Geobacter Autogenically Secretes Fulvic Acid to Facilitate the Dissimilated Iron Reduction and Vivianite Recovery

Mechanistic insight into enhancement of undissolved rice husk biochar on Tetracycline biodegradation by strain Serratia marcescens basing on electron transfer response

Undissolved biochar (UBC) plays a key role in persistently affecting bacterial characteristics after loss of dissolved biochar. However, its potential role as electron shuttle mediating tetracycline (TC) removal by bacteria is less understood. Result demonstrated UBC (700°C) coupled strain MSM2304 resulted in 72.19 % of TC biodegradation (37.76 % in free cells). UBC improved nutrients usage of TOC and TN to enhance cells proliferation, and facilitated biofilms formation and secretion of redox-active-related extracellular polymeric substances (EPS) including protein (40 % higher) and humus (30 % higher). Moreover, UBC optimized cells oxidative stress indicators including reactive oxygen species (40 % lower), total antioxidant capacity (30 % higher), superoxide dismutase (35 % higher), and catalase (30 % higher) during TC exposure. Importantly, UBC not only accelerated electron transfer from intracellular into extracellular by stimulating cytochrome C reductase activity and cytochrome C development, also decreased extracellular electron transfer resistance between MSM2304 and TC from 231.7 to 109.5 Ω, proved by cyclic voltammetry and electrochemical impedance spectra of EPS, and helped quinone moieties formation on UBC through CO and CC or CO production determined by FTIR and XPS. These findings indicate UBC could be as electron shuttle and contribute to provide a better understanding of interactions between biochar and microorganism.

Iron at the helm: Steering arsenic speciation through redox processes in soils

The toxicity and bioavailability of arsenic (As) in soils are largely determined by its speciation. Iron (Fe) is widely present in soils with a strong affinity for As, and therefore the environmental behaviors of As and Fe oxides (including oxides, hydrates and hydrated oxides) are closely correlated with each other. The redox fluctuations of Fe driven by changes in the environment can significantly affect As speciation and its fate in soils. The interaction between Fe and As has garnered widespread attention, and the adsorption mechanisms of As by Fe oxides have also been well-documented. However, there is still a lack of systematic understanding of how Fe redox dynamics affects As speciation depending on the soil environmental conditions. In this review, we summarize the mechanisms for As speciation transformation and redistribution, as well as the role of environmental factors in the main Fe redox processes in soils. These processes include the biotic Fe oxidation mediated by Fe-oxidizing bacteria, abiotic Fe oxidation by oxygen or manganese oxides, dissimilatory Fe reduction mediated by Fe-reducing bacteria, and Fe(II)-catalyzed transformation of Fe oxides. This review contributes to a deeper understanding of the environmental behaviors of Fe and As in soils, and provides theoretical guidance for the development of remediation strategies for As-contaminated soils.

The effect of auxiliary conditioning on humification of high-solids anaerobic digestion residues in aerobic composting processes

This study investigated the impact of cornstalk, bagasse, and spent mushroom substrate (SMS) as auxiliary materials on the nitrogen cycling and humification during the aerobic composting of high-solids anaerobically digested residues using high-throughput 16S rRNA sequencing and PICRUSt2 functional prediction. Results showed that cornstalk and SMS accelerated compost warming and upregulated the expression of nitrogen-cycling-related genes (e.g. ureC, narH, and narG), thereby significantly reducing (P < 0.05) N2O and NH3 emissions and increasing the NO3--N content in the compost. Furthermore, cornstalk enriched the microbial diversity and abundance of key bacteria involved in degradation and humification (e.g. Sphingobacterium and Moheibacter), which increased the humic acid content (HA) (78.4 g/kg DM). Although bagasse promoted aerobic conditions, it had less effect on nitrogen cycling and humification. The study highlights the intricate relationship between nitrogen metabolism and humification, demonstrating how selection of auxiliary materials can optimize composting for environmental sustainability.

Acetochlor degradation in anaerobic microcosms with hyporheic sediments: Insights from biogeochemical data, transformation products, and isotope analysis

Steep redox gradients and diverse microbial communities in the anaerobic hyporheic zone create complex pathways for the degradation of herbicides, often linked to various terminal electron-accepting processes (TEAPs). Identifying the degradation pathways and their controlling factors under various TEAPs is of great significance for understanding mechanisms of water purification in the hyporheic zone. However, current research on herbicides in this area remains insufficient. Acetochlor, a commonly detected herbicide in aquatic environments, was the target contaminant in this study. Biogeochemical data, transformation products examination, and compound-specific isotope analysis (CSIA) were used to elucidate the degradation mechanisms of acetochlor under various TEAPs in anaerobic microcosms with hyporheic sediments. Results showed that carbon isotope fractionation of acetochlor during abiotic reduction by reduced sulfur species (εbulk,C = -16.4 ± 0.4‰), such as HS- and Sn2-, was significantly larger than that observed during anaerobic biodegradation (εbulk,C = -3.7 ± 0.4‰). This suggested the utility of CSIA in identifying biotic/abiotic degradation pathways of acetochlor in anaerobic environments. CSIA and transformation products examination revealed that biodegradation under Fe(III) reducing conditions and abiotic reduction by reduced sulfur species under SO42- reducing conditions were the main pathways for acetochlor degradation in anaerobic hyporheic sediments. TEAPs controlled the abilities and mechanisms of acetochlor degradation in different hyporheic sediments, which were highly associated with terminal electron acceptors (Fe(III) and SO42-), Fe(III) reducing bacteria (Geobacter and Anaerolinea), SO42- reducing bacteria (Bacteroidetes_vadinHA17), and tryptophan-like substances. This study provides important insights into the mechanisms of herbicides degradation in the hyporheic zone.

Cometabolism of ferrihydrite reduction and methyl-dismutating methanogenesis by Methanosarcina mazei

Recent discoveries have shown that some Methanosarcina species can reduce Fe(III), reshaping our understanding of Methanosarcina ecophysiology. However, the specific minerals reduced, the products formed, and the underlying metabolic mechanisms remain elusive. Here, we report on the cometabolic process of Fe(III) reduction and methylotrophic methanogenesis in Methanosarcina mazei zm-15. Biogeochemical and mineralogical analyses were conducted to investigate Fe(III) reduction from three mineral preparations-ferrihydrite, goethite, and hematite. The results revealed that 38% of the 6 mM Fe(III) in ferrihydrite was reduced within 4 days, and this percentage increased to 75% with the addition of 100 µM anthraquinone-2,6-disulfonate (AQDS). Active Fe(III) reduction occurred immediately and preceded rapid methanogenesis. The addition of ferrihydrite and AQDS together significantly enhanced the maximal CH₄ production rate. However, Fe(III) reduction did not occur in goethite or hematite, even with the addition of 100 µM AQDS. Vivianite was identified as the major product from ferrihydrite reduction. Transcriptomic analysis revealed that gene expression related to the oxidation branch of the methyl-dismutating pathway and the membrane-associated electron transport chain (ETC) was significantly upregulated, whereas the expressions of genes associated with the reduction branch of the methyl-dismutating pathway were downregulated. In conclusion, M. mazei zm-15 demonstrates a strong ability to reduce poorly crystalline ferrihydrite, but not highly crystalline goethite and hematite. During the cometabolism of Fe(III) reduction and CH₄ production from methanol, the methyl-oxidation and membrane ETC pathways are enhanced, while the methyl-reduction pathway is downregulated. The mechanism of electron relay from cells to ferrihydrite, however, remains unclear and warrants further investigation.IMPORTANCEThe recent discovery that certain Methanosarcina species can grow by reducing Fe(III) challenges the traditional understanding of methanogens. However, the underlying metabolic mechanisms remain largely unexplored. Using a combination of biogeochemical, mineralogical, and microbiological approaches, we investigated the ability of Methanosarcina mazei zm-15. It exhibited a strong capacity to reduce poorly crystalline ferrihydrite but not highly crystalline goethite and hematite. The formation of vivianite from ferrihydrite reduction is likely due to the high rate of Fe(III) reduction and the presence of excess phosphorus in incubations. During the cometabolism of Fe(III) reduction and CH4 production from methanol, the methyl-oxidation and membrane electron transport pathways are upregulated, while the methyl-reduction pathway is downregulated. Our research uncovers a differential regulation of metabolic pathways during the cometabolism of Fe(III) reduction and CH4 production from methanol. The findings shed new light on the adaptive strategies employed by M. mazei in environments with the presence of Fe(III) and suggestthat Methanosarcina can play a significant role in methane production and iron cycling in natural environments.

Low-molecular weight organic acids can enhance the microbial reduction of iron oxide nanoparticles and pollutants by improving electrons transfer

The combined application of dissimilatory iron-reducing bacteria (DIRB) and Fe(III) nanoparticles has garnered widespread interest in the contaminants transformation and removal. The efficiency of this composite system relies on the extracellular electron transfer (EET) process between DIRB and Fe(III) nanoparticles. While modifications to Fe(III) nanoparticles have demonstrated improvements in EET, enhancing DIRB activity also shows potential for further EET enhancement, meriting further investigation. In this study, we demonstrated that the addition of low-molecular organic acids (LMWOAs) (oxalate, pyruvate, malate, citrate, or fumarate) can improve the reduction of Fe2O3 nanoparticles by Geobacter sulfurreducens PCA through three pathways: increasing intracellular electron production, enhancing the reductive activity of extracellular metabolites, and improving the electron-donating capacity of extracellular polymeric substances. The maximum reduction of Fe2O3 nanoparticles reached up to 72 %. Our results further showed that LMWOAs significantly boosted the removal rate and ratio of Cr(VI) and hexachlorobenzene (HCB) by accelerating the EET process. Following the introduction of LMWOAs, the maximum reduction ratio of Cr(VI) reached 98 ± 0.05 % within 24 h, while the degradation efficiency of HCB reached 92 ± 0.06 % within 9 h. Overall, our study provided a precise mechanism of the role of LMWOAs on the EET process and a new strategy for reductive bioremediation of pollutants.

A chemically enhanced primary treatment and anammox-based process for sustainable municipal wastewater treatment: The advantage and application prospects

Low-carbon nitrogen removal, bioenergy production, and phosphorus recovery are key goals for sustainable municipal wastewater treatment. Traditional activated sludge processes face an energy demand conflict. Anaerobic ammonium oxidation (Anammox) offers a solution to this issue, with the A-B process providing a sustainable approach. Stable and cost-effective nitrite supply for mainstream anammox has gained attention, while the interactions between A-B stage processes are also crucial. This paper reviews the benefits and challenges of mainstream anammox, bioenergy, and phosphorus recovery. A combined process of chemically enhanced primary treatment, partial denitrification and anammox is identified as effective for sustainable treatment. Additionally, the stable nitrite supply from the sidestream partial nitrification provides a 54% nitrogen removal contribution to the mainstream anammox. Anaerobic digestion with sulfate reduction is proposed as an efficient method for simultaneous bioenergy and phosphorus recovery from iron-enhanced primary sludge. The recycling of iron and sulfate reduces excess sludge and cuts costs. A novel wastewater treatment scheme, supported by a mass balance analysis, is presented; the proposed process is capable of recovering >50% of the carbon and phosphorus, while reduced 40% dosing of Fe and S chemicals, reducing the cost of chemical dosing and treatment of the digestate while meeting the high-quality effluent. The paper also explores the potential for transitioning from conventional activated sludge processes and suggests directions for future research.

Biogenic amorphous FeOOH activated additional intracellular electron flow pathways for accelerating reductive dechlorination of tetrachloroethylene

Dissimilatory iron-reducing bacteria (DIRB) with extracellular electron transfer (EET) capabilities have shown significant potential for bioremediating halogenated hydrocarbon contaminated sites rich in iron and humic substances. However, the role and microbial molecular mechanisms of iron-humic acid (Fe-HA) complexes in the reductive dehalogenation process of DIRB remains inadequately elucidated. In this study, we developed a sustainable carbon cycling approach using Fe-HA complexes to modulate the electron flux from sawdust (SD), enabling almost complete reductive dechlorination by most DIRB (e.g., Shewanella oneidensis MR-1) that lack complex iron-sulfur molybdo enzymes. The SD-Fe-HA/MR-1 system achieved a 96.52% removal efficiency of tetrachloroethylene (PCE) at concentrations up to 250 μmol/L within 60 days. Material characterization revealed that DIRB facilitated the hydrolysis of macromolecular carbon sources by inducing the formation of amorphous ferrihydrite (FeOOH) in Fe-HA complexes. More importantly, the bioavailable FeOOH activated additional intracellular electron flow pathways, increasing the activity of potential dehalogenases. Transcriptome further highlight the innovative role of biogenic amorphous FeOOH in integrating intracellular redox metabolism with extracellular charge exchange to facilitate reductive dechlorination in DIRB. These findings provide novel insights into accelerating reductive dechlorination in-situ contaminated sites lacking obligate dehalogenating bacteria.

Biogeochemical cycles of iron: Processes, mechanisms, and environmental implications

The iron (Fe) biogeochemical cycle is critical for abiotic and biological environmental processes that overlap spatially and may compete with each other. The development of modern molecular biology technologies promoted the understanding of the electron transport mechanisms of Fe-cycling-related microorganisms. Recent studies have revealed a novel pathway for microaerophilic ferrous iron (Fe(II))-oxidizers in extracellular Fe(II) oxidation. In addition, OmcS, OmcZ, and OmcE nanowires on the cell surface have been shown to promote electron transfer between microorganisms and their environment. These processes affect the fate of pollutants in directly or indirectly ways, such as greenhouse gas emissions. In this review, these advances and the environmental implications of the Fe cycle process were discussed, with a particular focus on the mechanisms of intracellular or extracellular electron transport in microorganisms.

The screening of iron oxides for long-term transformation into vivianite to recover phosphorus from sewage

The reducibility of iron oxides, depending on their properties, influences the kinetics of dissimilatory iron reduction (DIR) during vivianite recovery in sewage. This study elucidated the correlation between properties of iron oxides and kinetics of DIR during the long-term transformation into vivianite, mediated by Geobacter sulfurreducens PCA and sewage. The positive correlation between surface reactivity of iron oxides and reduction rate constant (k) influenced the terminal vivianite recovery efficiency. Akaganeite with the highest adhesion work and surface energy required the lowest reduction energy (Ea), obtained the highest k of 1.36 × 10-2 day-1 and vivianite recovery efficiency of 43 %. The vivianite yield with akaganeite as iron source was 76-164 % higher than goethite, hematite, feroxyhyte, and ferrihydrite in sewage. The distribution of P with akaganeite during DIR in sewage further suggested a more efficient pathway of direct vivianite formation via bio-reduced Fe(II) rather than indirect reduction of ferric phosphate precipitates. Thus, akaganeite was screened out as superior iron source among various iron oxides for vivianite recovery, which provided insights into the fate of iron sources and the cycle of P in sewage.

Nano-magnetite enhances dissimilated iron reduction to vivianite from sewage by structuring an enormous and compact electron transfer network

Acting as both terminal and conductor of extracellular electron transfer (EET), little studies were focused on how nano-magnetite participated in the dissimilated iron reduction (DIR), especially the synthesis of vivianite, which was the typical DIR products from sewage. In this study, nano-magnetite was confirmed to enhance DIR of ferrihydrite and akaganeite for vivianite recovery from sewage. Nano-magnetite incorporation enriched Comamonas and Geobacter in sewage, and microbial protein content was increased by 123 % and 57 % in ferrihydrite and akaganeite batches, respectively. In Geobacter sulfurreducens PCA pure culture, vivianite yield was promoted by 21 % and 37 % in ferrihydrite and akaganeite batches in the presence of nano-magnetite, respectively. Due to its nanoscale size and superior electrical conductivity, nano-magnetite embedded in the gaps formed by the microorganisms and electron acceptor, and architected coherent conductive pathways to promote EET. Simultaneously, the addition of nano-magnetite stimulated the secretion of proteins, polysaccharides, and humic acids in the extracellular polymeric substances. Nano-magnetite addition structured an enormous and compact electron transfer network, thus enhanced DIR and vivianite formation. Our study proposed a new strategy to promote iron-reduction-coupled phosphorus recovery with natural DIR products, and provided theoretical support for clarifying the interaction between minerals and microorganisms.

Characteristic and mechanism of biological nitrogen and phosphorus removal facilitated by biogenic manganese oxides (BioMnOx) at various concentrations of Mn(II)

Biogenic manganese oxides (BioMnOx) have attracted considerable attention as active oxidants, adsorbents, and catalysts. However, characteristics and mechanisms of nitrification-denitrification in biological redox reactions mediated by different concentrations of BioMnOx are still unclear. Fate of nutrients (e.g., NH4+-N, TP, NO3--N) and COD were investigated through different concentrations of BioMnOx produced by Mn(II) in the moving bed biofilm reactor (MBBR). 34% and 89.2%, 37.8% and 89.8%, 57.3% and 88.9%, and 62.1% and 90.4% of TN and COD by MBBR were synchronously removed in four phases, respectively. The result suggested that Mn(II) significantly improved the performance of simultaneous nitrification and denitrification (SND) and TP removal based on manganese (Mn) redox cycling. Characteristics of glutathione peroxidase (GSH-Px), reactive oxygen species (ROS), and electron transfer system activity (ETSA) were discussed, demonstrating that ROS accumulation reduced the ETSA and GSH-Px activities when Mn(II) concentration increased. Extracellular polymeric substance (EPS) function and metabolic pathway of Mn(II) were explored. Furthermore, effect of cellular components on denitrification was evaluated including BioMnOx performances, indicating that Mn(II) promoted the non-enzymatic action of cell fragments. Finally, mechanism of nitrification and denitrification, denitrifying phosphorus and Mn removal was further elucidated through X-ray photoelectron spectroscopy (XPS), high throughput sequencing, and fourier transform infrared reflection (FTIR). This results can bringing new vision for controlling nutrient pollution in redox process of Mn(II).

Anaerobic oxidation of methane in landfill and adjacent groundwater environments: Occurrence, mechanisms, and potential applications

Landfills remain the predominant means of solid waste management worldwide. Widespread distribution and significant stockpiles of waste in landfills make them a significant source of methane emissions, exacerbating climate change. Anaerobic oxidation of methane (AOM) has been shown to play a critical role in mitigating methane emissions on a global scale. The rich methane and electron acceptor environment in landfills provide the necessary reaction conditions for AOM, making it a potentially low-cost and effective strategy for reducing methane emissions in landfills. However, compared to other anaerobic habitats, research on AOM in landfill environments is scarce, and there is a lack of analysis on the potential application of AOM in different zones of landfills. Therefore, this review summarizes the existing knowledge on AOM and its occurrence in landfills, analyzes the possibility of AOM occurrence in different zones of landfills, discusses its potential applications, and explores the challenges and future research directions for AOM in landfill management. The identification of research gaps and future directions outlined in this review encourages further investigation and advancement in the field of AOM, paving the way for more effective waste stabilization, greenhouse gas reduction, and pollutant mitigation strategies in landfills.

Iron-electrolysis assisted anammox/denitrification system for intensified nitrate removal and phosphorus recovery in low-strength wastewater treatment

Two iron-electrolysis assisted anammox/denitrification (EAD) systems, including the suspended sludge reactor (ESR) and biofilm reactor (EMR) were constructed for mainstream wastewater treatment, achieving 84.51±4.38 % and 87.23±3.31 % of TN removal efficiencies, respectively. Sludge extracellular polymeric substances (EPS) analysis, cell apoptosis detection and microbial analysis demonstrated that the strengthened cell lysate/apoptosis and EPS production acted as supplemental carbon sources to provide new ecological niches for heterotrophic bacteria. Therefore, NO3--N accumulated intrinsically during anammox reaction was reduced. The rising cell lysis and apoptosis in the ESR induced the decline of anammox and enzyme activities. In contrast, this inhibition was scavenged in EMR because of the more favorable environment and the significant increase in EPS. Moreover, ESR and EMR achieved efficient phosphorus removal (96.98±5.24 % and 96.98±4.35 %) due to the continued release of Fe2+ by the in-situ corrosion of iron anodes. The X-ray diffraction (XRD) indicated that vivianite was the dominant P recovery product in EAD systems. The anaerobic microenvironment and the abundant EPS in the biofilm system showed essential benefits in the mineralization of vivianite.

Incorporating microbial inoculants to reduce nitrogen loss during sludge composting by suppressing denitrification and promoting ammonia assimilation

To address the challenge of increasing nitrogen retention in compost, this study investigated the effects of microbial communities on denitrification and ammonia assimilation during sludge composting by inoculating microbial inoculants. The results showed that the retention rates of total Kjeldahl nitrogen (TKN) and humic acid (HA) in MIs group (with microbial inoculants) were 4.94 % and 18.52 % higher than those in the control group (CK), respectively. Metagenomic analysis showed that Actinobacteria and Proteobacteria were identified as main microorganisms contributing to denitrification and ammonia assimilation. The addition of microbial agents altered the structure of the microbial community, which in turn stimulated the expression of functional genes. During cooling period, the ammonia assimilation genes glnA, gltB and gltD in MIs were 15.98 %, 24.84 % and 32.88 % higher than those in CK, respectively. Canonical correspondence analysis revealed a positive correlation between the dominant bacterial genera from the cooling stage to the maturity stage and the levels of NO3--N, NH4+-N, HA, and TKN contents. NH4+-N was positively correlated with HA, indicating NH4+-N might be incorporated into HA. Heat map and network analyses revealed NH4+-N as a key factor affecting functional genes of denitrification and ammonia assimilation, with Nitrospira identified as the core bacteria in the microbial network. Therefore, the addition of microbial agents could increase nitrogen retention and improve compost product quality.

Response of fulvic acid linking to redox characteristics on methane and short-chain fatty acids in anaerobic digestion of chicken manure

Fulvic acids (FAs) is formed during the bioconversion of organic matter (OM) to biogas during anaerobic digestion (AD) and has a complex structure and redox function. However, the evolutionary mechanisms of FAs during AD and its interactions with acid and methane production have not been sufficiently investigated, especially at different stages of AD. Intermittent AD experiments by chicken manure and rice husk showed significant structural changes and reduced aromatization of FAs (e.g., O-H stretch6, 14.10-0%; SR, 0.22-0.60). The electron donating capacity (EDC) [9.76-45.39 μmole-/(g C)] and electron accepting capacity (EAC) [2.55-5.20 μmole-/(g C)] of FAs showed a tendency of decreasing and then increasing, and FAs had a stronger electron transfer capacity (ETC) in the methanogenic stage. Correlation analysis showed that the EDC of FAs was influenced by their own structure (C-O stretch2, C-H bend1, C-H bend4, and N-H bend) and also had an inhibitory effect on propionic production, which further inhibited acetic production. The EAC of FAs was affected by molecular weight and had a promoting effect on methane production. Structural equation modelling identified three possible pathways for AD. The C-O stretch2 structure of FAs alone inhibits the production of propionic. In addition, pH can directly affect the EDC of FAs. This study provides a theoretical basis for the structural and functional evolution of FAs in AD of chicken manure on the mechanism of methane production.

Strategies for optimizing biovivianite production using dissimilatory Fe(III)-reducing bacteria

Vivianite (Fe3(PO4)2·8H2O), a sink for phosphorus, is a key mineralization product formed during the microbial reduction of phosphate-containing Fe(III) minerals in natural systems, and also in wastewater treatment where Fe(III)-minerals are used to remove phosphate. As biovivianite is a potentially useful Fe and P fertiliser, there is much interest in harnessing microbial biovivianite synthesis for circular economy applications. In this study, we investigated the factors that influence the formation of microbially-synthesized vivianite (biovivianite) under laboratory batch systems including the presence and absence of phosphate and electron shuttle, the buffer system, pH, and the type of Fe(III)-reducing bacteria (comparing Geobacter sulfurreducens and Shewanella putrefaciens). The rate of Fe(II) production, and its interactions with the residual Fe(III) and other oxyanions (e.g., phosphate and carbonate) were the main factors that controlled the rate and extent of biovivianite formation. Higher concentrations of phosphate (e.g., P/Fe = 1) in the presence of an electron shuttle, at an initial pH between 6 and 7, were needed for optimal biovivianite formation. Green rust, a key intermediate in biovivianite production, could be detected as an endpoint alongside vivianite and metavivianite (Fe2+Fe3+2(PO4)2.(OH)2.6H2O), in treatments with G. sulfurreducens and S. putrefaciens. However, XRD indicated that vivianite abundance was higher in experiments containing G. sulfurreducens, where it dominated. This study, therefore, shows that vivianite formation can be controlled to optimize yield during microbial processing of phosphate-loaded Fe(III) materials generated from water treatment processes.

Remobilization of Cd caused by iron oxide phase transformation and Mn2+ competition after stabilization by nano zero valent iron

Stabilization techniques are vital in controlling Cd soil pollution. Nano zero valent iron (nZVI) has been extensively utilized for Cd remediation owing to its robust adsorption and reactivity. However, the environmental stress-induced stability of Cd after nZVI addition remains unclear. A pot experiment was conducted to evaluate the Cd bioavailability in continuously flooded (130 d) soil after stabilization with nZVI. The findings indicated that nZVI application did not result in a decline in Cd concentration in rice, as compared to the no-nZVI control. Additionally, nZVI simultaneously increased the available Cd concentration, iron-manganese oxide-bound (OX) Mn fraction, and relative abundance of Fe(III)-reducing bacteria, but it decreased OX-Cd and Mn availability in soil. Cadmium in rice tissues was positively correlated with the available Cd in soil. The results of subsequent adsorption tests demonstrated that CdO was the product of Cd adsorption by the nZVI aging products. Conversely, Mn2+ decreased the adsorption capacity of Cd-containing solutions. These results underscore the crucial role of both biotic and abiotic factors in undermining the stabilization of nZVI under continuous flooding conditions. This study offers novel insights into the regulation of nZVI-mediated Cd stabilization efficiency in conjunction with biological inhibitors and functional modification techniques.

Fe(0)-Dissimilatory Nitrate Reduction to Ammonium for Autotrophic Recovery of Reactive Nitrogen

Bioreduction of nitrate to value-added ammonium is a potentially sustainable strategy to recycle nutrients from wastewater. Here, we have proven the feasibility of the reduction of autotrophic nitrate to ammonium with electrons extracted from Fe(0). Using a Geobacter-dominated anodic biofilm as an inoculum, we achieved nitrate-to-ammonium efficiency up to 90 ± 3% with a nitrate reduction rate of 35 ± 1.3 mg N/d/L. An electron acceptor instead of an inoculum greatly influenced the Fe(0)-dissimilatory nitrate reduction to ammonium (DNRA), where nitrite as the electron acceptor provided an effective selective pressure to enrich Geobacter from initial 5 to 56%. The DNRA repressing denitrification was demonstrated by the reverse tendencies of upregulated nrfA and downregulated nirS gene transcription. This finding provides a new route for autotrophic nitrate removal and recycling from water, which has a broader implication on biogeochemical nitrogen and iron cycling.

Deciphering the interaction of heavy metals with Geobacter-induced vivianite recovery from wastewater

Vivianite recovery from wastewater driven by Geobacter is one of the promising approaches to address the challenges of phosphorus (P) resource shortage and eutrophication. However, the interfere of heavy metals which are prevalent in many actual wastewater with this process is rarely reported. In this study, we investigated the impact of heavy metals (i.e., Cu and Zn ions) on microbial activity, Fe reduction, P recovery efficiency, and their fate during Geobacter-induced vivianite recovery process. The experimental results showed that low and medium concentrations of Cu and Zn prolonged the Fe reduction and P recovery time but had little effect on the final P recovery efficiency. However, high concentrations of Cu and Zn ultimately inhibit vivianite formation. In addition, the different concentrations of Cu and Zn showed different effects on the morphology of the recovered vivianite. The migration of Cu and Zn was analysed by stepwise extraction of heavy metals in the vivianite. Medium concentrations of Cu and Zn were more likely to co-precipitate with vivianite, while adsorption was the primary mechanism at low concentrations. Furthermore, there were differences in the fate of Cu and Zn, and a competition mechanism was observed. Finally, we found that increasing the Fe/P ratio can significantly reduce the residues of heavy metals in vivianite. It also increased the adsorbed Cu and Zn proportion and reduced co-precipitation. These results provide insights into improving the efficiency of vivianite recovery and managing the environmental risks of heavy metal in the recovered product.

Sub-MIC antibiotics affect microbial ferrihydrite reduction by extracellular membrane vesicles

Environmental concentrations of antibiotics, usually below MIC, have significant biological effects on bacterial cells. Sub-MIC antibiotics exposure induces bacteria to produce outer membrane vesicles (OMVs). Recently, OMVs is discovered as a novel pathway for dissimilatory iron reducing bacteria (DIRB) to mediate extracellular electron transfer (EET). Whether and how the antibiotic-induced OMVs modulate iron oxides reduction by DIRB have not been studied. This study showed the sub-MIC antibiotics (ampicillin or ciprofloxacin) increased OMVs secretion in Geobacter sulfurreducens, and the antibiotic-induced OMVs contained more redox active cytochromes facilitating iron oxides reduction, especially for the ciprofloxacin-induced OMVs. Deduced from a combination of electron microscopy and proteomic analysis, the influence of ciprofloxacin on SOS response triggered prophage induction and led to the formation of outer-inner membrane vesicles (OIMVs) in, which was a first report in Geobacter species. While ampicillin disrupting cell membrane integrity resulted in more formation of classic OMVs from outer membrane blebbing. The results indicated that the different structure and composition of vesicles were responsible for the antibiotic-dependent regulation on iron oxides reduction. This newly identified regulation on EET-mediated redox reactions by sub-MIC antibiotics expands our knowledge about the impact of antibiotics on microbial processes or "non-target" organisms.

Retracted: The bidirectional extracellular electron transfer process aids iron cycling by Geoalkalibacter halelectricus in a highly saline-alkaline condition

Bidirectional extracellular electron transfer (EET) is crucial to upholding microbial metabolism with insoluble electron acceptors or donors in anoxic environments. Investigating bidirectional EET-capable microorganisms is desired to understand the cell-cell and microbe-mineral interactions and their role in mineral cycling besides leveraging their energy generation and conversion, biosensing, and bio-battery applications. Here, we report on iron cycling by haloalkaliphilic Geoalkalibacter halelectricus via bidirectional EET under haloalkaline conditions. It efficiently reduces Fe3+ oxide (Fe2O3) to Fe0 at a 0.75 ± 0.08 mM/mgprotein/d rate linked to acetate oxidation via outward EET and oxidizes Fe0 to Fe3+ at a 0.24 ± 0.03 mM/mgprotein/d rate via inward EET to reduce fumarate. Bioelectrochemical cultivation confirmed its outward and inward EET capabilities. It produced 895 ± 23 µA/cm2 current by linking acetate oxidation to anode reduction via outward EET and reduced fumarate by drawing electrons from the cathode (‒2.5 ± 0.3 µA/cm2) via inward EET. The cyclic voltammograms of G. halelectricus biofilms revealed redox moieties with different formal potentials, suggesting the involvement of different membrane components in bidirectional EET. The cyclic voltammetry and GC-MS analysis of the cell-free spent medium revealed the lack of soluble redox mediators, suggesting direct electron transfer by G. halelecctricus in achieving bidirectional EET. By reporting on the first haloalkaliphilic bacterium capable of oxidizing and reducing insoluble Fe0 and Fe3+ oxide, respectively, this study advances the limited understanding of the metabolic capabilities of extremophiles to respire on insoluble electron acceptors or donors via bidirectional EET and invokes the possible role of G. halelectricus in iron cycling in barely studied haloalkaline environments. IMPORTANCE Bidirectional extracellular electron transfer (EET) appears to be a key microbial metabolic process in anoxic environments that are depleted in soluble electron donor and acceptor molecules. Though it is an ecologically important and applied microbial phenomenon, it has been reported with a few microorganisms, mostly from nonextreme environments. Moreover, direct electron transfer-based bidirectional EET is studied for very few microorganisms with electrodes in engineered systems and barely with the natural insoluble electron acceptor and donor molecules in anoxic conditions. This study advances the understanding of extremophilic microbial taxa capable of bidirectional EET and its role in barely investigated Fe cycling in highly saline-alkaline environments. It also offers research opportunities for understanding the membrane components involved in the bidirectional EET of G. halelectricus. The high rate of Fe3+ oxide reduction activity by G. halelectricus suggests its possible use as a biocatalyst in the anaerobic iron bioleaching process under neutral-alkaline pH conditions.

A neglected contributor of thermal hydrolysis to sludge anaerobic digestion: Fulvic acids release and their influences

Fulvic acids (FAs) belong to inert organic matters in sewage sludge and their influences are often overlooked during convectional anaerobic digestion (AD). Currently, thermal hydrolysis (TH) has been widely applied on sludge pretreatment before AD processes, which makes FAs undergo drastic evolutions and aggressive to sludge AD. Results in the present study indicated that FAs concentration in the liquid was elevated by over incredible 150 folds during sludge TH, from 3.4 mg/L in raw sludge to 590.0 mg/L in hydrolyzed sludge at 180 °C. Moreover, during sludge TH, the chemical structures of FAs, including aromatic condensation degree, elemental composition and functional group, also underwent significant changes, which enhanced FAs electron transfer capability, reduced their biodegradability and promoted their roles on sludge AD. Furthermore, fortunately, the evolutions of FAs were favorable to sludge AD in general. Methane production could be promoted by about 20% under FAs concentration of 0.6 g/L, and the FAs extracted from hydrolyzed sludge presented higher promotion performances than that of the raw FAs, in which 180 °C FAs were particularly conspicuous. Furthermore, FAs evolutions would present differential influences on each phase of sludge AD, promotional to acidogenesis and methanogenesis but inhibitory to hydrolysis. Pearson correlation analysis indicated FAs influences on sludge AD, particularly the hydrolysis phase, were not only related to their concentration, but also chemical structure. The findings of this study demonstrated that FAs influences should not be negligible anymore during sludge AD with TH pretreatment. Meanwhile, since FAs promotion on sludge AD was closely related to their concentration and chemical structure, it would be significant to take FAs evolutions as auxiliary indexes for the regulations of sludge TH.

Long-term transformation of nanoscale zero-valent iron explains its biological effects in anaerobic digestion: From ferroptosis-like death to magnetite-enhanced direct electron transfer networks

Nanoscale zero-valent iron (nZVI) has been extensively used for environmental remediation and wastewater treatment. However, the biological effects of nZVI remain unclear, which is no doubt a result of the complexity of iron species and the dynamic succession of microbial community during nZVI aging. Here, the aging effects of nZVI on methanogenesis in anaerobic digestion (AD) were consecutively investigated, with an emphasis on deciphering the causal relationships between nZVI aging process and its biological effects. The addition of nZVI in AD led to ferroptosis-like death with hallmarks of iron-dependent lipid peroxidation and glutathione (GSH) depletion, which inhibited CH₄ production during the first 12 days of exposure. With prolonged exposure time, a gradual recovery (12-21 days) and even better performance (21-27 days) in AD were observed. The recovery performance of AD was mainly attributed to nZVI-enhanced membrane rigidity via forming siderite and vivianite on the outer surface of cells, protecting anaerobes against nZVI-induced toxicity. At the end of 27-days exposure, the significantly increased amount of conductive magnetite simulated direct interspecies electron transfer among syntrophic partners, improving CH₄ production. Metagenomic analysis further revealed that microbial cells gradually adapted to the aging of nZVI by upregulating functional genes related to chemotaxis, flagella, conductive pili and riboflavin biosynthesis, in which electron transfer networks likely thrived and the cooperative behaviors between consortium members were promoted. These results unveiled the significance of nZVI aging on its biological effects toward multiple microbial communities and provided fundamental insights into the long-term fates and risks of nZVI for in situ applications.

Dual-roles of carbon black to accelerate phosphorus recovery as vivianite

Carbon materials have been confirmed to promote phosphorus recovery as vivianite through enhancing dissimilatory iron reduction (DIR), which alleviates phosphorus crisis. Carbon black (CB) exhibits contradictory dual roles of cytotoxicity inducer and electron transfer bridge towards extracellular electron transfer (EET). Herein, the effect of CB on vivianite biosynthesis was investigated with dissimilatory iron reduction bacteria (DIRB) or sewage. With Geobacter sulfurreducens PCA as inoculum, the vivianite recovery efficiency increased accompanied with CB concentrations and enhanced by 39 % with 2000 mg·L^-1 CB. G. sulfurreducens PCA activated the adaptation mechanism of secreting extracellular polymeric substance (EPS) to resist cytotoxicity of CB. While in sewage, the highest iron reduction efficiency of 64 % was obtained with 500 mg·L^-1 CB, which was appropriate for functional bacterial selectivity like Proteobacteria and bio-transformation from Fe(III)-P to vivianite. The balance of CB's dual roles was regulated by inducing the adaptation of DIRB to gradient CB concentrations. This study provide an innovative perspective of carbon materials with dual roles for vivianite formation enhancement.

Charging-discharging cycles of geobattery activated carbon enhance iron reduction and vivianite recovery from wastewater

Vivianite as a significant secondary mineral of dissimilatory iron reduction (DIR) exhibits marvelous potential to solve eutrophication as well as phosphorus shortage. Geobattery represents by natural organic matters (NOM) with rich functional groups influences bioreduction of natural iron mineral. Activated carbon (AC) which contains abundant functional groups is expected to serve as geobattery, but there remains insufficient understanding on its geobattery mechanism and how it benefits the vivianite formation. In this study, the charging and discharging cycle of "geobattery" AC enhanced extracellular electron transfer (EET) and vivianite recovery was demonstrated. Feeding with ferric citrate, AC addition increased vivianite formation efficiency by 141 %. The enhancement was attributed to the electron shuttle capacity of storage battery AC, which was contributed by the redox cycle between CO and O-H. Feeding with iron oxides, huge gap of redox potential between AC and Fe(III) minerals broke through the reduction energy barrier. Therefore the iron reduction efficiency of four Fe(III) minerals was accelerated to the same high level around 80 %, and the vivianite formation efficiency were increased by 104 %-256 % in pure culture batches. Except acting as storage battery, AC as a dry cell contributed 80 % to the whole enhancement towards iron reduction, in which O-H groups were the dominant driver. Due to the rechargeable nature and considerable electron exchange capacity, AC served as geobattery playing the role of both storage battery and dry cell on electron storaging and transferring to influence biogeochemical Fe cycle and vivianite recovery.

Anaerobic syntrophic system composed of phosphate solubilizing bacteria and dissimilatory iron reducing bacteria induces cadmium immobilization via secondary mineralization

Secondary mineralization is a promising method for remediating cadmium (Cd) pollution in sediments, but the poor stability of Cd-containing secondary minerals is a bottleneck that limits the development of this approach. The existence of phosphate can enhance the formation of stable secondary minerals and points a new direction for Cd immobilization. In this research, a novel syntrophic system composed of phosphate solubilizing bacteria (PSB) and dissimilatory iron reducing bacteria (DIRB) was established and the effect and mechanism of Cd immobilization in the system were also explored. The results showed that under the conditions of DIRB:PSB (V:V)= 3:1, syntrophic bacteria dosage of 5% and glucose dosage of 5 g/L, Cd incorporated in the secondary minerals could account for about 60% of the total Cd. In the pH range of 5-9, alkaline environment was conducive to the immobilization of Cd and the percentage of combined Cd was up to 58%, while the combined Cd in secondary minerals decreased from 62% to 56% with the increase of initial Cd concentration from 0.1 to 0.3 mmol/L. In addition, XRD, XPS, Mössbauer and other characterization results showed that secondary minerals, such as Cd exchange hydroxyapatite (Cd-HAP) and kryzhanovskite (Fe₃(PO₄)₂(OH)₃) were formed in this new system. The established syntrophic system of PSB and DIRB is thus a prospective bioremediation technology for Cd immobilization in sediments and can avoid the potential risk might be caused by the addition of phosphorus-containing materials.

Humic acid and fulvic acid facilitate the formation of vivianite and the transformation of cadmium via microbially-mediated iron reduction

The effects of humic acids (HA) and fulvic acids (FA) on the fate of Cd in anaerobic environment upon microbial reduction of Cd-bearing ferrihydrite (Fh) with Geobacter metallireducens were investigated. The results showed that HA and FA could promote the reductive dissolution of Fh and the formation of vivianite. After incubation of 38 d, vivianite accounted for 47.19%, 59.22%, and 48.53% of total Fe in biological control batch (BCK), HA and FA batches (C/Fe molar ratio of 1.0), respectively, by Mössbauer spectroscopy analysis. In terms of Cd, HA and FA could promote the release of adsorbed Cd during the initial bioreduction process, but reassuringly, after 38 d the dissolved Cd with HA and FA addition batches were 0.58-0.91 and 0.99-1.08 times of the BCK, respectively. The proportions of residual Cd in HA batches were higher than FA and BCK batches, indicating that HA was better than FA in immobilizing Cd. This might be because the quinone groups in HA could act as electron shuttle. This study showed that HA facilitated the transformation of vivianite better than FA, and Cd can be stabilized by resorption or co-precipitation with vivianite, providing a theoretical support for the translocation of Cd in sediment-water interface.

Facet Dependence of Biosynthesis of Vivianite from Iron Oxides by Geobacter sulfurreducens

Vivianite plays an important role in alleviating the phosphorus crisis and phosphorus pollution. The dissimilatory iron reduction has been found to trigger the biosynthesis of vivianite in soil environments, but the mechanism behind this remains largely unexplored. Herein, by regulating the crystal surfaces of iron oxides, we explored the influence of different crystal surface structures on the synthesis of vivianite driven by microbial dissimilatory iron reduction. The results showed that different crystal faces significantly affect the reduction and dissolution of iron oxides by microorganisms and the subsequent formation of vivianite. In general, goethite is more easily reduced by Geobacter sulfurreducens than hematite. Compared with Hem_{100} and Goe_L{110}, Hem_{001} and Goe_H{110} have higher initial reduction rates (approximately 2.25 and 1.5 times, respectively) and final Fe(II) content (approximately 1.56 and 1.20 times, respectively). In addition, in the presence of sufficient PO₄^3-, Fe(II) combined to produce phosphorus crystal products. The final phosphorus recoveries of Hem_{001} and Goe_H{110} systems were about 5.2 and 13.6%, which were 1.3 and 1.6 times of those of Hem_{100} and Goe_L{110}, respectively. Material characterization analyses indicated that these phosphorous crystal products are vivianite and that different iron oxide crystal surfaces significantly affected the size of the vivianite crystals. This study demonstrates that different crystal faces can affect the biological reduction dissolution of iron oxides and the secondary biological mineralization process driven by dissimilatory iron reduction.

Vivianite recovery from high concentration phosphorus wastewater with mine drainage as iron sources

High concentration phosphorus wastewater has attracted much attention due to the safety of water ecology and the potential crisis of phosphorus resource, which is caused by large amounts of phosphorus discharging into natural water bodies. Vivianite (Fe₃(PO₄)₂·8H₂O) crystallization has been considered as an effective technology for phosphorus recovery. In this study, we develop a potentially low-cost, sustainable approach to recover phosphorus from high concentration phosphorus wastewater using mine drainage as iron source. Inoculated with both sewage and Geobacter, mine drainage was suitable for vivianite recovery from high concentration phosphorus wastewater with PO₄^3- concentration between 6 and 18 mM. When the PO₄^3- concentration increased gradually, both phosphorus removal efficiency (R_P) and vivianite recovery efficiency (R_V) decreased significantly. The highest R_V of 48 % was obtained with 9 mM PO₄^3- in Geobacter batches (CJ2 batches), which was 15 % higher than that in the paralleled sewage batches (33 % in HJ2). Simultaneously, vivianite accounted for 91 % of the solid phosphate compounds in CJ2 batches due to the enhancement of Geobacter.

Applying fulvic acid for sediment metals remediation: Mechanism, factors, and prospect

Fulvic acid (FA) has been shown to play a decisive role in controlling the environmental geochemical behavior of metals. As a green and natural microbial metabolite, FA is widely used in environmental remediation because of its good adsorption complexation and redox ability. This paper introduces the reaction mechanism and properties of FA with metals, and reviews the progress of research on the remediation of metal pollutant by FA through physicochemical remediation and bioremediation. FA can control the biotoxicity and migration ability of some metals, such as Pb, Cr, Hg, Cd, and As, through adsorption complexation and redox reactions. The concentration, molecular weight, and source are the main factors that determine the remediation ability of FA. In addition, the ambient pH, temperature, metal ion concentrations, and competing components in sediment environments have significant effects on the extent and rate of a reaction between metals and FA during the remediation process. Finally, we summarize the challenges that this promising environmental remediation tool may face. The research directions of FA in the field of metals ecological remediation are also prospected. This review can provide new ideas and directions for the research of remediation of metals contaminants in sediments.

Influence of biocurrent self-generated by indigenous microorganisms on soil quality

The redox process driven by anaerobic respiration is a link between matter conversion and energy exchange in soil biogeochemistry. Microbial extracellular electron transfer forming biocurrents is a force in element cycling and community living in soil. However, the effect of indigenous microorganisms generating biocurrents on soil quality is unclear. We found that soil biocurrent showed little adverse influence on soil pH, cation exchange capacity, and available nitrogen, phosphorus and potassium and deblocked sequestered organic matter (29%). In addition, the bioelectric field derived from biocurrent obviously forced the migration of mineral elements, which was a supplement to the theory of water-salt transport, providing a new perspective on element transport. Moreover, the soil biocurrent directly regulated the availability of Ca and Fe (increase of 7-fold), indicating that electron transfer plays an important role in weathering and mineralization and thus pedogenesis. From a microbial ecology point of view, the soil bacterial richness and diversity were perfectly restored to their original state when the biocurrent stopped; including bacterial functions; although a temporary enrichment of certain species was observed. The above results provide new insights into the interactions between electron transfer and soil quality and confirm the safety of soil bioelectrochemical technology.

Carbon nanotubes accelerates the bio-induced vivianite formation

Vivianite widely existed in digested sludge and activated sludge as a potential phosphate resource recovered from wastewater treatment plants (WWTPs). As an important product of extracellular electron transfer (EET) and biological iron reduction, the production of vivianite can be enhanced by conductive materials. Carbon nanotubes (CNTs) with excellent electrical conductivity have been reported to promote electron transfer, which was applied in wastewater treatment to accelerate the degradation of the contaminants. However, the impact of CNTs on vivianite formation was barely reported. In this study, the iron reduction, vivianite recovery, and the biotoxicity of CNTs were investigated in order to determine the influence of CNTs towards the vivianite production. The enhancement of vivianite production after CNTs adding reached up to 17 % by promoting the electron transfer between dissimilative iron-reducing bacteria (DIRB) and Fe(III). However, at the initial stage (0-24 h), Fe(III) reduction efficiency decreased by 81 % after inoculating with sewage sludge, which was attributed to CNTs destroying of the cell membrane (as indicated by SEM, CLSM and AFM analysis). The biotoxicity of CNTs stimulated DIRB to secret extracellular polymeric substances (EPS) and form bio-flocs to resist the physical puncture. After 48 h, the proportion of living DIRB in 1000 mg/L CNTs batch increased to 98 %, which was 79 % higher than 12 h. As a result, the vivianite recovery of raw sewage with 1000 mg/L CNTs increased to 44 ± 1 %, which was 33 % higher than that in the CNT-0.

The enhancement on biohydrogen production by the driving forces from extracellular iron oxide respiration

Biohydrogen productions from xylose and glucose under dark condition were enhanced by the presence of natural Fe₃O₄. The electron equivalent of H₂ fractions accounted for 4.55 % and 5.69 % of the total given xylose and glucose in the experiments without Fe₃O₄, and that were correspondingly increased to 5.14 % and 6.50 % in the experiments with 100 mg/L of Fe₃O₄, respectively. Moreover, Fe₃O₄ increased the total intracellular NAD(H) concentrations by 8.84 % and 8.37 %, and boosted the ratios of NADH/NAD⁺ by 8.33 % and 17.72 % in xylose and glucose fermentation, respectively, comparing to the corresponding control experiments. The formation of electron couples of Fe(III)/Fe(II) during the iron oxide respiration and more generation of active extracellular polymeric substances components were determined as the important reasons for the improved biohydrogen production performance. Thus, a promotion mechanism of the internal "driving forces" from extracellular iron oxide respiration on the biohydrogen production was proposed.

Synchronous N and P Removal in Carbon-Coated Nanoscale Zerovalent Iron Autotrophic Denitrification─The Synergy of the Carbon Shell and P Removal

Fe⁰ is a promising electron donor for autotrophic denitrification in the simultaneous removal of nitrate and phosphorus in low C/N wastewater. However, P removal may inevitably inhibit bio-denitrification. It has not been well recognized and led to an overdose of iron materials. This study employed carbon-coated zerovalent iron (Fe⁰@C) to support autotrophic denitrification to mitigate the inhibition effects of P removal and enhance both N and P removal. The critical role of the carbon shell in Fe⁰@C was to block the direct contact between Fe⁰ and P and NO₃^--N, to maintain the Fe⁰ activity. Besides, P inhibited the chemical reduction of NO₃^--N by competing for Fe⁰ active sites. This indirectly boosted H₂ generation and promoted bio-denitrification. P removal displayed negligible effects on microbial species but indirectly enhanced the nitrogen metabolic activities because of promoted H₂ in Fe⁰@C-based autotrophic denitrification. Bio-denitrification, in turn, strengthened Fe-P co-precipitation by promoting the formation of ferric hydroxide as a secondary adsorbent for P removal. This study demonstrated an efficient method for simultaneous N and P removal in autotrophic denitrification and revealed the synergistic interactions among N and P removal processes.

A manganese-oxidizing bacterium-Enterobacter hormaechei strain DS02Eh01: Capabilities of Mn(II) immobilization, plant growth promotion and biofilm formation

While biogenic Mn oxides (BioMnO_x) generated by Mn(II)-oxidizing bacteria (MOB) have attracted increasing attention, a MOB strain isolated from Mn-polluted sediments was identified and assigned as Enterobacter hormaechei DS02Eh01. Its Mn(II) immobilization activity, plant growth-promoting traits, and biofilm formation capability were investigated. The results showed that strain DS02Eh01 was found to be able to tolerate Mn(II) up to 122 mM. The strain immobilized Mn(II) in aquatic media mainly through extracellular adsorption, bio-oxidation and pH-induced precipitation as well as manganese oxidation. DS02Eh01-derived BioMnO_x are negatively charged and have a larger specific surface area (86.70 m²/g) compared to the previously reported BioMnO_x. The strain can immobilize Mn(II) at extreme levels, for instance, when it was exposed to 20 mM Mn(II), about 59% of Mn(II) were found immobilized and 17% of Mn(II) were converted to MnO_x. The SEM and TEM observation revealed that the DS02Eh01-derived BioMnO_x were aggregates doped with granules and microbial pellets. The precipitated Mn(II) and the Mn(III)/Mn(IV) oxides co-existed in BioMnO_x, in which Mn(II) and Mn(IV) were found dominant with Mn(II) accounting for 49.6% and Mn(IV) accounting for 41.3%. DS02Eh01 possesses plant growth-promoting traits and biofilm formation capacity even under Mn(II) exposure. Mn(II) exposure at 5 mM was found to stimulate strain DS02Eh01 to form biofilms, from which, the extracted EPS was mainly composed of aromatic proteins. This study reveals that E. hormaechei strain DS02Eh01 possesses the potential in environmental ecoremediation via coupling processes of macrophytes extraction, biochemical immobilization and biosorption.

Performance and mechanism of tetracycline removal by the aerobic nitrate-reducing strain Pseudomonas sp. XS-18 with auto-aggregation

The coexistence of multiple pollutants has become a distinctive feature of water pollution. However, there are a few strains that can remove nitrate and tetracycline (TC). Here, the efficiency of strain XS-18 in removing nitrate and TC was analyzed, and the mechanism of tolerance and removal of TC was investigated by infrared spectroscopy, three-dimensional fluorescence spectroscopy, and genome analysis. XS-18 could efficiently remove TC (0.40 mg·L^-1·h^-1) at pH 7.0-11.0 with auto-aggregation. TC was removed via extracellular polymeric substance (EPS) (55.90%) and cell surface (44.10%) adsorption. TC (10 mg/L) could stimulate XS-18 to secrete more polysaccharides and hydrophobic proteins to improve its auto-aggregation ability. The findings also confirmed that TC resistance genes were present. Furthermore, the bacterial flagellum, signal transduction of the chemotactic system and regulatory genes were shown to be related to the auto-aggregation of the strain. XS-18 has potential applications in the treatment of wastewater containing nitrate and TC.

Enhancing methanogenesis of anaerobic granular sludge by incorporating Fe/Fe oxides nanoparticles aided with biofilm disassembly agents and mediating redox activity of extracellular polymer substances

Anaerobic granular sludge (AGS) is a promising technology for organic wastewater treatment and energy recovery. In this study, three different kinds of Fe and Fe oxides nanoparticles (Fe₃O₄, Fe₂O₃ and ZVI) were tried to be incorporated into AGS through direct loading or aided with biofilm disassembly agents of norspermidine and D-tyrosine, which was aimed to enhance methane production capacity of AGS via increasing redox activity of extracellular polymer substance (EPS) and interspecies electron transfer. Despite the loading methods, incorporation of Fe and Fe oxides nanoparticles into AGS increased methane production capacity remarkably, with an enhancement of 36.49-85.17%, 20.37-204.95% and 189.71-243.32%, respectively, for the Fe₃O₄, Fe₂O₃ and ZVI loaded AGS. Pretreatment of AGS using biofilm disassembly agents helped to incorporate more Fe and Fe oxides into the inner structure of AGS, which further enhanced methane production capacity by 48.68% and 184.58%, respectively, for the Fe₃O₄ and Fe₂O₃ loaded AGS. Loading Fe and Fe oxides into AGS not only introduced exogenous conductive substances and Fe(III)/Fe(II) redox couples into EPS matrix of AGS, but also stimulated the production of redox active components of flavins and c-Cyts. All these factors may contribute to the reduced resistance of EPS, enhanced interspecies electron transfer and methane production capacity of AGS. This study provides a novel strategy and facile method to accelerate interspecies electron transfer and enhance methane production for matured AGS.

Synergistic Effect of Soil Organic Matter and Nanoscale Zero-Valent Iron on Biodechlorination

Nanoscale zero-valent iron (nZVI) provides a promising solution for organochlorine (OC)-contaminated soil remediation. However, the interactions among nZVI, soil organic matter (SOM), and indigenous dechlorinating bacteria are intricate, which may result in unascertained effects on the reductive degradation of OCs and merits specific investigation. Herein, we isolated an indigenous dehalogenation bacterium (Burkholderia ambifaria strain L3) from a paddy soil and further investigated the biodechlorination of pentachlorophenol (PCP) with individual and a combination of SOM and nZVI. In comparison with individual-strain L3 treatment, the cotreatment with nZVI or SOM increased the removal efficiency of PCP from 34.4 to 44.3-54.2% after 15 day cultivation. More importantly, a synergistic effect of SOM and nZVI was observed on the PCP removal by strain L3, and the PCP removal efficiency reached up to 75.3-84.5%. Other than the biodegradation through ortho- and meta-substitution under the individual application of SOM or nZVI, PCP was further biodegraded to 2,4,6-trichlorophenol (TCP) through para-substitution by the isolated bacteria with the cotreatment of SOM and nZVI. The main roles of the nZVI-SOM cotreatment in the biodegradation included the SOM-facilitated microbial proliferation, the nZVI-promoted microbial transformation of SOM, and the induced higher electron transport capacity of redox Fe-PCP biocycling. These findings provide a novel insight into the action of nZVI in environmental remediations.

Effects of biochar/AQDS on As(III)-adsorbed ferrihydrite reduction and arsenic (As) and iron (Fe) transformation: Abiotic and biological conditions

Microbe induced iron (Fe) reduction play an important role in arsenic (As) transformation and the related secondary mineral formation. Meanwhile biochar could react as electron shuttle for this process. Impact of biochar and model electron shuttle anthraquinone-2,6-disulfonate (AQDS) on the chemical/biological iron reduction of As(III)-adsorbed ferrihydrite and the solid-liquid redistribution of As in M1 buffer were studied. Fe reduction results in the release of As adsorbed on ferrihydrite into the solution. Under abiogenic conditions, both biochar and AQDS promoted ferrous production, the chemical oxidation of As(III) and As release. Inoculate with Shewanella oneidensis MR-1, AQDS has greater electronic shuttle function than biochar (with the maximum Fe(II) contents: 154 mg/L > 76.6 mg/L respectively). However, only 12.8 mg/L As was released in the presence of AQDS, which was much lower than that in the presence of biochar (21.6 mg/L), and may be associated with the transformation of As speciation and the formation of secondary minerals. XRD and EDX-SEM confirmed that the As could be fixed by the generated secondary mineral vivianite. The relative contents of vivianite in biological control and AQDS addition were 2.7% and 18.4%, respectively. This study provides information on the transformation and migration of As and Fe with the addition of biochar under anaerobic conditions, which is potential to understand the mechanism of As(III)-contaminated soil remediation.

Light-driven carbon dioxide reduction to methane by Methanosarcina barkeri in an electric syntrophic coculture

The direct conversion of CO₂ to value-added chemical commodities, thereby storing solar energy, offers a promising option for alleviating both the current energy crisis and global warming. Semiconductor-biological hybrid systems are novel approaches. However, the inherent defects of photocorrosion, photodegradation, and the toxicity of the semiconductor limit the application of these biohybrid systems. We report here that Rhodopseudomonas palustris was able to directly act as a living photosensitizer to drive CO₂ to CH₄ conversion by Methanosarcina barkeri under illumination after coculturing. Specifically, R. palustris formed a direct electric syntrophic coculture with M. barkeri. Here, R. palustris harvested solar energy, performed anoxygenic photosynthesis using sodium thiosulfate as an electron donor, and transferred electrons extracellularly to M. barkeri to drive methane generation. The methanogenesis of M. barkeri in coculture was a light-dependent process with a production rate of 4.73 ± 0.23 μM/h under light, which is slightly higher than that of typical semiconductor-biohybrid systems (approximately 4.36 μM/h). Mechanistic and transcriptomic analyses showed that electrons were transferred either directly or indirectly (via electron shuttles), subsequently driving CH₄ production. Our study suggests that R. palustris acts as a natural photosensitizer that, in coculture with M. barkeri, results in a new way to harvest solar energy that could potentially replace semiconductors in biohybrid systems.

Long-term effect of fulvic acid amendment on the anammox biofilm system at 15 ℃: performance, microbial community and metagenomics analysis

The role of fulvic acid (FA) on the anammox system at 15 ℃ was investigated. After operation for 113 days, total inorganic nitrogen removal efficiency in FA amendment reactor achieved to 58.6% on average, higher than that of control group (42.1%). Anammox-related functional genes, i.e., hzo and hzs, also demonstrated higher expression level after introduction of FA. It was observed that Candidatus Kuenenia became more competitive than Candidatus Brocadia with the existence of FA at 15 ℃. Also, co-occurrence analysis showed that FA stimulated the complexity and interactive relationship of microbial communities in the anammox system. Metagenomics analysis revealed that FA introduction stimulated relative abundances of genes in central pathway of tricarboxylic acid cycle such as ACO, IDH, OGDH, SCS, FUM, and MDH. Meanwhile, metabolomics analysis revealed that metabolites related to amino sugar metabolic pathways (glucose 1-phosphate, UDP-D-glucuronate, UDP) and redox reactions (NAD⁺ and NADH) improved in the FA amendment reactor.

Enhancing phosphorus recovery from sewage sludge using anaerobic-based processes: Current status and perspectives

Anaerobic-based processes are green and sustainable technologies for phosphorus (P) recovery from sewage sludges economically and are promising in practical application. However, the P release efficiency is always not satisfied. In this paper, the P release mechanisms (regarding to different P species) from sewage sludge using anaerobic-based processes are systematically summarized. The obstacles of P release and the updated achievements of enhancing P release from sewage sludges are analyzed and discussed. It can be concluded that different P species can release from sewage sludge via different anaerobic-based processes. Extracellular polymeric substances and excessive metal ions are the two main limiting factors to P release. Acid fermentation and anaerobic fermentation with sulfate reduction could be two promising ways, with P release efficiencies of up to 64% and 63%. Based on the summarization and discussion, perspectives on practical application of P recovery from sewage sludge using anaerobic-based processes are proposed.

Anaerobic oxidation of methane mediated by microbial extracellular respiration

Anaerobic oxidation of methane (AOM) can be microbially mediated by the reduction of different terminal electron acceptors. AOM coupled to reduction of sulfate, manganese/iron oxides, humic substances, selenate, arsenic and other artificial extracellular electron acceptors are recognized as processes associated with microbial extracellular respiration. In these processes, methane-oxidizing archaea transfer electrons to external electron acceptors or to interdependent microbial species, which are mechanistically dependent on versatile extracellular electron transfer (EET) pathways. This review compiles recent progress in the research of electromicrobiology of AOM based on the catalogue of different electron acceptors. Naturally distributed and artificially constructed EET-mediated AOM is summarized, with the discussion of their environmental importance and application potentials. The diversity of responsible microorganisms involved in EET-mediated AOM is discussed with both methane-oxidizing archaea and their putative bacterial partners. More importantly, the review highlights progress and deficiencies in our understanding of EET pathways in EET-mediated AOM, raising open research questions for future research.

Orbitrap ESI-MS evaluation of solvent extractable organics from a crude oil release site

The concentrations of oxygen-containing organic compounds (OCOC), measured as dissolved organic carbon (DOC), in groundwater exceeds those of dissolved hydrocarbons, measured as total petroleum hydrocarbons (TPH), at a crude oil release site. Orbitrap mass spectrometry was used to characterize OCOC in samples of the oil, water from upgradient of the release, source area, and downgradient wells, and a local lake. Chemical characterization factors included carbon number, oxygen number, formulae similarity, double bond equivalents (DBE) and radiocarbon dating. Oil samples were dominated by formulae with less than 30 carbons, four or fewer oxygens, and a DBE of less than four. In water samples, formulae were identified with more than 30 carbons, more than 10 oxygens, and a DBE exceeding 30. These characteristics are consistent with DOC found in unimpacted water. Between 65% and 92% of the formulae found in samples collected within the elevated OCOC plume were also found in the upgradient or surface water samples. Evidence suggests that many of the OCOC are not petroleum degradation intermediates, but microbial products generated as a result of de novo synthesis by organisms growing on carbon supplied by the oil. Implications of these results for understanding the fate and managing the risk of hydrocarbons in the subsurface are discussed.

Condition-Specific Molecular Network Analysis Revealed That Flagellar Proteins Are Involved in Electron Transfer Processes of Shewanella piezotolerans WP3

Because of the ability to metabolize a large number of electron acceptors such as nitrate, nitrite, fumarate, and metal oxides, Shewanella species have attracted much attention in recent years. Generally, the use of these electron acceptors is mainly achieved through electron transfer proteins and their interactions which will dynamically change across different environmental conditions in cells. Therefore, functional analysis of condition-specific molecular networks can reveal biological information on electron transfer processes. By integrating expression data and molecular networks, we constructed condition-specific molecular networks for Shewanella piezotolerans WP3. We then identified condition-specific key genes and studied their potential functions with an emphasis on their roles in electron transfer processes. Functional module analysis showed that different flagellar assembly modules appeared under these conditions and suggested that flagellar proteins are important for these conditions. We also identified the electron transfer modules underlying these various environmental conditions. The present results could help with screening electron transfer genes and understanding electron transfer processes under various environmental conditions for the Shewanella species.

Activating soil microbial community using bacillus and rhamnolipid to remediate TPH contaminated soil

Soil petroleum contamination has become a global environmental problem. In order to develop a new soil remediation technology, this study established bacteria isolation, surfactant toxicity matching and petroleum contaminated soil remediation practice. The simulated field remediation showed that inoculating the soil with Bacillus methylotrophicus and adding 500 mg kg^-1 rhamnolipid (N + RL) to soil can remove 80.24% of aged total petroleum hydrocarbons (TPHs) within 30 days. In particular, although the remediated soil has inoculated sufficient bacterial suspension, the microbial abundance of Bacillus was not a significantly dominant genus after remediation, especially in N + RL (0.73% of the total), but the colonies of indigenous petroleum-degrading bacteria (such as Massilia and Streptomyces) increased significantly. The interaction among genera has been further proved to drive soil non-specific oxidases (such as polyphenol oxidase, laccase and catalase) to remove TPHs. This indicates that the interaction among microorganisms, rather than the degradability of exogenous degrading bacteria, plays more critical role in the degradation of organic pollutants, which enriches the traditional understanding of micro-remediation of contaminated soil. It can be concluded from the obtained results that the remediation of pollutants can be achieved by adjusting the purification capacity of the microbial community and the natural environment.

Fulvic Acid-Mediated Interfacial Reactions on Exposed Hematite Facets during Dissimilatory Iron Reduction

Although the dual role of natural organic matter (NOM) as an electron shuttle and an electron donor for dissimilatory iron (Fe) reduction has been extensively investigated, the underlying interfacial interactions between various exposed facets and NOM are poorly understood. In this study, fulvic acid (FA), as typical NOM, was used and its effect on the dissimilatory reduction of hematite {001} and {100} by Shewanella putrefaciens CN-32 was investigated. FA accelerates the bioreduction rates of hematite {001} and {100}, where the rate of hematite {100} is lower than that of hematite {001}. Secondary Fe minerals were not observed, but the HR-TEM images reveal significant defects. The ATR-FTIR results demonstrate that facet-dependent binding mainly occurs via surface complexation between the surface iron atoms and carboxyl groups of NOM. The spectroscopic and mass spectrometry analyses suggest that organic compounds with large molecular weight, highly aromatic and unsaturated structures, and lower H/C ratios are easily adsorbed on Fe oxides or decomposed by bacteria in FA-hematite {001} treatment after iron reduction. Due to the metabolic processes of cells, a significant number of compounds with higher H/C and medium O/C ratios appear. The Tafel curves show that hematite {100} possessed higher resistance (4.1-2.6 Ω) than hematite {001} (3.5-2.2 Ω) at FA concentrations ranging from 0 to 500 mg L^-1, indicating that hematite {100} is less conductive during the electron transfer from reduced FA or cells to Fe oxides than hematite {001}. Overall, the discrepancy in the iron bioreduction of two exposed facets is attributed to both the different electrochemical activities of the Fe oxides and the different impacts on the properties and composition of OM. Our findings shed light on the molecular mechanisms of mutual interactions between FA and Fe oxides with various facets.

Enhancement of short-chain fatty acids production from microalgae by potassium ferrate addition: Feasibility, mechanisms and implications

Anaerobic fermentation of microalgae was always hindered by its rigid cell wall structure. This paper reports a novel technique, i.e., adding potassium ferrate (K₂FeO₄) into microalgae fermentation systems to enhance short-chain fatty acids (SCFAs) production. The results showed that the maximum SCFAs production and acetic acid proportion were 732.6 mg COD/g VS and 54.6% at a dosage of 112.8 mg Fe(VI)/g VS, which were 168% and 208% of those in the control, respectively. Mechanism studies revealed that K₂FeO₄ effectively destroyed surface morphology and cell structure, and thus facilitated microalgae solubilization, providing a large number of biodegradable substrates for subsequent SCFA production. Although K₂FeO₄ inhibited all the microbial activities relevant to hydrolysis, acidification and methanogenesis processes to some degree, its inhibition to methanogens was much severer than that to other microbes. Illumina MiSeq sequencing analyses revealed that K₂FeO₄ addition increased the relative abundance (from 9.45% to 50.4%) of hydrolytic and SCFAs-forming bacteria.