Role of microbial exopolymeric substances (EPS) on chromium sorption and transport in heterogeneous subsurface soils: I. Cr(III) complexation with EPS in aqueous solution

Mechanism and application of bacterial exopolysaccharides: An advanced approach for sustainable heavy metal abolition from soil

The escalation of heavy metal pollutants in soils and effluents, driven by industrialization and human activities, poses significant environmental and health risks. Conventional remediation methods are often costly and ineffective, prompting a shift towards sustainable alternatives such as biological treatments. Natural biosorbents, including microbial cells and their byproducts, have emerged as promising solutions. One such approach involves leveraging exopolysaccharides (EPS), complex high-molecular-weight biopolymers synthesized by microbes under environmental stress conditions. EPS are intricate organic macromolecules comprising proteins, polysaccharides, uronic acids, humic compounds, and lipids, either located within microbial cells or secreted into their surroundings. Their anionic functional groups enable efficient electrostatic binding of cationic heavy metals, making EPS effective biosorbents for soil remediation. This review thoroughly explores the pivotal role of bacterial EPS in the removal of heavy metals, focusing on EPS biosynthesis mechanisms, the dynamics of interaction with heavy metals, and case studies that illustrate their effectiveness in practical remediation strategies. By highlighting these aspects, the review underscores the innovation and practical implications of EPS-based bioremediation technologies, demonstrating their potential to address critical environmental challenges effectively while paving the way for sustainable environmental management practices. Key findings reveal that EPS exhibit robust metal-binding capacities, facilitated by their anionic functional groups, thereby offering a promising solution for mitigating metal pollution in diverse environmental matrices.

Enhancement of Mn2+, Fe2+ and NH4+-N removal by biochar synergistic strains combined with activated sludge in real wastewater treatment

Acinetobacter sp. AL-6 combining with biochar was adapted in activated sludge (AS & co-system) to decontaminate Mn2+, Fe2+ and NH4+-N, and treat activated sludge (AS) for its activity and settling performance improvement. Specifically, the co-system promoted the growth of bacteria in the activated sludge, thus increasing its ability to nitrify and adsorb Mn2+ and Fe2+, resulting in the removal of high concentrations of NH4+-N, Mn2+, Fe2+ and COD in the reactor by 100%, 100%, 100%, and 96.8%, respectively. And the pH of wastewater was increased from 4 to 8.5 by co-system also facilitated the precipitation of Mn2+ and Fe2+. The MLVSS/MLSS ratio increased from 0.64 to 0.95 and SVI30 decreased from 92.54 to 1.54 after the addition of co-system, which indicated that biochar helped to improve the activity and settling performance of activated sludge and prevented it from being damaged by the compound Mn2+ and Fe2+. In addition, biochar promoted the increase of the tyrosine-like protein substance and humic acid-like organic matter in the sludge EPS, thus enhanced the ability of sludge to adsorb Mn2+ and Fe2+. Concretely, compared with AS group, the proteins content and polysaccharides content of the AS & co-system group were increased by 13.14 times and 6.30 times respectively. Further, microbial diversity analysis showed that more resistant bacteria and dominant bacteria Acinetobacter sp. AL-6 in sludge enhanced the nitrification and adsorption of manganese and iron under the promotion of biochar. Pre-eminently, the more effective AS & co-system were applied to the removal of actual electrolytic manganese slag leachate taken from the contaminated site, and the removal of NH4+-N, Mn2+, Fe2+ and COD remained high at 100%, 100%, 71.82% and 94.72%, respectively, revealing advanced value for high engineering applications of AS & co-system.

Microbial Exudates as Biostimulants: Role in Plant Growth Promotion and Stress Mitigation

Microbes hold immense potential, based on the fact that they are widely acknowledged for their role in mitigating the detrimental impacts of chemical fertilizers and pesticides, which were extensively employed during the Green Revolution era. The consequence of this extensive use has been the degradation of agricultural land, soil health and fertility deterioration, and a decline in crop quality. Despite the existence of environmentally friendly and sustainable alternatives, microbial bioinoculants encounter numerous challenges in real-world agricultural settings. These challenges include harsh environmental conditions like unfavorable soil pH, temperature extremes, and nutrient imbalances, as well as stiff competition with native microbial species and host plant specificity. Moreover, obstacles spanning from large-scale production to commercialization persist. Therefore, substantial efforts are underway to identify superior solutions that can foster a sustainable and eco-conscious agricultural system. In this context, attention has shifted towards the utilization of cell-free microbial exudates as opposed to traditional microbial inoculants. Microbial exudates refer to the diverse array of cellular metabolites secreted by microbial cells. These metabolites enclose a wide range of chemical compounds, including sugars, organic acids, amino acids, peptides, siderophores, volatiles, and more. The composition and function of these compounds in exudates can vary considerably, depending on the specific microbial strains and prevailing environmental conditions. Remarkably, they possess the capability to modulate and influence various plant physiological processes, thereby inducing tolerance to both biotic and abiotic stresses. Furthermore, these exudates facilitate plant growth and aid in the remediation of environmental pollutants such as chemicals and heavy metals in agroecosystems. Much like live microbes, when applied, these exudates actively participate in the phyllosphere and rhizosphere, engaging in continuous interactions with plants and plant-associated microbes. Consequently, they play a pivotal role in reshaping the microbiome. The biostimulant properties exhibited by these exudates position them as promising biological components for fostering cleaner and more sustainable agricultural systems.

Biodegradation of polyurethanes by Serratia liquefaciens L135 and its polyurethanase: In silico and in vitro analyses

Polyurethanes (PUs) are found in many everyday products and their disposal leads to environmental accumulation. Therefore, there is an urgent need to develop ecologically sustainable techniques to biodegrade and recycle this recalcitrant polymer and replace traditional methods that form harmful by-products. Serratia liquefaciens L135 secretes a polyurethanase with lipase activity, and this study explores the biodegradation of PUs by this bacterium and its enzyme through in silico and in vitro analyses. PUs monomers and tetramers were constructed in silico and tested with modeled and validated structure of the polyurethanase from S. liquefaciens. The molecular docking showed that all PUs monomers presented favorable interactions with polyurethanase (values of binding energy between -84.75 and -121.71 kcal mol-1), including PU poly[4,4'-methylenebis (phenyl isocyanate)-alt-1,4-butanediol/di (propylene glycol)/polycaprolactone] (PCLMDI). Due to repulsive steric interactions, tetramers showed less favorable interactions (values between 24.26 and -45.50 kcal mol-1). In vitro analyses evaluated the biodegradation of PUs: Impranil® and PCLMDI; this latter showed high binding energy with this polyurethanase in silico. The biodegradation of Impranil® by S. liquefaciens and its partially purified polyurethanase was confirmed in agar by forming a transparent halo. Impranil® disks inoculated with S. liquefaciens and incubated at 30 °C for six days showed rupture of the PU structure, possibly due to the formation of cracks visualized by scanning electron microscopy (SEM). PCLMDI films were also biodegraded by S. liquefaciens after 60 days of incubation, with the formation of pores and cracks visualized by SEM. The biodegradation may have occurred due to the action of polyurethanase produced by this bacterium. This work provides essential information on the potential of S. liquefaciens to biodegrade PUs through in silico analyses combined with in vitro analyses.

Promotion effects and mechanisms of molybdenum disulfide on the propagation of antibiotic resistance genes in soil

The rapid development of nanotechnology has aroused considerable attentions toward understanding the effects of engineered nanomaterials (ENMs) on the propagation of antibiotic resistance. Molybdenum disulfide (MoS₂) is an extensively used ENM and poses potential risks associated with environmental exposure; nevertheless, the role of MoS₂ toward antibiotic resistance genes (ARGs) transfer remains largely unknown. Herein, it was discovered that MoS₂ nanosheets accelerated the horizontal transfer of RP4 plasmid across Escherichia coli in a dose-dependent manner (0.5-10 mg/L), with the maximum transfer frequency 2.07-fold higher than that of the control. Integration of physiological, transcriptomics, and metabolomics analyses demonstrated that SOS response in bacteria was activated by MoS₂ due to the elevation of oxidative damage, accompanied by cell membrane permeabilization. MoS₂ promoted bacterial adhesion and intercellular contact via stimulating the secretion of extracellular polysaccharides. The ATP levels were maximally increased by 305.7 % upon exposure to MoS₂, and the expression of plasmid transfer genes was up-regulated, contributing to the accelerated plasmid conjugation and increased ARG abundance in soil. Our findings highlight the roles of emerging ENMs (e.g., MoS₂) in ARGs dissemination, which is significant for the safe applications and risk management of ENMs under the development scenarios of nanotechnology.

Exploring the role of extracellular polymeric substances in the antimony leaching of tailings by Acidithiobacillus ferrooxidans

The concentration of Sb bearing tailings in water located in abandoned antimony mines was found to be a big problem, as they contaminate other water resources and entire food chain. Microorganisms were found to be key in tailing leaching and reaction speeding in the presence of extracellular polymeric substances (EPS) produced by bacteria. Herein, we investigated the pattern of the Sb leaching from Sb bearing tailings using Acidithiobacillus ferrooxidans, and analyzed the mechanism of EPS in the leaching process of Sb. To completely and deeply understand the functions of EPS in the bioleaching of antimony tailings, the generation behavior of EPS produced by Acidithiobacillus ferrooxidans (A. ferrooxidans) during bioleaching was characterized by three-dimensional excitation-emission matrix (3D-EEM). Meanwhile, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) were used to show the changes of EPS functional groups before and after leaching. Compared with the functional groups in EPS produced by A. ferrooxidans before leaching, the content of hydroxyl and amino groups that reduce high-valent metals to low-valent metals in EPS decreases after leaching, and the carbonyl content increases, corresponding to the ratio of trivalent antimony increased, indicating that EPS could reduce the risk of pentavalent antimony to trivalent one. At the same time, with biological scanning electron microscopy and energy spectrum scanning, the observation of EPS on the mineral surface showed that Sb was adsorbed in the EPS, and the XPS of Sb was fine. Spectral analysis showed that the Sb adsorbed in EPS contained both Sb(III) and Sb(V). Besides, for revealing the influence of EPS in the leaching process of Sb from tailings, this work provided an in-depth understanding of the mechanism of Sb released from tailings under the action of A. ferrooxidans and further provides a basis for the biogeochemical cycle of Sb.

Role of extracellular polymeric substances in metal sequestration during mangrove restoration

Extracellular polymeric substances (EPS) are widely observed in aquatic ecosystems, however the potential function of EPS on metal sequestration in mangrove wetlands is unclear. Thus, an ecological restoration area (including Sonneratia apetala, Kandelia obovata and unvegetated mudflat) was employed to assess the effect of mangrove reforestation on metal sequestration and the underlying roles played by EPS. The results showed that mangrove restoration directly promoted metal accumulation (e.g., Cr, Cu, Ni, Pb, and Zn) in sediments. However, alleviated metal bioavailability was detected after mangrove reforestation. The changes in metal accumulation and bioavailability were highly correlated with EPS and microbial composition. Mangrove restoration (especially for K. obovata reforestation) also significantly promoted EPS production, in which multiple metal-chelating functional groups (e.g., hydroxyl, carboxyl, and imino) were identified by Fourier infrared spectra. Moreover, the contents of EPS were positively correlated with metal accumulation but negatively correlated with metal bioavailability. The present data further illustrated that the enhancements of Gammaproteobacteria, Bacteroidia, Desulfobulbia, and Desulfobacteria might be important for EPS production. In summary, this is the first study to reveal that the presence of artificial mangroves might act as an efficient barrier in metal sequestration and immobilization by enhancing inherent microbial EPS.

A Review of the Role of Extracellular Polymeric Substances (EPS) in Wastewater Treatment Systems

A review of the characterization and functions of extracellular polymeric substances (EPS) of microbial aggregates in biological wastewater treatment systems is presented in this paper. EPS represent the complex high-molecular-weight mixture of polymers excreted by microorganisms generated from cell lysis as well as adsorbed inorganic and organic matter from wastewater. EPS exhibit a three-dimensional, gel-like, highly hydrated matrix that facilitates microbial attachment, embedding, and immobilization. EPS play multiple roles in containments removal, and the main components of EPS crucially influence the properties of microbial aggregates, such as adsorption ability, stability, and formation capacity. Moreover, EPS are important to sludge bioflocculation, settleability, and dewatering properties and could be used as carbon and energy sources in wastewater treatment. However, due to the complex structure of EPS, related knowledge is incomplete, and further research is necessary to understand fully the precise roles in biological treatment processes.

Complexation behaviour and removal of organic-Cr(III) complexes from the environment: A review

Chromium (Cr) is mainly found in the form of organic-Cr(III) complexes in the natural environment and industrial waste. The widespread existence of composite contaminants composed of organic matter (OM) and Cr pose a serious ecological threat, and its potential interaction and removal need to be further summarised. Organic ligands, such as carbohydrates, nitrogen compounds, phenolic compounds, humus substances (HS), and low molecular weight organic acids (LMWOAs), play an important role in governing the speciation, mobility, and absorption and desorption of Cr in the environment. Moreover, growing evidence indicates that oxygen-containing functional groups (e.g., carboxyl, hydroxyl, and phosphate) are closely related to the complexation of Cr(III). Advanced oxidation processes (AOPs) are efficient and widely applicable technologies. However, the re-complexation of oxidation intermediates with Cr(III) and the formation and accumulation of much more toxic Cr(VI) species hinder the possible utilisation of AOPs. In this paper, the sources and harmful effects of organic-Cr(III) complexes are reported in detail. The complexation behaviour and structure of the organic-Cr(III) complexes are also described. Subsequently, the application of AOPs in the decomplexation and degradation of organic-Cr(III) complexes is summarised. This review can be helpful for developing technologies that are more efficient for organic-Cr(III) complex removal and establishing the scientific background for reducing Cr discharge Cr into the environment.

Bioreduction performance of Cr(VI) by microbial extracellular polymeric substances (EPS) and the overlooked role of tryptophan

Extracellular polymeric substances (EPS) have exhibited promising advantages in mitigating heavy metal contamination, e.g., single-valent silver (Ag(I)), trivalent gold (Au(III)), and hexavalent chromium (Cr(VI)). However, knowledge of the specific substrate in EPSs that supports Cr(VI) reduction has remained elusive. Here, we isolated a novel Cr(VI)-reducing strain with self-mediating properties in an aquatic environment with various pH values to investigate the mechanisms. After analysis by a batch assay coupled with X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) spectroscopic techniques, it was found that Cr(VI) was reduced by the strain and soluble-EPS (S-EPS), and then, organo-trivalent chromium (organo-Cr(III)) was successfully formed. In addition, compared with other components of the strain, the strain and S-EPS completely removed Cr(VI), and the S-EPS exhibited a positive effect on Cr(VI) reduction with a strong monotonic correlation (R² = 0.999, p = 9.03 × 10^-5), indicating that the reduction is an EPS-dependent process. Specifically, the Cr(VI) reduction efficiency was enhanced to 48.85% and 99.4% after EPS and EPS plus tryptophan were added; their respective efficiencies were 3.94 and 8.02 times higher than that of the control assay in which the reductant was depleted. High-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) analysis showed that the tryptophan concentration concomitantly decreased by 61.54%. These findings highlighted the importance of S-EPS and tryptophan and improved our understanding of EPS for Cr(VI) reduction, which might provide a novel strategy for decontaminating targeted heavy metals in future applications.

Phylogenetic diversity contributes more to sediment magnetism than abundance during incubation of iron-reducing sediment from a non-active volcanic lake in Northeast China

AIM

This study aimed to analyse bacterial community and biomineralization products from Wudalianchi non-active volcanic field and the relationship between magnetization and bacterial community.

METHODS AND RESULTS

Eighteen sediment samples obtained from Wenbo Lake, high-throughput sequencing and quantitative PCR (qPCR) were separately employed to investigate the bacterial community composition dynamics and abundance variation of the sediment sample with the highest iron-reducing capacity during incubation. The mineralization products were characterized by transmission electron microscopy, scanning electron microscopy, X-ray diffraction (XRD), Raman spectroscopy, vibrating sample magnetometer (VSM) and variable-temperature magnetism analyses. The results showed that the highest iron reduction rate was 98·06%. Seven phyla were identified as dominant bacterial phyla during the incubation process. Iron-reducing bacteria (FeRB) including Geobacter, Desulfosporosinus and Clostridium were involved in the iron mineralization process. The 16S rDNA copy numbers of sediment decreased quickly and then stayed steady during the incubation. Bacteria with rod-shaped and spheroid species were involved in extracellular iron reduction to produce magnetic particles with massive aggregation and columnar structures on the mineral surface morphologies. The materials produced by the microbial community over the incubation period were sequentially identified as siderite, magnetite and maghemite. The magnetism of the mineral samples gradually increased from 0·31748 to 33·58423 emu g^-1 with increased incubation time. The final products showed relatively stable magnetism under 0-400 K. Meanwhile, the saturation magnetization (M_S ) of the mineralized substance was tightly associated with bacterial diversity (P < 0·05).

CONCLUSIONS

Bacterial community varied during incubation of iron-reducing sediment of volcanic lake. Various iron mineral crystals were in turn formed extracellularly by FeRB. The magnetism of mineralized products was tightly associated with bacterial community.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results not only help us to better understand the iron mineralization of FeRB in the volcanic lake sediments but also provide basic information for the future application of FeRB in environmental bioremediation.

Insights into solubility of soil humic substances and their fluorescence characterisation in three characteristic soils

Soil humic substances (HS) are involved in almost all biogeochemical processes and functions in soils, thus their extraction from aiming to their characterization is very important. However, many factors that influence HS extraction from soil still need further studies. The aim of this work was to assess and quantify comparatively the solubility of soil HS as a function of extraction time, various extractants, solid to liquid ratio and sequential extraction. In this work three different soils, i.e. a forest, a maize and a paddy soil, were examined to assess the solubility of soil HS based on their fluorescence (excitation-emission matrix, EEM) features and changes in nutrient (NO₃^--N, PO₄^3--P and dissolved Si) contents using multiple extraction approaches (time-dependent, various extractants, solid to liquid ratio, and sequential extraction). Three fluorescent components, i.e. humic acids-like (HA-like), fulvic acids-like (FA-like), and protein-like fluorophores (PLF), were identified by parallel factor (PARAFAC) analysis of EEM spectra of the various soil extracts. The solubility of HS, dissolved organic carbon (DOC) and nutrients were shown to increase with extraction time, except for PLF. The FA-like fraction disappeared completely in KCl extracts of all three soils, suggesting the inefficiency of salt extraction. Conversely, HS and nutrients solubility substantially increased in alkaline extracts, and dissolved Si was correlated significantly with the fluorescent intensities of HA-like and FA-like, thus confirming the well-known typical process of alkaline dissolution of HS bound to phytolith and silicate minerals. The relative solubility of HS and nutrients was higher at lower solid to liquid ratio (1:250-1:100), whereas their maximum yields was achieved at high solid to liquid ratio (1:10) for all three soils. Sequential extraction results showed that the first water extraction step contributed 42-55% of HS, which suggested that a single extraction was insufficient to recover HS. In conclusion, water and alkaline extraction could provide, respectively, the labile and insoluble complexed HS existing in soil.

Mechanisms of Cr(VI) reduction by Bacillus sp. CRB-1, a novel Cr(VI)-reducing bacterium isolated from tannery activated sludge

Cr(VI) reduction by microorganisms has been extensively reported, however, the mechanism of Cr(VI) reduction varies among different microorganisms. In this study, a Cr(VI)-reducing bacterium identified as Bacillus sp. was isolated from tannery activated sludge, strain CRB-1 was able to completely reduce 50 mg/L of Cr(VI) within 24 h under aerobic conditions and exhibited considerable Cr(VI) removal efficiency in the pH range from 7.0 to 9.0, temperature 24-42 °C. Cr(VI) reduction assays with resting cells, permeabilized cells, and subcellular fractions suggested that Cr(VI) reduction mainly occurred in the cytoplasm. According to qRT-PCR analysis, a chrA gene and a nitR2 gene were up-regulated under Cr(VI) stress. Heterologous expression of the chrA gene and the nitR2 gene indicated that ChrA was associated with Cr(VI) resistance, while NitR2 was responsible for Cr(VI) reduction. Furthermore, soluble end products were detected. On the basis of FTIR, it was speculated that the formation of soluble end products may be due to the complexation of EPS with Cr(III). Consequently, the Cr(VI)-reducing ability of strain CRB-1 and its chromate reductases enables CRB-1 a potential candidate for Cr(VI) bioremediation.

Microbial chromate reduction coupled with anaerobic oxidation of methane in a membrane biofilm reactor

It has been reported that microbial reduction of sulfate, nitrite/nitrate and iron/manganese could be coupled with anaerobic oxidation of methane (AOM), which plays a significant role in controlling methane emission from anoxic niches. However, little is known about microbial chromate (Cr(VI)) reduction coupling with AOM. In this study, a microbial consortium was enriched via switching nitrate dosing to chromate feeding as the sole electron acceptor under anaerobic condition in a membrane biofilm reactor (MBfR), in which methane was continuously provided as the electron donor through bubble-less hollow fiber membranes. According to long-term reactor operation and chromium speciation analysis, soluble chromate could be reduced into Cr(III) compounds by using methane as electron donor. Fluorescence in situ hybridization and high-throughput 16S rRNA gene amplicon profiling further indicated that after feeding chromate Candidatus 'Methanoperedens' (a known nitrate-dependent anaerobic methane oxidation archaeon) became sole anaerobic methanotroph in the biofilm, potentially responsible for the chromate bio-reduction driven by methane. Two potential pathways of the microbial AOM-coupled chromate reduction were proposed: (i) Candidatus 'Methanoperedens' independently utilizes chromate as electron acceptor to form Cr(III) compounds, or (ii) Candidatus 'Methanoperedens' oxidizes methane to generate intermediates or electrons, which will be utilized to reduce chromate to Cr(III) compounds by unknown chromate reducers synergistically. Our findings suggest a possible link between the biogeochemical chromium and methane cycles.

Understanding and optimization of the flocculation process in biological wastewater treatment processes: A review

In the operation of biological wastewater treatment processes, fast sludge settling during liquid-solids disengagement is preferred as it affects effluent quality, treatment efficiency and plant operation economy. An important property of fast settling biological sludge is the ability to spontaneously form big and dense flocs (flocculation) that readily separates from water. Therefore, there had been much research to study the conditions that promote biological sludge flocculation. However, reported findings have often been inconsistent and this has possibly been due to the complex nature of the biological flocculation process. Thus, it has been challenging for wastewater treatment plant operators to extract practical information from the literature. The aim of this review is to summarize the current state of understanding of the factors that affect sludge flocculation so that evaluation of such information can be facilitated and strategize for intervention in the sludge flocculation and deflocculation process.

Advances in exopolysaccharides based bioremediation of heavy metals in soil and water: A critical review

Extracellular polysaccharides or Exopolysaccharides (EPS) are extensively studied bacterial byproducts with high molecular weight attributed to several applications. In spite of their application in the field of food, pharmaceutical, nutraceutical, herbicidal and cosmeceutical industries they were well known for their efficiency in the bioremediation of water and soil tainted with heavy metals. These heavy metals are comparatively high in density than water and are involved in several biological processes. But slight increase in levels can create toxicological bias. The techniques like electrodialysis, chemical precipitation, ion exchange and membrane separation have a lot of disadvantages akin to high energy consumption, high cost, partial exclusion, and creation of poisonous mire. In this context, EPS has a top role to play in the bioremediation of heavy metals. This review gives the critical assessment of the extensive work done to deal this issue by different groups in the last five years. It also explains how different natural circumstances have attributed to the advancement of EPS production, thereby increasing the capacity of bioremediation to deal the issue of heavy metal contamination in both soil and water. A detailed discussion of the EPS formation by bacteria and fungi with their applicability was reported.

Metal-tolerant thermophiles: metals as electron donors and acceptors, toxicity, tolerance and industrial applications

Metal-tolerant thermophiles are inhabitants of a wide range of extreme habitats like solfatara fields, hot springs, mud holes, hydrothermal vents oozing out from metal-rich ores, hypersaline pools and soil crusts enriched with metals and other elements. The ability to withstand adverse environmental conditions, like high temperature, high metal concentration and sometimes high pH in their niche, makes them an interesting subject for understanding mechanisms behind their ability to deal with multiple duress simultaneously. Metals are essential for biological systems, as they participate in biochemistries that cannot be achieved only by organic molecules. However, the excess concentration of metals can disrupt natural biogeochemical processes and can impose toxicity. Thermophiles counteract metal toxicity via their unique cell wall, metabolic factors and enzymes that carry out metal-based redox transformations, metal sequestration by metallothioneins and metallochaperones as well as metal efflux. Thermophilic metal resistance is heterogeneous at both genetic and physiology levels and may be chromosomally, plasmid or transposon encoded with one or more genes being involved. These effective response mechanisms either individually or synergistically make proliferation of thermophiles in metal-rich habitats possibly. This article presents the state of the art and future perspectives of responses of thermophiles to metals at genetic as well as physiological levels.

Characterization of Product and Potential Mechanism of Cr(VI) Reduction by Anaerobic Activated Sludge in a Sequencing Batch Reactor

Bioremediation of Cr(VI) and nitrate is considered as a promising and cost-effective alternative to chemical and physical methods. However, organo-Cr(III) complexes in effluent generally causes environmental concerns due to second-pollution. Here, Cr(VI) reduction and immobilization efficiencies of anaerobic activated sludge were investigated. Anaerobic activated sludge showed strong reduction ability of Cr(VI) and possessed a great potential of Cr(III) immobilization. Almost 100.0 mg l⁻¹ Cr(VI) could be completely reduced and immobilized by anaerobic activated sludge in a sequencing batch reactor in 24 h. And most generated Cr(III) was accumulated outside of sludge cells. Extracellular polymeric substances (EPS) could bind to Cr(VI) and form EPS-Cr(VI) interaction to reduce the toxic effect of Cr(VI) and promote the Cr(VI) reduction. Protein-like and humic-like substances were responsible for binding with Cr(VI), meanwhile the process was a thermodynamically favorable binding reaction. Then Cr(VI) was reduced to Cr(III) by membrane-associated chromate reductase of sludge. Eventually, the generated Cr(III) might exist as poly-nuclear Cr(III) complexes adhered to sludge surfaces.

Adsorption of Cu2+ and Zn2+ by extracellular polymeric substances (EPS) in different sludges: Effect of EPS fractional polarity on binding mechanism

Extracellular polymeric substances (EPS) in sludge samples played a major role in heavy metals removal during wastewater treatment. In this study, the binding quality, adsorption mechanism, as well as the chemical fractional contribution of the sludge EPS from activated sludge, anaerobic granular sludge and anaerobic flocculent sludge to the adsorption of Zn²⁺ and Cu²⁺ was investigated. For all three sludge samples, Cu²⁺ could be more easily adsorbed than Zn²⁺, and EPS extracted from the anaerobic granular sludge exhibited a relatively higher adsorption capacity than that of anaerobic flocculent sludge and activated sludge. Specifically, hydrophobic EPS of the activated sludge and anaerobic flocculent sludge was more efficient in adsorbing Cu²⁺ and Zn²⁺ than that of the hydrophilic EPS. However, hydrophilic EPS in anaerobic granular sludge played a greater role in heavy metals removal. The adsorption of those two heavy metals onto the unfractionated and hydrophobic EPS could be better described by the Langmuir isotherm, while Freundlich models fitted hydrophilic EPS. In addition, the effect of the heavy metals adsorption on the spectrum characteristics of the sludge EPS was also explored by analysis of FT-IR and fluorescent spectra.

Influence of bacterial extracellular polymeric substances on the sorption of Zn on γ-alumina: A combination of FTIR and EXAFS studies

Extracellular polymeric substances (EPS) isolated from bacteria, are abound of functional groups which can react with metals and consequently influence the immobilization of metals. In this study, we combined with Zn K-edge Extended X-ray Absorption Fine Structure (EXAFS), Fourier Transform Infrared (FTIR) spectroscopy, and High-Resolution Transmission Electron Microscopy (HRTEM) techniques to study the effects of EPS isolated from Bacillus subtilis and Pseudomonas putida on Zn sorption on γ-alumina. The results revealed that Zn sorption on aluminum oxide was pH-dependent and significantly influenced by bacterial EPS. At pH 7.5, Zn sorbed on γ-alumina was in the form of Zn-Al layered doubled hydroxide (LDH) precipitates, whereas at pH 5.5, Zn sorbed on γ-alumina was as a Zn-Al bidentate mononuclear surface complex. The amount of sorbed Zn at pH 7.5 was 1.3-3.7 times higher than that at pH 5.5. However, in the presence of 2 g L^-1 EPS, regardless of pH conditions and EPS source, Zn + EPS + γ-alumina ternary complex was formed on the surface of γ-alumina, which resulted in decreased Zn sorption (reduced by 8.4-67.8%) at pH 7.5 and enhanced Zn sorption (increased by 10.0-124.7%) at pH 5.5. The FTIR and EXAFS spectra demonstrated that both the carboxyl and phosphoryl moieties of EPS were crucial in this process. These findings highlight EPS effects on Zn interacts with γ-alumina.

Utilization of artificial recharged effluent as makeup water for industrial cooling system: corrosion and scaling

The secondary effluent from wastewater treatment plants was reused for industrial cooling water after pre-treatment with a laboratory-scale soil aquifer treatment (SAT) system. Up to a 95.3% removal efficiency for suspended solids (SS), 51.4% for chemical oxygen demand (COD), 32.1% for Cl(-) and 30.0% SO4(2-) were observed for the recharged secondary effluent after the SAT operation, which is essential for controlling scaling and corrosion during the cooling process. As compared to the secondary effluent, the reuse of the 1.5 m depth SAT effluent decreased the corrosion by 75.0%, in addition to a 55.1% decline of the scales/biofouling formation (with a compacted structure). The experimental results can satisfy the Chinese criterion of Design Criterion of the Industrial Circulating Cooling Water Treatment (GB 50050-95), and was more efficient than tertiary effluent which coagulated with ferric chloride. In addition, chemical structure of the scales/biofouling obtained from the cooling system was analyzed.

Quantification of the interactions between Ca²⁺, Hg²⁺ and extracellular polymeric substances (EPS) of sludge

The interactions between metals (Ca(2+) and Hg(2+)) and extracellular polymeric substances (EPS) extracted from the aerobic and anaerobic sludge in wastewater treatment reactors were investigated using a combination of zeta potential measurement and 3-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy with parallel factor (PARAFAC) analysis. Results show that Ca(2+) had no substantial effects on the EEM fluorescence spectra of the EPS, but their zeta potentials increased with the increasing Ca(2+) dosage. However, Hg(2+) had a significant effect on the EEM fluorescence spectra of the EPS, while their zeta potentials seemed not to be affected by the dose of Hg(2+). The interactions between Hg(2+) and EPS were elucidated using the fluorescence quenching with PARAFAC analysis, while the interactions between Ca(2+) and EPS were evaluated by the zeta potential technique. The binding constants for Hg(2+) and EPS were two orders of magnitude higher than those for Ca(2+) and EPS, suggesting that the binding mechanisms between Ca(2+) and EPS were different from those between Hg(2+) and EPS. The results might be useful for understanding the roles of EPS in bacterial self-protection against heavy metals and the aggregate formation mechanisms through ionic bridging interactions.

Protective effect of vanilloids against chemical stress on the white-rot fungus Ganoderma lucidum

Bioremediation of contaminated sites by biosorption of pollutants onto a wide range of materials has emerged as a promising treatment for recalcitrant aromatic compounds or heavy metals. When adsorption occurs on living white-rot fungi mycelia, the pollutants may be degraded by ligninolytic enzymes. However, the survival of mycelia in harsh conditions is one of the drawbacks of those methodologies. In this study, it was demonstrated that culture media supplemented with several guaiacol derivatives (vanilloids) increased the resistance of Ganoderma lucidum E47 cultures to chemical stress by enhancing the adsorptive capacity of the extracellular mucilaginous material (ECMM). The toxicity of the fungicides gentian violet (GV), malachite green (MG) and clotrimazole, and the heavy metal Cadmium was noticeably diminished in fungal cultures supplemented with the guaiacol derivative vanillic acid (VA). No degradation of the tested compounds was detected. The activity of the oxidative enzymatic systems like laccase, a well-known oxidase associated to dye degradation, was only detectable after complete growth on plates. Extremely low concentrations of VA caused a significant protective effect, radial extension of the growth halo in plates supplemented with 0.0001 mM of VA plus GV was up to 20% to that obtained in control plates (without addition of GV and VA). Therefore, the protective effect could not be attributable to VA per se. ECMM separated from the mycelium exhibited a much higher increase in the adsorptive capacity when isolated from liquid cultures containing VA, while that obtained from unsupplemented cultures showed an almost null adsorptive capacity.

Optimization of fermentation conditions and properties of an exopolysaccharide from Klebsiella sp. H-207 and application in adsorption of hexavalent chromium

The novel exopolysaccharide HZ-7 is produced by Klebsiella sp. H-207, and its fermentation conditions were optimized by response surface methodology (RSM). In this study, the optimized medium consisted of sucrose 31.93 g/L, KNO₃ 2.17 g/L and K₂HPO₄ 5.47 g/L; while the optimized culture conditions consisted of seed age 13 h, with an inoculum size of 10.6% and incubation temperature of 28.9°C. A maximum HZ-7 yield of about 15.05 g/L was achieved under the optimized conditions using RSM and single-factor experiments. Next the exopolysaccharide HZ-7 was partially purified and characterized. The resulting product showed good properties, such as high concentration of uronic acid (41.67%), low average molecular weight (about 1.94×10⁵ Da) and porous surface structure, were very advantageous to biosorption. Therefore HZ-7 was applied to absorb hexavalent chromium (Cr(VI)). The maximum adsorption efficiency (99.2%) which was obtained at an initial pH of 1.0 along with an initial Cr(VI) concentration of 20 mg/L, was not affected by ordinary metal ions and temperature. These data suggest Klebsiella sp. H-207 exopolysaccharide will be promising potential for industrial application.