Phosphatase mediated bioprecipitation of lead as pyromorphite by Achromobacter xylosoxidans

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.

Mechanistic and kinetic understanding of Pb-phosphate biomineralization from humic acid-bound Pb under active growth of phosphate solubilizing Enterobacter aerogenes W6

Humic acid-bound Pb (HA-Pb), as one of the representative solid-associated Pb species, plays important roles in Pb mobility and toxicity in aqueous environments. Stable Pb-phosphate minerals formation mediated by phosphate solubilizing bacteria (PSB) is a promising approach to immobilizing Pb in contaminated waters. However, the underlying processes and kinetics of Pb-phosphate biomineralization from labile HA-Pb species remain unclear. Here, experiments were conducted using dialysis bags to separate PSB Enterobacter Aerogenes W6 cells from HA-Pb and FePO4 solids, and the time-dependent evolutions of solution conditions, PSB metabolites, Pb species, and Pb-phosphate minerals were systematically analyzed. Acid-soluble Pb species (∼25%) in HA-Pb, mainly complexing with phenolic hydroxyl groups, served as the source for Pb-phosphate mineralization. Extracellular secreted small organic acids contributed to Pb solubilization from HA-Pb, while polysaccharide macromolecules played critical roles in promoting Pb-phosphate nucleation on PSB cell surfaces. By coupling time-dependent bacterial growth, an integrated kinetic model was developed and performed well in describing the underlying processes of Pb-phosphate biomineralization, including Pb solubilization from HA-Pb (RMSE<1%), Pb adsorption onto PSB cells (RMSE<20%), and Pb-phosphate mineral precipitation (RMSE<5%). The findings provide insights into the Pb-phosphate biomineralization of solid-associated Pb and could help to predict the fate of Pb in natural and engineering systems.

Mechanistic insights into tris(2-chloroisopropyl) phosphate biomineralization coupled with lead (II) biostabilization driven by denitrifying bacteria

The ubiquity and persistence of organophosphate esters (OPEs) and heavy metal (HMs) pose global environmental risks. This study explored tris(2-chloroisopropyl)phosphate (TCPP) biomineralization coupled to lead (Pb2+) biostabilization driven by denitrifying bacteria (DNB). The domesticated DNB achieved synergistic bioremoval of TCPP and Pb2+ in the batch bioreactor (efficiency: 98 %).TCPP mineralized into PO43- and Cl-, and Pb2+ precipitated with PO43-. The TCPP-degrading/Pb2+-resistant DNB: Achromobacter, Pseudomonas, Citrobacter, and Stenotrophomonas, dominated the bacterial community, and synergized TCPP biomineralization and Pb2+ biostabilization. Metagenomics and metaproteomics revealed TCPP underwent dechlorination, hydrolysis, the TCA cycle-based dissimilation, and assimilation; Pb2+ was detoxified via bioprecipitation, bacterial membrane biosorption, EPS biocomplexation, and efflux out of cells. TCPP, as an initial donor, along with NO3-, as the terminal acceptor, formed a respiratory redox as the primary energy metabolism. Both TCPP and Pb2+ can stimulate phosphatase expression, which established the mutual enhancements between their bioconversions by catalyzing TCPP dephosphorylation and facilitating Pb2+ bioprecipitation. TCPP may alleviate the Pb2+-induced oxidative stress by aiding protein phosphorylation. 80 % of Pb2+ converted into crystalized pyromorphite. These results provide the mechanistic foundations and help develop greener strategies for synergistic bioremediation of OPEs and HMs.

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.

Comparison of bio-beads combined with Pseudomonas edaphica and three phosphate materials for lead immobilization: Performance, mechanism and plant growth

Phosphate materials (PMs) combine with phosphate solubilizing bacteria play an essential roles in lead (Pb) immobilization, but their resulting ability to reduce Pb bioavailability may vary depending on PMs used. In this study, Pseudomonas edaphica GAU-665 and three PMs: tricalcium phosphate, calcium phytate and nano-hydroxyapatite were respectively encapsulated into bio-beads by sodium alginate, which immobilization efficiency of Pb2+ were 99.11%, 97.76% and 99.02% at initial Pb2+ concentration of 200 mg L-1, respectively. The Pb2+ immobilization performance of bio-beads under different conditions and their organic acids secreted were examined. Most Pb2+ was immobilized by bio-beads through combined functions of adsorption, precipitation, ion exchange and biomineralization, accompanied by the formation of more stable compounds such as Pb3(PO4)2, Pb5(PO4)3OH and Pb5(PO4)3Cl. Meanwhile, pot experimental results indicated that the inoculation of CPhy (calcium phytate) bio-beads with PSB have highest biomass and root growth of oat (Avena sativa L.) in Pb-stressed compared with CK, which increased the content of chlorophyll b (167.51%) in shoot. In addition, the CPhy bio-beads enhance the peroxidase, catalase activities and reduce the malondialdehyde content to alleviating lead physiological toxicity in oat, which reductions the Pb accumulation in shoot (52.06%) and root (81.04%), and increased the residual fraction of Pb by 165.80% in soil. These findings suggest the bio-beads combined with P. edaphica GAU-665 and calcium phytate is an efficient Pb immobilization material and provided feasible way to improve safety agricultural production and Pb-contaminated soil remediation.

Isolation and identification of a high-efficiency hexavalent uranium adsorption strain and preliminary study of the influencing factors and adsorption mechanism

In this study, a bacterial strain Chryseobacterium bernardetii WK-3 was isolated from the rhizosphere soil of a uranium tailings in Southern China. It can efficiently adsorb hexavalent uranium with an adsorption ratio of 92.3%. The influence of different environmental conditions on the adsorption ratio of Chryseobacterium bernardetii strain WK-3 was investigated, and the adsorption mechanism was preliminarily discussed by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS). The results showed that the optimal adsorption conditions for U(VI) by Chryseobacterium bernardetii strain WK-3 were pH = 5, temperature 30 ℃, NaCl concentration 1%, and inoculation volume 10%. When the initial concentration of U was 50 ~ 150 mg/L, the adsorption capacity of Chryseobacterium bernardetii strain WK-3 to U(VI) reached the maximum and maintained the equilibrium at 44 h. SEM-EDS results showed that phosphorus in cells participates in the interaction of uranyl ions, which may indicate that phosphate was produced during cell metabolism and was further combined to form U(VI)-phosphate minerals. In summary, Chryseobacterium bernardetii strain WK-3 would be a promising alternative for environmental uranium contamination remediation.

Bacterial tolerance and detoxification of cyanide, arsenic and heavy metals: Holistic approaches applied to bioremediation of industrial complex wastes

Cyanide is a highly toxic compound that is found in wastewaters generated from different industrial activities, such as mining or jewellery. These residues usually contain high concentrations of other toxic pollutants like arsenic and heavy metals that may form different complexes with cyanide. To develop bioremediation strategies, it is necessary to know the metabolic processes involved in the tolerance and detoxification of these pollutants, but most of the current studies are focused on the characterization of the microbial responses to each one of these environmental hazards individually, and the effect of co-contaminated wastes on microbial metabolism has been hardly addressed. This work summarizes the main strategies developed by bacteria to alleviate the effects of cyanide, arsenic and heavy metals, analysing interactions among these toxic chemicals. Additionally, it is discussed the role of systems biology and synthetic biology as tools for the development of bioremediation strategies of complex industrial wastes and co-contaminated sites, emphasizing the importance and progress derived from meta-omic studies.

Microbial functionalities and immobilization of environmental lead: Biogeochemical and molecular mechanisms and implications for bioremediation

The increasing environmental and human health concerns about lead in the environment have stimulated scientists to search for microbial processes as innovative bioremediation strategies for a suite of different contaminated media. In this paper, we provide a compressive synthesis of existing research on microbial mediated biogeochemical processes that transform lead into recalcitrant precipitates of phosphate, sulfide, and carbonate, in a genetic, metabolic, and systematics context as they relate to application in both laboratory and field immobilization of environmental lead. Specifically, we focus on microbial functionalities of phosphate solubilization, sulfate reduction, and carbonate synthesis related to their respective mechanisms that immobilize lead through biomineralization and biosorption. The contributions of specific microbes, both single isolates or consortia, to actual or potential applications in environmental remediation are discussed. While many of the approaches are successful under carefully controlled laboratory conditions, field application requires optimization for a host of variables, including microbial competitiveness, soil physical and chemical parameters, metal concentrations, and co-contaminants. This review challenges the reader to consider bioremediation approaches that maximize microbial competitiveness, metabolism, and the associated molecular mechanisms for future engineering applications. Ultimately, we outline important research directions to bridge future scientific research activities with practical applications for bioremediation of lead and other toxic metals in environmental systems.

Stabilization of Pb(II) in wastewater and tailings by commercial bacteria through microbially induced phosphate precipitation (MIPP)

Microbially induced phosphate precipitation (MIPP) is an effective and eco-friendly method for Pb(II) stabilization. The phosphate-solubilizing microorganisms (PSM) for MIPP are commonly isolated from Pb(II)-contaminated sites through a series of intricate and time-consuming enrichment and purification processes. This research used ready-made commercial bacteria to develop a simple MIPP process. Bacillus subtilis (BS, CCTCC AB 98002) was selected from two commercial PSM strains owing to more effective Pb(II) removal. Compared to the most isolated microorganisms, BS released more than twice as much inorganic phosphorus (Pi) as well as had a high-level Pb(II) tolerance. BS could remove >99% of Pb(II) from 500 mg/L Pb(II)-containing water at the optimal 0.05 M sodium glycerophosphate (SGP), pH 7-9, and ≤0.03 M MgCl₂, outperforming most isolated microorganisms. In addition, BS could mitigate the contamination risk of the lead‑zinc tailings, by reducing the readily leachable Pb(II) concentration from 0.81 mg/L (over the regulatory limit of 0.1 mg/L) to 0.00042 mg/L. The unstable Pb(II) in the solution and tailings was ultimately stabilized to Pb₅(PO₄)₃Cl after the SGP phosphorlysis and phosphate precipitation processes. In conclusion, commercial BS is a superior alternative to isolated microorganisms for MIPP on Pb(II) stabilization. The simple-processed and high-effective BS-based MIPP provides the MIPP method a new insight for widespread implementation in the remediation of heavy metals-containing wastewater, soil, and waste.

Periplasmic space is the key location for Pb(II) biomineralization by Burkholderia cepacia

Phosphate solubilizing bacteria (PSB) induced phosphate precipitation is considered as an effective method for Pb(II) removal through the formation of stable Pb(II)-phosphate compound, but the location of end-products is still unclear. Herein, the PSB strain of Burkholderia cepacia (B. cepacia) coupled with the hydroxyapatite (HAP) was used in this study to investigate the Pb(II) removal mechanism and the biomineralization location. The dissolving phosphate of three particle sizes of HAP and Pb(II) resistant capabilities, and the effect factors such as HAP dosage, initial concentrations of Pb(II), pH, temperature, and different treatments were determined. The results indicated that the highest soluble phosphate could reach 224.85 mg/L in a 200 nm HAP medium and the highest removal efficiency of Pb(II) was about 96.32 %. Additionally, it was interesting that Pb(II) was mainly located in the periplasmic space through the cellular distribution experiment, which was further demonstrated by scanning electron microscope (SEM) and transmission electron microscopy (TEM). Besides, the characterization results showed that the functional groups such as amide, hydroxy, carboxy and phosphate played an important role in Pb(II) biomineralization, and the free Pb(II) in aqueous solution could be transformed into pyromorphite through phosphate dissolution, extracellular adsorption/complexation, and intracellular precipitation.

Pb0 flue gas biosorption in denitrifying MBfR: Characteristics and binding ability of extracellular polymeric substances

Pb⁰ in flue gas which is ubiquitous in the environment, poses a certain threat to human and ecology, but the study on EPS-dependent stabilization of lead to remove Pb⁰ from flue gas remains insufficient. In this investigation, the characteristics and heavy metals-binding ability of four EPS fractions were evaluated. The EPS were extracted from denitrifying membrane biofilm reactor (MBfR) and divided into slime EPS (S-EPS), loosely-bound EPS (LB-EPS), tightly-bound EPS (TB-EPS) and EPS in circulating flow (Y-EPS). The S, LB, TB-EPS related to Pb stabilization on biofilm need more attention. Compared to Pb-S-EPS (0.013 mg g^-1) and Pb-LB-EPS (0.13 mg g^-1), the Pb-TB-EPS (0.26 mg g^-1) was mainly stable form of vapor Pb⁰, since TB-EPS's higher content (30.67-82.44 mg g^-1 VSS), proteins (13.47-36.32 mg g^-1 VSS) and polysaccharides (9.37-32.48 mg g^-1 VSS) concentration. Particularly, proteins related ligands were more effective in S, LB, TB-EPS dependent adsorption of Pb, complexing with hydrophobic acid ligands further strengthened in TB-EPS adsorption. The Pb-EPS complex formed via binding with functional groups (such as O-H, N-H, C-H and CC) on EPS, also facilitated by loose structure of proteins. This study enlightens the researchers on the bio-treatment and EPS-dependent biosorption of Pb⁰ in flue gas in denitrifying MBfR.

The Microbiology of Metal Mine Waste: Bioremediation Applications and Implications for Planetary Health

Mine wastes pollute the environment with metals and metalloids in toxic concentrations, causing problems for humans and wildlife. Microorganisms colonize and inhabit mine wastes, and can influence the environmental mobility of metals through metabolic activity, biogeochemical cycling and detoxification mechanisms. In this article we review the microbiology of the metals and metalloids most commonly associated with mine wastes: arsenic, cadmium, chromium, copper, lead, mercury, nickel and zinc. We discuss the molecular mechanisms by which bacteria, archaea, and fungi interact with contaminant metals and the consequences for metal fate in the environment, focusing on long-term field studies of metal-impacted mine wastes where possible. Metal contamination can decrease the efficiency of soil functioning and essential element cycling due to the need for microbes to expend energy to maintain and repair cells. However, microbial communities are able to tolerate and adapt to metal contamination, particularly when the contaminant metals are essential elements that are subject to homeostasis or have a close biochemical analog. Stimulating the development of microbially reducing conditions, for example in constructed wetlands, is beneficial for remediating many metals associated with mine wastes. It has been shown to be effective at low pH, circumneutral and high pH conditions in the laboratory and at pilot field-scale. Further demonstration of this technology at full field-scale is required, as is more research to optimize bioremediation and to investigate combined remediation strategies. Microbial activity has the potential to mitigate the impacts of metal mine wastes, and therefore lessen the impact of this pollution on planetary health.

Experimental and modeling studies of competitive Pb (II) and Cd (II) bioaccumulation by Aspergillus niger

Co-existence of toxic metals causes complex toxicity to microorganisms during bioremediation in water and soil. This study investigated the immobilization of Pb²⁺ and Cd²⁺ by fungus Aspergillus niger, which has been widely applied to environmental remediation. Five treatments were set, i.e., CK (no toxic metals), Pb²⁺ only, Cd²⁺ only, Pb²⁺/Cd²⁺ = 1:1(molar ratio), and Pb²⁺/Cd²⁺ = 2:1. Cadmium induced strong toxicity to the fungus, and maintained the high toxicity during incubation. However, as Pb/Cd ratio increased from 0 to 2, the removal rates of Cd²⁺ by A. niger were raised from 30 to 50%. The elevated activities of pyruvate dehydrogenase (PDH) and citrate synthetase (CS) enzymes confirmed that Pb addition could stimulate the growth of A. niger. For instance, citric acid concentrations and CS activities were 463.22 mg/L and 78.37 nmol/min/g, respectively, during 3-day incubation as Pb/Cd = 1. However, these two values were as low as ~ 50 with addition of only Cd. It was hence assumed that appropriate co-existence of Pb²⁺ enhanced microbial activity by promoting TCA cycle of the fungus. Moreover, the SEM analysis and geochemical modeling demonstrated that Pb²⁺ cations were more easily adsorbed and mineralized on A. niger with respect to Cd²⁺. Therefore, instead of intensifying metal toxicity, the addition of appropriate Pb actually weakened Cd toxicity to the fungus. This study sheds a bright future on application of A. niger to the remediation of polluted water with co-existence of Pb and Cd. KEY POINTS: • Cd²⁺ significantly inhibited P consumption, suggesting its high toxicity to A. niger. • Pb²⁺ stimulated the growth of A. niger by promoting TCA cycle in the cells. • Cd²⁺ removal by A. niger were improved with co-existence of Pb²⁺.

Isolation and Identification of Uranium Tolerant Phosphate-Solubilizing Bacillus spp. and Their Synergistic Strategies to U(VI) Immobilization

The remediation of uranium (U) through phosphate-solubilizing bacteria (PSB) is an emerging technique as well as an interesting phenomenon for transforming mobile U into stable minerals in the environment. While studies are well needed for in-depth understanding of the mechanism of U(VI) immobilization by PSB. In this study, two PSB were isolated from a U-tailing repository site. These bacterial strains (ZJ-1 and ZJ-3) were identified as Bacillus spp. by the sequence analysis of 16S ribosomal RNA (rRNA) genes. Incubation of PSB in liquid medium showed that the isolate ZJ-3 could solubilize more than 230 mg L^-1 P from glycerol-3-phosphate and simultaneously removed over 70% of 50 mg L^-1 U(VI) within 1 h. During this process, the rapid appearance of yellow precipitates was observed. The microscopic and spectroscopic analysis demonstrated that the precipitates were associated with U-phosphate compound in the form of saleeite-like substances. Besides, scanning electron microscopy coupled with energy-dispersive X-ray (SEM-EDS) and Fourier transform infrared spectroscopy (FTIR) analysis of the precipitates confirmed that the extracellular polymeric substances (EPS) might also play a key role in U sequestration. Furthermore, SEM and FTIR analysis revealed that part of U(VI) was adsorbed on the bacterial surface through cellular phosphate, hydroxy, carboxyl, and amide groups. This study provides new insights into the synergistic strategies enhancing U immobilization rates by Bacillus spp. that uses glycerol-3-phosphate as the phosphorus source, the process of which contributes to harmful pollutant biodegradation.

Exploring bacterial community composition in Mediterranean deep-sea sediments and their role in heavy metal accumulation

The role of microbial processes in bioaccumulation of major and trace elements has been broadly demonstrated. However, microbial communities from marine sediments have been poorly investigated to this regard. In marine environments, particularly under high anthropogenic pressure, heavy metal accumulation increases constantly, which may lead to significant environmental issues. A better knowledge of bacterial diversity and its capability to bioaccumulate metals is essential to face environmental quality assessment. The oligotrophic westernmost Mediterranean, which is highly sensitive to environmental changes and subjected to increasing anthropogenic pressure, was selected for this study. A sediment core spanning the last two millennia was sampled at two intervals, with ages corresponding to 140 (S1) and 1400 (S2) yr BP. High-throughput sequencing showed an abundance of Bacillus, Micrococcus, unclassified members of Planococcaceae, Anaerolineaceae, Planctomycetaceae, Microlunatus, and Microbacterium in both intervals, with slight differences in their abundance, along with newly detected ones in S2, i.e., Propionibacterium, Fictibacillus, Thalassobacillus, and Bacteroides. Canonical correspondence analysis (CCA) and co-occurrence patterns confirmed strong correlations among the taxa and the environmental parameters, suggesting either shared and preferred environmental conditions, or the performance of functions similar to or complementary to each other. These results were further confirmed using culture-dependent methods. The diversity of the culturable bacterial community revealed a predominance of Bacillus, and Micrococcus or Kocuria. The interaction of these bacterial communities with selected heavy metals (Cu, Cr, Zn and Pb) was also investigated, and their capacity of bioaccumulating metals within the cells and/or in the extracellular polymeric substances (EPS) is demonstrated. Interestingly, biomineralization of Pb resulted in the precipitation of Pb phosphates (pyromorphite). Our study supports that remnants of marine bacterial communities can survive in deep-sea sediments over thousands of years. This is extremely important in terms of bioremediation, in particular when considering possible environmentally friendly strategies to bioremediate inorganic contaminants.

Fungal mediated biotransformation reduces toxicity of arsenic to soil dwelling microorganism and plant

Rhizospheric and plant root associated microbes generally play a protective role against arsenic toxicity in rhizosphere. Rhizospheric microbial interaction influences arsenic (As) detoxification/mobilization into crop plants and its level of toxicity and burden. In the present investigation, we have reported a rhizospheric fungi Aspergillus flavus from an As contaminated rice field, which has capability to grow at high As concentration and convert soluble As into As particles. These As particles showed a reduced toxicity to soil dwelling bacteria, fungi, plant and slime mold. It does not disrupt membrane potential, inner/outer membrane integrity and survival of the free N₂ fixating bacteria. In arbuscular mycorrhiza like endophytic fungi Piriformospora indica, these As particles does not influence mycelial growth and plant beneficial parameters such as phosphate solubilizing enzyme rAPase secretion and plant root colonization. Similarly, it does not affect plant growth and chlorophyll content negatively in rice plant. However, these As particles showed a poor absorption and mobilization in plant. These As particle also does not affect attachment process and survival of amoeboid cells in slime mold, Dictyostelium discoideum. This study suggests that the process of conversion of physical and chemical properties of arsenic during transformation, decides the toxicity of arsenic particles in the rhizospheric environment. This phenomenon is of environmental significance, not only in reducing arsenic toxicity but also in the survival of healthy living organism in arsenic-contaminated rhizospheric environment.

Contrasting the Pb (II) and Cd (II) tolerance of Enterobacter sp. via its cellular stress responses

Successful application of microorganisms to heavy metal remediation depends on their resistance to toxic metals. This study contrasted the differences of tolerant mechanisms between Pb²⁺ and Cd²⁺ in Enterobacter sp. Microbial respiration and production of formic acid showed that Enterobacter sp. had a higher tolerant concentration of Pb (>1000 mg l^-1 ) than Cd (about 200 mg l^-1 ). Additionally, SEM confirmed that most of Pb and Cd nanoparticles (NPs) were adsorbed onto cell membrane. The Cd stress, even at low concentration (50 mg l^-1 ), significantly enlarged the sizes of cells. The cellular size raised from 0.4 × 1.0 to 0.9 × 1.6 μm on average, inducing a platelet-like shape. In contrast, Pb cations did not stimulate such enlargement even up to 1000 mg l^-1 . Moreover, Cd NPs were adsorbed homogeneously by almost all the bacterial cells under TEM. However, only a few cells work as 'hot spots' on the sorption of Pb NPs. The heterogeneous sorption might result from a 'self-sacrifice' mechanism, i.e., some cells at a special life stage contributed mostly to Pb sorption. This mechanism, together with the lower mobility of Pb cations, caused higher microbial tolerance and removal efficiency towards Pb²⁺ . This study sheds evident contrasts of bacterial resistance to the two most common heavy metals.