A hotspot of diversity: novel Shewanella species isolated from Baltic Sea sediments delineate a sympatric species complex.

Two bacterial strains, SP1S1-4T and SP2S1-2T, were isolated from sediment samples collected in the Stockholm archipelago in November 2021. Following whole-genome sequencing, these strains were identified as tentatively belonging to two novel Shewanella genospecies, based on digital DNA-DNA hybridization, as implemented in the Type Strain Genome Server. Shewanella septentrionalis, Shewanella baltica and Shewanella hafniensis were, in this order and within a narrow genomic relatedness range, their closest genotypic relatives. Additional sampling and sequencing efforts led to the retrieval of distinct isolates that were monophyletic with SP1S1-4T and SP2S1-2T, respectively, based on phylogenomic analysis of whole-genome sequences. Comparative analyses of genome sequence data, which included blast-based average nucleotide identity, core genome-based and core proteome-based phylogenomics, in addition to MALDI-TOF MS-based protein profiling, confirmed the distinctness of the putative novel genospecies with respect to their closest genotypic relatives. A comprehensive phenotypic characterisation of SP1S1-4T and SP2S1-2T revealed only minor differences with respect to the type strains of S. septentrionalis, S. baltica and S. hafniensis. Based on the collective phylogenomic, proteomic, and phenotypic evidence presented here, we describe two novel genospecies within the genus Shewanella, for which the names Shewanella scandinavica sp. nov. and Shewanella vaxholmensis sp. nov. are proposed. The type strains are, respectively, SP2S1-2T (=CCUG 76457T=CECT 30688T), with a draft genome sequence of 5 041 805 bp and a G+C content of 46.3 mol%, and SP1S1-4T (=CCUG 76453T=CECT 30684T), with a draft genome sequence of 4 920147 bp and a G+C content of 46.0 mol%. Our findings suggest the existence of a species complex formed by the species S. baltica, S. septentrionalis, S. scandinavica sp. nov., and S. vaxholmensis sp. nov., with S. hafniensis falling in the periphery, where distinct genomic species clusters could be identified. However, this does not exclude the possibility of a continuum of genomic diversity within this sedimental ecosystem, as discussed herein with additional sequenced isolates.

Pneumonia in an elderly Tibetan male caused by Shewanella algae: A case report.

RATIONALE: Shewanella algae are Gram-negative bacteria that are widely found in aquatic habitats and rarely cause lung infections in inland areas. PATIENT CONCERNS: Cough with light-yellow phlegm for 2 weeks. DIAGNOSES: The final diagnosis was bacterial pneumonia. INTERVENTIONS: The patient was treated with ceftazidime (2 g, every 12 h) for 1 week. OUTCOMES: The patient's lung infection improved and he was discharged. LESSONS: This case highlights a rare occurrence of lung infection caused by Shewanella algae in elderly Tibetan men residing in non-marine environments.

Implications of ammonia stress for the pathogenicity of Shewanella spp. in Oreochromis niloticus: effects on hematological, biochemical, immunological, and histopathological parameters.

Environmental stressors (such as ammonia) in aquaculture could increase the risk of pathogenicity, posing a more severe threat to farmed fish. The aim of this study was to investigate the effects of ammonia stress on the pathogenicity of Shewanella spp. in Oreochromis niloticus. First, a 96-hour static test was used to determine the median lethal concentration (LC50) of unionized ammonia to Nile tilapia. After 96 h of exposure, the Un-ionized ammonia (UIA) LC50 was estimated to be 4.26 mg/L. Second, an experiment was conducted to test the effect of unionized ammonia stress on the pathogenicity of Shewanella spp. in O. niloticus for 30 days. A study involved 180 fish divided into six groups, with the first group serving as a control. The second group (AMN1/10) and the third group (AMN1/20) were not challenged and were exposed to 1/10 (0.42 mg/L) and 1/20 (0.21 mg/L) of the 96-hour LC50 of UIA, respectively. Then 0.2 mL (0.14 × 105) of Shewanella spp. was intraperitoneally injected into the fourth (SH), fifth (SH + AMN1/10), and sixth (SH + AMN1/20) groups, which were subjected to 0, 1/10 (0.42 mg/L), and 1/20 (0.21 mg/L) of the 96-hour LC50 of UIA, respectively. The survival rate, hematological indices, immunological parameters, and antioxidant activity of the fish significantly decreased when they were exposed to ammonia and Shewanella infection separately or together. Histopathological changes were also observed in the kidney and liver. Furthermore, both individual and combined exposures significantly altered renal and hepatic function, with notable increases in glucose and cortisol levels, as well as in the expression of proinflammatory cytokine genes (TNF-α and IL-1ß). However, the detrimental effects of co-exposure to ammonia stress and Shewanella infection were greater than those of separate exposures. As a result, we may say that increased ammonia concentrations enhance the infection of Shewanella spp. These findings could contribute to a better understanding of Shewanella infection in Nile tilapia.

Review of cathodic electroactive bacteria: Species, properties, applications and electron transfer mechanisms.

Cathodic electroactive bacteria (C-EAB) which are capable of accepting electrons from solid electrodes provide fresh avenues for pollutant removal, biosensor design, and electrosynthesis. This review systematically summarized the burgeoning applications of the C-EAB over the past decade, including 1) removal of nitrate, aromatic derivatives, and metal ions; 2) biosensing based on biocathode; 3) electrosynthesis of CH4, H2, organic carbon, NH3, and protein. In addition, the mechanisms of electron transfer by the C-EAB are also classified and summarized. Extracellular electron transfer and interspecies electron transfer have been introduced, and the electron transport mechanism of typical C-EAB, such as Shewanella oneidensis MR-1, has been combed in detail. By bringing to light this cutting-edge area of the C-EAB, this review aims to stimulate more interest and research on not only exploring great potential applications of these electron-accepting bacteria, but also developing steady and scalable processes harnessing biocathodes.

Single-walled Carbon Nanotubes Wrapped with Charged Polysaccharides Enhance Extracellular Electron Transfer.

Microbial electrochemical systems (MESs) rely on the microbes' ability to transfer charges from their anaerobic respiratory processes to electrodes through extracellular electron transfer (EET). To increase the generally low output signal in devices, advanced bioelectrical interfaces tend to augment this problem by attaching conducting nanoparticles, such as positively charged multiwalled carbon nanotubes (CNTs), to the base carbon electrode to electrostatically attract the negatively charged bacterial cell membrane. On the other hand, some reports point to the importance of the magnitude of the surface charge of functionalized single-walled CNTs (SWCNTs) as well as the size of functional groups for interaction with the cell membrane, rather than their polarity. To shed light on these phenomena, in this study, we prepared and characterized well-solubilized aqueous dispersions of SWCNTs functionalized by either positively or negatively charged cellulose-derivative polymers, as well as with positively charged or neutral small molecular surfactants, and tested the electrochemical performance of Shewanella oneidensis MR-1 in MESs in the presence of these functionalized SWCNTs. By simple injection into the MESs, the positively charged polymeric SWCNTs attached to the base carbon felt (CF) electrode, and as fluorescence microscopy revealed, allowed bacteria to attach to these structures. As a result, EET currents continuously increased over several days of monitoring, without bacterial growth in the electrolyte. Negatively charged polymeric SWCNTs also resulted in continuously increasing EET currents and a large number of bacteria on CF, although SWCNTs did not attach to CF. In contrast, SWCNTs functionalized by small-sized surfactants led to a decrease in both currents and the amount of bacteria in the solution, presumably due to the detachment of surfactants from SWCNTs and their detrimental interaction with cells. We expect our results will help researchers in designing materials for smart bioelectrical interfaces for low-scale microbial energy harvesting, sensing, and energy conversion applications.

Combined metabolomics and proteomics to reveal the mechanism of S. oneidensis MR-1 degradation malathion enhanced by FeO/C.

Iron oxide @ biochar (FeO/C) promotes bacterial growth and facilitates electron transfer, thereby effectively promoting malathion degradation by Shewanella oneidensis MR-1 (S. oneidensis MR-1). This study elucidated the underlying mechanism of FeO/C-enhanced malathion degradation by S. oneidensis MR-1 through a combination of metabolomics and proteomics analysis. The kinetic fitting results from the degradation experiment indicated that 0.1 g/L FeO/C exerted the most significant enhancement effect on malathion degradation by S. oneidensis MR-1. Observations from Scanning Electron Microscopy and Laser Scanning Confocal Microscopy, along with physiological and biochemical analysis, showed that FeO/C enhanced the growth and oxidative response of S. oneidensis MR-1 under malathion stress. In addition, metabolomics and proteomics analysis revealed an increase in certain electron transfer related metabolites, such as coenzymes, and the upregulation of proteins, including coenzyme A, sdhD, and petC. Overall, spectroscopic analysis suggested that Fe2+, which was reduced from Fe3+ by S. oneidensis MR-1 in FeO/C, promoted electron transfer in S. oneidensis MR-1 to enhance the degradation of malathion. This study offers enhanced strategies for efficient removal of malathion contaminants.

Engineering bionanoreactor in bacteria for efficient hydrogen production.

Hydrogen production through water splitting is a vital strategy for renewable and sustainable clean energy. In this study, we developed an approach integrating nanomaterial engineering and synthetic biology to establish a bionanoreactor system for efficient hydrogen production. The periplasmic space (20 to 30 nm) of an electroactive bacterium, Shewanella oneidensis MR-1, was engineered to serve as a bionanoreactor to enhance the interaction between electrons and protons, catalyzed by hydrogenases for hydrogen generation. To optimize electron transfer, we used the microbially reduced graphene oxide (rGO) to coat the electrode, which improved the electron transfer from the electrode to the cells. Native MtrCAB protein complex on S. oneidensis and self-assembled iron sulfide (FeS) nanoparticles acted in tandem to facilitate electron transfer from an electrode to the periplasm. To enhance proton transport, S. oneidensis MR-1 was engineered to express Gloeobacter rhodopsin (GR) and the light-harvesting antenna canthaxanthin. This led to efficient proton pumping when exposed to light, resulting in a 35.6% increase in the rate of hydrogen production. The overexpression of native [FeFe]-hydrogenase further improved the hydrogen production rate by 56.8%. The bionanoreactor engineered in S. oneidensis MR-1 achieved a hydrogen yield of 80.4 µmol/mg protein/day with a Faraday efficiency of 80% at a potential of -0.75 V. This periplasmic bionanoreactor combines the strengths of both nanomaterial and biological components, providing an efficient approach for microbial electrosynthesis.

Heme d formation in a Shewanella benthica hemoglobin.

In our continued investigations of microbial globins, we solved the structure of a truncated hemoglobin from Shewanella benthica, an obligate psychropiezophilic bacterium. The distal side of the heme active site is lined mostly with hydrophobic residues, with the exception of a tyrosine, Tyr34 (CD1) and a histidine, His24 (B13). We found that purified SbHbN, when crystallized in the ferric form with polyethylene glycol as precipitant, turned into a green color over weeks. The electron density obtained from the green crystals accommodated a trans heme d, a chlorin-type derivative featuring a γ-spirolactone and a vicinal hydroxyl group on a pyrroline ring. In solution, exposure of the protein to one equivalent of hydrogen peroxide resulted in a similar green color change, but caused by the formation of multiple products. These were oxidation species released on protein denaturation, likely including heme d, and a species with heme covalently attached to the polypeptide. The Tyr34Phe replacement prevented the formation of both heme d and the covalent linkage. The ready modification of heme b by SbHbN expands the range of chemistries supported by the globin fold and offers a route to a novel heme cofactor.

Mechanistic insights into sulfadimethoxine degradation via microbially driven Fenton reactions.

Biodegradation, while cost-effective, is hindered by the requirement for specialized microorganisms and co-contaminants. Innovative biological technologies like the microbially driven Fenton reaction, hold promise for enhancing degradation efficiency. However, the intricate biochemical processes and essential steps for effective degradation in such systems have remained unclear. In this study, we harnessed the potential of the microbially driven Fenton reaction by employing Shewanella oneidensis MR-1 (MR-1). Our approach showcased remarkable efficacy in degrading a range of contaminants, including sulfadimethoxine (SDM), 4,4'-dibromodiphenyl ether (BDE-15) and atrazine (ATZ). Using SDM as a model contaminant of emergent contaminants (ECs), we unveiled that biodegradation relied on the generation of hydroxyl radicals (•OH) and involvement of oxidoreductases. Transcriptomic analysis shed light on the pivotal components of extracellular electron transfer (EET) during both anaerobic and aerobic periods. The presence of reactive oxidizing species induced cellular damage and impeded DNA repair, thereby affecting the Mtr pathway of EET. Moreover, the formation of vivianite hindered SDM degradation, underscoring the necessity of maintaining iron ions in the solution to ensure sustainable and efficient degradation. Overall, this study offers valuable insights into microbial technique for ECs degradation, providing a comprehensive understanding of degradation mechanisms during aerobic/anaerobic cycling.

Distinct influence of model electron shuttles on anaerobic mononitrophenols reduction in aquatic environments by Shewanella oneidensis MR-1.

The environmental fate and risks of mononitrophenols (mono-NPs), the simplest nitrophenols (NPs) often found in aquatic environments, are profoundly influenced by anaerobic bioreduction and co-existing electron shuttles (ESs), but little is known about the underlying mechanisms. Here, we elucidate the pathways of anaerobic mono-NPs bioreduction by Shewanella oneidensis MR-1 and assess the effect of model ESs on these processes. We found that all three mono-NPs isomers could be readily reduced to their corresponding aminophenols by S. oneidensis MR-1 under anaerobic conditions. CymA, a core component of the Mtr respiratory pathway, performs a dynamic role in these bioreduction, which is highly dependent on the bioreduction kinetics. The exogenous addition of quinones was found to accelerate the mono-NPs bioreduction through interactions with key outer-membrane proteins (e.g., OmcA and MtrC), and all these processes matched well to linear free energy relationships (LFERs). Surprisingly, adding riboflavin did not influence the bioreduction of all three mono-NPs isomers, which may be due to the contribution of OmcA and MtrC to these bioreduction processes and their downregulated expression. This study enhances our understanding of the environmental fate of mono-NPs and their bioconversion processes, providing valuable insights for the bioremediation of nitrophenol-contaminated sites.

Mixed Conducting Polymers Alter Electron Transfer Thermodynamics to Boost Current Generation from Electroactive Microbes.

Electroactive microbes that can release or take up electrons are essential components of nearly every ecological niche and are powerful tools for the development of alternative energy technologies. Small-molecule mediators are critical for this electron transfer but remain difficult to study and engineer because they perform concerted two-electron transfer in native systems but only individual, one-electron transfers in electrochemical studies. Here, we report that electrode modification with ion- and electron-conductive polymers yields biosimilar, concerted two-electron transfer from Shewanella oneidensis via flavin mediators. S. oneidensis biofilms on these polymers show significantly improved per-microbe current generation and morphologies that more closely resemble native systems, setting a new paradigm for the study and optimization of these electron transfer processes. The unprecedented concerted electron transfer was found to be due to altered mediator electron transfer thermodynamics, enabling biologically relevant studies of electroactive biofilms in the lab for the first time. These important findings pave the way for a complete understanding of the ecological role of electroactive microbes and their broad application in sustainable technologies.

Acrylate Reductase of an Anaerobic Electron Transport Chain of the Marine Bacterium Shewanella woodyi.

Many microorganisms are capable of anaerobic respiration in the absence of oxygen, by using different organic compounds as terminal acceptors in electron transport chain. We identify here an anaerobic respiratory chain protein responsible for acrylate reduction in the marine bacterium Shewanella woodyi. When the periplasmic proteins of S. woodyi were separated by ion exchange chromatography, acrylate reductase activity copurified with an ArdA protein (Swoo_0275). Heterologous expression of S. woodyi ardA gene (swoo_0275) in Shewanella oneidensis MR-1 cells did not result in the appearance in them of periplasmic acrylate reductase activity, but such activity was detected when the ardA gene was co-expressed with an ardB gene (swoo_0276). Together, these genes encode flavocytochrome c ArdAB, which is thus responsible for acrylate reduction in S. woodyi cells. ArdAB was highly specific for acrylate as substrate and reduced only methacrylate (at a 22-fold lower rate) among a series of other tested 2-enoates. In line with these findings, acrylate and methacrylate induced ardA gene expression in S. woodyi under anaerobic conditions, which was accompanied by the appearance of periplasmic acrylate reductase activity. ArdAB-linked acrylate reduction supports dimethylsulfoniopropionate-dependent anaerobic respiration in S. woodyi and, possibly, other marine bacteria.

Elucidation of high-pressure processing toward microbial inhibition, physicochemical properties, collagen fiber and muscle structure of blood clam edible portion.

Impact of high-pressure processing (HP-P) on microbial inactivation, protein oxidation, collagen fiber, and muscle structure of the edible portion (EP) of blood clams (BC) was investigated. Aerobic plate count, Vibrio parahaemolyticus, V. vulnificus, other Vibrio spp. and Shewanella algae counts were not detectable when HP-P pressure of ≥300 MPa was applied. Carbonyl, disulphide bond content, and surface hydrophobicity upsurged as HP-P with augmenting pressure was employed. Protein with ∼53 kDa appeared when HP-P at 100 and 200 MPa was implemented. Increased pressure enhanced gap formation and abnormal muscle cell structure arrangements. HP-P also affected connective tissue, causing size reduction and disruption of the collagen filament fibers. However, firmness and toughness of BC-EP with HP-P ≤ 300 MPa were comparable to those of the control. HP-P at 300 MPa was therefore appropriate for treatment of BC with maintained textural properties, while less protein oxidation, collagen fiber and muscle structure disruption occurred.

Evolution of paralogous multicomponent systems for site-specific O-sialylation of flagellin in Gram-negative and Gram-positive bacteria.

Many bacteria glycosylate flagellin on serine or threonine residues using pseudaminic acid (Pse) or other sialic acid-like donor sugars. Successful reconstitution of Pse-dependent sialylation by the conserved Maf-type flagellin glycosyltransferase (fGT) may require (a) missing component(s). Here, we characterize both Maf paralogs in the Gram-negative bacterium Shewanella oneidensis MR-1 and reconstitute Pse-dependent glycosylation in heterologous hosts. Remarkably, we uncovered distinct acceptor determinants and target specificities for each Maf. Whereas Maf-1 uses its C-terminal tetratricopeptide repeat (TPR) domain to confer flagellin acceptor and O-glycosylation specificity, Maf-2 requires the newly identified conserved specificity factor, glycosylation factor for Maf (GlfM), to form a ternary complex with flagellin. GlfM orthologs are co-encoded with Maf-2 in Gram-negative and Gram-positive bacteria and require an invariant aspartate in their four-helix bundle to function with Maf-2. Thus, convergent fGT evolution underlies distinct flagellin-binding modes in tripartite versus bipartite systems and, consequently, distinct O-glycosylation preferences of acceptor serine residues with Pse.

A large scale bacterial attraction assay: A new quantitative bacterial migration assay suitable for genetic screens.

Bacteria use various motility mechanisms to explore their environments. Chemotaxis is the ability of a motile bacterial cell to direct its movement in response to chemical gradients. A number of methods have been developed and widely used to study chemotactic responses to chemoeffectors including capillary, agar plug, microscopic slide, and microfluidic assays. While valuable, these assays are primarily designed to monitor rapid chemotactic responses to chemoeffectors on a small scale, which poses challenges in collecting large quantities of attracted bacteria. Consequently, these setups are not ideal for experiments like forward genetic screens. To overcome this limitation, we developed the Large Scale Bacterial Attraction assay (LSBA), which relies on the use of a Nalgene™ Reusable Filter Unit and other materials commonly found in laboratories. We validate the LSBA by investigating chemoeffector kinetics in the setup and by using chemoattractants to quantify the chemotactic response of wild-type, and motility impaired strains of the plant pathogenic bacterium Xanthomonas campestris pv. campestris and the environmental bacterium Shewanella oneidensis. We show that the LSBA establishes a long lasting chemoeffector gradient, that the setup can be used to quantify bacterial migration over time and that the LSBA offers the possibility to collect high numbers of attracted bacteria, making it suitable for genetic screens.

Fabrication of bio-abiotic hybrid living hydrogel for bifunctional electrochemical conversion.

Design and intelligent use renewable natural bioenergy is an important challenge. Electric microorganism-based materials are being serve as an important part of bioenergy devices for energy release and collection, calling for suitable skeleton materials to anchor live microbes. Herein we verified the feasibility of constructing bio-abiotic hybrid living materials based on the combination of gelatin, Li-ions and exoelectrogenic bacteria Shewanella oneidensis manganese-reducing-1 (MR-1). The gelatin-based mesh contains abundant pores, allowing microbes to dock and small molecules to diffuse. The hybrid materials hold plentiful electronegative groups, which effectively anchor Li-ions and facilitate their transition. Moreover, the electrochemical characteristics of the materials can be modulated through changing the ratios of gelatin, bacteria and Li-ions. Based on the gelatin-Li-ion-microorganism hybrid materials, a bifunctional device was fabricated, which could play dual roles alternatively, generation of electricity as a microbial fuel cell and energy storage as a pseudocapacitor. The capacitance and the maximum voltage output of the device reaches 68 F g-1 and 0.67 V, respectively. This system is a new platform and fresh start to fabricate bio-abiotic living materials for microbial electron storage and transfer. We expect the setup will extend to other living systems and devices for synthetic biological energy conversion.

Chlorophyll a acts as a natural photosensitizer to drive nitrate reduction in nonphotosynthetic microorganisms.

With the death and decomposition of widely distributed photosynthetic organisms, free natural pigments are often detected in surface water, sediment and soil. Whether free pigments can act as photosensitizers to drive biophotoelectrochemical metabolism in nonphotosynthetic microorganisms has not been reported. In this work, we provide direct evidence for the photoelectrophic relationship between extracellular chlorophyll a (Chl a) and nonphotosynthetic microorganisms. The results show that 10 µg of Chl a can produce significant photoelectrons (∼0.34 A/cm2) upon irradiation to drive nitrate reduction in Shewanella oneidensis. Chl a undergoes structural changes during the photoelectric process, thus the ability of Chl a to generate a photocurrent decreases gradually with increasing illumination time. These changes are greater in the presence of microorganisms than in the absence of microorganisms. Photoelectron transport from Chl a to S. oneidensis occurs through a direct pathway involving the cytochromes MtrA, MtrB, MtrC and CymA but not through an indirect pathway involving riboflavin. These findings reveal a novel photoelectrotrophic linkage between natural photosynthetic pigments and nonphototrophic microorganisms, which has important implications for the biogeochemical cycle of nitrogen in various natural environments where Chl a is distributed.

The dmsEFABGH operon encodes an essential and modular electron transfer pathway for extracellular iodate reduction by Shewanella oneidensis MR-1.

Extracellular iodate reduction by Shewanella spp. contributes to iodide generation in the biogeochemical cycling of iodine. However, there is a disagreement on whether Shewanella spp. use different extracellular electron transfer pathways with dependence on electron donors in iodate reduction. In this study, a series of gene deletion mutants of Shewanella oneidensis MR-1 were created to investigate the roles of dmsEFABGH, mtrCAB, and so4357-so4362 operons in iodate reduction. The iodate-reducing activity of the mutants was tested with lactate, formate, and H2 as the sole electron donors, respectively. In the absence of single-dms gene, iodate reduction efficiency of the mutants was only 12.9%-84.0% with lactate at 24 hours, 22.1%-85.9% with formate at 20 hours, and 19.6%-57.7% with H2 at 42 hours in comparison to complete reduction by the wild type. Progressive inhibition of iodate reduction was observed when the dms homolog from the so4357-so4362 operon was deleted in the single-dms gene mutants. This result revealed complementation of dmsEFABGH by so4357-so4362 at the single-gene level, indicating modularity of the extracellular electron transfer pathway encoded by dmsEFABGH operon. Under the conditions of all electron donors, significant inhibition of iodate reduction and accumulation of H2O2 were detected for ΔmtrCAB. Collectively, these results demonstrated that the dmsEFABGH operon encodes an essential and modular iodate-reducing pathway without electron donor dependence in S. oneidensis MR-1. The mtrCAB operon was involved in H2O2 elimination with all electron donors. The findings in this study improved the understanding of molecular mechanisms underlying extracellular iodate reduction.IMPORTANCEIodine is an essential trace element for human and animals. Recent studies revealed the contribution of microbial extracellular reduction of iodate in biogeochemical cycling of iodine. Multiple reduced substances can be utilized by microorganisms as energy source for iodate reduction. However, varied electron transfer pathways were proposed for iodate reduction with different electron donors in the model strain Shewanella oneidensis MR-1. Here, through a series of gene deletion and iodate reduction experiments, we discovered that the dmsEFABGH operon was essential for iodate reduction with at least three electron donors, including lactate, formate, and H2. The so4357-so4362 operon was first demonstrated to be capable of complementing the function of dmsEFABGH at single-gene level.

Reductive dissolution of As-bearing iron oxides: Mediating mechanism of fulvic acid and dissimilated iron reducing bacteria.

Fulvic acid (FA) and iron oxides often play regulating roles in the geochemical behavior and ecological risk of arsenic (As) in terrestrial ecosystems. FA can act as electron shuttles to facilitate the reductive dissolution of As-bearing iron (hydr)oxides. However, the influence of FA from different sources on the sequential conversion of Fe/As in As-bearing iron oxides under biotic and abiotic conditions remains unclear. In this work, we exposed prepared As-bearing iron oxides to FAs derived from lignite (FAL) and plant peat (FAP) under anaerobic conditions, tracked the fate of Fe and As in the aqueous phase, and investigated the reduction transformation of Fe(III)/As(V) with or without the presence of Shewanella oneidensis MR-1. The results showed that the reduction efficiency of Fe(III)/As(V) was increased by MR-1, through its metabolic activity and using FAs as electron shuttles. The reduction of Fe(III)/As(V) was closely associated with goethite being more conducive to Fe/As reduction compared to hematite. It is determined that functional groups such as hydroxy, carboxy, aromatic, aldehyde, ketone and aliphatic groups are the primary electron donors. Their reductive capacities rank in the following sequence: hydroxy> carboxy, aromatic, aldehyde, ketone> aliphatic group. Notably, our findings suggest that in the biotic reduction, Fe significantly reduction precedes As reduction, thereby influencing the latter's reduction process across all incubation systems. This work provides empirical support for understanding iron's role in modulating the geochemical cycling of As and is of significant importance for assessing the release risk of arsenic in natural environments.