Electron transfer capacity dependence of quinone-mediated Fe(III) reduction and current generation by Klebsiella pneumoniae L17

Mechanism of extracellular electron transport and reactive oxygen mediated Sb(III) oxidation by Klebsiella aerogenes HC10

Microbial oxidation and the mechanism of Sb(III) are key governing elements in biogeochemical cycling. A novel Sb oxidizing bacterium, Klebsiella aerogenes HC10, was attracted early and revealed that extracellular metabolites were the main fractions driving Sb oxidation. However, linkages between the extracellular metabolite driven Sb oxidation process and mechanism remain elusive. Here, model phenolic and quinone compounds, i.e., anthraquinone-2,6-disulfonate (AQDS) and hydroquinone (HYD), representing extracellular oxidants secreted by K. aerogenes HC10, were chosen to further study the Sb(III) oxidation mechanism. N2 purging and free radical quenching showed that oxygen-induced oxidation accounted for 36.78% of Sb(III) in the metabolite reaction system, while hydroxyl free radicals (·OH) accounted for 15.52%. ·OH and H2O2 are the main driving factors for Sb oxidation. Radical quenching, methanol purification and electron paramagnetic resonance (EPR) analysis revealed that ·OH, superoxide radical (O2•-) and semiquinone (SQ-•) were reactive intermediates of the phenolic induced oxidation process. Phenolic-induced ROS are one of the main oxidants in metabolites. Cyclic voltammetry (CV) showed that electron transfer of quinone also mediated Sb(III) oxidation. Part of Sb(V) was scavenged by the formation of the secondary Sb(V)-bearing mineral mopungite [NaSb(OH)6] in the incubation system. Our study demonstrates the microbial role of oxidation detoxification and mineralization of Sb and provides scientific references for the biochemical remediation of Sb-contaminated soil.

Environmental implications of superoxide radicals: From natural processes to engineering applications

The roles of superoxide radical (O2•-) in the domains of physiological, physical, and material chemistry are becoming increasingly recognized. Although extensive efforts have been directed to understand O2•- functions in diverse aquatic systems, there is a lack of systematic and in-depth review for its kinetics and mechanisms in various environmental scenarios. This review aims to bridge this gap through discussion of O2•- generation pathways under both natural and controlled conditions. The merits and limitations of the generation and detection methods under various conditions are compared, with emphasis on different approaches for the determination of O2•--triggered reaction kinetics. We summarize the reaction rate constants of O2•- with organic contaminants covering a wide diversity of structures and reactivity. The comparison indicates that O2•- exhibits weak reactivity with most contaminants and lacks selectivity towards compounds with different functional groups, except with quinones which exhibit higher reactivity compared to non-quinones. Further, the reaction mechanisms, namely single electron transfer, nucleophilic substitution, hydrogen atom abstraction, and radical-adduct formation, are critically evaluated. Various environmental implications of O2•- are highlighted including maintenance of biogeochemical iron cycle, synthesis of nanoparticles for antibacterial purposes, desorption of contaminants from heterogeneous interfaces, and synergetic degradation of contaminants.

AQDS-functionalized biochar enhances the bioreduction of Cr(VI) by Shewanella putrefaciens CN32

The bioreduction of toxic chromium(VI) to sparingly soluble chromium(III) represents an environmentally friendly and cost-effective method for remediating Cr contamination. Usually, this bioreduction process is slow and requires the addition of quinone compounds as electron shuttles to enhance the reaction rate. However, the dissolved quinone compounds are susceptible to loss with water flow, thereby limiting their effectiveness. To address this challenge, this study loaded anthraquinone-2,6-disulfonate (AQDS), a typical quinone compound, onto biochar (BC) to create a novel solid-phase electron mediator (BC-AQDS) that can sustainably promote Cr(VI) bioreduction. The experimental results demonstrated that BC-AQDS significantly promoted the bioreduction of Cr(VI), where the reaction rate constant increased by 4.81 times, and the reduction extent increased by 38.31%. X-ray photoelectron spectroscopy and Fourier-Transform Infrared Spectroscopy analysis revealed that AQDS replaced the -OH functional groups on the BC surface to form BC-AQDS. Upon receiving electrons from Shewanella putrefaciens CN32, BC-AQDS was reduced to BC-AH2DS, which subsequently facilitated the reduction of Cr(VI) to Cr(III). This redox cycle between BC-AQDS and BC-AH2DS effectively enhanced the bioreduction rate of Cr(VI). Our study also found that a lower carbonization temperature of BC resulted in a higher surface -OH functional group content, enabling a greater load of AQDS and a more pronounced enhancement effect on the bioreduction of Cr(VI). Additionally, a smaller particle size of BC and a higher dosage of BC-AQDS further contributed to the enhancement of Cr(VI) bioreduction. The preparation of BC-AQDS in this study effectively improve the utilization of quinone compounds and offer a promising approach for enhancing the bioreduction of Cr(VI). It provides a more comprehensive reference for understanding and solving the problem of Cr pollution in groundwater.

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.

Enriching electroactive microorganisms from ferruginous lake waters - Mind the sulfate reducers!

Electroactive microorganisms are pivotal players in mineral transformation within redox interfaces characterized by pronounced oxygen and dissolved metal gradients. Yet, their systematic cultivation from such environments remains elusive. Here, we conducted an anodic enrichment using anoxic ferruginous waters from a post-mining lake as inoculum. Weak electrogenicity (j = ∼5 µA cm-2) depended on electroactive planktonic cells rather than anodic biofilms, with a preference for formate as electron donor. Addition of yeast extract decreased the lag phase but did not increase current densities. The enriched bacterial community varied depending on the substrate composition but mainly comprised of sulfate- and nitrate-reducing bacteria (e.g., Desulfatomaculum spp. and Stenotrophomonas spp.). A secondary enrichment strategy resulted in different bacterial communities composed of iron-reducing (e.g., Klebsiella spp.) and fermentative bacteria (e.g., Paeniclostridium spp.). Secondary electron microscopy and energy-dispersive X-ray spectroscopy results indicate the precipitation of sulfur- and iron-rich organomineral aggregates at the anode surface, presumably impeding current production. Our findings indicate that (i) anoxic waters containing geogenically derived metals can be used to enrich weak electricigens, and (ii) it is necessary to specifically inhibit sulfate reducers. Otherwise, sulfate reducers tend to dominate over EAM during cultivation, which can lead to anode passivation due to biomineralization.

Thermodynamic considerations on the combined effect of electron shuttles and iron(III)-bearing clay mineral on Cr(VI) reduction by Shewanella oneidensis MR-1

Electron shuttles (ESs) and Fe-bearing clay minerals are commonly found in subsurface environments and have shown potential in enhancing the bioreduction of Cr(VI). However, the synergistic effect of ESs at different redox potentials and Fe-bearing clay minerals on Cr(VI) bioreduction, as well as the fundamental principles governing this process, remain unclear. In our study, we investigated the role of ESs and Fe(III) in Cr(VI) bioreduction. We found that the acceleration of ESs and Fe(III) are crucial factors in this process. Interestingly, the promotion of ESs on Cr(VI) and Fe(III) showed opposite trends. Electrochemical methods confirmed the limited steps are the extent of reduced ESs and the redox potential difference between ESs and Fe(III), separately. Furthermore, we investigated the combined effect of ESs and NAu-2 on Cr(VI) bioreduction. Our results revealed two segments: in the first segment, the ES (5-HNQ) and NAu-2 did not synergistically enhance Cr(VI) reduction. However, in the second segment, ESs and NAu-2 demonstrated a synergistic effect, significantly increasing Cr(VI) reduction by MR-1. These bioreduction processes all follow linear free energy relationships (LFERs). Overall, our study highlights the fundamental principles governing multivariate systems and presents a promising approach for the remediation of Cr(VI)-contaminated sites.

Quantification of Quinones in Environmental Media by Chemical Tagging with Cysteine-Containing Peptides Coupled to Size Exclusionary Separation

Quinones are one of the most important redox-reactive organic compounds in natural environments, such as soil, water, and sediment, playing an important role in regulating the environmental processes and biogeochemical cycles of critical elements under climate change, including the influences of extreme events such as wildfires. However, to date, no existing methods can quantify quinones in complex environmental media. To overcome this challenge, a quantification method was developed by coupling chemical tagging of quinones by cysteine-containing nonaromatic peptides (Cpep) through a Michael addition reaction with size exclusionary chromatography (SEC) separation and ultraviolet (UV) analysis─leveraging on the characteristic absorbance of aromatic rings at 254 nm and molecular size of peptide. The method was demonstrated using model quinones, including 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ), and 1,4-anthraquinone (AQ), with a detection limit of 3.3, 0.7, and 0.2 μM, respectively. Concentrations of quinones in water extractions of biochars, soils, and wildfire-derived ashes were determined to range from 0.8 to 14 μM and were positively correlated with their redox reactivity determined by a chemical assay. This method provides a novel rapid quantification of quinones in complex environmental media as well as a quick assessment for redox reactivity and opens up new avenues for studying environmental transformation and remediation of contaminants.

Enhanced microbial reduction of Cr(VI) in soil with biochar acting as an electron shuttle: Crucial role of redox-active moieties

Biochar has been proven to participate in the biotic reduction of hexavalent chromium (Cr(VI)) in environment since its involvement may accelerate the extracellular electron transfer (EET). However, roles of the redox-active moieties and the conjugated carbon structure of biochar in this EET process remain unclear. In this study, 350 °C and 700 °C were selected to produce biochar with more O-containing moieties (BC350) or more developed conjugated structures (BC700), and their performances in the microbial reduction of soil Cr(VI) were investigated. Our results showed that BC350 presented a 241% increase of Cr(VI) microbial reduction after 7-day incubation, much higher than that of BC700 (39%), suggesting that O-containing moieties might play more important roles in accelerating the EET process. Biochar, especially BC350 could serve as an electron donor for microbial anaerobic respiration, but its contribution (73.2%) as an electron shuttle for EET was dominant to the enhanced Cr(VI) reduction. The positive correlation between electron exchange capacities (EECs) of pristine and modified biochars and the corresponding maximum reduction rates of Cr(VI) evidenced the crucial role of redox-active moieties in electron shuttling. Moreover, EPR analysis suggested the nonnegligible contribution of semiquinone radicals in biochars to the accelerated EET process. This study demonstrates the crucial role of redox-active moieties, i.e., O-containing moieties in mediating the EET process during the microbial reduction of Cr(VI) in soil. Findings obtained will advance the current understanding of biochar as an electron shuttle participating in the biogeochemical processes of Cr(VI).

Exogenous Electroactive Microbes Regulate Soil Geochemical Properties and Microbial Communities by Enhancing the Reduction and Transformation of Fe(III) Minerals

Electroactive microbes can conduct extracellular electron transfer and have the potential to be applied as a bioresource to regulate soil geochemical properties and microbial communities. In this study, we incubated Fe-limited and Fe-enriched farmland soil together with electroactive microbes for 30 days; both soils were incubated with electroactive microbes and a common iron mineral, ferrihydrite. Our results indicated that the exogenous electroactive microbes decreased soil pH, total organic carbon (TOC), and total nitrogen (TN) but increased soil conductivity and promoted Fe(III) reduction. The addition of electroactive microbes also changed the soil microbial community from Firmicutes-dominated to Proteobacteria-dominated. Moreover, the total number of detected microbial species in the soil decreased from over 700 to less than 500. Importantly, the coexistence of N-transforming bacteria, Fe(III)-reducing bacteria and methanogens was also observed with the addition of electroactive microbes in Fe-rich soil, indicating the accelerated interspecies electron transfer of functional microflora.

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.

The Differing Roles of Flavins and Quinones in Extracellular Electron Transfer in Lactiplantibacillus plantarum

Lactiplantibacillus plantarum is a lactic acid bacterium that is commonly found in the human gut and fermented food products. Despite its overwhelmingly fermentative metabolism, this microbe can perform extracellular electron transfer (EET) when provided with an exogenous quinone, 1,4-dihydroxy-2-naphthoic acid (DHNA), and riboflavin. However, the separate roles of DHNA and riboflavin in EET in L. plantarum have remained unclear. Here, we seek to understand the role of quinones and flavins in EET by monitoring iron and anode reduction in the presence and absence of these small molecules. We found that addition of either DHNA or riboflavin can support robust iron reduction, indicating electron transfer to extracellular iron occurs through both flavin-dependent and DHNA-dependent routes. Using genetic mutants of L. plantarum, we found that flavin-dependent iron reduction requires Ndh2 and EetA, while DHNA-dependent iron reduction largely relies on Ndh2 and PplA. In contrast to iron reduction, DHNA-containing medium supported more robust anode reduction than riboflavin-containing medium, suggesting electron transfer to an anode proceeds most efficiently through the DHNA-dependent pathway. Furthermore, we found that flavin-dependent anode reduction requires EetA, Ndh2, and PplA, while DHNA-dependent anode reduction requires Ndh2 and PplA. Taken together, we identify multiple EET routes utilized by L. plantarum and show that the EET route depends on access to environmental biomolecules and on the electron acceptor. This work expands our molecular-level understanding of EET in Gram-positive microbes and provides additional opportunities to manipulate EET for biotechnology. IMPORTANCE Lactic acid bacteria are named because of their nearly exclusive fermentative metabolism. Thus, the recent observation of EET activity-typically associated with anaerobic respiration-in this class of organisms has forced researchers to rethink the rules governing microbial metabolic strategies. Our identification of multiple routes for EET in L. plantarum that depend on two different redox active small molecules expands our understanding of how microbes metabolically adapt to different environments to gain an energetic edge and how these processes can be manipulated for biotechnological uses. Understanding the role of EET in lactic acid bacteria is of great importance due to the significance of lactic acid bacteria in agriculture, bioremediation, food production, and gut health. Furthermore, the maintenance of multiple EET routes speaks to the importance of this process to function under a variety of environmental conditions.

Reduction capacity of humic acid and its association with the evolution of redox structures during composting

The reducing capacity (RC) of compost-derived humic acid (HA) is related to the type and number of redox-active functional moieties in its structure and has a considerable environmental influence on its geochemical redox cycle. Composting treatment can affect the redox-active fractions of organic substances through microbial transformation and degradation. However, the relationship between the RC of compost-derived HA and its fluorescence component and infrared spectra remains unclear. In this study, we assessed the response of the organic reducing capacity (ORC) and inorganic reducing capacity (IRC) of compost-derived HA to the stabilization of organic solid waste materials by analyzing the redox-active functional groups of HA extracted at different composting times. The results demonstrated that the RC of compost-derived HA continuously increased during composting because of the formation of fulvic- and humic-like fluorescent components, which consist of amide, phenolic hydroxyl, quinone, and aromatic groups. Adsorption occurred between HA and FeCit by aliphatic and out-of-plane aromatic CH, which released free hydrogen and increased the Fe-binding site; consequently, ORC was obviously higher than IRC. The results of this study could provide an understanding of the transformation of the fluorescent substances and functional groups that affect redox properties during composting; therefore, this study has considerable significance for exploring the application of compost products.

Defined and unknown roles of conductive nanoparticles for the enhancement of microbial current generation: A review

The ability of various bacteria to make use of solid substrates through extracellular electron transfer (EET) or extracellular electron uptake (EEU) has enabled the development of valuable biotechnologies such as microbial fuel cells (MFCs) and microbial electrosynthesis (MES). It is common practice to use metallic and semiconductive nanoparticles (NPs) for microbial current enhancement. However, the effect of NPs is highly variable between systems, and there is no clear guideline for effectively increasing the current generation. In the present review, the proposed mechanisms for enhancing current production in MFCs and MES are summarized, and the critical factors for NPs to enhance microbial current generation are discussed. Implications for microbially induced iron corrosion, where iron sulfide NPs are proposed to enhance the rate of EEU, photochemically driven MES, and several future research directions to further enhance microbial current generation, are also discussed.

Sb(III) resistance mechanism and oxidation characteristics of Klebsiella aerogenes X

Resistant bacteria are potential natural materials for the bioremediation of soil metalloid pollution. A strain isolated from farmland soil chronically exposed to Sb was identified as K. aerogenes X with high antimonite [Sb(III)] tolerance and oxidation ability. The resistance mechanism of K. aerogenes X and its extracellular polymeric substances (EPS), antioxidant enzymes, and oxidation characteristics in Sb(III) stress were investigated in this study by stress incubation experiments and FTIR. The biotoxicity of Sb was limited by the binding of the organic compounds in EPS, and the anionic functional groups (e.g., amino, carboxyl and hydroxyl groups, etc.) present in the cell envelope were the components primarily responsible for the metalloid-binding capability of K. aerogenes X. The K. aerogenes X can oxidize Sb(III), and its metabolites induce changes in reactive oxygen species (ROS), catalase (CAT), total superoxide dismutase (SOD) and glutathione s-transferase (GSH-S) activity, indicating that the resistance mechanisms of K. aerogenes X are mediated by oxidative stress, EPS restriction and cell damage. Oxidation of Sb(III) is driven by interactions in intracellular oxidation, cell electron transport, extracellular metabolism including proteins and low molecular weight components (LMWs). LMWs (molecular weight <3 kDa) are the main driving factor of Sb(III) oxidation. In addition, Sb resistance genes arsA, arsB, arsC, arsD and acr3 and potential oxidation gene arsH were identified in K. aerogenes X. Owing to its natural origin, high tolerance and oxidation ability, K. aerogenes X could serve as a potential bioremediation material for the mitigation of Sb(III) in contaminated areas.

Dielectric constants of organic pollutants determine their strength for enhancing microbial iron reduction

Physicochemical properties are essential characteristics of organic compounds, which not only impact the fate of organic pollutants but also determine their application in biological processes. Here, we first found that the dielectric constants (ɛ) of organic pollutants negatively correlated to their strength for enhancing microbial Fe(III) reduction. Those with lower ɛ values than 2.61 potentially promoted the above process following the sequence carbon tetrachloride (CT) > benzene > toluene > tetrachloroethylene (PCE) due to their different ability to deprotonate the phosphorus-related groups on the outer cell membrane of iron-reducing bacteria Shewanella oneidensis MR-1 (MR-1). The stronger deprotonation of phosphorus-related groups induced more negative charge of cell surface and more strongly increased cell membrane permeability and consequently stimulated faster release of flavin mononucleotide (FMN) as an electron shuttle/cofactor for Fe(III) reduction. These findings are significant for understanding the biogeochemistry in multi-organic contaminated subsurface and providing knowledge for remediation strategies and current production.

Carbon nanotubes mediating nano α-FeOOH reduction by Shewanella putrefaciens CN32 to enhance tetrabromobisphenol A removal

Carbon nanotubes (CNTs) mediation of the reduction of nano goethite (α-FeOOH) by Shewanella putrefaciens CN32 to improve the removal efficiency of tetrabromobisphenol A (TBBPA) was investigated in this study. The results showed that CNTs effectively promoted the biological reduction of α-FeOOH by strengthening the electron transfer process between Shewanella putrefaciens CN32 and α-FeOOH. After α-FeOOH was reduced to Fe(II), the adsorbed Fe(II) accounted for approximately 69.07% of the total Fe(II). And the secondary mineral vivianite was formed during the reduction of α-FeOOH, which was determined by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The vivianite (Fe^II₃(PO₄)₂·8H₂O) was formed by the reaction of PO₄^3- and Fe(II). The degradation effect of TBBPA was the best at pH 8. CNT-α-FeOOH reduced the toxicity of TBBPA to CN32 and had good stability and reusability. The byproduct bisphenol A was detected which indicated that the degradation pathway of TBBPA involved reductive debromination. Electrochemical experiments and EPR analysis showed that the electron transfer capacities (ETC) of CNTs was an important factor in the removal of TBBPA, and it may greatly depend on semiquinone radicals (CO). This study provided a new method and theoretical support for the removal of TBBPA in the environment.

Current production by non-methanotrophic bacteria enriched from an anaerobic methane-oxidizing microbial community

In recent years, the externalization of electrons as part of respiratory metabolic processes has been discovered in many different bacteria and some archaea. Microbial extracellular electron transfer (EET) plays an important role in many anoxic natural or engineered ecosystems. In this study, an anaerobic methane-converting microbial community was investigated with regard to its potential to perform EET. At this point, it is not well-known if or how EET confers a competitive advantage to certain species in methane-converting communities. EET was investigated in a two-chamber electrochemical system, sparged with methane and with an applied potential of +400 mV versus standard hydrogen electrode. A biofilm developed on the working electrode and stable low-density current was produced, confirming that EET indeed did occur. The appearance and presence of redox centers at −140 to −160 mV and at −230 mV in the biofilm was confirmed by cyclic voltammetry scans. Metagenomic analysis and fluorescence in situ hybridization of the biofilm showed that the anaerobic methanotroph ‘Candidatus Methanoperedens BLZ2’ was a significant member of the biofilm community, but its relative abundance did not increase compared to the inoculum. On the contrary, the relative abundance of other members of the microbial community significantly increased (up to 720-fold, 7.2% of mapped reads), placing these microorganisms among the dominant species in the bioanode community. This group included Zoogloea sp., Dechloromonas sp., two members of the Bacteroidetes phylum, and the spirochete Leptonema sp. Genes encoding proteins putatively involved in EET were identified in Zoogloea sp., Dechloromonas sp. and one member of the Bacteroidetes phylum. We suggest that instead of methane, alternative carbon sources such as acetate were the substrate for EET. Hence, EET in a methane-driven chemolithoautotrophic microbial community seems a complex process in which interactions within the microbial community are driving extracellular electron transfer to the electrode.

Effects of biofilm transfer and electron mediators transfer on Klebsiella quasipneumoniae sp. 203 electricity generation performance in MFCs

BACKGROUND

Extracellular electron transfer (EET) is essential in improving the power generation performance of electrochemically active bacteria (EAB) in microbial fuel cells (MFCs). Currently, the EET mechanisms of dissimilatory metal-reducing (DMR) model bacteria Shewanella oneidensis and Geobacter sulfurreducens have been thoroughly studied. Klebsiella has also been proved to be an EAB capable of EET, but the EET mechanism has not been perfected. This study investigated the effects of biofilm transfer and electron mediators transfer on Klebsiella quasipneumoniae sp. 203 electricity generation performance in MFCs.

RESULTS

Herein, we covered the anode of MFC with a layer of microfiltration membrane to block the effect of the biofilm mechanism, and then explore the EET of the electron mediator mechanism of K. quasipneumoniae sp. 203 and electricity generation performance. In the absence of short-range electron transfer, we found that K. quasipneumoniae sp. 203 can still produce a certain power generation performance, and coated-MFC reached 40.26 mW/m2 at a current density of 770.9 mA/m2, whereas the uncoated-MFC reached 90.69 mW/m2 at a current density of 1224.49 mA/m2. The difference in the electricity generation performance between coated-MFC and uncoated-MFC was probably due to the microfiltration membrane covered in anode, which inhibited the growth of EAB on the anode. Therefore, we speculated that K. quasipneumoniae sp. 203 can also perform EET through the biofilm mechanism. The protein content, the integrity of biofilm and the biofilm activity all proved that the difference in the electricity generation performance between coated-MFC and uncoated-MFC was due to the extremely little biomass of the anode biofilm. To further verify the effect of electron mediators on electricity generation performance of MFCs, 10 µM 2,6-DTBBQ, 2,6-DTBHQ and DHNA were added to coated-MFC and uncoated-MFC. Combining the time-voltage curve and CV curve, we found that 2,6-DTBBQ and 2,6-DTBHQ had high electrocatalytic activity toward the redox reaction of K. quasipneumoniae sp. 203-inoculated MFCs. It was also speculated that K. quasipneumoniae sp. 203 produced 2,6-DTBHQ and 2,6-DTBBQ.

CONCLUSIONS

To the best of our knowledge, the three modes of EET did not exist separately. K. quasipneumoniae sp.203 will adopt the corresponding electron transfer mode or multiple ways to realize EET according to the living environment to improve electricity generation performance.

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.

Humification in extracellular polymeric substances (EPS) dominates methane release and EPS reconstruction during the sludge stabilization of high-solid anaerobic digestion

High-solid anaerobic digestion (HAD) can directly treat dewatered sewage sludge (total solid content ≥15%) with superior volume efficiency. Sludge stabilization during HAD is expected to achieve by throughout organic degradation and conversion towards methane-rich biogas release and humic formation. Sewage sludge is the combination of microbial zoogleas and theirs adsorption of organic and inorganic matter, in which the extracellular polymeric substances (EPS) account 60-80% of total sludge organic matter, inevitably participating most extracellular metabolic pathways. The interactions between EPS transformation and genetically annotated metabolic pathways were found in this research. In brief, noticing the highly cross-linked structures in EPS with major active components of humic substances (HS) and protein (PN), as PN hydrolysis and decomposition in EPS were enhanced in the high-solid anaerobic condition, the exposure of aromatic groups and sites in HS were considerable. HS release was the main factor shifting the electron exchange capacity and activity, which aided in energy metabolism of sludge microorganisms involved in redox reactions, especially the methanogenesis, thus in turn facilitating the PN degradation; Then, the screened humic groups and active protein derives might act as the beneficial precursors to regenerate neo-humic structures, whose significant bridging effect and signal role on stimulating amino acid biosynthesis, member transport and metallic complexation could further contribute to proteolytic condensation and EPS reconstruction. Hence, the in-depth sludge stabilization mechanism during HAD process was established for developing enlightening strategies.

Response of nitrogen removal performance, functional genes abundances and N-acyl-homoserine lactones release to carminic acid of anammox biomass

Carminic acid (CA) can serve as a redox mediator and influence the electron transfer process. CA dosages of 0-5 mg/L were added to anaerobic ammonia oxidation (anammox) biomass. The results illustrated that CA not only reduced the inorganic nitrogen removal efficiency, but also decreased the nitrogen removal rate. The deterioration of nitrogen removal performance was due to the excess production of nitrate-nitrogen. The concentration of extracellular polymeric substances showed a decrease together with a decline in N-acyl-homoserine lactones release. CA addition decreased the activity of anammox bacteria while increasing the nitrifying potential. Quantitative reverse transcription polymerase chain reaction showed a decrease in anammox functional genes (nirS, hzo, and hzsB) and promotion of the expression of the nxrB gene, which corresponded with a decrease in anammox bacteria activity and the improvement of nitrifying potential. As a result, CA should not be added to anammox biomass.

Electrocatalysis via Intrinsic Surface Quinones Mediating Electron Transfer to and from Carbon Electrodes

The electrochemistry of the Fe²⁺/Fe³⁺ redox couple in aqueous solution at carbon electrodes is shown to be catalyzed by surface quinone groups intrinsically present in the carbon surface. Such mediation has long been speculated but hitherto unproven.

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.

Spatiotemporal evolvement and factors influencing natural and synthetic EDCs and the microbial community at different groundwater depths in the Chaobai watershed: A long-term field study on a river receiving reclaimed water

In this long-term field study, to restore a dried river ecosystem, reclaimed water was used as a supplementary water source. The main aim of this study was to investigate the accumulation and migration potential of EDCs in groundwater during long-term utilization of reclaimed water and the changes in microbial community during the removal of EDCs. A long-term field study was conducted in order to ascertain the temporal and spatial distribution of four selected endocrine-disrupting chemicals (EDCs) in an underground aquifer in the Chaobai watershed, where reclaimed water is the primary water source. Anew, the microbial community structure at different groundwater depths, along with related environmental factors were also determined. Based on the results obtained from this long-term study, it was found that the EDCs in the surface water of the Chaobai river have entered a depth of 80 m in the groundwater aquifers, within a distance of 360 m from the river. The vertical profiles of the concentrations of bisphenol A (BPA), 4-nonylphenol (NP), estrone (E1), and estriol (E3) decreased significantly from the surface to different groundwater depths with first-order attenuation rates of 0.0416, 0.0343, 0.0498, and 0.0173 m^-1. The aquifer depth, water temperature, conductivity, and coexisting anions correlated well with the distribution of EDCs in groundwater.

Effects and mechanism of riboflavin on the growth of Alcaligenes faecalis under bias conditions

Some microorganisms can utilize photoelectrons and electrode electrons. Exogenous electrons generate enough energy for growth, and electron shuttles may accelerate this process. This research data supported photoelectron-responsive microorganism Alcaligenes faecalis was effected by the growth metabolism due to bias and electron shuttle riboflavin (RF) with an adaptive screening voltage under oligotrophic conditions. A slight change was observed in the redox property of RF. RF played the role of an electron shuttle. Microbial extracellular metabolites could bind additional nicotinamide adenine dinucleotide (NADH) species with RF. The intracellular protein content in the group of RF-Bias was 1.94, 1.93 and 4.02 times higher than those in the RF, bias and control groups, respectively, while the corresponding intracellular contents of humus were 1.10, 0.93 and 1.42 times higher. The content of CoA in RF-Bias, RF and bias increased to 116.0%, 108.5% and 103.8%, respectively. The organic acids of the RF-Bias group in the Krebs cycle are more advanced than those of other groups. Overall, in the Krebs cycle, RF and bias facilitated the growth and metabolism of A. faecalis. Finally, a mechanism was proposed, showing that the electron transfer chain and the Krebs cycle are stimulated by RF and bias.

Biogeochemical fate of ferrihydrite-model organic compound complexes during anaerobic microbial reduction

Associations of organic carbon (OC) with iron (Fe) oxide minerals play an important role in regulating the stability of OC in soil environments. Knowledge about the fate and stability of Fe-OC complexes is impaired by the heterogeneity of OC. Additional biogeochemical variables in soil environments, such as redox conditions and microbes, further increase complexity in understanding the stability of mineral-associated soil OC. This study investigated the fate and stability of model organic compounds, including glucose (GL), glucosamine (GN), tyrosine (TN), benzoquinone (BQ), amylose (AM), and alginate (AL), complexed with an Fe oxide mineral, ferrihydrite (Fh), during microbial reduction. During a 25-d anaerobic incubation with Shewanella putrefaciens CN32, the reduction of Fe followed the order of Fh-BQ > Fh-GL > Fh-GN > Fh-TN > Fh-AL > Fh-AM. In terms of OC released during the anaerobic incubation, Fh-GN complexes released the highest amount of OC while Fh-AM complexes released the lowest. Organic carbon regulated the reduction of Fe by acting as an electron shuttle, affecting microbial activities, and associating with Fh. Benzoquinone had the highest electron accepting capacity, but potentially can inhibit microbial activity. These findings provide insights into the roles of different organic functional groups in regulating Fe reduction and the stability of Fh-bound OC under anaerobic conditions.

Bioremediation of Cr (VI) contaminated groundwater by Geobacter sulfurreducens: Environmental factors and electron transfer flow studies

In this study, the removal of Cr (VI) was examined in the presence of bio-produced Fe (II) from hematite, sulfate and dissolved organic matter by Geobacter sulfurreducens. The adaptation results of G. sulfurreducens showed that cells growth was stimulated up to 576 μM of Cr (VI) concentration. The first-order rate and electron transfer rate in each step during Cr (VI) reduction by G. sulfurreducens in the presence of hematite was clearly modeled and calculated. For Cr (VI) reduction rate, both separately dissolved and adsorbed bio-produced Fe (II) were faster than G. sulfurreducens although bio-produced Fe (II) contributed only 20% to total Cr (VI) removal in a combined system containing Cr (VI), hematite and G. sulfurreducens. The electron transfer rate from G. sulfurreducens to hematite (R₂) to produce Fe (II) was a limited step and electron transfer rate from acetate to Cr (VI) (1.8 μeq L^-1 h^-1) by G. sulfurreducens was much higher than that to hematite (0.272 μeq L^-1 h^-1, producing Fe (II)). Cr (VI) reduction was enhanced in the presence of SO₄^2- due to sulfate boost cells growth. AQDS enhanced Cr (VI) reduction by serving as an electron shuttle thus accelerating the electron transfer rate.

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.

No evidential correlation between veterinary antibiotic degradation ability and resistance genes in microorganisms during the biodegradation of doxycycline

Biodegradation of antibiotic residues in the environment by microorganisms may lead to the generation of antibiotic resistance genes (ARGs), which are of great concern to human health. The aim of this study was to determine whether there is a relationship between the ability to degrade antibiotic doxycycline (DOX) and the development of resistance genes in microorganisms. We isolated and identified ten bacterial strains from a vegetable field that had received long-term manure application as fertilizer and were capable of surviving in a series of DOX concentrations (25, 50, 80, and 100mg/L). Our results showed no evidential correlation between DOX degradation ability and the development of resistance genes among the isolated microorganisms that had high DOX degradation capability (P > 0.05). This was based on the fact that Escherichia sp. and Candida sp. were the most efficient bacterial strains to degrade DOX (92.52% and 91.63%, respectively), but their tetracycline resistance genes showed a relatively low risk of antibiotic resistance in a 7-day experiment. Moreover, the tetM of the ribosomal protection protein genes carried by these two preponderant bacteria was five-fold higher than that carried by other isolates (P < 0.05). Pearson correlations between the C_t/C₀ of DOX and tet resistance genes of three isolates, except for Escherichia sp. and Candida sp., showed remarkable negative correlations (P < 0.05), mainly because tetG markedly increased during the DOX degradation process. Our results concluded that the biodegradation of antibiotic residues may not necessarily lead to the development of ARGs in the environment. In addition, the two bacteria that we isolated, namely, Escherichia sp. and Candida sp., are potential candidates for the engineering of environmentally friendly bacteria.

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.

The Functional Mechanisms and Application of Electron Shuttles in Extracellular Electron Transfer

Electron shuttles extensively exist in various environments. Some kinds of organic substances can be applied by microorganisms to produce electrons, and then the electrons can be transferred to other substances or microorganisms through electron shuttles, resulting in coexistence and interaction of diverse species of microbes. In this review, the functional mechanisms of extracellular electron transfer mediated by different electron shuttles are described. And different subtypes as well as the application of electron shuttles in microbial degradation of pollutants, microbial electricity, and the promotion of energy generation are also discussed. Summary results show that extracellular electron transfer is based on the electrogenesis microorganism with the structure of cytochromes or pili. Materials were usually used in long-distance electron transfer because of their widespread presence and abundance. Therefore, the review is beneficial to perceive the pathways of extracellular electron transfer mediated by electron shuttles and explore the contribution of different electron shuttles in extracellular electron transfer.

The characteristics and two-step reaction model of p-nitroacetophenone biodegradation mediated by Shewanella decolorationis S12 and electron shuttle in the presence/absence of goethite

The current study mainly focused on the biodegradation process of p-nitroacetophenone (NP) in the presence and absence of goethite mediated by iron-reducing microbe (Shewanella decolorationis S12) and electron shuttle. The results showed that introduction of electron shuttle could obviously lead to an accumulation of biodegradation intermediate, especially in reaction systems containing high content of electron shuttle in the absence of goethite. Goethite could enhance the degree and rate of NP biodegradation. The microbial reductively generated Fe(II) played an active role in the biodegradation process. The relationship between the concentrations of biodegradation end product and the reaction times could be fitted by a consecutive reaction model with correlation coefficients (adjusted R(2)) in the range from 0.9241 to 0.9831 during the biodegradation stage from the beginning to about 250 h of incubation. However, during the subsequent biodegradation stages, in the presence and absence of goethite, transitions from the consecutive reaction model to zero-order reaction model and from the consecutive reaction model to exponential growth reaction model were observed, respectively. The newly proposed two-step reaction model will help understand the mechanism of the biodegradation process of nitroaromatic compounds and related pollutants.

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

Despite the importance of exogenous electron shuttles (ESs) in extracellular electron transfer (EET), a lack of understanding of the key properties of ESs is a concern given their different influences on EET processes. Here, the ES-mediated EET capacity of Shewanella putrefaciens 200 (SP200) was evaluated by examining the electricity generated in a microbial fuel cell. The results indicated that all the ESs substantially accelerated the current generation compared to only SP200. The current and polarization parameters were linearly correlated with both the standard redox potential (E(ES)(0)) and the electron accepting capacity (EAC) of the ESs. A thermodynamic analysis of the electron transfer from the electron donor to the electrode suggested that the EET from c-type cytochromes (c-Cyts) to ESs is a crucial step causing the differences in EET capacities among various ESs. Based on the derived equations, both E(ES)(0) and EAC can quantitatively determine potential losses (ΔE) that reflect the potential loss of the ES-mediated EET. In situ spectral kinetic analysis of ES reduction by c-Cyts in a living SP200 suspension was first investigated with the E(ES), E(c-Cyt), and ΔE values being calculated. This study can provide a comprehensive understanding of the role of ESs in EET.

Sulfur species as redox partners and electron shuttles for ferrihydrite reduction by Sulfurospirillum deleyianum

Iron(III) (oxyhydr)oxides can represent the dominant microbial electron acceptors under anoxic conditions in many aquatic environments, which makes understanding the mechanisms and processes regulating their dissolution and transformation particularly important. In a previous laboratory-based study, it has been shown that 0.05 mM thiosulfate can reduce 6 mM ferrihydrite indirectly via enzymatic reduction of thiosulfate to sulfide by the sulfur-reducing bacterium Sulfurospirillum deleyianum, followed by abiotic reduction of ferrihydrite coupled to reoxidation of sulfide. Thiosulfate, elemental sulfur, and polysulfides were proposed as reoxidized sulfur species functioning as electron shuttles. However, the exact electron transfer pathway remained unknown. Here, we present a detailed analysis of the sulfur species involved. Apart from thiosulfate, substoichiometric amounts of sulfite, tetrathionate, sulfide, or polysulfides also initiated ferrihydrite reduction. The portion of thiosulfate produced during abiotic ferrihydrite-dependent reoxidation of sulfide was about 10% of the total sulfur at maximum. The main abiotic oxidation product was elemental sulfur attached to the iron mineral surface, which indicates that direct contact between microorganisms and ferrihydrite is necessary to maintain the iron reduction process. Polysulfides were not detected in the liquid phase. Minor amounts were found associated either with microorganisms or the mineral phase. The abiotic oxidation of sulfide in the reaction with ferrihydrite was identified as rate determining. Cysteine, added as a sulfur source and a reducing agent, also led to abiotic ferrihydrite reduction and therefore should be eliminated when sulfur redox reactions are investigated. Overall, we could demonstrate the large impact of intermediate sulfur species on biogeochemical iron transformations.