Exogenous electron shuttle-mediated extracellular electron transfer of Shewanella putrefaciens 200: electrochemical parameters and thermodynamics

Syntrophic methane production from volatile fatty acids: Focus on interspecies electron transfer

Methane is a renewable biomass energy source produced via anaerobic digestion (AD). Interspecies electron transfer (IET) between methanogens and syntrophic bacteria is crucial for mitigating energy barriers in this process. Understanding IET is essential for enhancing the efficiency of syntrophic methanogenesis in anaerobic digestion. Interspecies electron transfer mechanisms include interspecies H2/formate transfer, direct interspecies electron transfer (DIET), and electron-shuttle-mediated transfer. This review summarizes the mechanisms, developments, and research gaps in IET pathways. Interspecies H2/formate transfer requires strict control of low H2 partial pressure and involves complex enzymatic reactions. In contrast, DIET enhances the electron transfer efficiency and process stability. Conductive materials and key microorganisms can be modulated to stimulate the DIET. Electron shuttles (ES) allow microorganisms to interact with extracellular electron acceptors without direct contact; however, their efficiency depends on various factors. Future studies should elucidate the key functional groups, metabolic pathways, and regulatory mechanisms of IET to guide the optimization of AD processes for efficient renewable energy production.

Induced domestication of humic reduction-denitrification coupled bacteria improved treatment of sediment: Performance, remediation effect, and metabolic mechanisms

The high organic matter in river sediment primarily induces black and odorous rebound. Traditional humic-reducing bacteria demonstrate relatively single metabolic functions and restrain the remediation within complex sediment environments. In addition, Ca(NO3)2 is commonly utilized in synergistic with bioremediation to improve the reducing environment of sediments. In this study, a multifunctional bacterial community with humic reduction-denitrification coupled bacteria was domesticated by the step-feeding strategy in an anaerobic baffle reactor (ABR). The performance, remediation effect, and metabolic mechanisms were analyzed. The results indicated that humic-reducing bacteria (HRB) and denitrifying-humic-reducing bacteria (DF/HRB) have quinone-reduction and denitrification capabilities. The synergistic effect of DF/HRBs and Ca(NO3)2 was superior to HRBs and Ca(NO3)2 on the removal of total organic matter(TOM). Microbial community structure analysis revealed an enhanced relative abundance of denitrification and humic-reducing bacteria (e.g., Thauera, Pseudomonas, Sulfurospirillum, Desulfovibrio, Geobacter) in the DF/HRB, resulting in a superior synergistic effect of DF/HRBs with Ca(NO3)2. This work helps to present an innovative approach to domesticate humic-reducing bacteria suited for the remediation environment effectively. It also expands the application of humic-reducing bacteria for in-situ anaerobic remediation of river sediments.

A bibliography study of Shewanella oneidensis biofilm

This study employs a bibliography study method to evaluate 472 papers focused on Shewanella oneidensis biofilms. Biofilms, which are formed when microorganisms adhere to surfaces or interfaces, play a crucial role in various natural, engineered, and medical settings. Within biofilms, microorganisms are enclosed in extracellular polymeric substances (EPS), creating a stable working environment. This characteristic enhances the practicality of biofilm-based systems in natural bioreactors, as they are less susceptible to temperature and pH fluctuations compared to enzyme-based bioprocesses. Shewanella oneidensis, a nonpathogenic bacterium with the ability to transfer electrons, serves as an example of a species isolated from its environment that exhibits extensive biofilm applications. These applications, such as heavy metal removal, offer potential benefits for environmental engineering and human health. This paper presents a comprehensive examination and review of the biology and engineering aspects of Shewanella biofilms, providing valuable insights into their functionality.

Lumichrome from the photolytic riboflavin acts as an electron shuttle in microbial photoelectrochemical systems

Riboflavin has been proposed to serve as an electron shuttle in photoelectrochemical systems. However, riboflavin was also observed for abiotic photolysis under illumination. Such conflicting reports raise the necessity for further investigation. In this study, riboflavin secreted by Rhodopseudomonas palustris was studied to clarify its stability and electron shuttle function under illumination. The data of high-performance liquid chromatography-mass spectrometry showed that the riboflavin was photolyzed to lumichrome in microbial photoelectrochemical systems. In addition, the anodic current increased by 75% after adding lumichrome compared with that of the control; it further demonstrated that lumichrome, not riboflavin, as an electron shuttle could facilitate microbial electron transfer. This study clarifies the mechanism of the interface process in microbial photoelectrochemical systems.

A New Electron Shuttling Pathway Mediated by Lipophilic Phenoxazine via the Interaction with Periplasmic and Inner Membrane Proteins of Shewanella oneidensis MR-1

Although it has been established that electron mediators substantially promote extracellular electron transfer (EET), electron shuttling pathways are not fully understood. Here, a new electron shuttling pathway was found in the EET process by Shewanella oneidensis MR-1 with resazurin, a lipophilic electron mediator. With resazurin, the genes encoding outer-membrane cytochromes (mtrCBA and omcA) were downregulated. Although cytochrome deletion substantially reduced biocurrent generation to 1-12% of that of wild-type (WT) cells, the presence of resazurin restored biocurrent generation to 168 μA·cm^-2 (ΔmtrA/omcA/mtrC), nearly equivalent to that of WT cells (194 μA·cm^-2), indicating that resazurin-mediated electron transfer was not dependent on the Mtr pathway. Biocurrent generation by resazurin was much lower in ΔcymA and ΔmtrA/omcA/mtrC/fccA/cctA mutants (4 and 6 μA·cm^-2) than in WT cells, indicating a key role of FccA, CctA, and CymA in this process. The effectiveness of resazurin in EET of Mtr cytochrome mutants is also supported by cyclic voltammetry, resazurin reduction kinetics, and in situ c-type cytochrome spectroscopy results. The findings demonstrated that low molecular weight, lipophilic electron acceptors, such as phenoxazine and phenazine, may facilitate electron transfer directly from periplasmic and inner membrane proteins, thus providing new insight into the roles of exogenous electron mediators in electron shuttling in natural and engineered biogeochemical systems.

New insight and enhancement mechanisms for Feammox process by electron shuttles in wastewater treatment - A systematic review

Ammonium oxidation coupled to Fe(III) reduction (Feammox) is a newly discovered iron-nitrogen cycle process of microbial catalyzed NH₄⁺ oxidation coupled with iron reduction. Fe(III) often exists in the form of insoluble iron minerals resulting in reduced microbial availability and low efficiency of Feammox. Electron shuttles(ESs) can be reversibly oxidized and reduced which has the potential to improve Feammox efficiency. This review summarizes the discovery process, electron transfer mechanism, influencing factors and driven microorganisms of Feammox, ang expounds the possibility and potential mechanism of ESs to enhance Feammox efficiency. Based on an in-depth analysis of the current research situation of Feammox for nitrogen removal, the knowledge gaps and future research directions including how to apply ESs enhanced Feammox to promote nitrogen removal in practical wastewater treatment have been highlighted. This review can provide new ideas for the engineering application research of Feammox and strong theoretical support for its development.

Electron shuttle-dependent biofilm formation and biocurrent generation: Concentration effects and mechanistic insights

Introduction

Electron shuttles (ESs) play a key role in extracellular electron transfer (EET) in Shewanella oneidensis MR-1. However, the quantification relationship between ES concentration, biofilm formation, and biocurrent generation has not been clarified.

Methods

In this study, 9,10-anthraquinone-2-sulfonic acid (AQS)-mediated EET and biofilm formation were evaluated at different AQS concentrations in bioelectrochemical systems (BESs) with S. oneidensis MR-1.

Results and discussion

Both the biofilm biomass (9- to 17-fold) and biocurrent (21- to 80-fold) were substantially enhanced by exogenous AQS, suggesting the dual ability of AQS to promote both biofilm formation and electron shuttling. Nevertheless, biofilms barely grew without the addition of exogenous AQS, revealing that biofilm formation by S. oneidensis MR-1 is highly dependent on electron shuttling. The biofilm growth was delayed in a BES of 2,000 μM AQS, which is probably because the redundant AQS in the bulk solution acted as a soluble electron acceptor and delayed biofilm formation. In addition, the maximum biocurrent density in BESs with different concentrations of AQS was fitted to the Michaelis-Menten equation (R ² = 0.97), demonstrating that microbial-catalyzed ES bio-reduction is the key limiting factor of the maximum biocurrent density in BESs. This study provided a fundamental understanding of ES-mediated EET, which could be beneficial for the enrichment of electroactive biofilms, the rapid start-up of microbial fuel cells (MFCs), and the design of BESs for wastewater treatment.

Multivariate landscapes constructed by Bayesian estimation over five hundred microbial electrochemical time profiles

Data science emerges as a promising approach for studying and optimizing complex multivariable phenomena, such as the interaction between microorganisms and electrodes. However, there have been limited reports on a bioelectrochemical system that can produce a reliable database until date. Herein, we developed a high-throughput platform with low deviation to apply two-dimensional (2D) Bayesian estimation for electrode potential and redox-active additive concentration to optimize microbial current production (I c ). A 96-channel potentiostat represents <10% SD for maximum I c . 576 time-I c profiles were obtained in 120 different electrolyte and potentiostatic conditions with two model electrogenic bacteria, Shewanella and Geobacter. Acquisition functions showed the highest performance per concentration for riboflavin over a wide potential range in Shewanella. The underlying mechanism was validated by electrochemical analysis with mutant strains lacking outer-membrane redox enzymes. We anticipate that the combination of data science and high-throughput electrochemistry will greatly accelerate a breakthrough for bioelectrochemical technologies.

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.

Extracellular proteins of Desulfovibrio vulgaris as adsorbents and redox shuttles promote biomineralization of antimony

Biomineralization is the key process governing the biogeochemical cycling of multivalent metals in the environment. Although some sulfate-reducing bacteria (SRB) are recently recognized to respire metal ions, the role of their extracellular proteins in the immobilization and redox transformation of antimony (Sb) remains elusive. Here, a model strain Desulfovibrio vulgaris Hildenborough (DvH) was used to study microbial extracellular proteins of functions and possible mechanisms in Sb(V) biomineralization. We found that the functional groups (N-H, CO, O-CO, NH₂-R and RCOH/RCNH₂) of extracellular proteins could adsorb and fix Sb(V) through electrostatic attraction and chelation. DvH could rapidly reduce Sb(V) adsorbed on the cell surface and form amorphous nanometer-sized stibnite and/or antimony trioxide, respectively with sulfur and oxygen. Proteomic analysis indicated that some extracellular proteins involved in electron transfer increased significantly (p < 0.05) at 1.8 mM Sb(V). The upregulated flavoproteins could serve as a redox shuttle to transfer electrons from c-type cytochrome networks to reduce Sb(V). Also, the upregulated extracellular proteins involved in sulfur reduction, amino acid transport and protein synthesis processes, and the downregulated flagellar proteins would contribute to a better adaption under 1.8 mM Sb(V). This study advances our understanding of how microbial extracellular proteins promote Sb biomineralization in DvH.

Stimulation of pyrolytic carbon materials as electron shuttles on the anaerobic transformation of recalcitrant organic pollutants: A review

Pyrolytic carbon materials (PCMs) with various surface functionalities are widely used as environmentally friendly and cost-efficient adsorbents for the removal of organic and inorganic pollutants. Recent studies have illustrated that PCMs as electron shuttles (ESs) could also show excellent performances in promoting the anaerobic transformation of recalcitrant organic pollutants (ROPs). Numerous studies have demonstrated the excellent electron-shuttle capability (ESC) of PCMs to stimulate the anaerobic reductive transformation of ROPs. However, there is a lack of consistent understanding of the mechanism of ESC formation in PCMs and the stimulation mechanism for ROPs anaerobic transformation. To gain a more comprehensive understanding of the latest developments in the study of PCMs as ESs for ROPs anaerobic transformation, this review summarizes the formation mechanism, influencing factors, and stimulation mechanisms of ESC. ESC benefits from redox functional groups (quinone and phenol groups), persistent free radicals (PFRs), redox-active metal ions, conductive graphene phase, and porous nature of their surface. The factors influencing ESC include the highest treatment temperature (HTT), feedstocks, modification methods, and environmental conditions, of which, the HTT is the key factor. PCMs promote the reductive transformation of ROPs under anaerobic conditions via abiotic and biotic pathways. Eventually, the prospects for the ROPs anaerobic transformation enhanced by PCMs are proposed.

Extracellular electron transfer via multiple electron shuttles in waterborne Aeromonas hydrophila for bioreduction of pollutants

Members of the genus Aeromonas prevail in aquatic habitats and have a great potential in biological wastewater treatment because of their unique extracellular electron transfer (EET) capabilities. However, the mediated EET mechanisms of Aeromonas have not been fully understood yet, hindering their applications in biological wastewater treatment processes. In this study, the electron shuttles in Aeromonas hydrophila, a model and widespread strain in aquatic environments and wastewater treatment plants, were explored. A. hydrophila was found to produce both flavins and 2-amino-3-carboxy-1,4-naphthoquinone (ACNQ) as electron shuttles and utilize them to accelerate its EET for the bioreduction of various pollutants. The Mtr-like respiratory pathway was essential for the reduction of flavins, but not involved in the ACNQ reduction. The electron shuttle activity of ACNQ for pollutant bioreduction involved the redox reactions that occurred inside the cell. These findings deepen our understanding about the underlying EET mechanisms in dissimilatory metal reducing bacteria and provide new insights into the roles of the genus Aeromonas in biological wastewater treatment.

Prompting the FDH/Hases-based electron transfers during Pt(IV) reduction mediated by bio-Pd(0)

Due to the excellent hydrogen affinity and high conductivity, palladium nanoparticles (Pd NPs) were considered as a potential strategy to regulate bacterial electron transfer and energy metabolism. Herein, Citrobacter freundii JH, capable of in-situ biosynthesizing Pd(0) NPs, was employed to promote Pt(IV) reduction. The results showed that the Pt(IV) reduction to Pt(II) was accomplished mainly via the flavins-mediated extracellular electron transfer (EET) process, while Pt(II) reduction to Pt(0) was limit step, and proceeded via two intracellular respiratory chains, including FDH/Hases-based short chain (S-chain) and typical CoQ-involved long respiratory chain (L-chain). Noteworthily, the incorporation of Pd(0) NPs mainly diverted the electrons to S-chain (as high as 71.7%-73.4%) by improving the hydrogenases (Hases) activity. Furthermore, Pd(0) NPs could stimulate the secreting of flavins and the combination between flavins and cytochrome c (c-Cyt), which converted electron transfer manner of L-chain. Additionally, Pd(0) NPs might also act as alternative proton channels to improve the energy metabolism. These findings provided significant insights into the promotion by Pd(0) NPs in terms of electron generation, electron consumption and proton translocation.

Tuning Redox Potential of Anthraquinone-2-Sulfonate (AQS) by Chemical Modification to Facilitate Electron Transfer From Electrodes in Shewanella oneidensis

Bioelectrochemical systems (BESs) are emerging as attractive routes for sustainable energy generation, environmental remediation, bio-based chemical production and beyond. Electron shuttles (ESs) can be reversibly oxidized and reduced among multiple redox reactions, thereby assisting extracellular electron transfer (EET) process in BESs. Here, we explored the effects of 14 ESs on EET in Shewanella oneidensis MR-1, and found that anthraquinone-2-sulfonate (AQS) led to the highest cathodic current density, total charge production and reduction product formation. Subsequently, we showed that the introduction of -OH or -NH₂ group into AQS at position one obviously affected redox potentials. The AQS-1-NH₂ exhibited a lower redox potential and a higher Coulombic efficiency compared to AQS, revealing that the ESs with a more negative potential are conducive to minimize energy losses and improve the reduction of electron acceptor. Additionally, the cytochromes MtrA and MtrB were required for optimal AQS-mediated EET of S. oneidensis MR-1. This study will provide new clues for rational design of efficient ESs in microbial electrosynthesis.

Biomedical Implants with Charge-Transfer Monitoring and Regulating Abilities

Transmembrane charge (ion/electron) transfer is essential for maintaining cellular homeostasis and is involved in many biological processes, from protein synthesis to embryonic development in organisms. Designing implant devices that can detect or regulate cellular transmembrane charge transfer is expected to sense and modulate the behaviors of host cells and tissues. Thus, charge transfer can be regarded as a bridge connecting living systems and human-made implantable devices. This review describes the mode and mechanism of charge transfer between organisms and nonliving materials, and summarizes the strategies to endow implants with charge-transfer regulating or monitoring abilities. Furthermore, three major charge-transfer controlling systems, including wired, self-activated, and stimuli-responsive biomedical implants, as well as the design principles and pivotal materials are systematically elaborated. The clinical challenges and the prospects for future development of these implant devices are also discussed.

Comparative Genomics Revealing Insights into Niche Separation of the Genus Methylophilus

The genus Methylophilus uses methanol as a carbon and energy source, which is widely distributed in terrestrial, freshwater and marine ecosystems. Here, three strains (13, 14 and QUAN) related to the genus Methylophilus, were newly isolated from Lake Fuxian sediments. The draft genomes of strains 13, 14 and QUAN were 3.11 Mb, 3.02 Mb, 3.15 Mb with a G+C content of 51.13, 50.48 and 50.33%, respectively. ANI values between strains 13 and 14, 13 and QUAN, and 14 and QUAN were 81.09, 81.06 and 91.46%, respectively. Pan-genome and core-genome included 3994 and 1559 genes across 18 Methylophilus genomes, respectively. Phylogenetic analysis based on 1035 single-copy genes and 16S rRNA genes revealed two clades, one containing strains isolated from aquatic and the other from the leaf surface. Twenty-three aquatic-specific genes, such as 2OG/Fe(II) oxygenase and diguanylate cyclase, reflected the strategy to survive in oxygen-limited water and sediment. Accordingly, 159 genes were identified specific to leaf association. Besides niche separation, Methylophilus could utilize the combination of ANRA and DNRA to convert nitrate to ammonia and reduce sulfate to sulfur according to the complete sulfur metabolic pathway. Genes encoding the cytochrome c protein and riboflavin were detected in Methylophilus genomes, which directly or indirectly participate in electron transfer.

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.

Distinct biofilm formation regulated by different culture media: Implications to electricity generation

Biofilm of Shewanella oneidensis MR-1 is extensively studied as it can transform organic compounds directly into electricity. Although revealing the biofilm regulation mechanism is crucial for enhancing bio-current, studies regarding the mechanism by which the culture condition affects biofilm formation are still lacking. The biofilm formation of S. oneidensis MR-1 in two typical media with same electron donor was investigated in this study. Bio-electricity increased 1.8 times in medium with phosphate-buffered saline (PBS) than in piperazine-1,4-bisethanesulfonic acid (PIPES). Biofilm total protein has 1.5-fold of difference between two media at day 3, and biofilm structures also differed; a fluffy biofilm with curled cells was formed in medium with PBS, whereas a compact, ordered, and closely attached biofilm was formed in medium with PIPES. Transcriptome studies clarified that the expression of genes beneficial for cell aggregation [e.g., aggA (2.3 fold), bpfA (2.8 fold) and csgB (3.9 fold)] in medium with PIPES was significantly upregulated, thus provided an explanation for the specific biofilm structure. Buffer concentration was proved to be a critical factor impacted cell morphology and current generation. The maximum current density in 30 mM of PBS and PIPES is 165 and 159 μA·cm^-2 respectively, but it increased to 327 and 274 μA·cm^-2 in 200 mM of PBS and PIPES. This study provides new insights into the mechanism of medium-dependent biofilm regulation, which will be beneficial for developing simple and efficient strategies to enhance bio-electricity generation.

Microbial Electrochemical Systems: Principles, Construction and Biosensing Applications

Microbial electrochemical systems are a fast emerging technology that use microorganisms to harvest the chemical energy from bioorganic materials to produce electrical power. Due to their flexibility and the wide variety of materials that can be used as a source, these devices show promise for applications in many fields including energy, environment and sensing. Microbial electrochemical systems rely on the integration of microbial cells, bioelectrochemistry, material science and electrochemical technologies to achieve effective conversion of the chemical energy stored in organic materials into electrical power. Therefore, the interaction between microorganisms and electrodes and their operation at physiological important potentials are critical for their development. This article provides an overview of the principles and applications of microbial electrochemical systems, their development status and potential for implementation in the biosensing field. It also provides a discussion of the recent developments in the selection of electrode materials to improve electron transfer using nanomaterials along with challenges for achieving practical implementation, and examples of applications in the biosensing field.

Microbial Cd(II) and Cr(VI) resistance mechanisms and application in bioremediation

The heavy metals cadmium (Cd) and chromium (Cr) are extensively used in industry and result in water and soil contamination. The highly toxic Cd(II) and Cr(VI) are the most common soluble forms of Cd and Cr, respectively. They enter the human body through the food chain and drinking water and then cause serious illnesses. Microorganisms can adsorb metals or transform Cd(II) and Cr(VI) into insoluble or less bioavailable forms, and such strategies are applicable in Cd and Cr bioremediation. This review focuses on the highlighting of novel achievements on microbial Cd(II) and Cr(VI) resistance mechanisms and their bioremediation applications. In addition, the knowledge gaps and research perspectives are also discussed in order to build a bridge between the theoretical breakthrough and the resolution of Cd(II) and Cr(VI) contamination problems.

Oxidation of Fe(II) by Flavins under Anoxic Conditions

Flavin-mediated electron transfer is an important pathway for Fe(III) reduction by dissimilatory iron-reducing bacteria. Although the mechanisms and kinetics of Fe(III) reduction by reduced flavins have been widely studied, the reaction between Fe(II) and oxidized flavins is rarely investigated. Results of this study show that under anoxic conditions, Fe(II) can be oxidized by the oxidized forms of riboflavin (RBF) and flavin mononucleotide (FMN) at pH 7-9. For instance, at pH 9, 73% of 17.8 μM Fe(II) was oxidized by 10 μM RBF within 20 min. Both the rate and extent of oxidation increased with increasing concentrations of oxidized flavins and increasing solution pH. Thermodynamic calculations and kinetic analyses implied that the oxidation of Fe(II) proceeded predominantly via the autodecomposition of Fe²⁺-RBF^- and Fe²⁺-FMN^- complexes, along with minor contributions from direct oxidation of Fe(II) by flavins and flavin radicals. Our findings suggest that the reoxidation of Fe(II) by oxidized flavins may be a rate-controlling factor in microbial Fe(III) reduction via flavin-mediated electron transfer.

Extracellular Electron Shuttling Mediated by Soluble c-Type Cytochromes Produced by Shewanella oneidensis MR-1

How metal-reducing bacteria transfer electrons during dissimilatory energy generation under electron acceptor-limited conditions is poorly understood. Here, we incubated the iron and manganese-reducing bacterium Shewanella oneidensis MR-1 without electron acceptors. Removal of soluble extracellular organic compounds (EOCs) dramatically retarded transfer of electrons to an experimental electron acceptor, Cr(VI), by MR-1. However, the return of either high MW (>3000 Da) or low MW (<3000 Da) soluble EOCs produced by MR-1 to washed cells restored Cr(VI) reduction though Cr(VI) reduction was fastest when both size fractions were added together. Spectral and electrochemical characterization of EOCs indicated the presence of flavins and c-type cytochromes (c-Cyts). A model of the kinetics of individual elementary reactions between cells, flavins, released c-Cyts, and Cr(VI), including the direct reduction of flavins, released c-Cyts, and Cr(VI) by cells and the indirect reduction of Cr(VI) by reduced forms of flavins and released c-Cyts, was developed. Model results suggest that released c-Cyts could act as electron mediators to accelerate electron transfer from cells to Cr(VI), and the relative contribution of this pathway was higher than that mediated by flavins. Hence, extracellular c-Cyts produced by MR-1 likely play a role in extracellular electron transfer under electron acceptor-limited conditions. These findings provide new insights into extracellular electron shuttling and the metabolic strategy of metal-reducing bacteria under electron acceptor-limited conditions.

Molecular density regulating electron transfer efficiency of S. oneidensis MR-1 mediated roxarsone biotransformation

Efficient extracellular electron transport is a key for sufficient bioremediation of organoarsenic pollutants such as 4-hydroxy-3-nitrobenzenearsonic acid (roxarsone). The related apparent kinetics characteristics are essential for engineering practice of bioremediation activities and for full understanding the environmental fate of roxarsone, yet remains poorly understood. We report, to our knowledge, the first study of the electron transfer characteristics between roxarsone and participating S. oneidensis MR-1. The electron transfer rate during roxarsone biotransformation was estimated up to 3.1 × 10⁶ electrons/cell/s, with its value being clearly associated with the apparent roxarsone concentration. Lowing roxarsone concentration extended the average separation distance between cells and neighboring roxarsone molecules and thereby augmented electric resistance as well as extended cell movement for foraging, thus reduced electron transfer rate. In addition, the presence of roxarsone significantly stimulated population growth of S. oneidensis MR-1 with nearly doubled maximum specific growth rate, albeit with clearly increased lag time, as compared with that of none-roxarsone scenario. These findings provide, at the first time, basic biostoichiometry of S. oneidensis MR-1 induced roxarsone biotransformation, which may shed lights for full understanding of roxarsone transformation process in waste treatment systems that are necessary for engineering practice and/or environmental risks assessment.

Enhanced Current Production by Exogenous Electron Mediators via Synergy of Promoting Biofilm Formation and the Electron Shuttling Process

Exogenous electron mediators (EMs) can facilitate extracellular electron transfer (EET) via electron shuttling processes, but it is still unclear whether and how biofilm formation is affected by the presence of EMs. Here, the impacts of EMs on EET and biofilm formation were investigated in bioelectrochemical systems (BESs) with Shewanella oneidensis MR-1, and the results showed that the presence of five different EMs led to high density current production. All the EMs substantially promoted biofilm formation with 15-36 times higher total biofilm DNA with EMs than without EMs, and they also increased the production of extracellular polymeric substances, which was favorable for biofilm formation. The current decreased substantially after removing EMs from the medium or by replacing electrodes without biofilm, suggesting that both biofilm and EMs are required for high density current production. EET-related gene expression was upregulated with EMs, resulting in the high flux of cell electron output. A synergistic mechanism was proposed: EMs in suspension were quickly reduced by the cells and reoxidized rapidly by the electrode, resulting in a microenvironment with sufficient oxidized EMs for biofilm formation, and thus, besides the well-known electron shuttling process, the EM-induced high biofilm formation and high Mtr gene expression could jointly contribute to the EET and subsequently produce a high density current. This study provides a new insight into EM-enhanced current production via regulating the biofilm formation and EET-related gene expression.

Conduction Band of Hematite Can Mediate Cytochrome Reduction by Fe(II) under Dark and Anoxic Conditions

While it was recently reported that the conduction band of iron minerals can mediate electron transfer between Fe(II) and different Fe(III) lattice sites during Fe(II)-catalyzed mineral transformation, it is unclear whether such a conduction band mediation pathway occurs in the microbial Fe(II) oxidation system under dark and anoxic subsurface conditions. Here, using cytochrome c (c-Cyts) as a model protein of microbial Fe(II) oxidation, the in vitro kinetics and thermodynamics of c-Cyts reduction by Fe(II) were studied. The results showed that the rates of c-Cyts reduction were greatly enhanced in the presence of the semiconducting mineral hematite (Hem, α-Fe₂O₃). The electrochemical experiments separating Fe(II) and c-Cyts demonstrated that electrons from Fe(II) to the electrode or from the electrode to c-Cyts could directly penetrate hematite, resulting in enhanced current. Independent photochemical and photoluminescence experiments indicated that c-Cyts could be directly reduced by the conduction band electrons of hematite which were generated under light illumination. In c-Cyts+Fe(II)+Hem, the redox potential of Fe(II)-Hem was shifted from -0.15 to -0.18 V and that of c-Cyts+Hem changed slightly from -0.05 to -0.04 V. For the bulk hematite, Mott-Schottky plots illustrated that the flat band was shifted negatively and positively in the presence of Fe(II) and oxidized c-Cyts, respectively, and the surface electron/charge density was higher in the presence of Fe(II)/c-Cyts. As a consequence, the redox gradients from adsorbed Fe(II) to adsorbed c-Cyts allow electron transfer across the conduction band of hematite and facilitate c-Cyts reduction. This mechanistic study on conduction band-mediating electron transfer could help interpret the role of semiconducting minerals in the microbial Fe(II) oxidation process under dark anoxic conditions.

Potassium channel blocker inhibits the formation and electroactivity of Geobacter biofilm

Bacteria in biofilms are able to utilize potassium ion channel-mediated electrical signaling to achieve cell-cell communication. However, it remains unclear whether these signals play a role in Geobacter sp. when surrounded by an intense electric field. This study used a potassium channel blocker (tetraethylammonium, TEA) that interfered with the release of K⁺ but not bacterial growth to demonstrate that potassium ion channel-mediated electrical signaling affected the formation and electroactivity of Geobacter sulfurreducens. The results showed that 5 mM TEA slowed the formation of Geobacter sulfurreducens biofilm, and the current density was ~50% lower than in the control. The electrochemical analyses showed that the electroactivity of the biofilms with TEA addition was inferior. In particular, the micrometer- scale biofilm with TEA exhibited fewer high current peaks, and the species of outermost groups that participated in the electron transfer in Geobacter sulfurreducens biofilms was different from the control. This work provides initial evidence to reveal the role of potassium channels in Geobacter sulfurreducens electroactive biofilms.

Electron shuttles enhance anaerobic oxidation of methane coupled to iron(III) reduction

Anaerobic oxidation of methane (AOM) has recently been coupled with the reduction of insoluble electron acceptors such as iron minerals. However, effects of electron shuttles (ESs) on this process and the underlying coupling mechanisms remain not well understood. Here, we evaluated AOM-coupled ferrihydrite reduction by a mixed culture in the absence and presence of ESs. The results showed that ESs (AQS, flavin, HA and AQDS) significantly enhanced the rate (up to 7.4 times) of AOM-dependent ferrihydrite reduction compared with the control. The enhancements were linearly related with the electron transfer capacity of ESs. Illumina high-throughput sequencing and DNA-based stable isotope probing revealed that the AOM-coupled iron reduction depended on the syntrophic interaction of Methanobacterium and the partner bacteria. Methanobacterium as the dominant microorganism, did not assimilate methane into its biomasses. However, it played a crucial role in the partial oxidation of methane into an intermediate (i.e. propionate), which was then assimilated by the partner bacteria (e.g. Cellulomonas, Desulfovibrio, Actinotalea, etc.) for ferrihydrite reduction. This work suggests that ESs in natural environments can mitigate the methane emissions by facilitating the AOM process and biogeochemical cycles of iron.

Determination and toxicity evaluation of the generated byproducts from sulfamethazine degradation during catalytic oxidation process

Sulfamethazine (SMZ), a kind of sulfonamide antibiotics, can exist for a long periods of time and has been widely detected in the environment, which could pose a potential health threat to human beings. In this study, sludge-derived carbon (SC) catalyst was modified and applied to degrade SMZ during catalytic oxidation process. Degradation products and possible transformation pathways were investigated based on data of GC-MS. The toxicity evolution of SMZ degradation after catalytic oxidation process was tested with zebrafish and microbial degradation respirometer. As a consequence, SC modified with nitric acid (SCHNO₃) exhibited highly catalytic efficiency reached 92.2% SMZ conversion and 75.2% total organic carbon (TOC) removal rate after 480 min. Ten kinds of possible products were identified by GC-MS during degradation process of SMZ, indicating two possible pathways. No pronounced malformation was observed in the toxicity experiments with zebrafish until 120 h post fertilization (hpf). However, further analysis showed that zebrafish incubated with SMZ solution had higher mortality, lower hatching rate, slower spontaneous movement and shorter body length, compared with the group used normal nutrient solution, while the water after treatment had lower toxicity effects on zebrafish. The toxicity experiments with microbial degradation respirometer showed that SMZ solution had lower value of oxygen uptake, which indicated that SMZ solution had higher values of toxicity and inhibition of pharmaceutical compounds. This study provides a catalyst with low cost and high catalytic efficiency for degradation process of SMZ and gives a deeper insight into the ecotoxicity of treated water.

Multi-omics response of Pannonibacter phragmitetus BB to hexavalent chromium

The release of hexavalent chromium [Cr(VI)] into water bodies poses a major threat to the environment and human health. However, studies of the biological response to Cr(VI) are limited. In this study, a toxic bacterial mechanism of Cr(VI) was investigated using Pannonibacter phragmitetus BB (hereafter BB), which was isolated from chromate slag. The maximum Cr(VI) concentrations with respect to the resistance and reduction by BB are 4000 mg L^-1 and 2500 mg L^-1, respectively. In the BB genome, more genes responsible for Cr(VI) resistance and reduction are observed compared with other P. phragmitetus strains. A total of 361 proteins were upregulated to respond to Cr(VI) exposure, including enzymes for Cr(VI) uptake, intracellular reduction, ROS detoxification, DNA repair, and Cr(VI) efflux and proteins associated with novel mechanisms involving extracellular reduction mediated by electron transfer, quorum sensing, and chemotaxis. Based on metabolomic analysis, 174 metabolites were identified. Most of the upregulated metabolites are involved in amino acid, glucose, lipid, and energy metabolisms. The results show that Cr(VI) induces metabolite production, while metabolites promote Cr(VI) reduction. Overall, multi-enzyme expression and metabolite production by BB contribute to its high ability to resist/reduce Cr(VI). This study provides details supporting the theory of Cr(VI) reduction and a theoretical basis for the efficient bioremoval of Cr(VI) from the environment.

Influence of Incubation Temperature on 9,10-Anthraquinone-2-Sulfonate (AQS)-Mediated Extracellular Electron Transfer

The electron shuttling process has been recognized as an important microbial respiration process. Because the incubation temperature can influence both the reactivity of electron mediators and cell growth, it may also affect the electron-shuttle-mediated extracellular electron transfer (EET) process. Here, the effect of incubation temperature (22-38°C) was investigated in a bioelectrochemical system (BES) using Shewanella oneidensis MR-1 and 50 μM of 9,10-anthraquinone-2-sulfonate (AQS). We found that current generation increased as the temperature was increased from 22 to 34°C and then decreased sharply at 38°C. The biofilm biomass, as indicated by the total protein extracted from the electrode, increased as the temperature increased from 22 to 34°C and then decreased at 38°C, mirroring the current generation results. These results were further confirmed by increasing the temperature slowly, step-by-step, in a single BES with a constant biofilm biomass, suggesting that the EET rates could be substantially influenced by temperature, even with the same biofilm. The effects of temperature on the AQS bioreduction rate, c-type cytochrome (c-Cyts)-bound-cofactor-mediated EET, the AQS mid-point potential, and the AQS diffusion coefficient were studied. From these results, we were able to conclude that temperature influenced the EET rates by changing the c-Cyts-bound-cofactor-mediated EET process and the AQS bioreduction rate, and that the change in biofilm formation was a dominant factor influencing the overall EET rates. These findings should contribute to the fundamental understanding of EET processes. Moreover, optimization of the operating parameters for current generation will be helpful for the practical application of bioelectrochemical techniques.

Electrodes Donate Electrons for Nitrate Reduction in a Soil Matrix via DNRA and Denitrification

Microbial strains and indigenous microbiota in soil slurries have been reported to use electrons from electrodes for nitrate (NO₃^-) reduction. However, few studies have confirmed this in a soil matrix hitherto. This study investigated if, and how, an electric potential affected NO₃^- reduction in a soil matrix. The results showed that, compared to a control treatment, applying an electric potential of -0.5 V versus the standard hydrogen electrode (SHE) significantly increased the relative abundance of NO₃^--reducing microbes (e.g., Alcaligenaceae and Pseudomonadaceae) and the abundances of the nrfA, nirK, nirS, and nosZ genes in soil matrices. Meanwhile, the electric potential treatment doubled the NO₃^- reduction rate and significantly increased the rates of production of ammonium (NH₄⁺), dinitrogen (N₂), and nitrous oxide (N₂O). The amount of NO₃^--N reduced under the electric potential treatment was comparable to the sum of the amounts of N observed in the increased N₂O, N₂, NH₄⁺, and nitrite (NO₂^-) pools. An open-air experiment showed that the electric potential treatment promoted soil NO₃^- reduction with a spatial scale of at least 38 cm. These results demonstrated that an electric potential treatment could enhance NO₃^- reduction via both denitrification and dissimilatory NO₃^- reduction to ammonium (DNRA) in the soil matrix. The mechanisms revealed in this study have implications for the future development of potential techniques for enhancing NO₃^- reduction in the vadose zone and consequently reducing the risk of NO₃^- leaching.

Novel Gas Diffusion Cloth Bioanodes for High-Performance Methane-Powered Microbial Fuel Cells

Microbial fuel cells (MFCs) are a promising technology that converts chemical energy into electricity. However, up to now only few MFCs have been powered by gas fuels, such as methane, and their limited performance is still challenged by the low solubility and bioavailability of gases. Here, we developed a gas diffusion cloth (GDC) anode to significantly enhance the performance of methane-powered MFCs. The GDC anode was constructed by simply coating waterproof GORE-TEX cloth with conductive carbon cloth in one step. After biofilm enrichment, the GDC anodes obtained a methane-dependent current up to 1130.2 mA m^-2, which was 165.2 times higher than conventional carbon cloth (CC) anodes. Moreover, MFCs equipped with GDC anodes generated a maximum power density of 419.5 mW m^-2. Illumina high-throughput sequencing revealed that the GDC anode biofilm was dominated mainly by Geobacter, in contrast with the most abundant Methanobacterium in planktonic cells. It is hypothesized that Methanobacterium reversed the methanogenesis process by transferring electrons to the anodes, and Geobacter generated electricity via the intermediates (e.g., acetate) of anaerobic methane oxidation. Overall, this work provides an effective route in preparing facile and cost-effective anodes for high-performance methane MFCs.

The dynamic role of pH in microbial reduction of uranium(VI) in the presence of bicarbonate

The negative effect of carbonate on the rate and extent of bioreduction of aqueous U(VI) has been commonly reported. The solution pH is a key chemical factor controlling U(VI)_aq species and the Gibbs free energy of reaction. Therefore, it is interesting to study whether the negative effect can be diminished under specific pH conditions. Experiments were conducted using Shewanella putrefaciens under anaerobic conditions with varying pH values (4-9) and bicarbonate concentrations ( [Formula: see text] , 0-50 mmol/L). The results showed a clear correlation between the pH-bioreduction edges of U(VI)_aq and the [Formula: see text] . The specific pH at which the maximum bioreduction occurred (pH_mbr) shifted from slightly basic to acidic pH (∼7.5-∼6.0) as the [Formula: see text] increased (2-50 mmol/L). At [Formula: see text]  = 0, however, no pH_mbr was observed in terms of increasing bioreduction with pH (∼100%, pH > 7). In the presence of [Formula: see text] , significant bioreduction was observed at pH_mbr (∼100% at 2-30 mmol/L [Formula: see text] , 93.7% at 50 mmol/L [Formula: see text] ), which is in contrast to the previously reported infeasibility of bioreduction at high [Formula: see text] . The pH-bioreduction edges were almost comparable to the pH-biosorption edges of U(VI)_aq on heat-killed cells, revealing that biosorption is favorable for bioreduction. The end product of U(VI)_aq bioreduction was characterized as insoluble nanobiogenic uraninite by HRTEM. The redox potentials of the master complex species of U(VI)_aq, such as [Formula: see text] , [Formula: see text] , and [Formula: see text] , were calculated to obtain insights into the thermodynamic reduction mechanism. The observed dynamic role of pH in bioreduction suggests the potential for bioremediation of uranium-contaminated groundwater containing high carbonate concentrations.

Positive effects of bio-nano Pd (0) toward direct electron transfer in Pseudomona putida and phenol biodegradation

This study constructed a biological-inorganic hybrid system including Pseudomonas putida (P. putida) and bioreduced Pd (0) nanoparticles (NPs), and inspected the influence of bio-nano Pd (0) on the direct electron transfer and phenol biodegradation. Scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDX) showed that bio-nano Pd (0) (~10 nm) were evenly dispersed on the surface and in the periplasm of P. putida. With the incorporation of bio-nano Pd (0), the redox currents of bacteria in the cyclic voltammetry (CV) became higher and the oxidation current increased as the addition of lactate, while the highest increase rates of two electron transfer system (ETS) rates were 63.97% and 33.79%, respectively. These results indicated that bio-nano Pd (0) could directly promote the electron transfer of P. putida. In phenol biodegradation process, P. putida-Pd (0)- 2 showed the highest k (0.2992 h^-1), μ_m (0.035 h^-1) and K_i (714.29 mg/L) and the lowest apparent K_s (76.39 mg/L). The results of kinetic analysis indicated that bio-nano Pd (0) markedly enhanced the biocatalytic efficiency, substrate affinity and the growth of cells compared to native P. putida. The positive effects of bio-nano Pd (0) to the electron transfer of P. putida would promote the biodegradation of phenol.

Intracellular versus extracellular accumulation of Hexavalent chromium reduction products by Geobacter sulfurreducens PCA

Hexavalent chromium (Cr(VI)) reduction by Geobacter sulfurreducens PCA was evaluated in batch experiments, and the form and amounts of intracellular and extra-cellular Cr(VI) reduction products were determined over time. The first-order Cr(VI) reduction rate per unit mass of cells was consistent for different initial cell concentrations, and approximately equal to (2.065 ± 0.389) x 10^-9 mL CFU^-1 h^-1. A portion of the reduced Cr(VI) products precipitated on Geobacter cell walls as Cr(III) and was bound via carboxylate functional groups, a portion accumulated inside Geobacter cells, and another portion existed as soluble Cr(III) or organo-Cr(III) released to solution. A mass balance analysis of total chromium in aqueous media, on cell walls, and inside cells was determined as a function of time, and with different initial cell concentrations. Mass balances were between 92% and 98%, and indicated Cr(VI) reduction products accumulate more on cell walls and inside cells with time and with increasing initial cell concentration, as opposed to particulates in aqueous solution. Reduced Cr(VI) products both in solution and on cell surfaces appear to form organo-Cr(III) complexes, and our results suggest that such complexes are more stable to reoxidation than aqueous Cr(III) or Cr(OH)₃. Chromium inside cells is also likely more stable to reoxidation, both because it can form organic complexes, and it is separated by the cell membrane from solution conditions. Hence, Cr(VI) reduction products in groundwater during bioremediation may become more stable against re-oxidation, and may pose a lower risk to human health, over time and with greater initial biomass densities.

Interaction between in vivo bioluminescence and extracellular electron transfer in Shewanella woodyi via charge and discharge

Extracellular electron transfer (EET) and bioluminescence are both important for microbial growth and metabolism, but the mechanism of interaction between EET and bioluminescence is poorly understood. Herein, we demonstrate an exclusively respiratory luminous bacterium, Shewanella woodyi, which possesses EET ability and electron communication at the interface of S. woodyi and solid substrates via charge and discharge methods. Using an electro-chemiluminescence apparatus, our results confirmed that the FMN/FMNH₂ content and the redox status of cytochrome c conjointly regulated the bioluminescence intensity when the potential of an indium-tin oxide electrode was changed. More importantly, this work revealed that there is an interaction between the redox reaction of single cells and bioluminescence of group communication via the EET pathway.

Electron transfer mechanism of biocathode in a bioelectrochemical system coupled with chemical absorption for NO removal

A biocathode with the function of Fe(III)EDTA and Fe(II)EDTA-NO reduction was applied in a microbial electrolysis cell coupled with chemical absorption for NO removal from flue gas. As the mediated electron transfer was excluded by the same electrochemical characterizations of the biocathodes before and after a 48 h continuous operation, the profiles of reduction experiments indicated that direct electron transfer was the main mechanism of Fe(III)EDTA reduction, while Fe(III)EDTA-NO was mainly reduced via Fe(II)-assisted autotrophic denitrification. The microscopy of the biocathode confirmed the existence of pili, which was supposed to be bacterial nanowires for electron transfer. The analysis of microbial community revealed that iron-reducing bacteria, including Escherichia coli, had the possibility of electron uptake from electrode via physical contact. These results first time gave us in-depth understanding of the electron transfer in the multifunctional biocathode and mechanism for further enhancement of the bioreduction processes.

Effects of riboflavin and AQS as electron shuttles on U(vi) reduction and precipitation byShewanella putrefaciens

Understanding the mechanisms for electron shuttles (ESs) in microbial extracellular electron transfer (EET) is important in biogeochemical cycles, bioremediation applications, as well as bioenergy strategies.

Chitin degradation and electricity generation by Aeromonas hydrophila in microbial fuel cells

Chitin is one of the most abundant biopolymers in nature and the main composition of shrimp and crab shells (usually as food wastes). Thus it is essential to investigate the potential of degrading chitin for energy recovery. This study investigated the anaerobic degradation of chitin by Aeromonas hydrophila, a chitinolytic and popular electroactive bacterium, in both fermentation and microbial fuel cell (MFC) systems. The primary chitin metabolites produced in MFC were succinate, lactate, acetate, formate, and ethanol. The total metabolite concentration from chitin degradation increased seven-fold in MFC compared to the fermentation system, as well as additional electricity generation. Moreover, A. hydrophila degraded GlcNAc (the intermediate of chitin hydrolysis) significantly faster (0.97 and 0.94 mM C/d/mM-GlcNAc) than chitin (0.13 and 0.03 mM C/d/mM-GlcNAc) in MFC and fermentation systems, indicating that extracellular hydrolysis of chitin was the rate-limiting step and this step could be accelerated in MFC. Furthermore, more chemicals produced by the addition of exogenous mediators in MFC. This study proves that the chitin could be degraded effectively by an electroactive bacterium in MFC, and our results suggest that this bioelectrochemical system might be useful for the degradation of recalcitrant biomass to recover energy.

Dissolved organic matter as a terminal electron acceptor in the microbial oxidation of steroid estrogen

Steroid estrogen in natural waters may be biodegraded by quinone-reducing bacteria, dissolved organic matter (DOM) may serve as a terminal electron acceptor in this process. The influence of temperature, pH, dissolved oxygen and light illumination on the reduction efficiency of anthraquinone-2-disulfonate (AQS) was investigated using 17β-estradiol (E2) as the target species. The optimum reduction conditions were found to be in the dark under anaerobic conditions at pH 8.0 and 30 °C. Quinone-reducing bacteria can use the quinone structure of DOM components as a terminal electron acceptor coupling with microbial growth to promote biodegradation. Compared with other DOM models, AQS best stimulated E2 biodegradation and the mediating effect was improved as the AQS concentration increased from 0 to 0.5 mM. However, further increase had an inhibiting effect. Natural DOM containing lake humic acid (LHA) and lake fulvic acid (LFA) had a very important accelerating effect on the degradation of E2, the action mechanism of which was consistent with that defined using DOM models. The natural DOM contained more aromatic compounds, demonstrating their greater electron-accepting capacity and generally more effective support for microorganism growth and E2 oxidation than Aldrich humic acid (HA). These results provide a more comprehensive understanding of microbial degradation of steroid estrogens in anaerobic environments and confirm DOM as an important terminal electron acceptor in pollutant transformation.