microbacterium sp.-SURC140 (MB) mitigates the Cr toxicity resulting in improved root length (92 %), plant height (86 %), dry root (74 %) and herb biomass (99 %) as compared with control positive (CP; uninoculated plants grown in Cr(VI) contaminated soil). The maximum reduction in bioavailability (82 %) of Cr(VI) in soil and its uptake (50 %) by the plant were also observed in MB-treated plants. However, All Cr(VI)-reducing PGPRs failed to decrease the translocation of Cr to the aerial parts. Moreover, the plant treated with MB observed diminution in relative water content (13 %), electrolyte leakage (16%) and lipid peroxidation (38 %) as well as higher chlorophyll (37 %) carotenoids (17 %) contents and antioxidants (18%) potential.

CONCLUSION

This study demonstrates that MB can lower the Cr(VI) toxicity to the plant by reducing the bioavailable Cr(VI), consequently reducing the Cr(VI) toxicity level in soil and helping in improving the growth and yield of fenugreek. Additionally, Cr(III) uptakes and translocation may improve the effectiveness of fenugreek in treating diabetes.

Effects of Cr Stress on Bacterial Community Structure and Composition in Rhizosphere Soil of Iris tectorum under Different Cultivation Modes

With the rapid development of industry, Cr has become one of the major heavy metal pollutants in soil, severely impacting soil microecology, among which rhizosphere microorganisms can improve the soil microenvironment to promote plant growth. However, how rhizosphere bacterial communities respond to Cr stress under different cultivation modes remains to be further studied. Therefore, in this study, a greenhouse pot experiment combined with 16S rRNA high-throughput sequencing technology was used to study the effects of Cr stress at 200 mg kg−1 on the bacterial community structure and diversity in the rhizosphere soil of Iris tectorum under different cultivation modes. The results showed that the rhizosphere bacterial community diversity index (Shannon and Simpson) and abundance index (Ace and Chao) increased significantly with wetland plant diversity under Cr stress. Moreover, the bacterial community changed by 20.1% due to the addition of Cr, further leading to a 15.9% decrease in the common species of the bacterial community, among which Proteobacteria, Actinobacteria, Chloroflexi, and Acidobacteriota accounted for more than 74.8% of the total sequence. However, with the increase in plant diversity, the abundance of rhizosphere-dominant bacteria and plant growth-promoting bacteria communities increased significantly. Meanwhile, the symbiotic network analysis found that under the two cultivation modes, the synergistic effect between the dominant bacteria was significantly enhanced, and the soil microenvironment was improved. In addition, through redundancy analysis, it was found that C, N, and P nutrients in uncontaminated soil were the main driving factors of bacterial community succession in the rhizosphere of I. tectorum, and Cr content in contaminated soil was the main driving factor of bacterial community succession in I. tectorum rhizosphere. In summary, the results of this study will provide a basis for the response of the rhizosphere bacterial community to Cr and the interaction between wetland plants and rhizosphere bacteria in the heavy metal restoration of wetland plants under different cultivation modes.

Chromate Removal by Enterobacter cloacae Strain UT25 from Tannery Effluent and Its Potential Role in Cr (VI) Remediation

An indigenous chromate-resistant bacterial strain isolated from tannery effluent was identified based on morphological, biochemical, and 16S rRNA gene sequencing, as Enterobacter cloacae UT25. It was found to resist heavy metal ions such as Cr (VI), Pb (II), Cu (II), Co (II), Ni (II), Hg (II), and Zn (II) and antibiotics. The strain was able to remove 89 and 86% chromate, after 24 h of incubation in a Luria-Bertani (LB) medium at an initial Cr (VI) concentration of 1000 and 1500 µg/ml, respectively. Minimum inhibitory concentration (MIC) was observed for chromate to be 80,000 and 1850 µg/ml, after 48 h of incubation in LB and acetate minimal media (AMM), respectively. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) analysis showed discrete cells with intact and smooth cell walls and homogenous cytoplasm in the absence of metal stress, whereas chromate stress caused cell lysis and reduction in size, which was a characteristic response to Cr (VI) toxicity. Energy Dispersive X-Ray Spectroscopy (EDX) confirmed the adsorption of oxyanions to the cell wall which was one of the Cr (VI) removal mechanisms by the bacterium. Atomic Force Microscopy (AFM) micrographs of chromate-untreated and treated cells revealed Root Mean Square roughness (Rq) values of 16.25 and 11.26 nm, respectively, indicating less roughness in the presence of stress. The partial gene sequence of class 1 integrons (intI1) of strain UT25 showed 94% homology with intI1 gene of strain Enterobacter hormaechei strain ECC59 plasmid pECC59-1. The present analysis highlighted the potential of E. cloacae UT25 as a promissory bacterium that could be applied in removing chromate from polluted environments.

Heavy Metal Contamination in the Aquatic Ecosystem: Toxicity and Its Remediation Using Eco-Friendly Approaches

Urbanization and industrialization are responsible for environmental contamination in the air, water, and soil. These activities also generate large amounts of heavy metal ions in the environment, and these contaminants cause various types of health issues in humans and other animals. Hexavalent chromium, lead, and cadmium are toxic heavy metal ions that come into the environment through several industrial processes, such as tanning, electroplating, coal mining, agricultural activities, the steel industry, and chrome plating. Several physical and chemical methods are generally used for the heavy metal decontamination of wastewater. These methods have some disadvantages, including the generation of secondary toxic sludge and high operational costs. Hence, there is a need to develop a cost-effective and eco-friendly method for the removal of heavy metal ions from polluted areas. Biological methods are generally considered eco-friendly and cost-effective. This review focuses on heavy metal contamination, its toxicity, and eco-friendly approaches for the removal of heavy metals from contaminated sites.

Chromium toxicity, speciation, and remediation strategies in soil-plant interface: A critical review

In recent decades, environmental pollution with chromium (Cr) has gained significant attention. Although chromium (Cr) can exist in a variety of different oxidation states and is a polyvalent element, only trivalent chromium [Cr(III)] and hexavalent chromium [Cr(VI)] are found frequently in the natural environment. In the current review, we summarize the biogeochemical procedures that regulate Cr(VI) mobilization, accumulation, bioavailability, toxicity in soils, and probable risks to ecosystem are also highlighted. Plants growing in Cr(VI)-contaminated soils show reduced growth and development with lower agricultural production and quality. Furthermore, Cr(VI) exposure causes oxidative stress due to the production of free radicals which modifies plant morpho-physiological and biochemical processes at tissue and cellular levels. However, plants may develop extensive cellular and physiological defensive mechanisms in response to Cr(VI) toxicity to ensure their survival. To cope with Cr(VI) toxicity, plants either avoid absorbing Cr(VI) from the soil or turn on the detoxifying mechanism, which involves producing antioxidants (both enzymatic and non-enzymatic) for scavenging of reactive oxygen species (ROS). Moreover, this review also highlights recent knowledge of remediation approaches i.e., bioremediation/phytoremediation, or remediation by using microbes exogenous use of organic amendments (biochar, manure, and compost), and nano-remediation supplements, which significantly remediate Cr(VI)-contaminated soil/water and lessen possible health and environmental challenges. Future research needs and knowledge gaps are also covered. The review's observations should aid in the development of creative and useful methods for limiting Cr(VI) bioavailability, toxicity and sustainably managing Cr(VI)-polluted soils/water, by clear understanding of mechanistic basis of Cr(VI) toxicity, signaling pathways, and tolerance mechanisms; hence reducing its hazards to the environment.

Hexavalent-Chromium-Induced Oxidative Stress and the Protective Role of Antioxidants against Cellular Toxicity

Hexavalent chromium is a highly soluble environmental contaminant. It is a widespread anthropogenic chromium species that is 100 times more toxic than trivalent chromium. Leather, chrome plating, coal mining and paint industries are the major sources of hexavalent chromium in water. Hexavalent chromium is widely recognised as a carcinogen and mutagen in humans and other animals. It is also responsible for multiorgan damage, such as kidney damage, liver failure, heart failure, skin disease and lung dysfunction. The fate of the toxicity of hexavalent chromium depends on its oxidation state. The reduction of Cr (VI) to Cr (III) is responsible for the generation of reactive oxygen species (ROS) and chromium intermediate species, such as Cr (V) and Cr (IV). Reactive oxygen species (ROS) are responsible for oxidative tissue damage and the disruption of cell organelles, such as mitochondria, DNA, RNA and protein molecules. Cr (VI)-induced oxidative stress can be neutralised by the antioxidant system in human and animal cells. In this review, the authors summarise the Cr (VI) source, toxicity and antioxidant defence mechanism against Cr (VI)-induced reactive oxygen species (ROS).

Nano zero-valent iron loaded corn-straw biochar for efficient removal of hexavalent chromium: remediation performance and interfacial chemical behaviour

To improve the poor stability of nano zero-valent iron (nZVI), corn-straw biochar (BC) was used as a support for the synthesis of composites of nZVI-biochar (nZVI/BC) in different mass ratios. After a thorough characterization, the obtained nZVI/BC composite was used to remove hexavalent chromium [Cr(vi)] in an aquatic system under varying conditions including composite amount, Cr(vi) concentration, and pH. The obtained results show that the treatment efficiency varied in the following order: nZVI-BC (1 : 3) > nZVI-BC (1 : 5) > nZVI alone > BC alone. This order indicates the higher efficiency of composite material and the positive effect of nZVI content in the composite. Similarly, the composite dosage and Cr(vi) concentration had significant effects on the removal performance and 2 g L-1 and 6 g L-1 were considered to be the optimum dose at a Cr(vi) concentration of 20 mg L-1 and 100 mg L-1, respectively. The removal efficiency was maximum (100%) at pH 2 whereas solution pH increased significantly after the reaction (from 2 to 4.13). The removal kinetics of Cr(vi) was described by a pseudo-second-order model which indicated that the removal process was mainly controlled by the rate of chemical adsorption. The thermodynamics was more in line with the Freundlich model which indicated that the removal was multi-molecular layer adsorption. TEM-EDS, XRD, and XPS were applied to characterize the crystal lattice and structural changes of the material to specify the interfacial chemical behaviour on the agent surface. These techniques demonstrate that the underlying mechanisms of Cr(vi) removal include adsorption, chemical reduction-oxidation reaction, and co-precipitation on the surface of the nZVI-BC composite. The results indicated that the corn-straw BC as a carrier material highly improved Cr(vi) removal performance of nZVI and offered better utilization of the corn straw.

Chromium toxicity and its remediation by using endophytic bacteria and nanomaterials: A review

Chromium (Cr) is a crucial element for all life forms. Various anthropogenic activities have been responsible for environmental contamination with Cr (VI) in recent years. For this review, articles were collected using electronic databases such as Web of Science, Pubmed, ProQuest, and Google Scholar as per the guidelines of PRISMA-2015, applying the Boolean search methods. Chromium can cause severe health complications in humans and animals and threatens the surrounding environment, with negative impacts on crop yield, development, and quality. Hence, monitoring Cr contamination is essential, and various remediation technologies have emerged in the past 50 years to reduce the amount of Cr in the environment. This review focuses on chromium exposure and the associated environmental health risks. We also reviewed sustainable remediation processes, with emphasis on nanoparticle and endophytic remediation processes.

Chromium contamination in paddy soil-rice systems and associated human health risks in Pakistan

Chromium (Cr) contamination in paddy soil-rice systems threatens human health through the food chain. This study used a new dataset of 500 paddy soil and plant tissue samples collected in the rice-growing regions of Sindh and Punjab Provinces of Pakistan. Overall, 97.4% of grain samples exceeded the Cr threshold values of 1.0 mg kg^-1, determined by the China National Food Standard (CNFS). The Cr in paddy soil, 62.6% samples exceeding the China natural background threshold value (90 mg kg^-1) for Cr concentration in paddy soil, and lower than the (pH-dependant > 7.5 threshold value for Cr 350 mg kg^-1) as determined by China Environmental Quality Standards (EQSs) for paddy soil (GB15618-2018). Geographically weighted regression (GWR) modelling showed spatially nonstationary correlations, confirming the heterogeneous relationship between dependent (rice grain Cr) and independent paddy soil (pH, SOM, and paddy soil Cr) and plant tissue variables (shoot Cr and root Cr) throughout the study area. The GWR model was then used to determine the critical threshold (CT) for the measured Cr concentrations in the paddy soil system. Overall, 38.4% of paddy soil samples exceeding CT values confirm that the paddy soil Cr risk prevails in the study area. Furthermore, the GWR model was applied to assess the loading capacity (LC), the difference between the CT, and the actual concentration of Cr in paddy soil. Loading capacity identified potential paddy soil Cr pollution risk to rice grain and assessed the risk areas. Overall LC% of samples paddy soil Cr risk areas grade: low-risk grade I (34.6%); moderate-risk grade II (15.8%); high-risk grade III (11.2%); and very high-risk grade IV (38.4%) have been assessed in the study area. The human health index, total hazard quotient (THQ ≪ 1), indicates no potential health risk originating from Cr exposure to the population. However, the excess Cr level in paddy soil and rice grain is still a concern. The current study's results are also valuable for the national decision-making process regarding Cr contamination in the paddy soil-rice system.

Method and mechanism of chromium removal from soil: a systematic review

Heavy metal pollution has increasingly affected human life, and the treatment of heavy metal pollution, especially chromium pollution, is still a major problem in the field of environmental governance. As a commonly used industrial metal, chromium can easily enter the environment with improperly treated industrial waste or wastewater, then pollute soil and water sources, and eventually accumulate in the human body through the food chain. Many countries and regions in the world are threatened by soil chromium pollution, resulting in the occurrence of cancer and a variety of metabolic diseases. However, as a serious threat to agriculture, food, and human health. Notwithstanding, there are limited latest and systematic review on the removal methods, mechanisms, and effects of soil chromium pollution in recent years. Hence, this article outlines some of the methods and mechanisms for the removal of chromium in soil, including physical, chemical, biological, and biochar methods, which provide a reference for the treatment and research on soil chromium pollution drawn from existing publications.

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.

Plant Growth-Promoting Bacteria (PGPB) integrated phytotechnology: A sustainable approach for remediation of marginal lands

Land that has little to no utility for agriculture or industry is considered marginal land. This kind of terrain is frequently found on the edge of deserts or other arid regions. The amount of land that can be used for agriculture continues to be constrained by increasing desertification, which is being caused by climate change and the deterioration of agriculturally marginal areas. Plants and associated microorganisms are used to remediate and enhance the soil quality of marginal land. They represent a low-cost and usually long-term solution for restoring soil fertility. Among various phytoremediation processes (viz., phytodegradation, phytoextraction, phytostabilization, phytovolatilization, phytofiltration, phytostimulation, and phytodesalination), the employment of a specific mechanism is determined by the state of the soil, the presence and concentration of contaminants, and the plant species involved. This review focuses on the key economically important plants used for phytoremediation, as well as the challenges to plant growth and phytoremediation capability with emphasis on the advantages and limits of plant growth in marginal land soil. Plant growth-promoting bacteria (PGPB) boost plant development and promote soil bioremediation by secreting a variety of metabolites and hormones, through nitrogen fixation, and by increasing other nutrients' bioavailability through mineral solubilization. This review also emphasizes the role of PGPB under different abiotic stresses, including heavy-metal-contaminated land, high salinity environments, and organic contaminants. In our opinion, the improved soil fertility of marginal lands using PGPB with economically significant plants (e.g., Miscanthus) in dual precession technology will result in the reclamation of general agriculture as well as the restoration of native vegetation.

Differential strategies of two species of arbuscular mycorrhizal fungi in the protection of maize plants grown in chromium-contaminated soils

Chromium (Cr) is a nonessential element for plants that is extremely toxic at high concentrations. Zea mays L. is a species of plant that has developed adaptive mechanisms to increase its tolerance and absorption capacity for this metal. One effective mechanism is to form associations with arbuscular mycorrhizal fungi (AMF), which help the plant cope with stress from heavy metals such as Cr. However, it is still not clear which species of AMF are more efficient as bioremediating agents for plants of agricultural importance. Here, we evaluated the effect of Gigaspora gigantea and Rhizophagus irregularis as protective agents of maize plants in soils contaminated with Cr at concentrations of 0, 48.5, 97 and 194 mg kg^-1 under greenhouse conditions. Maize tolerance to Cr was corroborated, as well as increased absorption of this element by plants associated with both species of AMF. G. gigantea caused an increase in nitrogen content and greater translocation of Cr to the aerial part of the plant; R. irregularis registered an increase in the formation of arbuscules and vesicles with increasing metal concentration and greater retention of Cr in the roots of the plants. Based on these results, we can conclude that the analyzed species of fungi use different strategies, with similar effectiveness, to enhance the absorption capacity of Cr by the plant and influence the differential deposition of this metal in various parts of the plant.

Nitrogen Effect on Growth-Related Parameters and Evaluation of Portulaca oleracea as a Phytoremediation Species in a Cr(VI)-Spiked Soil

In a pot experiment, we assessed the potential of purslane (Portulaca oleracea) as a phytoremediation species in Cr(VI)-contaminated soils. We focused on the evaluation of phytotoxic Cr(VI) effects at concentrations reaching 150 mg Cr(VI) kg−1 and the possible stress amelioration effect of nitrogen on Cr(VI)-stressed plants. Treatments were T-0 (control), T-1 (25 mg Cr(VI) kg−1), T-2 = 50 mg kg−1, T-3 = 100 mg kg−1, and T-4 = 150 mg kg−1. We measured Cr(VI) concentration in aerial and root tissues, a series of parameters related to photosynthesis and plant growth, phosphorus aerial plant tissue content, and we also calculated indices (ratios) related to leaf growth and above ground tissue water content. Cr(VI) almost exclusively was found in root tissues; all physiological and growth parameters studied were severely affected and plants selectively accumulated phosphorus in aerial plant tissues with increasing Cr(VI) soil concentrations. On the other hand, N amendment resulted in improved plant features in some of the measured parameters: chlorophyll index was improved with added N at T-2, plant height was significantly higher at T-0, T-1, and T-2, and aerial dry weight and leaf area was higher at T-0; these effects indicate that added N did increase P. oleracea potential to ameliorate Cr(VI) toxic effects. We conclude that purslane showed a potential as a possible species to be successfully introduced to Cr(VI)-laden soils, but more research is certainly necessary.

Amelioration of disulfonated Acid Red and hexavalent chromium phytotoxic effects on Triticum aestivum using bioremediating and plant growth-promoting Klebsiella pneumoniae SK1

Bacterial strain identified as Klebsiella pneumoniae SK1, based on 16S rDNA sequence study, was isolated from a textile mill discharge point at Dada Nagar industrial area in Kanpur, India in media containing 100 µg/ml each of disulfonated azo dye Acid Red 249 (AR) and hexavalent chromium [Cr(VI)]. SK1 is efficient in the concomitant decolorization and reduction of 100 µg/ml toxic AR and Cr(VI) respectively in 48 h under microaerophilic conditions. The efficacy of AR decolorization was found to be preeminent in static conditions, 37°C, 2-4% salinity and 7-9 pH range. 99% decolorization for initial AR concentration of 100 µg/ml in 48 h, and 91% for 250 µg/ml and 77% for 500 µg/ml was recorded in 72 h respectively in the presence of 100 µg/ml Cr(VI). Significant reductive changes in spectroscopic absorption spectra were observed for SK1 treated AR+Cr(VI) amended media with respect to controls. FITR spectroscopy was used to ascertain the breakage of the azo dye bond and the formation of biodegradative metabolites. Additionally, SK1 was found to be positive for indole acetic acid, ammonia, phosphate and potassium solubilization and biofilm formation. In plant bioassay, in vitro SK1 treated AR+Cr(VI) TSB media was used to treat Triticum aestivum in the soil environment. In comparison to untreated control, plants treated with bioremediation media shows increased percent germination, root and shoot length with a complete reversal of phytotoxic effects of the AR+Cr(VI) treated plants. In this study, environmental K. pneumoniae SK1 shows concomitant azo dye and Cr(VI) remediation with plant growth-promoting activity.

Effects of chromium stress on the rhizosphere microbial community composition of Cyperus alternifolius

Wetland plants are often used as the main body of soil, and the rhizosphere is a hot spot migration and transformation. Response mechanism to rhizosphere microorganisms on chromium(Cr) stressing could help improve the phytoremediation system. Cyperus alternifolius(CA) is selected as the research object by Cr-stress treatments and uncontaminated treatments with different cultivated pattern, included sole cultivated pattern(CAI), two-cultivated pattern (CAII), three-cultivated pattern (CAIII), and the un-planted blank samples (CK). 16s rRNA gene sequencing and metagenomic sequencing are performed to measure rhizosphere microbial community. And Five common enzymes in rhizosphere soils were observed: β-1,4-glucosidase (BG), β-N-acetylglucosaminidase (NAG), β-1,4-xylosidase (BX), cellobiohydrolase (CBH) and Leucine amino peptidase (LAP) in the rhizosphere. The results show that Gammaproteobacteria, Actinobacteria, Alphaproteobacteria, Gemmatimonadetes, Deltaproteobacteria are top five (63.97%) of the total sequence number. Wetland plants enriched a large amount of soil Cr in themselves, and the rhizosphere microorganisms don't show significant difference in community structure after affecting. 10.48% variation of microbial community is caused by Cr-stress. Acidovorax showed a great potential for chromium resistance. BX involvement in tolerance processes indirectly affects microbial communities (P < 0.01), there is a strong linear relationship between enzyme activity and the plants accumulating Cr and microbial community within 15.58% variant. The material accumulation and microbial quantity of CAIII are relatively low, but high biodiversity remains after affecting. These results provide references for in-depth understanding of rhizosphere microbial response to heavy metal pollution in wetland phytoremediation and interaction between wetland plants and rhizosphere microorganisms.

Exogenous application of black cumin (Nigella sativa) seed extract improves maize growth under chromium (Cr) stress

Accumulation of non-essential heavy metals like chromium (Cr) is among major abiotic stresses, which adversely affect crop growth. Hexavalent chromium [Cr(VI)] is the most dangerous form negatively affecting the growth and productivity of crops. This study evaluated the role of black cumin extracts (BCE) in improving growth and productivity of maize genotypes under different concentrations of Cr(VI). Two maize genotypes ("Neelum" and "P1543") were grown under 0, 4, 8 and 12 mg Cr(VI) kg^-1 concentrations. The BCE was applied as foliar spray at three concentrations (0, 10 and 20%) at 25 and 45 days after sowing. Increasing Cr(VI) concentration significantly (p < 0.05) reduced seed germination, root and allometric traits, gas exchange attributes and relative water contents of tested genotypes. Hybrid maize genotype better tolerated tested Cr(VI) concentrations than synthetic genotype with lower Cr accumulation and better allometric and gas exchange traits. Exogenous application of 20% BCE proved effective in lowering the adverse effects of Cr(VI) toxicity on maize genotypes. It is concluded that 20% BCE could be used to improve maize performance through better allometric and gas exchange traits under different Cr(VI) concentrations. Nonetheless, actual mechanisms involved in improved Cr(VI)-tolerance of maize with BCE application must be explored. Novelty statement Black cumin has been widely used to reduce Cr toxicity in animals. However, the role of black cumin in reducing Cr toxicity in plants has never been studied. The present study was conducted to infer the role of different concentrations of black cumin extract in improving the growth of synthetic and hybrid maize genotypes under different levels of Cr stress. It is concluded that black cumin extract could be used to lower Cr toxicity in maize grown under Cr-contaminated soils.

Effect of hexavalent chromium on the environment and removal techniques: A review

Despite the importance of chromium (Cr) in most anthropogenic activities, the subsequent environmental adulteration is now a source of major concern. Cr occurs in numerous oxidation states, with the furthermost stable and frequently occur states being Cr(0), Cr(III) and Cr(VI). Cr(0) and Cr(III) are vital trace elements while Cr(VI) is dispensable and noxious to living organisms. Predominantly in plants, Cr at low concentrations of about 0.05-1 mg/L assist to boost growth as well as increase productivity. However, accumulation of Cr could represent a potential threat to living organisms. Cr absorption, displacement and accretion depend on its speciation, which also determines its toxicity which is often diverse. Indications of its toxicity include; reduction of seed germination, retardation of growth, reduction of yield, inhibition of enzymatic activities, weakening of photosynthesis, nutrient, oxidative disparities and genetic mutation in plants as well as several injurious diseases in animals and humans. In this study, we have presented a comprehensive review as well as an informative account of the influence of Cr on the environment drawn from researches carried out over the years following an analytical approach. Uniquely, this work presents a review of the effects and remediation of Cr from soil and wastewater drawn from several evidence and meta-data-based articles and other publications. Accordingly, the write-up is intended to appeal to the consciousness of the general public that the significance of Cr notwithstanding, its environmental toxicity should not be taken for granted.

Unique biocenosis as a foundation to develop a phytobial consortium for effective bioremediation of Cr(VI)-polluted waters and sediments

This paper analyzes a unique, aquatic phytobial biocenosis that has been forming naturally for over 20 years and operating as a filter for Cr(VI)-polluted groundwater. Our study presents a thorough taxonomic analysis of the biocenosis, including filamentous algae, vascular plants, and microbiome, together with the analysis of Cr accumulation levels, bioconcentration factors and other environmentally-significant parameters: siderophore production by bacteria, biomass growth of the plants or winter hardiness. Among 67 species identified in the investigated reservoir, 13 species were indicated as particularly useful in the bioremediation of Cr(VI)-polluted water and sediment. Moreover, three species of filamentous algae, Tribonema sp., and three easily culturable bacterial species were for the first time shown as resistant to Cr concentration up to 123 mg/dm3, i.e. 6150 times over the permissible level. The work presents a modern holistic phytobial consortium indispensable for the remediation of Cr(VI)-contaminated aquatic environment in temperate zones worldwide.

In-site and Ex-site Date Palm Exposure to Heavy Metals Involved Infra-Individual Biomarkers Upregulation

As a tree of considerable importance in arid regions-date palm, Phoenix dactylifera L. survival in contaminated areas of Sfax city has drawn our attention. Leaf samples of the plants grown in the study area showed high levels of cadmium (Cd), copper (Cu), and chromium (Cr). On the basis of this finding, the cellular mechanisms that explain these metal accumulations were investigated in controlled conditions. After four months of exposure to Cd, Cr, or Cu, high bioconcentration and translocation factor (TF>1) have been shown for date palm plantlets exposed to Cd and low TF values were obtained for plantlets treated with Cr and Cu. Moreover, accumulation of oxidants and antioxidant enzyme activities occurred in exposed roots to Cu and Cd. Secondary metabolites, such as polyphenols and flavonoids, were enhanced in plants exposed at low metal concentrations and declined thereafter. Accumulation of flavonoids in cells may be correlated with the expression of the gene encoding Pdmate5, responsible for the transport of secondary metabolites, especially flavonoids. Other transporter genes responded positively to metal incorporation, especially Pdhma2, but also Pdabcc and Pdnramp6. The latter would be a new candidate gene sensitive to metallic stress in plants. Expressions of gene coding metal chelators were also investigated. Pdpcs1 and Pdmt3 exhibited a strong induction in plants exposed to Cr. These modifications of the expression of some biochemical and molecular based-markers in date palm helped to better understand the ability of the plant to tolerate metals. They could be useful in assessing heavy metal contaminations in polluted soils and may improve accumulation capacity of other plants.

Mechanisms in plant growth-promoting rhizobacteria that enhance legume-rhizobial symbioses

Nitrogen fixation is an important biological process in terrestrial ecosystems and for global crop production. Legume nodulation and N₂ fixation have been improved using nodule-enhancing rhizobacteria (NER) under both regular and stressed conditions. The positive effect of NER on legume-rhizobia symbiosis can be facilitated by plant growth-promoting (PGP) mechanisms, some of which remain to be identified. NER that produce aminocyclopropane-1-carboxylic acid deaminase and indole acetic acid enhance the legume-rhizobia symbiosis through (i) enhancing the nodule induction, (ii) improving the competitiveness of rhizobia for nodulation, (iii) prolonging functional nodules by suppressing nodule senescence and (iv) upregulating genes associated with legume-rhizobia symbiosis. The means by which these processes enhance the legume-rhizobia symbiosis is the focus of this review. A better understanding of the mechanisms by which PGP rhizobacteria operate, and how they can be altered, will provide opportunities to enhance legume-rhizobial interactions, to provide new advances in plant growth promotion and N₂ fixation.

Phytoremediation—From Environment Cleaning to Energy Generation—Current Status and Future Perspectives

Phytoremediation is a technology based on the use of green plants to remove, relocate, deactivate, or destroy harmful environmental pollutants such as heavy metals, radionuclides, hydrocarbons, and pharmaceuticals. Under the general term of phytoremediation, several processes with distinctively different mechanisms of action are hidden. In this paper, the most popular modes of phytoremediation are described and discussed. A broad but concise review of available literature research with respect to the dominant process mechanism is provided. Moreover, methods of plant biomass utilization after harvesting, with particular regard to possibilities of “bio-ore” processing for metal recovery, or using energy crops as a valuable source for bio-energy production (bio-gas, bio-ethanol, bio-oil) are analyzed. Additionally, obstacles hindering the commercialization of phytoremediation are presented and discussed together with an indication of future research trends.

A review of chromite mining in Sukinda Valley of India: impact and potential remediation measures

Sukinda Valley, one of the highly polluted areas of the world is generating tons of mining waste and causing serious health and environmental issues in its surroundings. Several reports are available reporting the severity of hexavalent chromium, yet little efforts have been made to address the pollution and its remediation due to a lack of proper remedial measures. The review highlights the pros and cons of various physical, chemical and biological techniques used worldwide for the treatment of chromium waste and also suggests better and reliable bioremediation measures. Microbes such as Acidophilium and Acidithiobacillus caldus (Bioleaching), Pseudomonas, Micrococcus and Bacillus (Bioreduction), Aereobacterium and Saccharomyces (Biosorption), are widely used for bioremediation of hexavalent chromium owing to their unique metabolic activities, ionic movement through an extracellular membrane, and other cellular adsorptions and reduction properties. The use of native and hybrid combinations of microbes supported by organic supplements is projected as a fast and efficient technique that not only reduces chromium quantity but also maintains the integrity of the microbial sources. Innovation and emphasis on nano-based products like nanocomposite, nano adsorbent, nanoscale zerovalent iron (nZVI) particles and multifunctional plant-growth-promoting bacteria (PGPB) will serve as the next generation environmental remediation technologies in the near future.

Improvement of rice plant productivity by native Cr(VI) reducing and plant growth promoting soil bacteria Enterobacter cloacae

Rapid industrialization and anthropogenic activities have produced huge amount of noxious Cr(VI), which accumulate in the soil for longer period. As a consequence, that decreases rice plant productivity in contiguous agricultural field of Sukinda mining area, Odisha. Thus, the high Cr(VI) resistant native bacterial strain CTWI-06 was selected for the study, which depicted resistance to 3500 ppm of Cr(VI) and wide array of other metals. Under optimized condition, the multi-metal resistant bacteria reduced 94% Cr(VI) within 92 h and Cr(VI) reduction was confirmed by FTIR and XRD analysis. Plant growth promoting traits like N₂ fixation; phosphate (146.87 ppm), potassium (12.55 ppm) and Zn solubilization; ammonification; IAA production (114 μg mL^-1) and suppression of fungal phytopathogens such as Rhizoctonia solani (ITCC 2060) and Phytium debaryanum (ITCC 5488) were also recorded. The bacterial strain was identified as Enterobacter cloacae CTWI-06 by 16S rDNA sequence (Accession No. MG757378). It significantly improved growth traits as well as productivity of Mahalakshmi rice variety in pot culture. Thus, the potential Cr(VI) reducing and PGPB strain may be utilized for long term bioremediation of Cr(VI) in chromium contaminated soil and to maintain soil fertility.

Phytoremediation and detoxification of xenobiotics in plants: herbicide-safeners as a tool to improve plant efficiency in the remediation of polluted environments. A mini-review

Phytoremediation is a widely studied and applied technology, based on the use of plants and their associated microorganisms to decontaminate polluted sites. In recent years, different strategies have been investigated to improve the phytoremediation efficiency of the selected plants. In this context, some studies have shown that herbicide-safeners, chemicals applied to crops to enhance their tolerance to herbicides, can increase the phytoremediation of soils and water polluted by organic and inorganic contaminants. Safeners, by inducing the xenobiotic detoxification and the antioxidant metabolism in plants, can enhance their removal potential in the cleaning process. In this review, after a short survey of phytoremediation technologies and the biochemical mechanisms activated by plants to tolerate and detoxify heavy metals and herbicides, the use of herbicide-safeners as a tool to increase the phytoremediation performance is reviewed and discussed.

A comparative evaluation towards the potential of Klebsiella sp. and Enterobacter sp. in plant growth promotion, oxidative stress tolerance and chromium uptake in Helianthus annuus (L.)

Prevalence of metal pollutants exerts negative effects on human health and environment, thus propounding an urgent need for a safer substitute. This study was conducted to compare the chromium bioremediation and plant growth promotion ability of two bacterial strains, Klebsiella sp. strain CPSB4 (MH266218) and Enterobacter sp. strain CPSB49 (MH532567), isolated from the rhizospheric soils. A pot scale experiment was setup with Helianthus annuus (L.) as a test plant to compare the efficiency of both isolates in enhancement of plant growth, nutrients uptake, anti-oxidative enzymes production, lipid peroxidation, and chromium bioremediation. Inoculation of strains, CPSB4 and CPSB49 enhanced plant biomass, plant growth, nutrient uptake, anti-oxidative enzymes, and chromium bioremediation, while reduction in lipid peroxidation was observed compared to uninoculated control under chromium stress. The maximum increase in plant growth and nutrient uptake was found in treatments inoculated with CPSB49, while maximum chromium uptake by sunflower was observed in treatments inoculated with CPSB4. Moreover, an increase in anti-oxidative enzyme production and decrease in lipid peroxidation was observed on inoculation of the selected strains. Thus, the strains Klebsiella sp. and Enterobacter sp. can be effectively used in chromium bioremediation and plant growth promotion under chromium stress conditions.

Exploring the survival tactics and plant growth promising traits of root-associated bacterial strains under Cd and Pb stress: A modelling based approach

The study represents a microbial method for reducing heavy metal stress in terrestrial environment. Two rhizobacterial strains Pantoea agglomerance (PC1) and Pseudomonas aeruginosa (SA) having the ability to tolerate Cd²⁺ and Pb²⁺ ions stress, were employed in this study. The growth promotion and survival tactics of the strains under metal stress were explored through kinetic growth model using logistic equation, Luedeking-Piret model and Box Behnken design. Study also involves the interaction of strains with Zea mays L. under Cd²⁺ and Pb²⁺ ions stress. Results revealed that both strains have the potential to tolerate 500 mg L^-1 of Cd²⁺ and Pb²⁺, ions and maintained the plant growth promoting traits. The Luedeking-Piret model estimated the maximum value of IAA on biomass growth (Y_P/X) 5.377 μg g^-1 and 10.3 μg g^-1 under Cd²⁺ ions, while 7.742 μg g^-1 and 18.071 μg g^-1 under Pb²⁺ ions stress for strains SA and PC1, respectively. Further, phosphate solubilization activity was optimized with the help of response surface methodology using Box Behnken Design. The optimum solubilization by strain PC1 and SA was achieved at 100 and 150 mg L^-1 of Cd²⁺, and 150 and 200 mg L^-1 of Pb²⁺ ion concentration at the pH range 6.75 and 7.5 respectively. The interactive study with Zea mays L. showed significant increase in seed germination in the presence of Cd²⁺ and Pb²⁺ ions thereby proving them as potent plant growth promoters and metal stress reducing biological agents. Hence, the findings of the study suggest that rhizobacterial strains could be a sustainable tool for restoration of metal contaminated sites.

A Bacillus strain TCL isolated from Jharia coalmine with remarkable stress responses, chromium reduction capability and bioremediation potential

Microbial reduction of Cr(VI) to Cr(III) can mitigate environmental chromium toxicity. A chromium, cadmium and nickel tolerating strain TCL with 97% 16S rRNA gene sequence homology to Bacillus cereus was isolated from a derelict open-cast, Tasra Coalmine Lake of Jharia, India. It could tolerate up to Cr₂₀₀₀ [2,000 mg L^-1 Cr(VI)] and completely reduce Cr₂₀₀ within 16 h under heterotrophic condition. TCL grown in ≥ Cr₅₀₀ exhibited multifarious stress responses particularly in its prolonged lag-phase, like cell aggregation, up to two-fold elongation, increased exopolysaccharide production, and stress enzyme activities. These were relieved by increasing inoculum size or nutrient content. Chromium reduction was constitutive, with maximum activities detected in loosely-bound exopolysaccharides and membrane fractions, followed by cytoplasm and spent media. Cr(VI) was efficiently reduced to Cr(III) and >90% was released in spent media. Cells also expressed Cr-induced active efflux pumps. Growing cells or its crude enzyme extracts could efficiently reduce Cr(VI) in diverse temperatures (15-45 °C), pH (5-9); and in presence of other metals (Cd, Cu, Mo, Ni, Pb), oxyanions (SO₄^-2, NO₂^-), and metabolic inhibitors (phenol, NaN₃, EDTA). Growth and reduction were also detected in nutrient-limited minimal salt media, and contaminated leather industry effluent thereby making TCL a potential candidate for bioremediation.

Augmentation with potential endophytes enhances phytostabilization of Cr in contaminated soil

The contamination of soil with heavy metals is a major environmental problem worldwide. The combined use of plants and their associated microbes has gained popularity in recent years for their potential to remediate heavy metal-contaminated soil. In the current study, the effect that augmentation of soil with plant growth-promoting endophytes has on the phytostabilization of chromium (Cr)-contaminated soil was investigated. Three potential endophytic bacterial strains (Enterobacter sp. HU38, Microbacterium arborescens HU33, and Pantoea stewartii ASI11) were inoculated individually as well as in combination to Leptochloa fusca and Brachiaria mutica vegetated in Cr-contaminated soil. The accumulation of Cr in the root and shoot of the plants was determined. Moreover, bacterial persistence in the rhizosphere and endosphere was determined. Augmentation with potential endophytes significantly increased root length (24-45%), shoot height (39-64%), chlorophyll content (20-55%), and the overall biomass (32-61%) of the plants. Although L. fusca and B. mutica showed potential to accumulate Cr in their root and shoot, endophytic augmentation increased uptake, translocation, and accumulation of Cr in the roots and shoots of both plant species. However, L. fusca showed more potential to phytostabilize Cr as compared to B. mutica. Furthermore, the potential endophytes showed more survival and persistence within the roots than in the rhizosphere and shoot interior. This study provides useful evidence of endophyte-assisted phytoremediation to be the most sustainable and affordable approach for in situ remediation of Cr-contaminated soil.

Reduction of Hexavalent Chromium and Detection of Chromate Reductase (ChrR) in Stenotrophomonas maltophilia

An Gram negative strain of S. maltophilia, indigenous to environments contaminated by Cr(VI) and identified by biochemical methods and 16S rRNA gene analysis, reduced chromate by 100%, 98-99% and 92% at concentrations in the 10-70, 80-300, and 500 mg/L range, respectively at pH 7 and temperature 37 °C. Increasing concentrations of Cr(VI) in the medium lowered the growth rate but could not be directly correlated with the amount of Cr(VI) reduced. The strain also exhibited multiple resistance to antibiotics and tolerance and resistance to various heavy metals (Ni, Zn and Cu), with the exception of Hg. Hexavalent chromium reduction was mainly associated with the soluble fraction of the cell evaluated with crude cell-free extracts. A protein of molecular weight around 25 kDa was detected on SDS-PAGE gel depending on the concentration of hexavalent chromium in the medium (0, 100 and 500 mg/L). In silico analysis in this contribution, revealed the presence of the chromate reductase gene ChrR in S. maltophilia, evidenced through a fragment of around 468 bp obtained experimentally. High Cr(VI) concentration resistance and high Cr(VI) reducing ability of the strain make it a suitable candidate for bioremediation.

Citric acid assisted phytoextraction of chromium by sunflower; morpho-physiological and biochemical alterations in plants

Soil and water contamination from heavy metals and metalloids is one of the most discussed and burning global issues due to its potential to cause the scarcity of healthy food and safe water. The scientific community is proposing a range of lab and field based physical, chemical and biological solutions to remedy metals and metalloids contaminated soils and water. The present study finds out a possibility of Chromium (Cr) extraction by sunflower from spiked soil under chelating role of citric acid (CA). The sunflower plants were grown under different concentrations of Cr (0, 5, 10 & 20mgkg^-1) and CA (0, 2.5 & 5mM). Growth, biomass, gas exchange, photosynthesis, electrolyte leakage (EL), reactive oxygen species (ROS; malondialdehyde (MDA), hydrogen peroxide (H₂O₂) and the activities of antioxidant enzymes such as, superoxide dismutase (SOD), guaiacole values peroxidase (POD), ascorbate peroxidase (APX), catalase (CAT) were measured. The results depicted a clear decline in plant height, root length, leaf area, number of leaves and flowers per plant along with fresh and dry biomass of all parts of plant with increasing concentration of Cr in soil. Similar reduction was observed in chlorophyll a and b, total chlorophyll, carotenoids, soluble protein, gas exchange attributes and SPAD. The increasing concentration of Cr also enhanced the Cr uptake and accumulation in plant roots, stem and leaves along with the production of ROS and EL. The activities of antioxidant enzymes increased with increasing Cr concentration from 0 to 10mg, but decreased at 20mgkg^-1 soil. The CA application significantly alleviated Cr-induced inhibition of plant growth, biomass, photosynthesis, gas exchange, soluble proteins and SPAD value. Presence of CA also enhanced the activities of all antioxidant enzymes and reduced the production of ROS and EL. The chelating potential of CA increased the concentration and accumulation of Cr in plant roots, stem and leaves. It is concluded that the sunflower can be a potential candidate for the remediation of Cr under CA treatment, while the possibility may vary with genotype, Cr level and CA concentration.