Metal tolerance and biosorption of Pb ions by Bacillus cereus RMN 1 (MK521259) isolated from metal contaminated sites

Lead (Pb2+) biosorption and bioaccumulation efficiency of Enterobacter chuandaensis DGI-2: Isotherm, kinetics and mechanistic study for bioremediation

Heavy metal (HM) contamination, particularly lead (Pb²⁺), threatens environmental and agricultural sustainability, necessitating effective remediation strategies. This study evaluates the Pb²⁺ sequestration potential of Enterobacter chuandaensis DGI-2, an HM-tolerant rhizobacterium isolated from a Pb-contaminated rhizosphere. DGI-2 exhibited high Pb²⁺ removal efficiency, achieving 94.73 % removal at 100 µg/mL and 69.09 % at 750 µg/mL over 96 h, primarily through cell surface and exopolysaccharide (EPS) adsorption. Biosorption studies demonstrated higher Pb²⁺ uptake in living biomass (102.95 mg/g, 68.63 %) than in dead biomass (98.61 mg/g, 65.74 %) under controlled conditions (0.5 g/L biomass, pH-6.5, 720 min). Mechanistic analyses revealed that Pb²⁺ adsorption primarily involved interactions with -OH, -COOH, and -PO₄³ ⁻ functional groups, facilitated by multilayer sorption, complexation, and ion exchange. Moreover, a 210.66 % increase in phosphatase activity promoted Pb²⁺ precipitation, forming stable Pb-phosphate minerals (e.g., Pb₅(PO₄)₃Cl, Pb₁₀(PO₄)₆(OH)₂), as confirmed by X-ray diffraction (XRD), significantly contributing to Pb sequestration. Regeneration studies demonstrated the biomass' reusability over four cycles. Soil microcosm experiments showed an 11.7-13.1 % reduction in bioavailable Pb, with greater stabilization in non-sterile soils, suggesting synergistic effects with native microbiota. Additionally, DGI-2 exhibited plant growth-promoting (PGP) traits, reducing phytotoxicity, enhancing soil health and phytostabilization potential, positioning it as a sustainable biosorbent for Pb²⁺ remediation.

Integrated analyses of characterization and transcriptome reveal the adaptive response mechanism of Bacillus cereus FCHN 7-1 in cadmium adsorption

Cadmium (Cd2+) pollution is a pressing environmental issue that seriously threatens human health. In recent years, microbial extracellular polymeric substances (EPS) have been developed as an eco-friendly and effective solution for heavy metal bioremediation. In this study, Bacillus cereus FCHN 7-1, which was isolated from highly Cd-polluted soils in Hunan Province, China, has strong resistance to Cd2+ and excellent Cd2+ adsorptive capacity. Microscopic analyses showed that B. cereus FCHN 7-1 mainly adsorbed Cd2+ on the surface of EPS via the formation of granule deposits. The Freundlich isotherm was proven to better describe the sorption data with a higher R2 of 0.958, and the pseudo-second-order model fitted the sorption kinetic processes well, with an adsorption capacity of 66.128 mg/g. The variations in zeta potential indicated the occurrence of electrostatic attraction during the Cd2+ adsorption process. XRD, FTIR, 3D-EEM, and XPS analyses revealed that the functional groups of the EPS involved in Cd2+ adsorption were mainly CO, N-H, and COO- groups of proteins in the EPS, which greatly facilitated the adsorption of Cd2+ by forming EPS-metal complexes. Transcriptome sequencing analysis showed that an adaptive response to Cd2+ increased the expression of genes involved in amino acid biosynthesis and metabolism (asnB, alsS, ablA), and ABC transporter pathways (brnQ, abc-f, U2I57_RS23145), which promoted the synthesis of proteins and secretion of abundant EPS. This study provides new insights into the biosorption of Cd2+ for future applications.

Molecular and eco-physiological responses of soil-borne lead (Pb2+)-resistant bacteria for bioremediation and plant growth promotion under lead stress

Lead (Pb) is the 2nd known portentous hazardous substance after arsenic (As). Being highly noxious, widespread, non-biodegradable, prolonged environmental presence, and increasing accumulation, particularly in arable land, Pb pollution has become a serious global health concern requiring urgent remediation. Soil-borne, indigenous microbes from Pb-polluted sites have evolved diverse resistance strategies, involving biosorption, bioprecipitation, biomineralization, biotransformation, and efflux mechanisms, under continuous exposure to Pb in human-impacted surroundings. These strategies employ a wide range of functional bioligands to capture Pb and render it inaccessible for leaching. Recent breakthroughs in molecular technology and understanding of lead resistance mechanisms offer the potential for utilizing microbes as biological tools in environmental risk assessment. Leveraging the specific affinity and sensitivity of bacterial regulators to Pb2+ ions, numerous lead biosensors have been designed and deployed worldwide to monitor Pb bioavailability in contaminated sites, even at trace levels. Besides, the ongoing degradation of croplands due to Pb pollution poses a significant challenge to meet the escalating global food demands. The accumulation of Pb in plant tissues jeopardizes both food safety and security while severely impacting plant growth. Exploring Pb-resistant plant growth-promoting rhizobacteria (PGPR) presents a promising sustainable approach to agricultural practices. The active associations of PGPR with host plants have shown enhancements in plant biomass and stress alleviation under Pb influence. They thus serve a dual purpose for plants grown in Pb-contaminated areas. This review aims to offer a comprehensive understanding of the role played by Pb-resistant soil-borne indigenous bacteria in expediting bioremediation and improving the growth of Pb-challenged plants essential for potential field application, thus broadening prospects for future research and development.

Enhancing La(III) biosorption and biomineralization with Micromonospora saelicesensis: Involvement of phosphorus and formation of monazite nano-minerals

The release of rare earth elements (REEs) from mining wastes and their applications has significant environmental implications, necessitating the development of effective prevention and reclamation strategies. The mobility of REEs in groundwater due to microorganisms has garnered considerable attention. In this study, a La(III) resistant actinobacterium, Micromonospora saelicesensis KLBMP 9669, was isolated from REE enrichment soil in GuiZhou, China, and evaluated for its ability to adsorb and biomineralize La(III). The findings demonstrated that M. saelicesensis KLBMP 9669 immobilized La(III) through the physical and chemical interactions, with immobilization being influenced by the initial La(III) concentration, biomass, and pH. The adsorption kinetics followed a pseudo-second-order rate model, and the adsorption isotherm conformed to the Langmuir model. La(III) adsorption capacity of this strain was 90 mg/g, and removal rate was 94 %. Scanning electron microscope (SEM) coupled with energy dispersive X-ray spectrometer (EDS) analysis revealed the coexistence of La(III) with C, N, O, and P. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) investigations further indicated that carboxyl, amino, carbonyl, and phosphate groups on the mycelial surface may participate in lanthanum adsorption. Transmission electron microscopy (TEM) revealed that La(III) accumulation throughout the M. saelicesensis KLBMP 9669, with some granular deposits on the mycelial surface. Selected area electron diffraction (SAED) confirmed the presence of LaPO4 crystals on the M. saelicesensis KLBMP 9669 biomass after a prolonged period of La(III) accumulation. This post-sorption nano-crystallization on the M. saelicesensis KLBMP 9669 mycelial surface is expected to play a crucial role in limiting the bioimmobilization of REEs in geological repositories.

Ecologically different earthworm species are the driving force of microbial hotspots influencing Pb uptake by the leafy vegetable Brassica campestris

Food chain contamination by soil lead (Pb), beginning with Pb uptake by leafy vegetables, is a threat to food safety and poses a potential risk to human health. This study highlights the importance of two ecologically different earthworm species (the anecic species Amynthas aspergillum and the epigeic species Eisenia fetida) as the driving force of microbial hotspots to enhance Pb accumulation in the leafy vegetable Brassica campestris at different Pb contamination levels (0, 100, 500, and 1,000 mg·kg-1). The fingerprints of phospholipid fatty acids (PLFAs) were employed to reveal the microbial mechanism of Pb accumulation involving earthworm-plant interaction, as PLFAs provide a general profile of soil microbial biomass and community structure. The results showed that Gram-positive (G+) bacteria dominated the microbial community. At 0 mg·kg-1 Pb, the presence of earthworms significantly reduced the total PLFAs. The maximum total of PLFAs was found at 100 mg·kg-1 Pb with E. fetida inoculation. A significant shift in the bacterial community was observed in the treatments with E. fetida inoculation at 500 and 1,000 mg·kg-1 Pb, where the G+/G- bacteria ratio was significantly decreased compared to no earthworm inoculation. Principal component analysis (PCA) showed that E. fetida had a greater effect on soil microbial hotspots than A. aspergillum, thus having a greater effect on the Pb uptake by B. campestris. Redundancy analysis (RDA) showed that soil microbial biomass and structure explained 43.0% (R2 = 0.53) of the total variation in Pb uptake by B. campestris, compared to 9.51% of microbial activity. G- bacteria explained 23.2% of the total variation in the Pb uptake by B. campestris, significantly higher than the other microbes. The Mantel test showed that microbial properties significantly influenced Pb uptake by B. campestris under the driving force of earthworms. E. fetida inoculation was favorable for the G- bacterial community, whereas A. aspergillum inoculation was favorable for the fungal community. Both microbial communities facilitated the entry of Pb into the vegetable food chain system. This study delivers novel evidence and meaningful insights into how earthworms prime the microbial mechanism of Pb uptake by leafy vegetables by influencing soil microbial biomass and community composition. Comprehensive metagenomics analysis can be employed in future studies to identify the microbial strains promoting Pb migration and develop effective strategies to mitigate Pb contamination in food chains.

Quantification of the antagonistic and synergistic effects of Pb2+, Cu2+, and Zn2+ bioaccumulation by living Bacillus subtilis biomass using XGBoost and SHAP

Bioaccumulation and adsorption are efficient methods for removing heavy metal ions (HMIs) from aqueous environments. However, methods to quantifiably characterize the removal selectivity for co-existing HMIs are limited. In this study, we applied Shapley additive explanations (SHAP) following extreme gradient boosting (XGBoost) modeling, to generate SHAP values. We used these values to create an affinity interference index (AII) that quantitatively represented the interference between metal ions in a multi-metal bioaccumulation system. The selectivity for simultaneous bioaccumulation of Pb²⁺, Cu²⁺, and Zn²⁺ by living Bacillus subtilis biomass was then characterized as a proof of concept. The AII indicated that the bioaccumulation of Zn²⁺ was more strongly inhibited by Pb²⁺/Cu²⁺ (AII = 1) than that of Pb²⁺/Cu²⁺ by Zn²⁺. Moreover, the presence of Zn²⁺ promoted the bioaccumulation of Pb²⁺ (AII = 0.39), which was confirmed in further experiments where the bioaccumulation of Pb²⁺ (300 μM) was increased by 38% with Zn²⁺ (300 μM). This study demonstrated that the combination of XGBoost and SHAP is effective in the quantifiable characterization of the antagonistic and synergistic effects in a multi-metal simultaneous bioaccumulation system. This method could also be generalized to similar tasks for analyzing the selectivity effects in a multi-component system.

Simultaneous removal of ternary heavy metal ions by a newly isolated Microbacterium paraoxydans strain VSVM IIT(BHU) from coal washery effluent

In the present work, the removal of Cr (VI), Cd (II) and Pb (II) at 50 mg/L of each metal ion concentration was investigated by Microbacterium paraoxydans strain VSVM IIT(BHU). The heavy metal binding on the bacterial cell surface was confirmed through X-ray photoelectron spectroscopy and energy dispersive X-ray. X-ray photoelectron spectroscopy analysis also confirmed the reduction of Cr (VI) to Cr (III). Heavy metal removal dynamics was investigated by evaluating dimensionless, and the value of N_k (9.49 × 10^-3, 9.92 × 10^-3 and 1.23 × 10^-2 for Cr (VI), Cd (II) and Pb (II) ions) indicated that the removal of heavy metals by bacterial isolate was mixed diffusion and transfer controlled. It was found that both the experimental and predicted values for isolated bacterial strain coincided with each other with a good R² value in the L-M Algorithm range of 0.94-0.98 for the ternary metal ion system. The bacterial isolate presented a maximum heavy metal ion removal efficiency of 91.62% Cr (VI), 89.29% Pb (II), and 83.29% Cd (II) at 50 mg/L.