Isolation and characterization of a heavy metal-resistant Burkholderia sp. from heavy metal-contaminated paddy field soil and its potential in promoting plant growth and heavy metal accumulation in metal-polluted soil

Two cadmium-resistant bacteria Burkholderia contaminans HA09 and Arthrobacter humicola improve phytoremediation efficiency of cadmium in Ageratum conyzoides L

To investigate the impact and mechanism of Cd-tolerant bacteria in soil on promoting Cd accumulation in Ageratum conyzoides L., we verified the impact of inoculating two strains, B-1 (Burkholderia contaminans HA09) and B-7 (Arthrobacter humicola), on Cd accumulation in A. conyzoides through a pot experiment. Additionally, we investigated the dissolution of CdCO3 and nutrient elements, as well as the release of indoleacetic acid (IAA) by the two strains. The results showed that both strains can significantly improve the dissolution of CdCO3. Strains B-1 and B-7 had obvious effect of dissolving phosphorus, which was 5.63 and 2.76 times higher than that of the control group, respectively. Strain B-7 had significant effect of dissolution potassium, which was 1.79 times higher than that of the control group. Strains B-1 and B-7 had significant nitrogen fixation effect, which was 29.53 and 44.39 times higher than that of the control group, respectively. In addition, inoculating with strain B-1 and B-7 significantly increased the Cd extraction efficiency of A. conyzoides (by 114% and 45% respectively) through enhancing Cd accumulation and the biomass of A. conyzoides. Furthermore, the inoculation of strain B-1 and B-7 led to a significant increase in the activities of CAT and SOD, as well as the content of chlorophyll a and total chlorophyll in the leaves of A. conyzoides. To sum up, strain B-1 and B-7 can promote the phytoremediation efficiency of A. conyzoides on Cd by promoting the biomass and Cd accumulation of A. conyzoides.

The quorum sensing SinI/R system contributes to cadmium immobilization in Ensifer adhaerens NER9 in the cadmium-contaminated solution

In this study, the cadmium (Cd)-tolerant Ensifer adhaerens strain NER9 with quorum sensing (QS) systems (responsible for N-acyl homoserine lactone (AHL) production) was characterized for QS system-mediated Cd immobilization and the underlying mechanisms involved. Whole-genome sequence analysis revealed that strain NER9 contains the QS SinI/R and TraI/R systems. Strains NER9 and the NER9∆sinI/R, NER9∆traI/R, and NER9∆sinI/R-traI/R mutants were constructed and compared for QS SinI/R and TraI/R system-mediated Cd immobilization in the solution and the mechanisms involved. After 24 h of incubation, strain NER9 significantly decreased the Cd concentration in the Cd-contaminated solution compared with the NER9∆sinI/R, NER9∆traI/R, and NER9∆sinI/R-traI/R mutants. The NER9∆sinI/R mutant had a greater impact on Cd immobilization and a lower impact on the activities of AHLs than did the NER9∆traI/R mutant. The NER9∆sinI/R mutant had significantly greater Cd concentrations and lower cell wall- and exopolysaccharide (EPS)-adsorbed Cd contents than did strain NER9. Furthermore, the NER9∆sinI/R mutant presented a decrease in the number of functional groups interacting with Cd, compared with strain NER9. These results suggested that the SinI/R system in strain NER9 contributed to Cd immobilization by mediating cell wall- and EPS-adsorption in Cd-containing solution.

In vitro and in vivo screening of bacterial species from contaminated soil for heavy metal biotransformation activity

Heavy metals (HMs) are widely used in various industries. High concentrations of HMs can be severely toxic to plants, animals and humans. Microorganism-based bioremediation has shown significant potential in degrading and detoxifying specific HM contaminants. In this study, we cultivated a range of bacterial strains in liquid and solid nutrient medium containing different concentrations of different HMs to select and analyze bacteria capable of transforming HMs. The bacterial strains most resistant to selected HMs and exhibiting the ability to remove HMs from contaminated soils were identified. Then, the bacterial species capable of utilizing HMs in soil model experiments were selected, and their ability to transform HMs was evaluated. This study has also generated preliminary findings on the use of plants for further removal of HMs from soil after microbial bioremediation. Alcaligenes faecalis, Delftia tsuruhatensis and Stenotrophomonas sp. were selected for their ability to grow in and utilize HM ions at the maximum permissible concentration (MPC) and two times the MPC. Lysinibacillus fusiformis (local microflora) can be used as a universal biotransformation tool for many HM ions. Brevibacillus parabrevis has potential for the removal of lead ions, and Brevibacillus reuszeri and Bacillus safensis have potential for the removal of arsenic ions from the environment. The bacterial species have been selected for bioremediation to remove heavy metal ions from the environment.

Plant growth-promoting bacteria isolated from earthworms enhance spinach growth and its phytoremediation potential in metal-contaminated soils

The aim of this study was to evaluate the impact of metal-tolerant plant growth-promoting bacteria (PGPB) isolated from the chloragogenous tissue of Aporrectodea molleri, which represents a unique habitat. Our objectives were to investigate their effects on the growth of Spinacia oleracea under heavy metal stress and assess their potential for enhancing phytoremediation capabilities. The experiment was conducted in an alkaline soil contaminated with 7 mg kg-1 of cadmium, 100 mg kg-1 of nickel, 150 mg kg-1 of copper, 300 mg kg-1 of Zinc, and mg kg-1 of 600 Manganese. The results showed that heavy metal stress considerably diminished root (42.8%) and shoot length (60.1%), biomass (80%), chlorophyll content (41%), soil alkaline (45%), and acid (51%) phosphatases (42%) and urease (42%). However, soil inoculation with bacterial isolates remarkably improved plant growth. Soil bioaugmentation increased spinach growth (up to 74.5% for root length, up to 106.3% for shoot length, and up to 5.5 folds for fresh biomass) while significantly increasing soil enzyme activity and NPK content. Multivariate data analysis indicated that soil inoculation with Bacillus circulans TC7 promoted plant growth while limiting metal bioaccumulation, whereas Pseudomonas sp. TC33 and Bacillus subtilis TC34 increased metal bioaccumulation in spinach tissues while minimizing their toxicity. Our study confirms that earthworms are a reservoir of multi-beneficial bacteria that can effectively improve phytoremediation efficiency and mitigate the toxic effects of heavy metals on plant growth. Further studies are needed to investigate the long-term effects and feasibility of using these isolates as a consortium in field applications.

Isolation and identification of metallotolerant bacteria with a potential biotechnological application

Mining has led to severe environmental pollution in countries with exhaustive mining production and inadequate industrial waste regulation. Microorganisms in contaminated sites, like mine tailings, have adapted to high concentrations of heavy metals, developing the capacity of reducing or removing them from these environments. Therefore, it is essential to thoroughly characterize bacteria present in these sites to find different ways of bioremediation. In this regard, in this study, an enrichment and isolation procedure were performed to isolate bacteria with lower nutritional requirements and high tolerance to Cu(II) and Fe(II) from two Sonoran River basin mining tails. Two Staphylococcus species and a Microbacterium ginsengisoli strain were isolated and identified from the San Felipe de Jesús mining tail. Also, three strains were isolated from the Nacozari de García mining tail: Burkholderia cenocepacia, Sphingomonas sp. and Staphylococcus warneri. Significant microbiological differences were found between the two sites. All these species exhibited tolerance up to 300 mg/L for Cu (II)-Fe (II) solutions, indicating their capacity to grow in these conditions. Moreover, a consortium of isolated bacteria was immobilized in two different biocomposites and the biocomposite with larger pore size achieved greater bacterial immobilization showcasing the potential of these bacteria in biotechnological applications.

Novel modified semi-carbonized fiber prepared using discarded clothes for derisking Cu(II) and Pb(II) contaminated water

We report a novel modified semi-carbonized fiber (CF) prepared using cotton and acrylic clothes for derisking contaminated water to realize the resource utilization of discarded clothes in wastewater treatment. In this study, amphoteric and auxiliary modifiers were used to modify CFs for preparing amphoteric and amphoteric-auxiliary CFs. The basic physicochemical properties of different modified CFs were determined, and the microscopic morphology of modified CFs was detected. The isothermal adsorption characteristics of Cu(II) and Pb(II) on different modified CFs were investigated by the batch method, and the effect mechanisms of temperature, pH, ionic strength, and material dose were compared. Physicochemical properties and microscopic morphology results proved that amphoteric and auxiliary modifiers were modified on the CF surface and changed the surface properties of CF. The adsorption capacities of Cu(II) and Pb(II) on modified CFs increased with the increase in equilibrium concentration of Cu(II) and Pb(II), and the isotherm was more suitable for Freundlich model fitting than that of the Langmuir model. The maximum adsorption capacities (qm) of Cu(II) and Pb(II) on different modified CFs were 60.72-81.26 mg/g and 102.58-161.72 mg/g, respectively, and presented the trend of amphoteric-auxiliary CFs > amphoteric CFs > CFs. Increasing pH and temperature and decreasing ionic strength and material dose were beneficial to Cu(II) and Pb(II) adsorption. The Cu(II) and Pb(II) adsorption process was a spontaneous, endothermic, and entropy-increasing reaction, and the adsorption rate was controlled by chemisorption. The adsorption amount of amphoteric-auxiliary CFs maintained about 65% of original materials after 3 times of regeneration. Electrostatic attraction, precipitation, complexation, and ion exchange were the main adsorption mechanisms. The cation exchange capacity and total pore volume of modified CFs were key to determining qm of Cu(II) and Pb(II).

Characterization of Rhizosphere Microbial Diversity and Selection of Plant-Growth-Promoting Bacteria at the Flowering and Fruiting Stages of Rapeseed

Plant rhizosphere microorganisms play an important role in modulating plant growth and productivity. This study aimed to elucidate the diversity of rhizosphere microorganisms at the flowering and fruiting stages of rapeseed (Brassica napus). Microbial communities in rhizosphere soils were analyzed via high-throughput sequencing of 16S rRNA for bacteria and internal transcribed spacer (ITS) DNA regions for fungi. A total of 401 species of bacteria and 49 species of fungi in the rhizosphere soil samples were found in three different samples. The composition and diversity of rhizosphere microbial communities were significantly different at different stages of rapeseed growth. Plant-growth-promoting rhizobacteria (PGPRs) have been widely applied to improve plant growth, health, and production. Thirty-four and thirty-one PGPR strains were isolated from the rhizosphere soil samples collected at the flowering and fruiting stages of rapeseed, respectively. Different inorganic phosphorus- and silicate-solubilizing and auxin-producing capabilities were found in different strains, in addition to different heavy-metal resistances. This study deepens the understanding of the microbial diversity in the rapeseed rhizosphere and provides a microbial perspective of sustainable rapeseed cultivation.

ABA-metabolizing bacteria and rhamnolipids as valuable allies for enhancing phytoremediation efficiency in heavy metal-contaminated soils

Microbial-assisted phytoremediation has great potential to improve the efficiency of phytoremediation in heavy metal (HM)-contaminated soils. In this study, the synergistic effects of rhamnolipids and the abscisic acid (ABA)-metabolizing bacterium Rhodococcus qingshengii on the phytoremediation efficiency of Indian mustard (Brassica juncea) in HM-contaminated soils were investigated. The Cd, Zn, and Pb contents in plants treated with a combination of rhamnolipids and R. qingshengii were 48.4-77.1 %, 14.6-40.4 %, and 16.1-20.0 % higher, respectively, than in those treated with R. qingshengii alone, and 42.8-59.2 %, 13.1-48.2 %, and 7.3-67.5 % higher, respectively, than in those treated with rhamnolipids alone. In addition, the bioconcentration factors of each metal were improved, and the biomass further increased by 36.6-65.7 % compared to that of single treatments. Pearson's correlation analysis showed that rhamnolipids and R. qingshengii enhanced the accumulation of HMs in B. juncea by activating the available forms of HMs in the soil and regulating the ABA and indole-3-acetic acid in plants, respectively. The structural equation model indicated that R. qingshengii had a larger path coefficient than rhamnolipids in terms of HM content and plant biomass, suggesting that R. qingshengii may have a greater contribution to promoting the extraction of HMs from the soil under synergistic conditions. In conclusion, the combination of rhamnolipids and R. qingshengii has great potential to enhance the phytoremediation efficiency of hyperaccumulating plants in HM-contaminated soils.

Plant growth-promoting rhizobacteria: A good companion for heavy metal phytoremediation

Phytoremediation is an environment-friendly approach regarded as a potential candidate for remediating heavy metal (HM)-contaminated soils. However, the low efficacy of phytoremediation is a major limitation that hampers its large-scale application. Therefore, developing strategies to enhance phytoremediation efficacy for contaminated soils is crucial. Plant growth-promoting rhizobacteria (PGPR) considerably contribute to phytoremediation intensification. To improve the efficiency of plant-microbe symbiosis for remediation, the mechanisms underlying PGPR-stimulated HM accumulation and tolerance in plants should be comprehensively understood. This review focuses on hyperaccumulators, PGPR, and the mechanisms by which PGPR enhance phytoremediation from four aspects: providing nutrients to plants, secreting plant hormones and specific enzymes, inducing systemic resistance, and altering the bioavailability of HMs in soils. It also provides a theoretical and technical basis for future research on PGPR synergism in promoting the phytoextraction efficiency in HM-contaminated soils.

Biofilm-forming plant growth-promoting rhizobacterial consortia isolated from mines and dumpsites assist green remediation of toxic metal (Ni and Pb) using Brassica juncea

To study how biofilm-forming rhizobacteria isolated from mines and dumpsites improved the phytoremediation efficacy of B. juncea in metal-contaminated soil. Out of 91 isolates, six were chosen for research based on their tolerance to metals, and their efficient PGPR properties, and subjected to the design of a consortium. A compatibility study revealed no antagonistic interaction between rhizobacterial-consortiums. The results of the biofilm formation and FEG-SEM studies revealed that a consortium-BC8 formed a strong biofilm on the root surface of B. juncea seedlings. Based on results obtained with the phytoextraction efficiency of B. juncea in consortium-BC8 (SMHMZ46 and SMHMP23), they were identified as Klebsiella variicola and Pseudomonas otitidis, respectively, and submitted to NCBI GenBank with accession numbers MZ145092 and OK560623. This rhizobacteria is the first to be reported as assisting Ni and Pb phytoremediation by employing B. juncea. Soil inoculation with consortium-BC8 increased the amount of soluble Ni and Pb by 13.25-fold and 10.69-fold, respectively, when compared to the control. These consortiums-BC8 significantly increased vegetative growth and metal accumulation in root and shoot with a translocation-factor of 1.58 for Ni and soil to root with a bioconcentration-factor of 1.3 for Pb in B. juncea grown in individual soil contamination with 96.05 mg/kg NiCl2 and 89.63 mg/kg Pb(NO3)2, which are significantly higher than other consortium treatments and the non-inoculated control. B. juncea amendments with a biofilm-forming consortium-BC8 having TF, BCF, and BAC > 1 for Ni, whereas BCF > 1, TF, and BAC < 1 for Pb, are appropriate for green remediation of Ni and phytostabilization of Pb.

Effect of the Combination of Phosphate-Solubilizing Bacteria with Orange Residue-Based Activator on the Phytoremediation of Cadmium by Ryegrass

Amendments with activators or microorganisms to enhance phytoremediation in toxic-metal-polluted soils have been widely studied. In this research, the production of indoleacetic acid, siderophore, and 1-aminocyclopropane-1-carboxylate (ACC) deaminase by phosphate-solubilizing bacteria was investigated during a pure culture experiment. Pot experiments were performed using Cd-polluted soil with the following treatments: control (CK, only ultrapure water), orange-peel-based activator (OG), and a combination of phosphate-solubilizing bacteria (Acinetobacter pitti) and OG (APOG). Ryegrass plant height and fresh weight, Cd content in ryegrass, total and available Cd soil content, soil enzyme activity, and soil bacterial diversity were determined in this work. The findings showed that the height of ryegrass in OG and APOG increased by 14.78% and 21.23%. In the APOG group, a decreased ratio of Cd was 3.37 times that of CK, and the bioconcentration factor was 1.28 times that of CK. The neutral phosphatase activity of APOG was 1.33 times that of CK and catalase activity was 1.95 times that of CK. The activity of urease was increased by 35.48%. APOG increased the abundance of beneficial bacteria and Proteobacteria was the dominant bacterium, accounting for 57.38% in APOG. Redundancy analysis (RDA) showed that nutrient elements were conducive to the propagation of the dominant bacteria, the secretion of enzymes, and the extraction rate of Cd in the soil. The possible enhancement mechanism of phytoremediation of cadmium by A. pitti combined with OG was that, on the one hand, APOG increased soil nutrient elements and enzyme activities promoted the growth of ryegrass. On the other hand, APOG activated Cd and boosted the movement of Cd from soil to ryegrass. This research offers insight for the combination of phosphate-solubilizing bacteria with an orange-peel-based activator to improve phytoremediation of Cd-contaminated soils and also provides a new way for the resource utilization of fruit residue.

Genome-resolved metagenomics provides insights into the ecological roles of the keystone taxa in heavy-metal-contaminated soils

Microorganisms that exhibit resistance to environmental stressors, particularly heavy metals, have the potential to be used in bioremediation strategies. This study aimed to explore and identify microorganisms that are resistant to heavy metals in soil environments as potential candidates for bioremediation. Metagenomic analysis was conducted using microbiome metagenomes obtained from the rhizosphere of soil contaminated with heavy metals and mineral-affected soil. The analysis resulted in the recovery of a total of 175 metagenome-assembled genomes (MAGs), 73 of which were potentially representing novel taxonomic levels beyond the genus level. The constructed ecological network revealed the presence of keystone taxa, including Rhizobiaceae, Xanthobacteraceae, Burkholderiaceae, and Actinomycetia. Among the recovered MAGs, 50 were associated with these keystone taxa. Notably, these MAGs displayed an abundance of genes conferring resistance to heavy metals and other abiotic stresses, particularly those affiliated with the keystone taxa. These genes were found to combat excessive accumulation of zinc/manganese, arsenate/arsenite, chromate, nickel/cobalt, copper, and tellurite. Furthermore, the keystone taxa were found to utilize both organic and inorganic energy sources, such as sulfur, arsenic, and carbon dioxide. Additionally, these keystone taxa exhibited the ability to promote vegetation development in re-vegetated mining areas through phosphorus solubilization and metabolite secretion. In summary, our study highlights the metabolic adaptability and ecological significance of microbial keystone taxa in mineral-affected soils. The MAGs associated with keystone taxa exhibited a markedly higher number of genes related to abiotic stress resistance and plant growth promotion compared to non-keystone taxa MAGs.

Current status and future prospect of managing lead (Pb) stress through microbes for sustainable agriculture

Soil is an important residence under various biotic and abiotic conditions. Contamination of soil by various means has hazardous effects on both plants and humans. Soil contamination by heavy metals occurs due to various man-made activities, including improper industrial and agricultural practices. Among the heavy metals, after arsenic, lead (Pb) was found to be the second most toxic metal and potent pollutants that accumulate in sediments and soils. Pb is not considered an essential element for promoting plant growth but is readily absorbed and accumulated in different plant parts. Many parameters such as pH, root exudation, soil particle size, cation exchange capacity, and other physicochemical parameters are involved in Pb uptake in plants. Excess amounts of Pb pose a threat to plant growth and cause toxicity such as chlorosis, blackening of the root system, and stunted growth. Pb toxicity may inhibit photosynthesis, disturb water balance and mineral nutrition, and alter the hormonal status, structure, and membrane permeability of plants. Therefore, this review addresses the effects of Pb toxicity and its impact on plant growth, including the morphological, physiological, and biological effects of Pb toxicity, the mechanisms behind different strategies promoting plant growth, and in combating Pb-induced stress. The bioremediation strategy for Pb removal from Pb-contaminated soil also plays an important role in combating Pb toxicity using bacterial community. Pb-contaminated soil may be remediated using different technologies such as rhizofiltration and phytoremediation, which tend to have a great capacity to curb Pb-contamination within the soil.

Cascading effects of Pb on the environmental and symbiotic microbiota and tadpoles' physiology based on field data and laboratory validation

Heavy metal pollution poses a serious threat to ecosystems. Currently, there is a lack of field data that would enable us to gain a systematic understanding of the influences of heavy metals on aquatic ecosystems, especially the interactions between environments and animals. We studied the relationships between the variations in heavy metal concentrations (10 species including Pb in sediments and surface water), the community structure of environmental and symbiotic microbiota, and the gut traits of Bufo gargarizans tadpoles across 16 sampling sites on the Chengdu Plain through rigorous statistical analysis and laboratory validation. The results show that heavy metal concentrations, especially the Pb concentration of the sediment, are linked to the variations in sediment and tadpoles' gut microbiomes but not to water microbiota. For the sediment microbiota, Pb causes a trade-off between the proportions of Burkholderiales and Verrucomicrobiae and affects the methane, sulfide, and nitrate metabolisms. For tadpoles, a high sediment Pb content leads to a low abundance of gut aerobic bacteria and a large relative gut weight under both field and laboratory conditions. In addition, Pb promotes the growth of B. gargarizans tadpoles under laboratory conditions. These effects seem to be beneficial to tadpoles. However, a high Pb content leads to a low abundance of probiotic bacteria (e.g., Verrucomicrobiae, Eubacteriaceae, and Cetobacterium) and a high abundance of pathogenic bacteria in the gut and environment, suggesting potential health risks posed by Pb. Interestingly, there is a causal relationship between Pb-induced variations in sediment and symbiotic microbiotas, and the latter is further linked to the variation in relative gut weight of tadpoles. This suggests a cascading effect of Pb on the ecosystem. In conclusion, our results indicate that among the heavy metals, the Pb in sediment is a critical factor affecting the aquatic ecosystem through an environment-gut-physiology pathway mediated by microbiota.

Phosphorus acquisition strategies of wheat are related to biochar types added in cadmium-contaminated soil: Evidence from soil zymography and root morphology

Biochar application for the remediation of cadmium (Cd)-contaminated soils may result in a relative deficiency of phosphorus (P) due to the disruption of soil nutrient balance. However, the P acquisition strategies of plants in such situation are still unclear. In this study, analyses on soil zymography and root morphology were combined for the first time to investigate the effects of pristine and P-modified biochars from apple tree branches on the P acquisition strategies of wheat under Cd stress. The results show that the application of pristine biochar exacerbated the soil's relative P deficiency. Wheat was forced to improve foraging for P by forming longer and thinner roots (average diameter 0.284 mm) as well as releasing more phosphatase to promote P mobilization in the soil. Moreover, bioavailable Cd affected the P acquisition strategies of wheat through stimulating the release of phosphatase from roots. The P-modified biochar maintained high levels of Olsen-P (>100 mg kg^-1) in the soil over time by slow release, avoiding the creation of relative P deficiency in the soil; and increased the average root diameter (0.338 mm) and growth performance index, which promoted shoot growth (length and biomass). Furthermore, the P-modified biochar reduced DTPA-extracted Cd concentration in soils by 79.8 % (pristine biochar by 26.9 %), and decreased the Cd translocation factor from root to shoot as well as Cd concentration in the shoots. Therefore, P-modified biochar has a great potential to regulate the soil element balance (carbon, nitrogen, and P), promote wheat growth, and remediate the Cd-contaminated soil.

Legacy metal contamination is reflected in the fish gut microbiome in an urbanised estuary

Estuaries are critical habitats subject to a range of stressors requiring effective management. Microbes are gaining recognition as effective environmental indicators, however, the response of host associated communities to stressors remains poorly understood. We examined microbial communities from seawater, sediments and the estuarine fish Pelates sexlineatus, in Australia's largest urbanised estuary, and hypothesised that anthropogenic contamination would be reflected in the microbiology of these sample types. The human faecal markers Lachno3 and HF183 were not detected, indicating negligible influence of sewage, but a gradient in copy numbers of the class 1 integron (intI-1), which is often used as a marker for anthropogenic contamination, was observed in sediments and positively correlated with metal concentrations. While seawater communities were not strongly driven by metal contamination, shifts in the diversity and composition of the fish gut microbiome were observed, with statistical links to levels of metal contamination (F_{2, 21} = 1.536, p < 0.01). Within the fish gut microbiome, we further report increased relative abundance of amplicon sequence variants (ASVs; single inferred DNA sequences obtained in sequencing) identified as metal resistant and potentially pathogenic genera, as well as those that may have roles in inflammation. These results demonstrate that microbial communities from distinct habitats within estuarine systems have unique response to stressors, and alterations of the fish gut microbiome may have implications for the adaptation of estuarine fish to legacy metal contamination.

Microbial induced phosphate precipitation accelerate lead mineralization to alleviate nucleotide metabolism inhibition and alter Penicillium oxalicum's adaptive cellular machinery

Microbial-induced phosphate (P) precipitation (MIPP) based on P-solubilizing microorganisms (PSM) is regarded as a promising approach to bioimmobilize environmental lead (Pb). Nevertheless, the underlying changes of Pb²⁺ biotoxicity in PSM during MIPP process were rarely discussed. The current study explored the Pb²⁺ immobilization and metabolic changes in PSM Penicillium oxalicum postexposure to Pb²⁺ and/or tricalcium phosphate (TCP). TCP addition significantly increased soluble P concentrations, accelerated extracellular Pb mineralization, and improved antioxidative enzyme activities in P. oxalicum during MIPP process. Secondary Pb²⁺ biomineralization products were measured as hydroxypyromorphite [Pb₁₀(PO₄)₆(OH)₂]. Using untargeted metabolomic and transcriptomics, we found that Pb²⁺ exposure stimulated the membrane integrity deterioration and nucleotide metabolism obstruction of P. oxalicum. Correspondingly, P. oxalicum could produce higher levels of gamma-aminobutyric acid (GABA) to enhance the adaptive cellular machineries under Pb²⁺ stress. While the MIPP process improved extracellular Pb²⁺ mineralization, consequently alleviating the nucleotide metabolism inhibition and membrane deterioration. Multi-omics results suggested that GABA degradation pathway was stimulated for arginine biosynthesis and TCA cycle after Pb²⁺ mineralization. These results provided new biomolecular information underlying the Pb²⁺ exposure biotoxicities to microorganisms in MIPP before the application of this approach in environmental Pb²⁺ remediation.

Hydrogen-oxidizing bacteria and their applications in resource recovery and pollutant removal

Hydrogen oxidizing bacteria (HOB), a type of chemoautotroph, are a group of bacteria from different genera that share the ability to oxidize H₂ and fix CO₂ to provide energy and synthesize cellular material. Recently, HOB have received growing attention due to their potential for CO₂ capture and waste recovery. This review provides a comprehensive overview of the biological characteristics of HOB and their application in resource recovery and pollutant removal. Firstly, the enzymes, genes and corresponding regulation systems responsible for the key metabolic processes of HOB are discussed in detail. Then, the enrichment and cultivation methods including the coupled water splitting-biosynthetic system cultivation, mixed cultivation and two-stage cultivation strategies for HOB are summarized, which is the critical prerequisite for their application. On the basis, recent advances of HOB application in the recovery of high-value products and the removal of pollutants are presented. Finally, the key points for future investigation are proposed that more attention should be paid to the main limitations in the large-scale industrial application of HOB, including the mass transfer rate of the gases, the safety of the production processes and products, and the commercial value of the products.

The potential of mineral weathering of halophilic-endophytic bacteria isolated from Suaeda salsa and Spartina anglica

Bacteria have the abilities of salt tolerant, mineral weathering and plant growth promoting can promote the growth of plants in saline lands. However, few reports of the mineral weathering capacity of halophilic-endophytic bacteria, raising the question of whether the halophilic-endophytic weathering bacteria are fundamentally distinct from those in plants communities. In this study, we isolated and characterized halophilic bacterial strains from the roots and leaves of Suaeda salsa and Spartina anglica with respect to their mineral weathering pattern, role in the promoting plant growth, community structure, and their changes in these two plants. Using improved Gibbson medium, we obtained 156 halophilic bacterial strains, among which 92 and 64 strains were isolated from the S. salsa and S. anglica samples, respectively. The rock weathering patterns of the isolates were characterized using batch cultures that measure the quantity of Si, Al, K, and Fe released from crystal biotite under aerobic conditions. Significantly, the biomass and capacity of the mineral weathering of the halophilic-endophytic bacteria were different in the plants. The abundance of the halophilic-endophytic bacterials in the Suaeda salsa was significantly greater than Spartina anglica, whereas the mineral weathering bacterial in the Suaeda salsa was similar to the Spartina anglica. Furthermore, the proportion of plant growth-promoting bacteria in the Suaeda salsa was higher than Spartina anglica. Phylogenetic analyses show that the weathered minerals were inhabited by specific functional groups of bacteria (Halomonas, Acinetobacter, Burkholderia, Alcaligenes, Sphingobium, Arthrobacter, Chryseobacterium, Paenibacillus, Microbacterium, Ensifer, Ralstonia and Enterobacter) that contribute to the mineral weathering. The changes in halophilic endophytes weathering communities between the two plants were attributable not only to major bacterial groups but also to a change in the minor population structure.

Claroideoglomus etunicatum affects the structural and functional genes of the rhizosphere microbial community to help maize resist Cd and La stresses

Arbuscular mycorrhizal fungi (AMF) and plant rhizosphere microbes reportedly enhance plant tolerance to abiotic stresses and promote plant growth in contaminated soils. The co-contamination of soil by heavy metals (e.g., Cd) and rare earth elements (e.g., La) represents a severe environmental problem. Although the influence of AMF in the phytoremediation of contaminated soils is well documented, the underlying interactive mechanisms between AMF and rhizosphere microbes are still unclear. We conducted a greenhouse pot experiment to evaluate the effects of AMF (Claroideoglomus etunicatum) on maize growth, nutrient and metal uptake, rhizosphere microbial community, and functional genes in soils with separate and combined applications of Cd and La. The purpose of this experiment was to explore the mechanism of AMF affecting plant growth and metal uptake via interactions with rhizosphere microbes. We found that C. etunicatum (i) significantly enhanced plant nutritional level and biomass and decreased metal concentration in the co-contaminated soil; (ii) significantly altered the structure of maize rhizosphere bacterial and fungal communities; (iii) strongly enriched the abundance of carbohydrate metabolism genes, ammonia and nitrate production genes, IAA (indole-3-acetic acid) and ACC deaminase (1-aminocyclopropane-1-carboxylate) genes, and slightly altered the abundance of P-related functional genes; (iv) regulated the abundance of microbial quorum sensing system and metal membrane transporter genes, thereby improving the stability and adaptability of the rhizosphere microbial community. This study provides evidence of AMF improving plant growth and resistance to Cd and La stresses by regulating plant rhizosphere microbial communities and aids our understanding of the underlying mechanisms.

Protein nanofibrils as versatile and sustainable adsorbents for an effective removal of heavy metals from wastewater: A review

Water is a valuable natural resource, which plays a crucial role in ecological survival as well as economic progress. However, the water quality has deteriorated in recent years as a result of urbanization, industrialization and human activities due to the uncontrolled release of industrial wastes, which can be extremely carcinogenic and non-degradable, in air, water and soil bodies. Such wastes showed the presence of organic and inorganic pollutants in high dosages. Heavy metals are the most obstinate contaminants, and they can be harmful because of having a variety of detrimental consequences to the ecosystem. The existing water treatment methods in many situations may not be sustainable or effective because of their high energy requirements and ecological impacts. In this review, state-of-the-art water treatment methods for the elimination of heavy metals with the help of protein nanofibrils are covered featuring a discussion on the strategies and possibilities of developing protein nanofibrils for the active elimination of heavy metals using kitchen waste as well as residues from the cattle, agriculture, and dairy industries. Further, the emphasis has been given to their environmental sustainability and economical aspects are also discussed.

Research Progress of Soil Microorganisms in Response to Heavy Metals in Rice

Soil heavy-metal pollution leads to excessive heavy metals in rice and other food crops, which has caused serious impacts on the ecological environment and on human health. In recent years, environmental friendly treatment methods that reduce the bioavailability of heavy metals in soil by soil microorganisms improving the tolerance of heavy metals in rice and reducing the transfer of heavy metals from the roots to the above-ground parts of rice have attracted much attention. This paper reviews the role and mechanism of soil microorganisms in alleviating heavy-metal stress in rice at home and abroad in recent years. At present, microorganisms tolerant to heavy metals mainly include bacteria and fungi, and their mechanisms include the adsorption of heavy metals by microorganisms, the secretion of growth-promoting substances (growth hormone, ACC deaminase, IAA), changing the physical and chemical properties of the soil and the composition of the microbial community, changing the transport mode of heavy metals in soil, the improvement of the antioxidant capacity of rice, etc. Hence, soil microorganisms have good application value and prospects in rice and other crops. However, the vast majority of current research focuses on a single strain, the screening principles of strains are limited, the pathogenicities of the strains have not been evaluated, and there are still few field experiments under natural conditions. In the future, we should strengthen the action of soil microorganisms on rice in response to the above problems in heavy metals, to better promote the microbial remediation technology.

Synergistic Impacts of Arsenic and Antimony Co-contamination on Diazotrophic Communities

Nitrogen (N) shortage poses a great challenge to the implementation of in situ bioremediation practices in mining-contaminated sites. Diazotrophs can fix atmospheric N₂ into a bioavailable form to plants and microorganisms inhabiting adverse habitats. Increasing numbers of studies mainly focused on the diazotrophic communities in the agroecosystems, while those communities in mining areas are still not well understood. This study compared the variations of diazotrophic communities in composition and interactions in the mining areas with different extents of arsenic (As) and antimony (Sb) contamination. As and Sb co-contamination increased alpha diversities and the abundance of nifH encoding the dinitrogenase reductase, while inhibited the diazotrophic interactions and substantially changed the composition of communities. Based on the multiple lines of evidence (e.g., the enrichment analysis of diazotrophs, microbe-microbe network, and random forest regression), six diazotrophs (e.g., Sinorhizobium, Dechloromonas, Trichormus, Herbaspirillum, Desmonostoc, and Klebsiella) were identified as keystone taxa. Environment-microbe network and random forest prediction demonstrated that these keystone taxa were highly correlated with the As and Sb contamination fractions. All these results imply that the above-mentioned diazotrophs may be resistant to metal(loid)s.

Whole genome sequencing and comparative genomic analyses of Pseudomonas aeruginosa strain isolated from arable soil reveal novel insights into heavy metal resistance and codon biology

Elevated concentration of non-essential persistent heavy metals and metalloids in the soil is detrimental to essential soil microbes and plants, resulting in diminished diversity and biomass. Thus, isolation, screening, and whole genomic analysis of potent strains of bacteria from arable lands with inherent capabilities of heavy metal resistance and plant growth promotion hold the key for bio remedial applications. This study is an attempt to do the same. In this study, a potent strain of Pseudomonas aeruginosa was isolated from paddy fields, followed by metabolic profiling using FTIR, metal uptake analysis employing ICP-MS, whole genome sequencing and comparative codon usage analysis. ICP-MS study provided insights into a high degree of Cd uptake during the exponential phase of growth under cumulative metal stress to Cd, Zn and Co, which was further corroborated by the detection of cadA gene along with czcCBA operon in the genome upon performing whole-genome sequencing. This potent strain of Pseudomonas aeruginosa also harboured genes, such as copA, chrA, znuA, mgtE, corA, and others conferring resistance against different heavy metals, such as Cd, Zn, Co, Cu, Cr, etc. A comparative codon usage bias analysis at the genomic and genic level, whereby several heavy metal resistant genes were considered in the backdrop of two housekeeping genes among 40 Pseudomonas spp. indicated the presence of a relatively strong codon usage bias in the studied strain. With this work, an effort was made to explore heavy metal-resistant bacteria (isolated from arable soil) and whole genome sequence analysis to get insight into metal resistance for future bio remedial applications.

Isolation, Identification, and Characterization of an Efficient Siderophore Producing Bacterium From Heavy Metal Contaminated Soil

An efficient siderophore producing strain, YQ9, was isolated from heavy metal contaminated soil and identified as Burkholderia vietnamiensis. To the best of our known, the strain owns the highest siderophore producing capacity among genus Burkholderia with 96.6% siderophore unit. Moreover, B. vietnamiensis YQ9 has good adaptability to different pH values, temperatures, NaCl, and Fe³⁺ concentrations. In addition, the minimum inhibitory concentration (MIC) of heavy metals and antibiotics were also tested. It was found that the MIC values of strain YQ9 to several major soil heavy metal pollutants, such as Pb²⁺, Zn²⁺, Cu²⁺, and Cd²⁺ reached 3000, 5000, 4500, and 1000 μmol·L^-1, respectively. And YQ9 was sensitive to 4 of 8 test antibiotics, including rifampicin, kanamycin, doxycycline hyclate, and gentamicin (25, 25, 30, and 30 μg·mL^-1, respectively). Strain YQ9 also owns the ability to produce indole-3-acetic acid (IAA) and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase and dissolve phosphorus. The IAA production capacity was 6.93 mg·L^-1, the ACC deaminase activity was 8.71 μmol α-KA·(h·mg)^-1, and the phosphorus dissolving capacity of YQ9 was 104.05 mg·L^-1. The traits were excellent, and the strain was qualified as a candidate for microbial reinforcement of phytoremediation in soil contaminated by heavy metals.

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.

Black soldier fly larvae vermicompost alters soil biochemistry and bacterial community composition

Black soldier fly larvae (Hermetia illucens L. BSFL) bioconversion is a promising biotechnology for food waste recycling, yet little is known about how BSFL vermicompost affects soil health in terms of element availability and related microbial response. In this work, a field soil experiment for luffa (Luffa cylindrica (L.) Roem.) growth was conducted to examine the impacts of BSFL vermicompost (BV, 9750 kg ha^-1, equal to total N input rate of chemically treated soil (CK)) on soil biochemistry and bacterial communities. Relative to CK, application of BV significantly increased total soil carbon by 149% and enhanced catalase and urease activity by 59.2% and 16.2%, respectively. BV increased the degree of aromaticity and humification in dissolved organic matter (DOM) in soil by 28.6% and 27.3%, respectively, compared to CK treatment. Among bacterial communities in soil, Bacteroidetes, Firmicutes, Proteobacteria, and Actinobacteria were the phyla that showed the most substantial alteration in response to BV. Redundancy analysis further revealed that the bacterial community structure was affected by DOM and total phosphorus. Functional analyses indicated that BV enhanced xylanolysis (55.4%) and nitrogen fixation (46.3%), but inhibited nitrification (59.8%). BSFL vermicompost input might effectively prevent the harm of soil borne pathogens (e.g., wilt). Moreover, these function groups strongly correlated with Clostridiales, Actinomycetales, and Nitrospirales. Our study reveals that BSFL vermicompost promoted soil nutrient availability, microbial community succession, and biochemical function optimization, which is conducive to the popularization and application of BSFL vermicompost in the field of soil health. KEY POINTS: • Vermicompost enhanced catalase and urease levels while increased DOM aromaticity. • Vermicompost enriched Bacteroidetes and Firmicutes and improved soil health.

Simultaneous biodegradation of λ-cyhalothrin pesticide and Vicia faba growth promotion under greenhouse conditions

λ-cyhalothrin is a widely used synthetic pyrethroid insecticide and its persistence in plant, soil and water exerts a detrimental effect on humans as well as the environment. There are many studies regarding isolated bacteria capable of degrading λ-cyhalothrin in vitro. However, limited work has been done examining the microbial degradation of λ-cyhalothrin together with plant growth promotion under greenhouse conditions. In this study, 43 bacterial strains were isolated from heavily polluted soil with λ-cyhalothrin by the enrichment technique. The plant growth promotion characteristics of all isolates were evaluated. The results revealed that five isolates were potential in λ-cyhalothrin biodegradation at high concentration (1200 mg/L) within only 24 h together with their high plant growth promotion abilities. The morphological, biochemical and 16S rDNA sequence analyses identified the isolates as Bacillus subtilis strains. The GC/MS analysis revealed that the selected isolates reached high levels of degradation after only two days, the degradation percentage ranged from 95.72 to 99.52% after 48 h of incubation. Furthermore, the degradation pathway for complete detoxification and metabolism of λ-cyhalothrin was established. Moreover, greenhouse experiment was conducted, the results indicate that the application of seed coat significantly enhanced Vicia faba seedling growth and caused an increase from 38.4 to 40.2% percentage of fresh and dry weight, respectively compared to untreated control. All isolates were effective to remove the pesticide residues in Vicia faba seedlings and recorded the highest degradation percentage of 83.79 under greenhouse conditions. Therefore, it can be concluded that the Bacillus subtilis strains isolated in this study have a dual potential role in complete mineralization of λ-cyhalothrin residues in vivo as well as effective biofertilization for future use in sustainable agriculture.

Cadmium-Tolerant Plant Growth-Promoting Bacteria Curtobacterium oceanosedimentum Improves Growth Attributes and Strengthens Antioxidant System in Chili (Capsicum frutescens)

The remediation of potentially toxic element-polluted soils can be accomplished through the use of microbial and plant-assisted bioremediation. A total of 32 bacteria were isolated from soil samples contaminated with potentially toxic elements. The isolated bacterial strain DG-20 showed high tolerance to cadmium (up to 18 mM) and also showed bioaccumulative Cd removal properties, as demonstrated by atomic absorption spectroscopy studies. By sequencing the 16S rRNA gene, this strain was identified as Curtobacterium oceanosedimentum. Under stress and normal conditions, isolate DG-20 also produced a wide range of plant growth promoting traits, including ammonia production (51–73 µg/mL) and IAA production (116–183 µg/mL), alongside siderophore production and phosphate solubilization. Additionally, pot experiments were conducted to determine whether the strain could promote Chili growth when Cd salts are present. Over the control, bacterial colonization increased root and shoot lengths significantly up to 58% and 60%, respectively. Following inoculation with the Cd-tolerant strain, the plants also increased in both fresh and dry weight. In both the control and inoculated plants, Cd was accumulated more in roots than in shoots, indicating that Chili was phytostabilizing Cd levels. Besides improving the plant attributes, Cd-tolerant bacteria were also found to increase the amount of total chlorophyll, proline, total phenol, and ascorbic acid in the soil when added to the soil. These results suggest that the inoculant provides protection to plants from negative effects. The results of the present study predict that the combined properties of the tested strain in terms of Cd tolerance and plant growth promotion can be exploited for the purpose of the bioremediation of Cd, and for the improvement of Chili cultivation in soils contaminated with Cd.

Role of ACC deaminase producing bacteria for abiotic stress management and sustainable agriculture production

Plants are immobile and are exposed to various biotic and abiotic stresses, including heat, cold, drought, flooding, nutrient deficiency, heavy metal exposure, phytopathogens, and pest attacks. The stressors significantly affect agricultural productivity when exceed a certain threshold. It has been reported that most of the stressed plants are reported to have increased ethylene synthesis from its precursor 1-aminocyclopropane-1-carboxylic acid (ACC). Ethylene is a plant hormone that plays a vital role in the regulation of various physiological processes, such as respiration, nitrogen fixation, and photosynthesis. The increment in the plant hormone ethylene would reduce plant growth and development, and if the ethylene level increased beyond the limit, it could also result in plant death. Therefore, plant growth-promoting bacteria (PGPB) possessing ACC deaminase activity play an essential role in the management of biotic and abiotic stresses by hydrolysing 1-aminocyclopropane-1-carboxylic acid using ACC deaminase. In this review, the importance of ACC deaminase-producing bacteria in promoting plant growth under various abiotic stressors is discussed.

Isolation, characterization, and evaluation of a high-siderophore-yielding bacterium from heavy metal-contaminated soil

Heavy metal-resistant siderophore-producing bacteria (SPB) with plant growth-promoting traits can assist in phytoremediation of heavy metal-contaminated soil. We isolated siderophore-producing bacteria from Pb and Zn mine soil in Shangyu, Zhejiang, China. The isolate with the highest siderophore production, strain SX9, was identified as Burkholderia sp. Burkholderia sp. SX9 produced catecholate-type siderophore, with the highest production at a pH range of 6.0 to 8.0, a temperature range of 20 to 30 °C and NaCl concentration below 2%. Siderophore production was highest without Fe³⁺ and became gradually lower with increasing Fe³⁺ concentration. Minimal inhibitory concentrations (MIC) of Pb²⁺, Zn²⁺, Cu²⁺, and Cd²⁺ were 4000, 22000, 5000, and 2000 μmol L^-1, respectively. The strain had a strong metal solubilization ability: the contents of Cu²⁺, Zn²⁺, and Cd²⁺ in the supernatant were 47.4%, 133.0%, and 35.4% higher, respectively, in strain SX9-inoculated cultures than in the not inoculated controls. The siderophore produced by strain SX9 could combine with Fe³⁺, Zn²⁺, and Cd²⁺ with good effectiveness. The plant growth-promoting traits of the strain included indole acetic acid (IAA) production, 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, and phosphate solubilization capability. Compared to the uninoculated growth medium and SX9 culture supernatant, the germination rate of Lolium perenne seeds was higher when inoculated with strain SX9 culture. In the experiment of seed germination, adding bacterial culture or supernatant could alleviate the toxicity of heavy metals to L. perenne seed germination. Under Cu²⁺ and Zn²⁺ stress, strain SX9 promoted the germination rate. Taken together, Burkholderia sp. SX9 had properties beneficial in the microbial enhancement of phytoremediation of soil contaminated with heavy metals.

Theobroma cacao L. agricultural soils with natural low and high cadmium (Cd) in Santander (Colombia), contain a persistent shared bacterial composition shaped by multiple soil variables and bacterial isolates highly resistant to Cd concentrations

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This study report statistically significant differences in beta-diversity correlated with soil properties, including Cd concentrations.

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Culture-dependent techniques allowed the isolation of bacterial strains tolerating high Cd concentrations up to 120 mgL⁻¹ for potencial Cd biosorption or intracellular sequestration.

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The combination of different approaches provides a baseline about the bacterial composition and Cd tolerant strains found in these soils influence for lower Cd accumulation in cocoa crops.

Heavy metals can be found in soil as natural components or as product of contaminations events; plants growing in soils are prone to bioaccumulate heavy metals on their biomass. Theobroma cacao L. can bioaccumulate cadmium (Cd) in the seed and could be in derived food products, it considered a human health risk; therefore, removal of Cd is desirable but not vet technically and economically feasible; only to avoid Cd in cocoa is by selecting lands plots exhibiting lower Cd concentrations in soils, imposing a serious limitation to farmers and regulators. The study of bacterial communities and isolation bacteria with tolerance and mechanisms to counteract the translocation of Cd to the parts of cocoa plant exhibits high relevance in Colombia economy and especially to companies producing chocolate and derivatives. Here, we explore bacterial communities associated with soils having relatively high natural Cd concentrations in a large agricultural cocoa plot located in the Santander region. We characterized the bacterial communities’ compositions by amplicon 16S rRNA sequencing from metagenomics soil DNA and by culturing-based enumeration and isolation approaches.

Culture-dependent techniques allowed the isolation of bacteria tolerant to Cd concentration, complement the information for Colombia, and expand the number of strains characterized with adaptive capacity against Cd with tolerance in a concentration of 120 mg/L, which represents the first capacity for Exiguobacterium sp., Ralstonia sp., Serratia sp., Dermacoccus sp., Klebsiella sp., Lactococcus sp. and Staphylococcus sp. In addition to confirming that there is a greater diversity of Cd-tolerant bacteria present in soils of farms cultivated with cocoa in Colombia. As for the results of new generation sequencing, they revealed that, the alpha-diversity in bacterial composition, according to the ANOVA, there are statistically significant differences of the bacterial communities present in the samples. Regarding Pearson correlation analysis, it was found the Shannon Simpson indices, have a positive correlation against OM, C, pH, Mn, C.E.C.I., Ca, P and negatively correlated with S; respect to bacterial community structure, a principal component analysis, which revealed that independent of the concentration of Cd present in soil samples, separates them according to pH value. Phyla to high abundance relative in all samples were Proteobacteria, Acidobacteriota, Actinobacteriota, Verrucomicrobiota, Myxococcota, Chloroflexi, Plactomycetota, Bacteroidota, Gemmatimonadota, Nitrospirota, Firmicutes and NB1_J; the bacteria genera with higher relative abundance (>0.5%) Nitrospira, candidatus Udaeobacter, Haliangium, Cupriavidus, MND1, Bacillus, Kitasatospora, Niveibacterium, Acidothermus, Burkholderia, Acidibacter, Terrimonas, Gaiella, candidatus Solibacter, Kitasatospora, Sphingomonas, Streptomyces, this genus with a relationship with the Cd tolerance process. After it, redundancy analysis was performed between the variation of the bacterial communities identified by dependent and independent techniques and edaphic soil variables, where their positive correlation was found against K, OM, C, Ca, pH (p<0.01) and P, C.E.C.I (p<0.05). For soil samples, the bacterial genera that make up the core community were identified, which are present in all samples as Nitrospira sp., Cupriavidus sp., Burkholderia sp., Haliangium sp., candidatus Udaeobacter, MND1, Kitasatospora, Acidothermus, Acidibacter, Streptomyces, Gaiella, candidatus Solibacter and Terramonas; the genera identified has a different and fundamental role in ecosystem functioning. The combination of different approaches offers new clues regarding the assessment of bacterial communities in soils cultivated with cocoa in soils with elevated Cd content in Colombia, and the ecological role and interplay of soil components and bacterial communities that contribute to modulate the effect of bioaccumulation in products.

Removal of phosphorus from wastewater by Diutina rugosa BL3: Efficiency and pathway

A novel phosphorus removal yeast BL3 was isolated from an alternating anaerobic/aerobic biofilter and identified as Diutina rugosa by 26S rDNA gene sequence analysis. Yeast BL3 could effectively remove phosphorus from synthetic wastewater containing 2-20 mg/L phosphorus under optimal environmental conditions. The highest phosphorus removal efficiency was above 70% under the conditions of DO 6.86 mg/L, C/P ratios of 60, N/P ratios of 3.3, pH 6.0-9.0, and at 25.0-35.0 °C. The phosphorus distribution in the aqueous solution and different components of yeast BL3 analysis indicated that around 55%-70% and 20%-40% of removed phosphorus were transferred into extracellular polymeric substances (EPS) and yeast cells, respectively. The plausible phosphorus transfer pathway was proposed based on the phosphorus distribution and species analysis, suggesting the important role of EPS as a phosphorus reservoir. These results indicate that yeast BL3 can efficiently remove phosphorus under aerobic conditions without alternating anaerobic/aerobic cycling, and thus has significant potential for practical application in wastewater phosphorus removal.

Microbial Communities and Functions in the Rhizosphere of Disease-Resistant and Susceptible Camellia spp

Oil tea (Camellia spp.) is endemic to the hilly regions in the subtropics. Camellia yuhsienensis is resistant to diseases such as anthracnose and root rot, while Camellia oleifera is a high-yield species but susceptible to these diseases. We hypothesize that differences in the rhizosphere microbial communities and functions will elucidate the resistance mechanisms of these species. We used high-throughput sequencing over four seasons to characterize the rhizosphere microbiome of C. oleifera (Rhizo-Sus) and C. yuhsienensis (Rhizo-Res) and of the bulk soil control (BulkS). In Rhizo-Res, bacterial richness and diversity (Shannon index) in autumn and winter were both higher than that in Rhizo-Sus. In Rhizo-Res, fungal richness in autumn and winter and diversity in summer, autumn, and winter were higher than that in Rhizo-Sus. The seasonal variations in bacterial community structure were different, while that of fungal community structure were similar between Rhizo-Res and Rhizo-Sus. Gram-positive, facultatively anaerobic, and stress-tolerant bacteria were the dominant groups in Rhizo-Sus, while Gram-negative bacteria were the dominant group in Rhizo-Res. The significant differences in bacterial and fungal functions between Rhizo-Sus and Rhizo-Res were as follows: (1) in Rhizo-Sus, there were three bacterial and four fungal groups with plant growth promoting potentials, such as Brevibacterium epidermidis and Oidiodendron maius, and one bacterium and three fungi with pathogenic potentials, such as Gryllotalpicola sp. and Cyphellophora sessilis; (2) in Rhizo-Res, there were also three bacteria and four fungal groups with plant-growth-promoting potentials (e.g., Acinetobacter lwoffii and Cenococcum geophilum) but only one phytopathogen (Schizophyllum commune). In summary, the rhizosphere microbiome of disease-resistant C. yuhsienensis is characterized by a higher richness and diversity of microbial communities, more symbiotic fungal communities, and fewer pathogens compared to the rhizosphere of high-yield but disease-susceptible C. oleifera.

An actinomycete strain of Nocardiopsis lucentensis reduces arsenic toxicity in barley and maize

Accumulation of arsenic in plant tissues poses a substantial threat to global crop yields. The use of plant growth-promoting bacterial strains to mitigate heavy metal toxicity has been illustrated before. However, its potential to reduce plant arsenic uptake and toxicity has not been investigated to date. Here, we describe the identification and characterization of a Nocardiopsis lucentensis strain isolated from heavy metal contaminated soil. Inoculation with this bioactive actinomycete strain decreased arsenic root and shoot bioaccumulation in both C3 and C4 crop species namely barley and maize. Upon arsenate treatment, N. lucentensis S5 stimulated root citric acid production and the plant's innate detoxification capacity in a species-specific manner. In addition, this specific strain promoted biomass gain, despite substantial tissue arsenic levels. Detoxification (metallothionein, phytochelatin, glutathione-S-transferase levels) was upregulated in arsenate-exposed shoot and roots, and this response was further enhanced upon S5 supplementation, particularly in barley and maize roots. Compared to barley, maize plants were more tolerant to arsenate-induced oxidative stress (less H₂O₂ and lipid peroxidation levels). However, barley plants invested more in antioxidative capacity induction (ascorbate-glutathione turnover) to mitigate arsenic oxidative stress, which was strongly enhanced by S5. We quantify and mechanistically discuss the physiological and biochemical basis of N. lucentensis-mediated plant biomass recovery on arsenate polluted soils. Our findings substantiate the potential applicability of a bactoremediation strategy to mitigate arsenic-induced yield loss in crops.

Castro I, Soares R, Brito DRA, Mbanze AA, Chaúque A, Máguas C, Ezeokoli OT, Ribeiro NS, Marques I, Ribeiro-Barros AI. The Nexus between Fire and Soil Bacterial Diversity in the African Miombo Woodlands of Niassa Special Reserve, Mozambique

(1) Background: the Miombo woodlands comprise the most important vegetation from southern Africa and are dominated by tree legumes with an ecology highly driven by fires. Here, we report on the characterization of bacterial communities from the rhizosphere of Brachystegia boehmii in different soil types from areas subjected to different regimes. (2) Methods: bacterial communities were identified through Illumina MiSeq sequencing (16S rRNA). Vigna unguiculata was used as a trap to capture nitrogen-fixing bacteria and culture-dependent methods in selective media were used to isolate plant growth promoting bacteria (PGPB). PGP traits were analysed and molecular taxonomy of the purified isolates was performed. (3) Results: Bacterial communities in the Miombo rhizosphere are highly diverse and driven by soil type and fire regime. Independent of the soil or fire regime, the functional diversity was high, and the different consortia maintained the general functions. A diverse pool of diazotrophs was isolated, and included symbiotic (e.g., Mesorhizobium sp., Neorhizobium galegae, Rhizobium sp., and Ensifer adhaerens), and non-symbiotic (e.g., Agrobacterium sp., Burkholderia sp., Cohnella sp., Microvirga sp., Pseudomonas sp., and Stenotrophomonas sp.) bacteria. Several isolates presented cumulative PGP traits. (4) Conclusions: Although the dynamics of bacterial communities from the Miombo rhizosphere is driven by fire, the maintenance of high levels of diversity and functions remain unchanged, constituting a source of promising bacteria in terms of plant-beneficial activities such as mobilization and acquisition of nutrients, mitigation of abiotic stress, and modulation of plant hormone levels.

Characterization of siderophore-producing microorganisms associated to plants from high-Andean heavy metal polluted soil from Callejón de Huaylas (Ancash, Perú)

Endophytic and rhizospheric microorganisms associated with six native plants adapted to heavy metal polluted soil from Punta Olímpica and Chahuapampa, located in Callejón de Huaylas mountains, were evaluated as potential candidates for technologies to clean polluted ecosystems. It was selected 14 bacteria and 9 fungi strains by their iron and/or aluminum siderophore production trait, where BEP17-Dm showed higher production. According to the 16S rDNA analysis, bacteria belong to Pseudomonas, Bacillus, and Achromobacter genera, whereas by ITS analysis fungi belong to Talaromyces, Hypoxylon, Tolypocladium, and Penicillium. All bacteria strains tolerated lead (2-8 mM) and eigth tolerated cadmium (1-6 mM); also all fungi tolerated lead (9-70 mM) and cadmium (3-10 mM). Two bacteria and six fungi solubilized cadmium carbonate, while eleven bacteria and two fungi solubilized tricalcium phosphate, where P. japonica BEP18-Dm and B. subtilis BRU16-Sr exhibited higher solubilization index. None strains solubilized lead carbonate. BEP18-Dm produced higher concentration of IAA (53.42 μgml^-1); while six bacteria and all fungi strains produced a low concentration of auxins. Medicago sativa seedlings inoculated with BEP17-Dm, BEP18-Dm, or BRU16-Sr showed more surviving percentage under in vitro culture in presence of Cd, Pb (0.5-1.0 mM), or Al (2.5-5.0 mM). Finally, it is the first report of siderophore-producing microorganisms from polluted soil of Callejón de Huaylas highlands, interestedly they displayed metabolic properties useful to enhance phytoremediation and biotechnology application.

Cadmium-resistant and arginine decarboxylase-producing endophytic Sphingomonas sp. C40 decreases cadmium accumulation in host rice (Oryza sativa Cliangyou 513)

In this study, an cadmium (Cd)-immobilizing and arginine decarboxylase-producing endophytic Sphingomonas sp. strain C40 obtained from the seeds of Oryza sativa Cliangyou 513 was characterized for its Cd availability and Cd uptake in host rice using hydroponic and soil experiments. The Cd concentration decreased by 51-95% compared to the control, while the spermidine concentration increased by 19-25% with Cd compared with no Cd in the strain C40-inoculated solution. Strain C40 decreased the above-ground tissue Cd content by 27-37% and increased spermine and spermidine contents by 28-67% and the expression levels of genes involved in spermine and spermidine production by 29-217% in rice roots compared to the controls. Furthermore, correlation analyses showed the significantly negative correlation between rice root spermine and spermidine contents and above-ground tissue Cd content. In the Cd-added soil, strain C40 promoted the rice biomass by 29-36% and decreased rice root, above-ground tissue, and grain Cd contents by 18, 16, and 33% and total grain Cd uptake by 14% compared with the controls at the maturity stage. Strain C40 decreased the exchangeable Cd content by 27% and increased the Fe and Mn oxides-bound Cd content by 45% in the rice rhizosphere soils at the maturity stage compared with the controls. These results suggested that the endophytic bacterial strain C40 increased rice root polyamine production and their related gene expression and the transformation of available Cd to unavailable Cd, leading to reduced Cd accumulation and translocation from the rice roots to grains.

Investigation of arsenic-resistant, arsenite-oxidizing bacteria for plant growth promoting traits isolated from arsenic contaminated soils

The problem of arsenic (As) pollution being severe warrants opting for low-cost microbial remediation strategies. The present study of identifying suitable bacterial strains led to the isolation of eleven As-tolerant strains from the As-contaminated rhizosphere soils of West Bengal, India. They were found to oxidize/reduce 55-31.6% of 5 mM As(III) and 73-37.6% of 5 mM As(V) within 12 h. The four isolates (BcAl-1, JN 73, LAR-2, and AR-30) had a high level of As(III) oxidase activity along with a higher level of As(V) and As(III) resistance. The agar diffusion assay of the isolates further confirmed their ability to endure As stress. The presence of aoxB gene was observed in these four As(III) oxidizing isolates. Evaluation of plant growth-promoting characteristics revealed that BcAl-1 (Burkholderia cepacia), JN 73 (Burkholderia metallica), AR-30 (Burkholderia cenocepacia), and LAR-2 (Burkholderia sp.) had significant plant growth-promoting characteristics (PGP), including the ability to solubilize phosphate, siderophore production, indole acetic acid-like molecules production, ACC deaminase production, and nodule formation under As stressed condition. BcAl-1 and JN 73 emerged as the most promising traits in As removal as well as plant growth promotion.

Long-term Impact of Gold and Platinum on Microbial Diversity in Australian Soils

The effects of platinum (Pt) and gold (Au) and on the soil bacterial community was evaluated in four different Australian soil types (acidic Burn Grounds (BGR), organic matter-rich Fox Lane, high silt/metal Pinpinio (PPN), and alkali Minnipa (MNP) spiked with either Pt or Au at 1, 25, and 100 mg kg^-1 using a next-generation sequencing approach (amplicon-based, MiSeq). Soil type and metal concentrations were observed to be key drivers of Pt and Au effects on soil microbial community structure. Different trends were therefore observed in the response of the bacterial community to Pt and Au amendments; however in each soil type, Pt and Au amendment caused a detectable shift in community structure that in most samples was positively correlated with increasing metal concentrations. New dominant groups were only observed in BGR and PPN soils at 100 mg kg^-1 (Kazan-3B-28 and Verrucomicrobia groups (BGR, Pt) and Firmicutes and Caldithrix groups (PPN, Pt) and WS2 (BGR, Au). The effects of Pt on soil microbial diversity were largely adverse at 100 mg kg^-1 and were pronounced in acidic, basic, and metal/silt-rich soils. However, this effect was concentration-related; Au appeared to be more toxic to soil bacterial communities than Pt at 25 mg kg^-1 but Pt was more toxic at 100 mg kg^-1. More bacterial groups such as those belonging to Burkholderiales/Burkholderiaceae, Alicyclobacillaceae, Rubrobacteraceae, Cytophagaceae, Oxalobacteraceae were selectively enriched by Pt compared to Au (Sphingomonadaceae and Rhodospirillaceae) amendments irrespective of soil type. The research outcomes have important implications in the management (remediation) of Pt- and Au-contaminated environments.

Reduced tillage, cover crops and organic amendments affect soil microbiota and improve soil health in Uruguayan vegetable farming systems

Conventional tillage and mineral fertilization (CTMF) jeopardize soil health in conventional vegetable production systems. Using a field experiment established in Uruguay in 2012, we aimed to compare the soil restoration potential of organic fertilization (compost and poultry manure) combined with conventional tillage and cover crop incorporated into the soil (CTOF) or with reduced tillage and the use of cover crop as mulch (RTOF). In 2017, table beet was cultivated under CTMF, CTOF and RTOF, and yields, soil aggregate composition and nutrients, as well as soil and table beet rhizosphere microbiota (here: bacteria and archaea) were evaluated. Microbiota was studied by high-throughput sequencing of 16S rRNA gene fragments amplified from total community DNA. RTOF exhibited higher soil aggregation, soil organic C, nutrient availability and microbial alpha-diversity than CTMF, and became more similar to an adjacent natural undisturbed site. The soil microbiota was strongly shaped by the fertilization source which was conveyed to the rhizosphere and resulted in differentially abundant taxa. However, 229 amplicon sequencing variants were found to form the core table beet rhizosphere microbiota shared among managements. In conclusion, our study shows that after only 5 years of implementation, RTOF improves soil health under intensive vegetable farming systems.

Bio-remediation approaches for alleviation of cadmium contamination in natural resources

Cadmium (Cd) is a harmful heavy metal that can cause potent environmental and health hazards at different trophic levels through food chain. Cd is relatively non-biodegradable and persists for a long time in the environment. Considering the potential toxicity and non-biodegradability of Cd in the environment as well as its health hazards, this is an urgent issue of international concern that needs to be addressed by implicating suitable remedial approaches. The current article specifically attempts to review the different biological approaches for remediation of Cd contamination in natural resources. Further, bioremediation mechanisms of Cd by microbes such as bacteria, fungi, algae are comprehensively discussed. Studies indicate that heavy metal resistant microbes can be used as suitable biosorbents for the removal of Cd (up to 90%) in the natural resources. Soil-to-plant transfer coefficient (TC) of Cd ranges from 3.9 to 3340 depending on the availability of metal to plants and also on the type of plant species. The potential phytoremediation strategies for Cd removal and the key factors influencing bioremediation process are also emphasized. Studies on molecular mechanisms of transgenic plants for Cd bioremediation show immense potential for enhancing Cd phytoremediation efficiency. Thus, it is suggested that nano-technological based integrated bioremediation approaches could be a potential futuristic path for Cd decontamination in natural resources. This review would be highly useful for the biologists, chemists, biotechnologists and environmentalists to understand the long-term impacts of Cd on ecology and human health so that potential remedial measures could be taken in advance.

Biological characteristics and salt-tolerant plant growth-promoting effects of an ACC deaminase-producing Burkholderia pyrrocinia strain isolated from the tea rhizosphere

Plant growth-promoting rhizobacteria that produce 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase can promote plant growth and enhance abiotic stress tolerance. In this study, Burkholderia pyrrocinia strain P10, with an ACC deaminase activity of 33.01-µmol/h/mg protein, was isolated from the tea rhizosphere and identified based on morphological, biochemical, and molecular characteristics. In addition to its ACC deaminase activity at pH 5.0-9.0 and in response to 5% NaCl and 20% polyethylene glycol, strain P10 can also solubilize phosphorus compounds, produce indole-3-acetic acid, and secrete siderophores. Pot experiments revealed that strain P10 can significantly enhance peanut seedling growth under saline conditions (100- and 170-mmol/L NaCl). Specifically, it increased the fresh weight and root length of plants by 90.12% and 79.22%, respectively, compared with high-salt stress. These results provide new insights into the biological characteristics of Burkholderia pyrrocinia, which may be useful as a bio-fertilizer.

Evaluation of direct and biochar carrier-based inoculation of Bacillus sp. on As- and Pb-contaminated technosol: effect on metal(loid) availability, Salix viminalis growth, and soil microbial diversity/activity

Phytomanagement manipulates the soil-plant system to lower the risk posed by contaminated soils. In this process, the addition of amendments, such as biochar, and bacteria can improve the fertility of poor contaminated soils and consequently ameliorate plant growth. A number of studies based on the inoculation of soil with microorganisms of the genus Bacillus, previously isolated from contaminated sites, revealed positive effects on soil properties and plant growth. Furthermore, when the Bacillus isolates were used in association with biochar, better results were obtained, as biochar can ameliorate soil properties and serve as habitat for microorganisms. Accordingly, a mesocosm study was set-up using a mining technosol amended with biochar and inoculated with an endogenous Bacillus isolate, to evaluate the effect of inoculation on soil properties, metal(loid) immobilization, and Salix viminalis growth. Two inoculation methods were compared: (1) direct inoculation of bacteria (Bacillus sp.) and (2) inoculation using biochar as a carrier. Results showed that the Bacillus isolate modified soil properties and ameliorated plant growth, while having a reduced effect on metal(loid) accumulation. The microbial activity was also stimulated, and the community composition was shifted, more importantly when biochar was used as a carrier. In conclusion, this research revealed an improvement of the plant growth and microbial activity after the addition of the endogenous bacterium to the analyzed former mining soil, with better results recorded when a carrier was used.

Two-stage enrichment of hydrogen-oxidizing bacteria as biofertilizers

Biofertilizers can replace chemical fertilizers to promote the plant growth without causing any pollution. The study of hydrogen-oxidizing bacteria (HOB) enrichment as biofertilizers from mixed culture is scarce. Our recent study shows that biofertilizing HOB are successfully enriched in a short hydraulic retention time of 10 h. While, the mechanism is unknown. This study intentionally used a two-stage method to enrich biofertilizing HOB specifically with nitrate as nitrogen source in Stage 1 and then 1-aminocyclopropane-1-carboxylate (ACC) as nitrogen source in Stage 2. It was found Pseudomonas (34.46%, reported HOB) predominated in Stage 1, while Azospirillum (59.35%), Azoarcus (36%) were dominant genera and Azospirillum lipoferum strain DSM 1691 (50%), Azoarcus olearius strain DQS-4 (50%) were dominant species in Stage 2. The enriched HOB of Stage 2 showed ACC deaminase activity. Furthermore, they could also fix N₂ and consume Ca₃(PO₄)₂. Thus, the two-stage method can be used as a specific enrichment for HOB as biofertilizers, which extends the application of HOB in agriculture.

Trifolium repens L. regulated phytoremediation of heavy metal contaminated soil by promoting soil enzyme activities and beneficial rhizosphere associated microorganisms

Rhizosphere soil physiochemical properties, enzyme activities and rhizosphere associated microbial communities are of the central importance for modulating phytoremediation in heavy metal contaminated soil. In this study, the rhizosphere micro-ecological characteristics of phytoremediation in seven groups of contaminated soil with different heavy metal species and concentrations were examined. The results showed that heavy metal-enrichment inhibited plant growth, but enhanced both anions (Cr₂O₇^2-) and cations (Cd²⁺ and Pb²⁺) uptake with corresponding mean values ranging from 19.37 to 168.74 mg/kg in roots and 10.89-86.53 mg/kg in shoots. Trifolium repens L. planting was able to compensate the lost caused by the heavy metal on the soil organic matter, available N, available P, available K and enzyme activities as well. According to the cluster, some species like Lysobacter, Kaistobacter and Pontibacter, was significantly related to heavy metal accumulation while others such as Flavisolibacter, Adhaeribacter and Bacillus promoted plant growth. The importance of root-associated microbial community could relatively regulate plant growth and heavy metal uptake. Our study not only illustrated the correlation among rhizosphere micro-ecological parameters, and the possible mechanisms of phytoremediation regulation, but also provide clear strategy for improving the phytoremediation efficiency.

Evaluation of interaction among indigenous rhizobacteria and Vigna unguiculata on remediation of metal-containing abandoned magnesite mine tailing

Abandoned magnesite mine heap causing pollution to nearby farmland and water reservoir. Thus the intention of this research was to screening metal mobilizing and absorbing bacteria from the rhizosphere section of V. unguiculata from farmland nearby to magnesite mine. Further, studied their stimulus effect on growth, biomass, and phytoextraction prospective of V unguiculata in mine tailing. The results of the physicochemical properties of mine tailing shows that four metals (Pb, Mn, Cd, and Zn) were crossing the permissible limit. Out of 27 isolates, 2 isolates (MMS15 and MMS17) were identified with maximum metal tolerance for up to 700 mg L^-1 (MIC) and metal mobilization (Pb 5.5 and 5.87, Mn 6.6 and 4.88, Cd 1.99 and 2.59, and Zn 6.55 and 6.94 mg kg^-1) and biosorption efficiency as Pb 3.74 and 3.74, Mn 4.9 and 4.7, Cd 2.41 and 3.96, and Zn 4.3 and 4.9 mg g^-1. These two strains were identified as members of B. cereus and Kosakonia sp. using 16S rRNA technique and labelled strains NDRMN001 and MGR1, respectively. The Kosakonia sp. MGR1 effectively fixes the nitrogen in the rate of 81.94% and B. cereus NDRMN001 solubilizes 69.98 ± 2.31 mg L^-1 of soluble phosphate. The experimental group's study results show that the group C (Kosakonia sp. MGR1 and B. cereus NDRMN001) has effectively stimulate the growth, biomass, and phytoextraction potential of V. unguiculata. The results conclude that the optimistic interaction between these two bacteria could be more significant to minimize the metal pollution in magnesite mine tailing.

Wheat-associated Pseudomonas taiwanensis WRS8 reduces cadmium uptake by increasing root surface cadmium adsorption and decreasing cadmium uptake and transport related gene expression in wheat

Metal-resistant bacteria can reduce Cd accumulation in plants, but mechanisms underlying this effect are poorly understood. In this study, a highly effective Cd-resistant WRS8 strain was obtained from the rhizoshere soil of Triticum aestivum L. Yangmai-13 and identified as Pseudomonas taiwanensis based on 16S rRNA gene sequence analysis. Strain WRS8 was investigated for its effects on Cd availability and wheat tissue Cd contents and the related mechanisms using a hydroponic culture experiment. In strain WRS8-inoculated solution, the Cd concentration reduced and the pH and cell-adsorbed Cd increased with time. Strain WRS8 increased the wheat root and above-ground tissue dry weights by 11-36% compared to the controls. In strain WRS8-inoculated wheat plants, the Cd contents of the roots and above-ground tissues decreased by 78-85% and 88-94% and the Cd bioconcentration and translocation factors decreased by 78-85% and 46-58% at days 3 and 10, respectively, compared with the controls. The root surface-adsorbed Cd contents increased by 99-121% in the WRS8 strain-inoculated wheat plants at days 3 and 10 compared to the controls. Furthermore, strain WRS8 colonized the wheat root surfaces and interiors and reduced the expression levels of the LCT1 and HMA2 genes involved in Cd accumulation and transport in wheat roots by 46% and 30%, respectively, compared to the controls. In the Cd-contaminated soils, strain WRS8 significantly reduced the available Cd content by 20-24% and increased the pH compared to the controls. These findings showed the important role of strain WRS8 in reducing solution and soil Cd availability and suggested that strain WRS8 reduced the wheat tissue Cd accumulation by increasing root surface Cd adsorption and decreasing wheat root Cd uptake and transport-related gene expression and may provide a new and effective wheat rhizobacteria-enhanced approach for reducing wheat Cd uptake in Cd-polluted environments.

Individual and combined application of Cu-tolerant Bacillus spp. enhance the Cu phytoextraction efficiency of perennial ryegrass

Forage grasses have recently received a remarkable amount of attention as promising candidates for decontaminating metal-polluted soils, but this strategy is time-consuming and inefficient. The present study aimed to address the beneficial effects of screened plant growth-promoting rhizobacteria (PGPR) strains Bacillus sp. EhS5 and EhS7 on perennial ryegrass and tall fescue. Single or combined inoculation considerably increased the biomass yield and Cu content of inoculated ryegrass compared with uninoculated plants, thereby enhancing the extraction efficiency at different Cu contamination levels. Bioaugmentation did not show a positive impact on the improvement of fescue's phytoextraction efficiency. Principal component analysis (PCA) and Pearson correlation coefficient results identified root development and photosynthesis as the key variables influencing ryegrass biomass. Antioxidant activities and Cu bioavailability are the key variables influencing Cu accumulation. The inoculated ryegrass showed improved photosynthetic status as the photosystem II system efficiency parameters increased and energy dissipation in the form of heat (DI_o/RC) decreased with the help of PGPR. The root length, diameter, surface area, and forks of inoculated ryegrass increased remarkably. The levels of scavengers of reactive oxygen species were enhanced in these plants. Moreover, PGPR significantly increased soil Cu bioavailability by secreting siderophores and organic acid and by increasing soil organic carbon content. Dual inoculation showed better results than individual inoculation in improving ryegrass growth and Cu translocation under high Cu contamination level according to PCA. This study systematically explored the effects and mechanisms of the Bacillus-ryegrass combined remediation and provided a novel method for cleaning Cu-contaminated sites.