The luxS deletion reduces the spoilage ability of Shewanella putrefaciens: An analysis focusing on quorum sensing and activated methyl cycle

The luxS mutant strains of Shewanella putrefaciens (SHP) were constructed to investigate the regulations of gene luxS in spoilage ability. The potential regulations of AI-2 quorum sensing (QS) system and activated methyl cycle (AMC) were studied by analyzing the supplementation roles of key circulating substances mediated via luxS, including S-adenosylmethionine (SAM), S-adenosylhomocysteine (SAH), methionine (Met), homocysteine (Hcy) and 4,5-dihydroxy-2,3-pentanedione (DPD). Growth experiments revealed that the luxS deletion led to certain growth limitations of SHP, which were associated with culture medium and exogenous additives. Meanwhile, the decreased biofilm formation and diminished hydrogen sulfide (H2S) production capacity of SHP were observed after luxS deletion. The relatively lower total volatile base nitrogen (TVB-N) contents and higher sensory scores of fish homogenate with luxS mutant strain inoculation also indicated the weaker spoilage-inducing effects after luxS deletion. However, these deficiencies could be offset with the exogenous supply of circulating substances mentioned above. Our findings suggested that the luxS deletion would reduce the spoilage ability of SHP, which was potentially attributed to the disorder of AMC and AI-2 QS system.

Coculture of Acinetobacter johnsonii and Shewanella putrefaciens Contributes to the ABC Transporter that Impacts Cold Adaption in the Aquatic Food Storage Environment

Acinetobacter johnsonii and Shewanella putrefaciens were identified as specific spoilage organisms in aquatic food. The interactions among specific spoilage organisms under cold stress have a significant impact on the assembly of microbial communities, which play crucial roles in the spoilage and cold adaptation processes. The limited understanding of A. johnsonii and S. putrefaciens interactions in the cold adaptation mechanism hinders the elucidation of their roles in protein and metabolism levels. 4D quantitative proteomic analysis showed that the coculture of A. johnsonii and S. putrefaciens responds to low temperatures through ABC transporter proteins, resulting in phospholipid transport and inner membrane components. SapA and FtsX proteins were significantly upregulated, while LolC, LolD, LolE, PotD, PotA, PotB, and PotC proteins were significantly downregulated. Metabolome assays revealed that metabolites of glutathione and spermidine/putrescin were significantly upregulated, while metabolites of arginine/lysine/ornithine were significantly downregulated and involved in the ABC transporter metabolism. The results of ultramicroscopic analyses showed that the coculture of A. johnsonii and S. putrefaciens surface combined with the presence of the leakage of intracellular contents, suggesting that the bacteria were severely damaged and wrinkled to absorb metabolic nutrients and adapt to cold temperatures.

Effects of exogenous riboflavin or cytochrome addition on the cathodic reduction of Cr(VI) in microbial fuel cell with Shewanella putrefaciens

Bioreduction of Cr(VI) is recognized as a cost-effective and environmentally friendly method, attracting widespread interest. However, the slow rate of Cr(VI) bioreduction remains a practical challenge. Additionally, the direct removal efficiency of microbes for high concentrations of Cr(VI) is not ideal due to the toxicity. Therefore, this study investigated the effects of exogenous riboflavin or cytochrome on the cathodic reduction of Cr(VI) in microbial fuel cells. The results demonstrated that the exogenous riboflavin or cytochrome effectively improved the voltage output of the cells, with riboflavin increasing the voltage by 52.08%. Within the first 24 h, the Cr(VI) removal ratio in the normal, cytochrome, and riboflavin groups was 14.3%, 29.3%, and 53.8%, respectively. And the final removal ratio was 55.1%, 69.1%, and 98.0%, respectively. These results showed different enhancement effects of riboflavin and cytochrome on Cr(VI) removal. The analysis of riboflavin and cytochrome contents revealed that the additions did not have a significant impact on the autocrine riboflavin of S. putrefaciens, but affected the autocrine cytochrome. SEM, XPS, and FTIR results confirmed the presence of reduced Cr(III) on the cathode, which formed precipitate and adhered to the cathode surface. The EDS analysis showed that the amount of Cr on the cathode in normal, cytochrome, and riboflavin groups was 4.71%, 6.37%, 7.56%, respectively, which was consistent with the voltage and Cr(VI) removal data. These findings demonstrated the significant enhancement of exogenous riboflavin or cytochrome on Cr(VI) reduction, thereby providing data reference for the future bio-assisted remediation of Cr(VI) pollution.

Chelating Effect of Siderophore Desferrioxamine-B on Uranyl Biomineralization Mediated by Shewanella putrefaciens

In contaminated water and soil, little is known about the role and mechanism of the biometabolic molecule siderophore desferrioxamine-B (DFO) in the biogeochemical cycle of uranium due to complicated coordination and reaction networks. Here, a joint experimental and quantum chemical investigation is carried out to probe the biomineralization of uranyl (UO22+, referred to as U(VI) hereafter) induced by Shewanella putrefaciens (abbreviated as S. putrefaciens) in the presence of DFO and Fe3+ ion. The results show that the production of mineralized solids {hydrogen-uranium mica [H2(UO2)2(PO4)2·8H2O]} via S. putrefaciens binding with UO22+ is inhibited by DFO, which can both chelate preferentially UO22+ to form a U(VI)-DFO complex in solution and seize it from U(VI)-biominerals upon solvation. However, with Fe3+ ion introduced, the strong specificity of DFO binding with Fe3+ causes re-emergence of biomineralization of UO22+ {bassetite [Fe(UO2)2(PO4)2·8(H2O)]} by S. putrefaciens, owing to competitive complexation between Fe3+ and UO22+ for DFO. As DFO possesses three hydroxamic functional groups, it forms hexadentate coordination with Fe3+ and UO22+ ions via these functional groups. The stability of the Fe3+-DFO complex is much higher than that of U(VI)-DFO, resulting in some DFO-released UO22+ to be remobilized by S. putrefaciens. Our finding not only adds to the understanding of the fate of toxic U(VI)-containing substances in the environment and biogeochemical cycles in the future but also suggests the promising potential of utilizing functionalized DFO ligands for uranium processing.

Genome analysis of Shewanella putrefaciens 4H revealing the potential mechanisms for the chromium remediation

Microbial remediation of heavy metal polluted environment is ecofriendly and cost effective. Therefore, in the present study, Shewanella putrefaciens stain 4H was previously isolated by our group from the activated sludge of secondary sedimentation tank in a dyeing wastewater treatment plant. The bacterium was able to reduce chromate effectively. The strains showed significant ability to reduce Cr(VI) in the pH range of 8.0 to 10.0 (optimum pH 9.0) and 25-42 ℃ (optimum 30 ℃) and were able to reduce 300 mg/L of Cr(VI) in 72 h under parthenogenetic anaerobic conditions. In this paper, the complete genome sequence was obtained by Nanopore sequencing technology and analyzed chromium metabolism-related genes by comparative genomics The genomic sequence of S. putrefaciens 4H has a length of 4,631,110 bp with a G + C content of 44.66% and contains 4015 protein-coding genes and 3223,  2414, 2343 genes were correspondingly annotated into the COG, KEGG, and GO databases. The qRT-PCR analysis showed that the expression of chrA, mtrC, and undA genes was up-regulated under Cr(VI) stress. This study explores the Chromium Metabolism-Related Genes of S. putrefaciens 4H and will help to deepen our understanding of the mechanisms of Cr(VI) tolerance and reduction in this strain, thus contributing to the better application of S. putrefaciens 4H in the field of remediation of chromium-contaminated environments.

Characterization of the Positive Transcription Regulator PfaR for Improving Eicosapentaenoic Acid Production in Shewanella putrefaciens W3-18-1

The biosynthetic pathway of eicosapentaenoic acid (EPA) has previously been reported in marine bacteria, while the regulatory mechanism remains poorly understood. In this study, a putative transcriptional regulator PfaR encoded adjacent to the PFA biosynthesis gene cluster (pfaEABCD) was computationally and experimentally characterized. Comparative analyses on the wild type (WT) strain, in-frame deletion, and overexpression mutants revealed that PfaR positively regulated EPA synthesis at low temperature. RNA-Seq and real-time quantitative PCR analyses demonstrated that PfaR stimulated the transcription of pfaABCD. The transcription start site of pfaR was mapped by using primer extension and highly conserved promoter motifs bound by the housekeeping Sigma 70 factor that were identified in the upstream of pfaR. Moreover, overexpression of PfaR in WT strain W3-18-1 at low temperature could improve EPA productivity from 0.07% to 0.13% (percentage of EPA to dry weight, mg/mg) of dry weight. Taken together, these findings could provide important implications into the transcriptional control and metabolic engineering in terms of EPA productivity for industrial strains. IMPORTANCE We have experimentally confirmed that PfaR is a positive transcription regulator that promotes EPA synthesis at low temperature in Shewanella putrefaciens W3-18-1. Overexpression of PfaR in WT strain W3-18-1 could lead to a 1.8-fold increase in EPA productivity at low temperature. It is further shown that PfaR may be regulated by housekeeping Sigma 70 factor at low temperature.

Effect of humic acid on bioreduction of facet-dependent hematite by Shewanella putrefaciens CN-32

Interfacial reactions between iron (Fe) (hydr)oxide surfaces and the activity of bacteria during dissimilatory Fe reduction affect extracellular electron transfer. The presence of organic matter (OM) and exposed facets of Fe (hydr)oxides influence this process. However, the underlying interfacial mechanism of facet-dependent hematite and its toxicity toward microbes during bioreduction in the presence of OM remains unknown. Herein, humic acid (HA), as typical OM, was selected to investigate its effect on the bioreduction of hematite {100} and {001}. When HA concentration was increased from 0 to 500 mg L^-1, the bioreduction rates increased from 0.02 h^-1 to 0.04 h^-1 for hematite {100} and from 0.026 h^-1 to 0.05 h^-1 for hematite {001}. Since hematite {001} owned lower resistance than hematite {100} irrespective of the HA concentration, and hematite {100} was less favorable for reduction. Microscopy-based analysis showed that more hematite {001} nanoparticles adhered to the cell surface and were bound more closely to the bacteria. Moreover, less cell damage was observed in the HA-hematite {001} treatments. As the reaction progressed, some bacterial cells died or were inactivated; confocal laser scanning microscopy showed that bacterial survival was higher in the HA-hematite {001} treatments than in the HA-hematite {100} treatments after bioreduction. Spectroscopic analysis revealed that facet-dependent binding was primarily realized by surface complexation of carboxyl functional groups with structural Fe atoms, and that the binding order of HA functional groups and hematite was affected by the exposed facets. The exposed facets of hematite could influence the electrochemical properties and activity of bacteria, as well as the binding of bacteria and Fe oxides in the presence of OM, thereby governing the extracellular electron transfer and concomitant bioreduction of Fe (hydr)oxides. These results provide new insights into the interfacial reactions between OM and facet-dependent Fe oxides in anoxic, OM-rich soil and sediment environments.

Single Strain-Triggered Biogeochemical Cycle of Arsenic

The microbial metabolism of arsenic plays a prominent role in governing the biogeochemical cycle of arsenic. Although diverse microbes are known to be involved in the redox transformation of inorganic arsenic, the underlying mechanisms about the arsenic redox cycle mediated by a single microbial strain remain unclear yet. Herein, we discover that Shewanella putrefaciens CN32, a well-known arsenate-respiring and dissimilatory metal-reducing bacterium, could mediate the reversible arsenic redox transformation under aerobic conditions. Genetic analysis shows that S. putrefaciens CN32 contains both ars and arr operon but lacks an As(III) oxidase encoding gene. Arsenic(V) reduction tests demonstrate that the ars operon is advantageous but not essential for As(V) respiration in S. putrefaciens CN32. The Arr complex encoded by the arr operon not only plays a crucial role in arsenate respiration under anaerobic conditions but also participates in the sequential process of As(V) reduction and As(III) oxidation under aerobic conditions. The Arr enzyme also contributes to the microbial As(III) resistance. The expression and catalysis directionality of Arr in S. putrefaciens CN32 are regulated by the carbon source types. Our results highlight the complexity of arsenic redox biotransformation in environments and provide new insights into the important contribution of Arr to the As biogeochemical cycle in nature.

Surface biomineralization of uranium onto Shewanella putrefaciens with or without extracellular polymeric substances

The influence of extracellular polymeric substances (EPS) on the interaction between uranium [U(VI)] and Shewanella putrefaciens (S. putrefaciens), especially the U(VI) biomineralization process occurring on whole cells and cell components of S. putrefaciens was investigated in this study. The removal efficiency of U(VI) by S. putrefaciens was decreased by 22% after extraction of EPS. Proteins were identified as the main components of EPS by EEM analysis and were determined to play a major role in the biosorption of uranium. SEM-EDS results showed that U(VI) was distributed around the whole cell as 500-nanometer schistose structures, which consisted primarily of U and P. However, similar uranium lamellar crystal were wrapped only on the surface of EPS-free S. putrefaciens cells. FTIR and XPS analysis indicated that phosphorus- and nitrogen-containing groups played important roles in complexing U (VI). XRD and U L_III-edge EXAFS analyses demonstrated that the schistose structure consisted of hydrogen uranyl phosphate [H₂(UO₂)₂(PO₄)₂•8H₂O]. Our study provides new insight into the mechanisms of induced uranium crystallization by EPS and cell wall membranes of living bacterial cells under aerobic conditions.

Release of Pb adsorbed on graphene oxide surfaces under conditions of Shewanella putrefaciens metabolism

In this study, Pb(II) was used as a target heavy metal pollutant, and the metabolism of Shewanella putrefaciens (S. putrefaciens) was applied to achieve reducing conditions to study the effect of microbial reduction on lead that was preadsorbed on graphene oxide (GO) surfaces. The results showed that GO was transformed to its reduced form (r-GO) by bacteria, and this process induced the release of Pb(II) adsorbed on the GO surfaces. After 72 hr of exposure in an S. putrefaciens system, 5.76% of the total adsorbed Pb(II) was stably dispersed in solution in the form of a Pb(II)-extracellular polymer substance (EPS) complex, while another portion of Pb(II) released from GO-Pb(II) was observed as lead phosphate hydroxide (Pb₁₀(PO₄)₆(OH)₂) precipitates or adsorbed species on the surface of the cell. Additionally, increasing pH induced the stripping of oxidative debris (OD) and elevated the content of dispersible Pb(II) in aqueous solution under the conditions of S. putrefaciens metabolism. These research results provide valuable information regarding the migration of heavy metals adsorbed on GO under reducing conditions due to microbial metabolism.

Comparative Transcriptome Analysis of Shewanella putrefaciens WS13 Biofilms Under Cold Stress

Shewanella putrefaciens is a Gram-negative bacterium that can cause seafood spoilage under low-temperature conditions. The bacterium easily forms biofilms to enhance its survival in challenging environments. Our previous research revealed that the biofilm formed by S. putrefaciens WS13 under the low temperature (4 °C) has larger biomass and tighter structure than at an optimum growth temperature (30 °C). In this study, comparative transcriptome analysis was further performed to get insights into the global-level of gene expression in the biofilm formed by S. putrefaciens WS13 under the refrigerating and optimal temperatures using Illumina RNA-Sequencing technique. The results revealed that a total of 761 genes were differentially expressed, of which 497 were significantly up-regulated and 264 were significantly down-regulated (p<0.05). The qRT-PCR results of randomly selected differentially expressed genes (DEGs) confirmed the RNA sequencing results. Comparison of transcriptome data revealed 28 significantly changed metabolic pathways under the cold stress, including the down-regulated chemotaxis, and motility, and up-regulated tryptophan metabolism, histidine biosynthesis, and quorum sensing, which benefited the biofilm formation of S. putrefaciens WS13 under the adverse circumstance. This study provided useful data for better understanding of the biofilm formation of S. putrefaciens, and also laid a theoretical foundation for novel vaccine and drug targets against the severe spoilage bacterium under the cold stress.

Plant growth-promoting rhizobacteria Shewanella putrefaciens and Cronobacter dublinensis enhance drought tolerance of pearl millet by modulating hormones and stress-responsive genes

Drought is a major abiotic stress that affects crop productivity. Endophytic bacteria have been found to alleviate the adverse effects of drought on plants. In the present study, we evaluated the effects of two endophytic bacteria Shewanella putrefaciens strain MCL-1 and Cronobacter dublinensis strain MKS-1 on pearl millet (Pennisetum glaucum (L.) R. Br.) under drought stress conditions. Pearl millet plants were grown under three water levels: field capacity (FC), mild drought stress (MD), and severe drought stress (SD). The effects of inoculation on plant growth, physiological attributes, phytohormone content, and drought stress-responsive genes were assessed. The inoculation of pearl millet seeds with endophytes significantly improved shoot and root dry weight and root architecture of plants grown under FC and drought stress conditions. There was a significant increase in relative water content and proline accumulation in the inoculated plants. Among the phytohormones analyzed, the content of ABA and IAA was significantly higher in endophyte-treated plants under all moisture regimes than in uninoculated plants. C. dublinensis-inoculated plants had higher GA content than uninoculated plants under all moisture regimes. The expression level of genes involved in phytohormone biosynthesis (SbNCED, SbGA20oX, and SbYUC) and coding drought-responsive transcription factors (SbAP2, SbSNAC1 and PgDREB2A) was significantly higher under SD in endophyte-inoculated plants than in uninoculated plants. Thus, these endophytic bacteria presumably enhanced the tolerance of pearl millet to drought stress by modulating root growth, plant hormones, physiology and the expression of genes involved in drought tolerance.

Antibacterial Activity and Mechanism of Linalool against Shewanella putrefaciens

The demand for reduced chemical preservative usage is currently growing, and natural preservatives are being developed to protect seafood. With its excellent antibacterial properties, linalool has been utilized widely in industries. However, its antibacterial mechanisms remain poorly studied. Here, untargeted metabolomics was applied to explore the mechanism of Shewanella putrefaciens cells treated with linalool. Results showed that linalool exhibited remarkable antibacterial activity against S. putrefaciens, with 1.5 µL/mL minimum inhibitory concentration (MIC). The growth of S. putrefaciens was suppressed completely at 1/2 MIC and 1 MIC levels. Linalool treatment reduced the membrane potential (MP); caused the leakage of alkaline phosphatase (AKP); and released the DNA, RNA, and proteins of S. putrefaciens, thus destroying the cell structure and expelling the cytoplasmic content. A total of 170 differential metabolites (DMs) were screened using metabolomics analysis, among which 81 species were upregulated and 89 species were downregulated after linalool treatment. These DMs are closely related to the tricarboxylic acid (TCA) cycle, glycolysis, amino acid metabolism, pantothenate and CoA biosynthesis, aminoacyl-tRNA biosynthesis, and glycerophospholipid metabolism. In addition, linalool substantially affected the activity of key enzymes, such as succinate dehydrogenase (SDH), pyruvate kinase (PK), ATPase, and respiratory chain dehydrogenase. The results provided some insights into the antibacterial mechanism of linalool against S. putrefaciens and are important for the development and application of linalool in seafood preservation.

Highly efficient removal of victoria blue R and bioelectricity generation from textile wastewater using a novel combined dual microbial fuel cell system

A novel combined dual microbial fuel cell (MFC) system was developed for the continuous removal of Victoria Blue R (VBR) and electricity generation. Anaerobic and aerobic VBR-degrading bacteria, Shewanella putrefaciens and Acinetobacter calcoaceticus, respectively, were applied simultaneously. The effects of various factors on the performance of the novel system in the continuous mode were investigated, and optimal operating parameters for the system were determined. The optimal liquid retention time for continuous treatment was 36 h. The optimal external resistances of connected MFCs were 390 Ω and 1300 Ω. When artificial wastewater containing 1000 mg/l of VBR was fed continuously into the system, the VBR removal efficiency achieved was 98.7%. In addition, the acute toxicity of the effluent was decreased by a factor of 21.1-22.3, indicating that the system could detoxify VBR intermediates. VBR degradation involved a stepwise demethylation process, which occurred mainly in the first MFC, whereas aromatic ring opening, sequential deamination reaction, and carbon oxidation occurred mainly in the second MFC. When actual VBR-containing wastewater (75-262 mg/l) was introduced, the removal efficiencies of VBR, chemical oxygen demand, colority, NH₃, and bioelectricity generation were >99.8%, >96.6%, >88.0%, 100%, and >194.8 mW/m², respectively and the original inoculated strains remained dominant. Therefore, the combined dual MFC system could be applied to the treatment of actual VBR-containing wastewater.

Quantitative Analysis of Cold Stress Inducing Lipidomic Changes in Shewanella putrefaciens Using UHPLC-ESI-MS/MS

Shewanella putrefaciens is a well-known specific spoilage organism (SSO) and cold-tolerant microorganism in refrigerated fresh marine fish. Cold-adapted mechanism includes increased fluidity of lipid membranes by the ability to finely adjust lipids composition. In the present study, the lipid profile of S. putrefaciens cultivated at 30, 20, 10, 4, and 0 °C was explored using ultra-high-pressure liquid chromatography/electrospray ionization tandem mass spectrometry (UHPLC-ESI-MS/MS) to discuss the effect of lipid composition on cold-adapted tolerance. Lipidomic analysis detected a total of 27 lipid classes and 606 lipid molecular species in S. putrefaciens cultivated at 30, 20, 10, 4, and 0 °C. S. putrefaciens cultivated at 30 °C (SP-30) had significantly higher content of glycerolipids, sphingolipids, saccharolipids, and fatty acids compared with that at 0 °C (SP-0); however, the lower content of phospholipids (13.97%) was also found in SP-30. PE (30:0), PE (15:0/15:0), PE (31:0), PA (33:1), PE (32:1), PE (33:1), PE (25:0), PC (22:0), PE (29:0), PE (34:1), dMePE (15:0/16:1), PE (31:1), dMePE (15:1/15:0), PG (34:2), and PC (11:0/11:0) were identified as the most abundant lipid molecular species in S. putrefaciens cultivated at 30, 20, 10, 4, and 0 °C. The increase of PG content contributes to the construction of membrane lipid bilayer and successfully maintains membrane integrity under cold stress. S. putrefaciens cultivated at low temperature significantly increased the total unsaturated liquid contents but decreased the content of saturated liquid contents.

γ-proteobacteria eject their polar flagella under nutrient depletion, retaining flagellar motor relic structures

Bacteria switch only intermittently to motile planktonic lifestyles under favorable conditions. Under chronic nutrient deprivation, however, bacteria orchestrate a switch to stationary phase, conserving energy by altering metabolism and stopping motility. About two-thirds of bacteria use flagella to swim, but how bacteria deactivate this large molecular machine remains unclear. Here, we describe the previously unreported ejection of polar motors by γ-proteobacteria. We show that these bacteria eject their flagella at the base of the flagellar hook when nutrients are depleted, leaving a relic of a former flagellar motor in the outer membrane. Subtomogram averages of the full motor and relic reveal that this is an active process, as a plug protein appears in the relic, likely to prevent leakage across their outer membrane; furthermore, we show that ejection is triggered only under nutritional depletion and is independent of the filament as a possible mechanosensor. We show that filament ejection is a widespread phenomenon demonstrated by the appearance of relic structures in diverse γ-proteobacteria including Plesiomonas shigelloides, Vibrio cholerae, Vibrio fischeri, Shewanella putrefaciens, and Pseudomonas aeruginosa. While the molecular details remain to be determined, our results demonstrate a novel mechanism for bacteria to halt costly motility when nutrients become scarce.

Microbial mediated sedimentary phosphorus mobilization in emerging and eroding wetlands of coastal Louisiana

The interactions between the microbial reduction of Fe (III) oxides and sediment geochemistry are poorly understood and mostly unknown for the Louisiana deltaic plain. This study evaluates the potential of P mobilization for this region during bacterially mediated redox reactions. Samples were collected from two wetland habitats (forested wetland ridge, and marsh) characterized by variations in vegetation structure and elevation in the currently prograding Wax Lake Delta (WLD) and two habitats (wetland marsh, and benthic channel) in degrading Barataria Bay in Lake Cataouatche (BLC). Our results show that PO₄^3- mobilization from WLD and BLC habitats were negligible under aerobic condition. Under anaerobic condition, there is a potential for significant release of PO₄^3- from sediment and wetland soils. PO₄^3- release in sediments spiked with Fe reducing bacteria Shewanella putrefaciens (Sp-CN32) were significantly higher in all cases with respect to a control treatment. In Wax Lake delta, PO₄^3- release from sediment spiked with Sp-CN32 increased significantly from 0.064±0.001 to 1.460±0.005μmolg^-1 in the ridge and from 0.079±0.007 to 2.407±0.001μmolg^-1 in the marsh substrates. In Barataria bay, PO₄^3- release increased from 0.103±0.006μmolg^-1 to 0.601±0.008μmolg^-1 in the channel and 0.050±0.000 to 0.618±0.026μmolg^-1 in marsh substrates. The PO₄^3- release from sediment slurries spiked with Sp-CN32 was higher in the WLD habitats (marsh 30-fold, ridge 22-fold) compared to the BLC habitats (marsh 12-fold, channel 6-fold). The increase in PO₄^3- release was significantly correlated with the Fe bound PO₄^3- in sediments from different habitats but not with their organic matter content. This study contributes to our understanding of the release mechanism of PO₄^3- during bacterial mediated redox reaction in wetland soils undergoing pulsing sediment deposition and loss.

Decolorization and detoxification of water-insoluble Sudan dye by Shewanella putrefaciens CN32 co-cultured with Bacillus circulans BWL1061

Effluents loaded with various synthetic dyes are considered as a huge burden to the surrounding ecosystems. Sudan dyes are relatively difficult to decolorize due to its water-insolubility. In the present study, the strain Shewanella putrefaciens CN32 was firstly applied to decolorize Sudan dyes under the anaerobic condition, and the physicochemical parameters on the decolorization were optimized. The results demonstrated that the suitable decolorization condition was temperature 26 °C, initial pH 7.0-8.0 and NaCl concentrations 0-20 g/L. Electron competitive acceptors including nitrite, nitrate, dimethyl sulphoxide and oxygen could cause the significant inhibition to the decolorization of Sudan dyes. Biosurfactant rhamnolipid played a positive role in enhancing the decolorization of Sudan I. The co-culture of S. putrefaciens CN32 and Bacillus circulans BWL1061 is reported for the first time to accelerate the decolorization through improving the synergistic effect of enzymatic degradation and biological reductive effect. The highest decolorization of 90.23% to Sudan I was achieved within 108 h, suggesting that co-culture technique has a good potential in the treatment of dyeing wastewater. Furthermore, the microbial toxicity tests indicated that the toxicity of Sudan I to Escherichia coli BL21 and Bacillus subtilis 168 was obviously decreased after the decolorization.

EDDS enhanced Shewanella putrefaciens CN32 and α-FeOOH reductive dechlorination of carbon tetrachloride

S,S-ethylenediamine-N,N-disuccinic acid (EDDS) enhanced reductive dissolution of α-FeOOH by Shewanella putrefaciens CN32 (CN32), resulting in formation of surface-bound Fe(II) species (Fe^IIEDDS) to improve reductive dechlorination of carbon tetrachloride (CT). The pseudo-first-order rate constants for bio-reduction extents of α-FeOOH by CN32 in the presence of 1.36 mM EDDS was 0.023 ± 0.0003 d^-1 which was higher than without EDDS. The enhancement mechanism of bio-reduction was attributed to the strong complexation ability of EDDS to formed Fe^IIIEDDS, which could be better utilized by CN32. The dechlorination kinetic of CT by Fe^IIEDDS (2.016 h^-1) in the presence of 1.36 mM EDDS was 24 times faster than without EDDS. Chloroform were detected as main products for the degradation of CT. The chemical analyses and morphological observation showed that combination between EDDS and Fe²⁺ produced Fe^IIEDDS complex, which had a reductive potential of -0.375 V and significantly enhanced CT dechlorination. The results showed that EDDS played an important role in enhancing the bio-reduction of α-FeOOH to accelerate reductive dechlorination of CT.

Characterising microbial reduction of arsenate sorbed to ferrihydrite and its concurrence with iron reduction

A series of model anoxic incubations were performed to understand the concurrence between arsenate and ferrihydrite reduction by Shewanella putrefaciens strain CN-32 at different concentrations of arsenate, ferrihydrite and lactate, and with given ΔG_rxn for arsenate and ferrihydrite reduction in non-growth conditions. The reduction kinetics of arsenate sorbed to ferrihydrite is predominately controlled by the availability of dissolved arsenate, which is measured by the integral of dissolved arsenate concentrations against incubation time and shown to correlate with the first order rate constants. High lactate concentrations slightly slowed down the rate of arsenate reduction due to the competition with arsenate for microbial contact. Under all experimental conditions, simultaneous arsenate and ferrihydrite reduction occurred following addition of S. putrefaciens inoculums and suggested no apparent competition between these two enzymatic reductions. Ferrous ions released from iron reduction might retard microbial arsenate reduction at high arsenate and ferrihydrite concentrations due to formation of ferrous arsenate. At high arsenate to ferrihydrite ratios, reductive dissolution of ferrihydrite shifted arsenate from sorption to dissolution and hence accelerated arsenate reduction. The interaction between microbial arsenate and ferrihydrite reduction did not correlate with ΔG_rxn, but instead was governed by other factors such as geochemical and microbial parameters.

Effects of humic acid concentration on the microbially-mediated reductive solubilization of Pu(IV) polymers

The role of humic acid concentration in the microbially-mediated reductive solubilization of Pu(IV) polymers remains unclear until now. The effects of humic concentration (0-150.5mg/L) on the rate and extent of reduction of polymeric Pu(IV) were studied under anaerobic and pH 7.2 conditions. The results show that Shewanella putrefaciens, secreting flavins as endogenous electron shuttles, cannot notably stimulate the reduction of polymeric Pu(IV). In the presence of humic acids, the reduction rate of polymeric Pu(IV) increased with increasing humic concentrations (0-15.0mg/L): e.g., a 102-fold increase from 4.1×10^-15 (HA=0) to 4.2×10^-13mol Pu(III)_aq/h (HA=15.0mg/L). The bioreduced humic acids by S. putrefaciens facilitated the extracellular electron transfer to Pu(IV) polymers and thus the reduction of polymeric Pu(IV) to Pu(III)_aq became thermodynamically favorable. However, the reduction rate did not increase but decrease with increasing humic concentrations from 15.0 to 150.5mg/L. Humic coatings formed on the polymer surfaces at relatively high humic concentrations limited the electron transfer to the polymers and thus decreased the reduction rate. The finding of the dynamic role of humic acids in the bioreductive solubilization may be helpful in evaluating Pu mobility in the geosphere.

Reductive dechlorination of carbon tetrachloride by bioreduction of nontronite

Reductive dechlorination of carbon tetrachloride (CT) was investigated during bioreduction of iron-containing clay mineral (i.e., nontronite) by iron-reducing bacteria (Shewanella putrefaciens CN32 (CN32)). In the absence of CT, the production of Fe(II) significantly increased in nontronite suspension with CN32 in 124 d (11.1% of Fe(III) reduction), resulting in formation of new secondary Fe(II) mineral phase (i.e., vivianite (Fe^II₃(PO₄)₂·8H₂O)). In the presence of CT, an acceleration of CT dechlorination was observed after 13 d and it reached almost 68% of removal efficiency at 32 d in nontronite suspension with CN32, which was 1.8 times higher than that by CN32 alone (37%). Significant amounts of formate (30.1%) and CO (2.4%) were measured during the CT dechlorination in the nontronite suspension with CN32. Results obtained from Fe(II) measurement and X-ray diffraction (XRD) showed the acceleration of Fe(II) production after 13 d and the formation of vivianite in the range of 13-25 d, suggesting that the biogenic vivianite enhanced the CT dechlorination in this study. Experimental results from batch kinetic tests, Fe(II) measurements, XRD analysis, and by-product study suggested that the formation of vivianite can play a crucial role for the enhanced reductive dechlorination of CT in phosphorous enriched subsurface environments with iron-containing clay minerals.

Impact of Bioreduction on Remobilization of Adsorbed Cadmium on Iron Minerals in Anoxic Condition

  The impact of bioreduction on the remobilization of adsorbed cadmium Cd(II) on minerals, including hematite, goethite, and two iron(III)-rich clay minerals nontronites (NAU-1 and NAU-2) under anoxic conditions was investigated. Langmuir isotherm equation better described the sorption of Cd(II) onto the all minerals. The maximum adsorption capacity was 6.2, 18.1, 3.6, and 4 mg g-1 for hematite, goethite, NAU-1 and NAU-2, respectively. The desorption of Cd(II) was due to the production of Fe(II) as a result of bioreduction of structural Fe(III) in the minerals by Shewanella putrefaciens. The bioreduction of Cd(II)-loaded Fe(III) minerals was negligible during the initial 5 days followed by a rapid increase up to 20 days. The amount of Cd(II) in solution phase at the end of 30 days increased up to 0.07 mmol L-1 for hematite, NAU-1, and NAU-2 and 0.02 mmol L-1 for goethite. The X-ray diffraction study showed negligible changes in bioreduced minerals phases.

Emerging investigator series: dual role of organic matter in the anaerobic degradation of triclosan

Triclosan (TCS), one of the most widely used antimicrobial agents, has been listed among the top 10 contaminants in US rivers. Environmental persistence, endocrine disruption effects, and the antibiotic resistance induction capacity of TCS attract interest in its environmental fate and degradation. Herein, we found that TCS can be anaerobically degraded at pH 9 by a metal-reducing bacterium, Shewanella putrefaciens CN32. The degradation was substantially facilitated by low-concentration (0-15 mg C per L) organic matter (OM) extracted from a peat soil, whereas TCS degradation was inhibited by further increased concentration (15-100 mg C per L) of OM. OM acted as both an electron shuttle and sorbent in regulating the degradation of TCS. The novel dual role of ubiquitous OM in the reaction of TCS governs the environmental degradation and persistence of TCS. Our study highlights the effects of OM on the reaction of emerging trace organic pollutants, with implications on their engineering treatment and environmental risk regulation.

Organoarsenical Biotransformations by Shewanella putrefaciens

Microbes play a critical role in the global arsenic biogeocycle. Most studies have focused on redox cycling of inorganic arsenic in bacteria and archaea. The parallel cycles of organoarsenical biotransformations are less well characterized. Here we describe organoarsenical biotransformations in the environmental microbe Shewanella putrefaciens. Under aerobic growth conditions, S. putrefaciens reduced the herbicide MSMA (methylarsenate or MAs(V)) to methylarsenite (MAs(III)). Even though it does not contain an arsI gene, which encodes the ArsI C-As lyase, S. putrefaciens demethylated MAs(III) to As(III). It cleaved the C-As bond in aromatic arsenicals such as the trivalent forms of the antimicrobial agents roxarsone (Rox(III)), nitarsone (Nit(III)) and phenylarsenite (PhAs(III)), which have been used as growth promoters for poultry and swine. S. putrefaciens thiolated methylated arsenicals, converting MAs(V) into the more toxic metabolite monomethyl monothioarsenate (MMMTAs(V)), and transformed dimethylarsenate (DMAs(V)) into dimethylmonothioarsenate (DMMTAs(V)). It also reduced the nitro groups of Nit(V), forming p-aminophenyl arsenate (p-arsanilic acid or p-AsA(V)), and Rox(III), forming 3-amino-4-hydroxybenzylarsonate (3A4HBzAs(V)). Elucidation of organoarsenical biotransformations by S. putrefaciens provides a holistic appreciation of how these environmental pollutants are degraded.

Magnetite as a precursor for green rust through the hydrogenotrophic activity of the iron-reducing bacteria Shewanella putrefaciens

Magnetite (Fe(II) Fe(III) 2 O4 ) is often considered as a stable end product of the bioreduction of Fe(III) minerals (e.g., ferrihydrite, lepidocrocite, hematite) or of the biological oxidation of Fe(II) compounds (e.g., siderite), with green rust (GR) as a mixed Fe(II) -Fe(III) hydroxide intermediate. Until now, the biotic transformation of magnetite to GR has not been evidenced. In this study, we investigated the capability of an iron-reducing bacterium, Shewanella putrefaciens, to reduce magnetite at circumneutral pH in the presence of dihydrogen as sole inorganic electron donor. During incubation, GR and/or siderite (Fe(II) CO3 ) formation occurred as secondary iron minerals, resulting from the precipitation of Fe(II) species produced via the bacterial reduction of Fe(III) species present in magnetite. Taking into account the exact nature of the secondary iron minerals and the electron donor source is necessary to understand the exergonic character of the biotic transformation of magnetite to GR, which had been considered to date as thermodynamically unfavorable at circumneutral pH. This finding reinforces the hypothesis that GR would be the cornerstone of the microbial transformations of iron-bearing minerals in the anoxic biogeochemical cycle of iron and opens up new possibilities for the interpretation of the evolution of Earth's history and for the understanding of biocorrosion processes in the field of applied science.

Functional roles of CymA and NapC in reduction of nitrate and nitrite by Shewanella putrefaciens W3-18-1

Shewanella putrefaciens W3-18-1 harbours two periplasmic nitrate reductase (Nap) gene clusters, NapC-associated nap-alpha (napEDABC) and CymA-dependent nap-beta (napDAGHB), for dissimilatory nitrate respiration. CymA is a member of the NapC/NirT quinol dehydrogenase family and acts as a hub to support different respiratory pathways, including those on iron [Fe(III)] and manganese [Mn(III, IV)] (hydr)oxide, nitrate, nitrite, fumarate and arsenate in Shewanella strains. However, in our analysis it was shown that another NapC/NirT family protein, NapC, was only involved in nitrate reduction, although both CymA and NapC can transfer quinol-derived electrons to a periplasmic terminal reductase or an electron acceptor. Furthermore, our results showed that NapC could only interact specifically with the Nap-alpha nitrate reductase while CymA could interact promiscuously with Nap-alpha, Nap-beta and the NrfA nitrite reductase for nitrate and nitrite reduction. To further explore the difference in specificity, site-directed mutagenesis on both CymA and NapC was conducted and the phenotypic changes in nitrate and nitrite reduction were tested. Our analyses demonstrated that the Lys-91 residue played a key role in nitrate reduction for quinol oxidation and the Asp-166 residue might influence the maturation of CymA. The Asp-97 residue might be one of the key factors that influence the interaction of CymA with the cytochromes NapB and NrfA.

Influence of clay minerals on sorption and bioreduction of arsenic under anoxic conditions

Adsorption of As(V) on various clay minerals including kaolinite (KGa-1), montmorillonite (SWy-1) and nontronites (NAU-1 and NAU-2), and subsequent bioreduction of sorbed As(V) to As(III) by bacterium Shewanella putrefaciens strain CN-32 were investigated. Nontronites showed relatively higher sorption capacity for As(V) primarily due to higher iron oxide content. Freundlich equation well described the sorption of As(V) on NAU-1, NAU-2 and SWy-1, while As(V) sorption isotherm with KGa-1 fitted well in the Langmuir model. The bacterium rapidly reduced 50% of dissolved As(V) to As(III) in 2 h, followed by its complete reduction (>ca. 98%) within 12 h. In contrast, sorption of As(V) to the mineral surfaces interferes with the activity of bacterium, resulting in low bioreduction of As(V) by 27% for 5 days of incubation. S. putrefaciens also promoted the reduction of Fe(III) present in the clay mineral to Fe(II). This study indicates that the sorption and subsequent bioreduction of As(V) on clay minerals can significantly influence the mobility of As(V) in subsurface environment.

The interactive biotic and abiotic processes of DDT transformation under dissimilatory iron-reducing conditions

The objective of the study was to elucidate the biotic and abiotic processes under dissimilatory iron reducing conditions involved in reductive dechlorination and iron reduction. DDT transformation was investigated in cultures of Shewanella putrefaciens 200 with/without α-FeOOH. A modified first-order kinetics model was developed and described DDT transformation well. Both the α-FeOOH reduction rate and the dechlorination rate of DDT were positively correlated to the biomass. Addition of α-FeOOH enhanced reductive dechlorination of DDT by favoring the cell survival and generating Fe(II) which was absorbed on the surface of bacteria and iron oxide. 92% of the absorbed Fe(II) was Na-acetate (1M) extractable. However, α-FeOOH also played a negative role of competing for electrons as reflected by the dechlorination rate of DDT was inhibited when increasing the α-FeOOH from 1 g L(-1) to 5 g L(-1). DDT was measured to be toxic to S. putrefaciens 200. The metabolites DDD, DDE and DDMU were recalcitrant to S. putrefaciens 200. The results suggested that iron oxide was not the key factor to promote the dissipation of DDX (DDT and the metabolites), whereas the one-electron reduction potential (E1) of certain organochlorines is the main factor and that the E1 higher than the threshold of the reductive driving forces of DIRB probably ensures the occur of reductive dechlorination.

Biological redox cycling of iron in nontronite and its potential application in nitrate removal

Biological redox cycling of structural Fe in phyllosilicates is an important but poorly understood process. The objective of this research was to study microbially mediated redox cycles of Fe in nontronite (NAu-2). During the reduction phase, structural Fe(III) in NAu-2 served as electron acceptor, lactate as electron donor, AQDS as electron shuttle, and dissimilatory Fe(III)-reducing bacterium Shewanella putrefaciens CN32 as mediator in bicarbonate- and PIPES-buffered media. During the oxidation phase, biogenic Fe(II) served as electron donor and nitrate as electron acceptor. Nitrate-dependent Fe(II)-oxidizing bacterium Pseudogulbenkiania sp. strain 2002 was added as mediator in the same media. For all three cycles, structural Fe in NAu-2 was able to reversibly undergo three redox cycles without significant dissolution. Fe(II) in bioreduced samples occurred in two distinct environments, at edges and in the interior of the NAu-2 structure. Nitrate reduction to nitrogen gas was coupled with oxidation of edge-Fe(II) and part of interior-Fe(II) under both buffer conditions, and its extent and rate did not change with Fe redox cycles. These results suggest that biological redox cycling of structural Fe in phyllosilicates is a reversible process and has important implications for biogeochemical cycles of carbon, nitrogen, and other nutrients in natural environments.

Linear free energy relationships for the biotic and abiotic reduction of nitroaromatic compounds

Nitroaromatic compounds (NACs) are ubiquitous environmental contaminants that are susceptible to biological and abiotic reduction. Prior works have found that for the abiotic reduction of NACs, the logarithm of the NACs’ rate constants correlate with one-electron reduction potential values of the NACs (EH,NAC1) according to linear free energy relationships (LFERs). Here, we extend the application of LFERs to the bioreduction of NACs and to the abiotic reduction of NACs by bioreduced (and pasteurized) iron-bearing clay minerals. A linear correlation (R2=0.96) was found between the NACs’ bioreduction rate constants (kobs) and EH,NAC1 values. The LFER slope of log kobs versus EH,NAC1/(2.303RT/F) was close to one (0.97), which implied that the first electron transfer to the NAC was the rate-limiting step of bioreduction. LFERs were also established between NAC abiotic reduction rate constants by bioreduced iron-bearing clay minerals (montmorillonite SWy-2 and nontronite NAu-2). The second-order NAC reduction rate constants (k) by bioreduced SWy-2 and NAu-2 were well correlated to EH,NAC1 (R2=0.97 for both minerals), consistent with bioreduction results. However, the LFER slopes of log k versus EH,NAC1/(2.303RT/F) were significantly less than one (0.48–0.50) for both minerals, indicating that the first electron transfer to the NAC was not the rate-limiting step of abiotic reduction. Finally, we demonstrate that the rate of 4-acetylnitrobenzene reduction by bioreduced SWy-2 and NAu-2 correlated to the reduction potential of the clay (EH,clay, R2=0.95 for both minerals), indicating that the clay reduction potential also influences its reactivity.

Pseudo-first-order reaction of chemically and biologically formed green rusts with HgII and C₁₅H₁₅N₃O₂: effects of pH and stabilizing agents (phosphate, silicate, polyacrylic acid, and bacterial cells)

The kinetics of Hg(II) and methyl red (MR) reduction by hydroxycarbonate green rust (GR1) and by hydroxysulfate green rust (GR2) were studied in the presence of naturally occurring organic and inorganic ligands (phosphate, polyacrylic acid, bacterial cells, silicate). The reducing ability of biogenic hydroxycarbonate green rust (GR1bio), obtained after microbial reduction of lepidocrocite by Shewanella putrefaciens, was also investigated and compared to those of chemically synthesized GR1 and GR2 (GR1ab and GR2ab). Pseudo first-order rate constants (kobs) of Hg(II) reduction (at pH 7.0, 8.2, and 9.5) and MR reduction (at pH 7.0) were determined and were normalized to the structural Fe(II) content of GRs (kFeII) and to the estimated concentration of surface Fe(II) sites (kS). The kS values ranged from 0.3 L mmol(-1) min(-1) to 43 L mmol(-1) min(-1) for the Hg reduction, and from 0.007 L mmol(-1) min(-1) to 3.4 L mmol(-1) min(-1) for the MR reduction. No significant discrepancy between GRab and GRbio was observed in term of reactivity. However, the reduction kinetics of MR was generally slower than the Hg(II) reduction kinetics for all tested GRs. While a slight difference in Hg(II) reduction rate was noted whatever the pH values (7.0, 8.2, or 9.5), the reduction of MR was significantly affected in the presence of ligands. A decrease by a factor of 2-200, depending on the type of ligand used, was observed. These data give new insights into the reactivity of GRs in the presence of co-occurring organic and inorganic ligands, and have major implications in the characterization of contaminated systems as well as water treatment processes.

Iron(III)-bearing clay minerals enhance bioreduction of nitrobenzene by Shewanella putrefaciens CN32

Iron-bearing clay minerals are ubiquitous in the environment, and the clay-Fe(II)/Fe(III) redox couple plays important roles in abiotic reduction of several classes of environmental contaminants. We investigated the role of Fe-bearing clay minerals on the bioreduction of nitrobenzene. In experiments with Shewanella putrefaciens CN32 and excess electron donor, we found that the Fe-bearing clay minerals montmorillonite SWy-2 and nontronite NAu-2 enhanced nitrobenzene bioreduction. On short time scales (<50 h), nitrobenzene reduction was primarily biologically driven, but at later time points, nitrobenzene reduction by biologically formed structural Fe(II) in the clay minerals became increasingly important. We found that chemically reduced (dithionite) iron-bearing clay minerals reduced nitrobenzene more rapidly than biologically reduced iron-bearing clay minerals despite the minerals having similar structural Fe(II) concentrations. We also found that chemically reduced NAu-2 reduced nitrobenzene faster as compared to chemically reduced SWy-2. The different reactivity of SWy-2 versus NAu-2 toward nitrobenzene was caused by different forms of structural clay-Fe(II) in the clay minerals and different reduction potentials (Eh) of the clay minerals. Because most contaminated aquifers become reduced via biological activity, the reactivity of biogenic clay-Fe(II) toward reducible contaminants is particularly important.

2-Nitrophenol reduction promoted by S. putrefaciens 200 and biogenic ferrous iron: the role of different size-fractions of dissolved organic matter

The reduction of nitroaromatic compounds (listed as a priority pollutant) in natural subsurface environments typically coexists with dissimilatory reduction of iron oxides effected by dissolved organic matter (DOM). Investigating the impact of the DOM that influences those reduction processes is crucial for understanding and predicting the geochemical fate of these environmental species. This study investigated the impact of different molecular weight DOM fractions (DMWDs) on the 2-nitrophenol (2-NP) reduction by S. putrefaciens 200 (SP200) and α-Fe2O3 with lactate (excluding electron donor interference). Kinetic measurements demonstrated that 2-NP reduction rates were affected by the redox reactivity of active species under DMWDs (denoted as L-DOM, M-DOM, and H-DOM). The enhanced reduction rates are consistent with the negative shifts in peak oxidation potential values, the increases in HA-like/FA-like values, aromaticity index values and electron transfer capacity values. L-DOM acted mainly as ligands to complex Fe(II), whereas the significant role of H-DOM in reductive reactions should be acting as an electron shuttle, transferring electrons from SP200 to Fe(III) and 2-NP and from biogenic Fe(II) to 2-NP, further accelerating the 2-NP reductions. Those observations provide valuable insights into the role of DOM in the biogeochemical redox processes and the remediation of contaminated soil in a natural environment.

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.

Arsenite oxidation and removal driven by a bio-electro-Fenton process under neutral pH conditions

The iron-catalyzed oxidation of arsenite (As(III)) associated with Fenton or Fenton-like reactions is one of the most efficient arsenic removal methods. However, the conventional chemical or electro-Fenton systems for the oxidation of As(III) are only efficient under acid conditions. In the present study, a cost-effective and efficient bio-electro-Fenton process was performed for As(III) oxidation in a dual-chamber microbial fuel cell (MFC) under neutral pH conditions. In such a system, the Fenton reagents, including H2O2 and Fe(II), were generated in situ by microbial-driven electro-reduction of O2 and γ-FeOOH, respectively, without an electricity supply. The results indicated that the process was capable of inducing As(III) oxidation with an apparent As(III) depletion first-order rate constant of 0.208 h(-1). The apparent oxidation current efficiency was calculated to be as high as 73.1%. The γ-FeOOH dosage in the cathode was an important factor in determining the system performance. Fourier-transform infrared spectroscopy (FT-IR) analysis indicated that As(V) was bound to the solid surface as a surface complex but not as a precipitated solid phase. The mechanism of bio-E-Fenton reaction for As(III) oxidation was also proposed. The bio-electro-Fenton system makes it potentially attractive method for the detoxification of As(III) from aqueous solution.

Riboflavin-mediated RDX transformation in the presence of Shewanella putrefaciens CN32 and lepidocrocite

The potential of riboflavin for the reductive degradation of a cyclic nitramine, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), was investigated in the presence of lepidocrocite and/or Shewanella putrefaciens CN32. RDX reduction by CN32 alone or CN32 with lepidocrocite was insignificant, while 110 μM RDX was completely reduced by CN32 with riboflavin in 78 h. The transformation products identified included nitroso metabolites, formaldehyde, and ammonium, indicating the ring cleavage of RDX. UV and visible light analysis revealed that riboflavin was microbially reduced by CN32, and that the reduced riboflavin was linked to the complete degradation of RDX. In the presence of both CN32 and lepidocrocite (γ-FeOOH), 100 μM-riboflavin increased the rate and extent of Fe(II) production as well as RDX reduction. An abiotic study also showed that Fe(II)-riboflavin complex, and Fe(II) adsorbed on lepidocrocite, reduced RDX by 48% and 21%, respectively. The findings in this study suggest that riboflavin-mediated RDX degradation pathways in subsurface environments are diverse and complex. However, riboflavin, either from bacteria or exogenous sources, can significantly increase RDX degradation. This will provide a sustainable clean-up option for explosive-contaminated subsurface environments.

Thermodynamic controls on the microbial reduction of iron-bearing nontronite and uranium

Iron-bearing phyllosilicate minerals help establish the hydrogeological and geochemical conditions of redox transition zones because of their small size, limited hydraulic conductivity, and redox buffering capacity. The bioreduction of soluble U(VI) to sparingly soluble U(IV) can promote the reduction of clay-Fe(III) through valence cycling. The reductive precipitation of U(VI) to uraninite was previously reported to occur only after a substantial percentage of clay-Fe(III) had been reduced. Using improved analytical techniques, we show that concomitant bioreduction of both U(VI) and clay-Fe(III) by Shewanella putrefaciens CN32 can occur. Soluble electron shuttles were previously shown to enhance both the rate and extent of clay-Fe(III) bioreduction. Using extended incubation periods, we show that electron shuttles enhance only the rate of reduction (overcoming a kinetic limitation) and not the final extent of reduction (a thermodynamic limitation). The first 20% of clay-Fe(III) in nontronite NAu-2 was relatively "easy" (i.e., rapid) to bioreduce; the next 15% of clay-Fe(III) was "harder" (i.e., kinetically limited) to bioreduce, and the remaining 65% of clay-Fe(III) was effectively biologically unreducible. In abiotic experiments with NAu-2 and biogenic uraninite, 16.4% of clay-Fe(III) was reduced in the presence of excess uraninite. In abiotic experiments with NAu-2 and reduced anthraquinone 2,6-disulfonate (AH2DS), 18.5-19.1% of clay-Fe(III) was reduced in the presence of excess and variable concentrations of AH2DS. A thermodynamic model based on published values of the nonstandard state reduction potentials at pH 7.0 (E'H) showed that the abiotic reactions between NAu-2 and uraninite had reached an apparent equilibrium. This model also showed that the abiotic reactions between NAu-2 and AH2DS had reached an apparent equilibrium. The final extent of clay-Fe(III) reduction correlated well with the standard state reduction potential at pH 7.0 (E°'H) of all of the reductants used in these experiments (AH2DS, CN32, dithionite, and uraninite).

Dissimilatory reduction and transformation of ferrihydrite-humic acid coprecipitates

Organic matter (OM) is present in most terrestrial environments and is often found coprecipitated with ferrihydrite (Fh). Sorption or coprecipitation of OM with Fe oxides has been proposed to be an important mechanism for long-term C preservation. However, little is known about the impact of coprecipitated OM on reductive dissolution and transformation of Fe(III) (oxyhydr)oxides. Thus, we study the effect of humic acid (HA) coprecipitation on Fh reduction and secondary mineral formation by the dissimilatory Fe(III)-reducing bacterium Shewanella putrefaciens strain CN32. Despite similar crystal structure for all coprecipitates investigated, resembling 2-line Fh, the presence of coprecipitated HA resulted in lower specific surface areas. In terms of reactivity, coprecipitated HA resulted in slower Fh bioreduction rates at low C/Fe ratios (i.e., C/Fe ≤ 0.8), while high C/Fe ratios (i.e., C/Fe ≥ 1.8) enhanced the extent of bioreduction compared to pure Fh. The coprecipitated HA also altered the secondary Fe mineralization pathway by inhibiting goethite formation, reducing the amount of magnetite formation, and increasing the formation of a green rust-like phase. This study indicates that coprecipitated OM may influence the rates, pathway, and mineralogy of biogeochemical Fe cycling and anaerobic Fe respiration within soils.

Power generation and nitrogen removal of landfill leachate using microbial fuel cell technology

Microbial fuel cell (MFC) technology has been practised in the treatment of landfill leachate. However, it is a big challenge for the usage of MFCs to treat landfill leachate with high ammonium content. The purpose of this study was to design and test two MFC reactors, i.e. an ammonium oxidation/MFC reactor and an MFC/Anammox reactor for the treatment of landfill leachate with high ammonium content in terms of power generation and nitrogen removal. Using the ammonium oxidation/MFC reactor, the landfill leachate collected from Leon County Landfill of Northwest Florida generated a power density of 8 mW/m2 together with 92% of nitrogen removal. For the MFC/Anammox reactor, a power density of 12 mW/m2 was achieved with 94% of nitrogen removal. Compared with the ammonium oxidation/MFC reactor, 50% more energy was generated because in the MFC/Anammox Reactor, nitrite served as the electron acceptor; while in the Ammonium Oxidation/MFC reactor, nitrate served as the electron acceptor. In this research, power generation was also found to be directly linked to the microbial species that were involved in organic decomposition, i.e. the greater the microbial concentration, the more the power generated.

Effects of bound phosphate on the bioreduction of lepidocrocite (γ-FeOOH) and maghemite (γ-Fe2O3) and formation of secondary minerals

Natural Fe(III) oxides typically contain a range of trace elements including P. Although solution phase and adsorbed P (as phosphate) have been shown to impact the bioreduction of Fe(III) oxides and the formation of "biogenic" secondary minerals, little is known about the potential effects of occluded/incorporated phosphate. We have examined the bioreduction of Fe(III) oxides (lepidocrocite (γ-FeOOH) and maghemite (γ-Fe2O3)) containing 0-3 mass% P as "bound" (a term we use to include both adsorbed and occluded/incorporated) phosphate. Kinetic dissolution studies showed congruent release of Fe and P, suggesting that the phosphate in these materials was incorporated within the particles; however, 53% or 86% of the total phosphate associated with the lepidocrocites containing 0.7 or 3 mass% P, respectively, was extracted with 0.1 M NaOH and can be considered to be adsorbed, both to exterior surfaces and within micropores. In the absence of phosphate, lepidocrocite was rapidly reduced to magnetite by Shewanella putrefaciens CN32, and over time the magnetite was partially transformed to ferrous hydroxy carbonate (FHC). The presence of 0.2-0.7 mass% P significantly inhibited the initial reduction of lepidocrocite but ultimately resulted in greater Fe(II) production and the formation of carbonate green rust. The bioreduction of maghemite with and without bound phosphate resulted in solid-state conversion to magnetite, with subsequent formation of FHC. We also examined the potential redox cycling of green rust under alternating Fe(III)-reducing and oxic conditions. Oxidation of biogenic green rust by O2 resulted in conversion to ferric green rust, which was readily reduced back to green rust by S. putrefaciens CN32. These results indicate the potential for cycling of green rust between reduced and oxidized forms under redox dynamics similar to those encountered in environments that alternate between iron-reducing and oxic conditions, and they are consistent with the identification of green rust in soils/sediments with seasonal redox cycling.

Bioreduction of hydrogen uranyl phosphate: mechanisms and U(IV) products

The mobility of uranium (U) in subsurface environments is controlled by interrelated adsorption, redox, and precipitation reactions. Previous work demonstrated the formation of nanometer-sized hydrogen uranyl phosphate (abbreviated as HUP) crystals on the cell walls of Bacillus subtilis, a non-U(VI)-reducing, Gram-positive bacterium. The current study examined the reduction of this biogenic, cell-associated HUP mineral by three dissimilatory metal-reducing bacteria, Anaeromyxobacter dehalogenans strain K, Geobacter sulfurreducens strain PCA, and Shewanella putrefaciens strain CN-32, and compared it to the bioreduction of abiotically formed and freely suspended HUP of larger particle size. Uranium speciation in the solid phase was followed over a 10- to 20-day reaction period by X-ray absorption fine structure spectroscopy (XANES and EXAFS) and showed varying extents of U(VI) reduction to U(IV). The reduction extent of the same mass of HUP to U(IV) was consistently greater with the biogenic than with the abiotic material under the same experimental conditions. A greater extent of HUP reduction was observed in the presence of bicarbonate in solution, whereas a decreased extent of HUP reduction was observed with the addition of dissolved phosphate. These results indicate that the extent of U(VI) reduction is controlled by dissolution of the HUP phase, suggesting that the metal-reducing bacteria transfer electrons to the dissolved or bacterially adsorbed U(VI) species formed after HUP dissolution, rather than to solid-phase U(VI) in the HUP mineral. Interestingly, the bioreduced U(IV) atoms were not immediately coordinated to other U(IV) atoms (as in uraninite, UO2) but were similar in structure to the phosphate-complexed U(IV) species found in ningyoite [CaU(PO4)2·H2O]. This indicates a strong control by phosphate on the speciation of bioreduced U(IV), expressed as inhibition of the typical formation of uraninite under phosphate-free conditions.

Abiotic reductive immobilization of U(VI) by biogenic mackinawite

During subsurface bioremediation of uranium-contaminated sites, indigenous metal and sulfate-reducing bacteria may utilize a variety of electron acceptors, including ferric iron and sulfate that could lead to the formation of various biogenic minerals in situ. Sulfides, as well as structural and adsorbed Fe(II) associated with biogenic Fe(II)-sulfide phases, can potentially catalyze abiotic U(VI) reduction via direct electron transfer processes. In the present work, the propensity of biogenic mackinawite (Fe 1+x S, x = 0 to 0.11) to reduce U(VI) abiotically was investigated. The biogenic mackinawite produced by Shewanella putrefaciens strain CN32 was characterized by employing a suite of analytical techniques including TEM, SEM, XAS, and Mössbauer analyses. Nanoscale and bulk analyses (microscopic and spectroscopic techniques, respectively) of biogenic mackinawite after exposure to U(VI) indicate the formation of nanoparticulate UO2. This study suggests the relevance of sulfide-bearing biogenic minerals in mediating abiotic U(VI) reduction, an alternative pathway in addition to direct enzymatic U(VI) reduction.

Influence of chelating agents on biogenic uraninite reoxidation by Fe(III) (Hydr)oxides

Microbially mediated reduction of soluble U(VI) to U(IV) with subsequent precipitation of uraninite, UO(2(S)), has been proposed as a method for limiting uranium (U) migration. However, microbially reduced UO(2) may be susceptible to reoxidation by environmental factors, with Fe(III) (hydr)oxides playing a significant role. Little is known about the role that organic compounds such as Fe(III) chelators play in the stability of reduced U. Here, we investigate the impact of citrate, DFB, EDTA, and NTA on biogenic UO(2) reoxidation with ferrihydrite, goethite, and hematite. Experiments were conducted in anaerobic batch systems in PIPES buffer (10 mM, pH 7) with bicarbonate for approximately 80 days. Results showed EDTA accelerated UO(2) reoxidation the most at an initial rate of 9.5 μM day(-1) with ferrihydrite, 8.6 μM day(-1) with goethite, and 8.8 μM day(-1) with hematite. NTA accelerated UO(2) reoxidation with ferrihydrite at a rate of 4.8 μM day(-1); rates were less with goethite and hematite (0.66 and 0.71 μM day(-1), respectively). Citrate increased UO(2) reoxidation with ferrihydrite at a rate of 1.8 μM day(-1), but did not increase the extent of reaction with goethite or hematite, with no reoxidation in this case. In all cases, bicarbonate increased the rate and extent of UO(2) reoxidation with ferrihydrite in the presence and absence of chelators. The highest rate of UO(2) reoxidation occurred when the chelator promoted both UO(2) and Fe(III) (hydr)oxide dissolution as demonstrated with EDTA. When UO(2) dissolution did not occur, UO(2) reoxidation likely proceeded through an aqueous Fe(III) intermediate with lower reoxidation rates observed. Reaction modeling suggests that strong Fe(II) chelators promote reoxidation whereas strong Fe(III) chelators impede it. These results indicate that chelators found in U contaminated sites may play a significant role in mobilizing U, potentially affecting bioremediation efforts.

Simultaneous release of Fe and As during the reductive dissolution of Pb-As jarosite by Shewanella putrefaciens CN32

Jarosites are produced during metallurgical processing, on oxidized sulfide deposits, and in acid mine drainage environments. Despite the environmental relevance of jarosites, few studies have examined their biogeochemical stability. This study demonstrates the simultaneous reduction of structural Fe(III) and aqueous As(V) during the dissolution of synthetic Pb-As jarosite (PbFe(3)(SO(4),AsO(4))(2)(OH)(6)) by Shewanella putrefaciens using batch experiments under anaerobic circumneutral conditions. Fe(III) reduction occurred immediately in inoculated samples while As(V) reduction was observed after 72 h. XANES spectra showed As(III) (14.7%) in the solid phase at 168 h coincident with decreased aqueous As(V). At 336 h, XANES spectra and aqueous speciation analysis demonstrated 20.2% and 3.0% of total As was present as As(III) in the solid and aqueous phase, respectively. In contrast, 12.4% of total Fe was present as aqueous Fe(II) and was below the detection limits of XANES in the solid phase. TEM-EDS analysis at 336 h showed secondary precipitates enriched in Fe and O with minor amounts of As and Pb. Based on experimental data and thermodynamic modeling, we suggest that structural Fe(III) reduction was thermodynamically driven while aqueous As(V) reduction was triggered by detoxification induced to offset the high As(V) (328 μM) concentrations released during dissolution.

Reductive dissolution of Tl(I)-jarosite by Shewanella putrefaciens: providing new insights into Tl biogeochemistry

Thallium (Tl) is emerging as a metal of concern in countries such as China due to its release during the natural weathering of Tl-bearing ore deposits and mining activities. Despite the high toxicity of Tl, few studies have examined the reductive dissolution of Tl mineral phases by microbial populations. In this study we examined the dissolution of synthetic Tl(I)-jarosite, (H(3)O)(0.29)Tl(0.71)Fe(2.74)(SO(4))(2)(OH)(5.22)(H(2)O)(0.78), by Shewanella putrefaciens CN32 using batch experiments under anaerobic circumneutral conditions. Fe(II) concentrations were measured over time and showed Fe(II) production (4.6 mM) in inoculated samples by 893 h not seen in mineral and dead cell controls. Release of aqueous Tl was enhanced in inoculated samples whereby maximum concentrations in inoculated and cell-free samples reached 3.2 and 2.1 mM, respectively, by termination of the experiment. Complementary batch Tl/S. putrefaciens sorption experiments were conducted under experimentally relevant pH (5 and 6.3) at a Tl concentration of 35 μM and did not show significant Tl accumulation by either live or dead cells. Therefore, in contrast to many metals such as Pb and Cd, S. putrefaciens does not represent a sink for Tl in the environment and Tl is readily released from Tl-jarosite during both abiotic and biotic dissolution.

Mercury capture into biogenic amorphous selenium nanospheres produced by mercury resistant Shewanella putrefaciens 200

Shewanella putrefaciens 200, resistant to high concentration of Hg(II), was selected for co-removal of mercury and selenium from aqueous medium. Biogenic Hg(0) reduced from Hg(II) by S. putrefaciens 200 was captured into extracellular amorphous selenium nanospheres, resulting in the formation of stable HgSe nanoparticles. This bacterial reduction could be a new strategy for mercury removal from aquatic environments without secondary pollution of mercury methylation or Hg(0) volatilization.

Microbial carbon capture cell using cyanobacteria for simultaneous power generation, carbon dioxide sequestration and wastewater treatment

Microbial carbon capture cells (MCCs) were constructed with cyanobacteria growing in a photo biocathode in dual-chambered flat plate mediator-less MFCs separated by an anion exchange membrane from the anode compartment containing Shewanella putrefaciens. The performance of the MCC with Anabaena sparged with CO(2)-air mixture was compared with that of a conventional cathode sparged with air only. The power densities achieved were 57.8 mW/m(2) for Anabaena sparged with a CO(2)-air mixture, 39.2 mW/m(2) for CO(2)-air mixture sparging only, 29.7 mW/m(2) for Anabaena sparged with air, and 19.6 mW/m(2) for air sparging only. The pH of the cathode containing Anabaena gradually increased from 7 to 9.12 and power generation decreased from 34.7 to 23.8 mW/m(2) 17 due to pH imbalance associated voltage losses without CO(2)-air mixture sparging. Sparging with a 5% CO(2)-air mixture produced maximum power of 100.1 mW/m(2). In addition, the power density of MCC increased by 31% when nitrate was added into the catholyte.

Removal of methylmercury and tributyltin (TBT) using marine microorganisms

Two marine species of bacteria were isolated that are capable of degrading organometallic contaminants: Pseudomonas balearica, which decomposes methylmercury; and Shewanella putrefaciens, which decomposes tributyltin. P. balearica decomposed 97% of methylmercury (20.0 μg/L) into inorganic mercury after 3 h, while S. putrefaciens decomposed 88% of tributyltin (55.3 μg Sn/L) in real wastewater after 36 h. These data indicate that the two bacteria efficiently decomposed the targeted substances and may be applied to real wastewater.

Influence of arsenate adsorption to ferrihydrite, goethite, and boehmite on the kinetics of arsenate reduction by Shewanella putrefaciens strain CN-32

The kinetics of As(V) reduction by Shewanella putrefaciens strain CN-32 was investigated in suspensions of 0.2, 2, or 20 g L(-1) ferrihydrite, goethite, or boehmite at low As (10 μM) and lactate (25 μM) concentrations. Experimental data were compared with model predictions based on independently determined sorption isotherms and rates of As(V) desorption, As(III) adsorption, and microbial reduction of dissolved As(V), respectively. The low lactate concentration was chosen to prevent significant Fe(III) reduction, but still allowing complete As(V) reduction. Reduction of dissolved As(V) followed first-order kinetics with a 3 h half-life of As(V). Addition of mineral sorbents resulted in pronounced decreases in reduction rates (32-1540 h As(V) half-life). The magnitude of this effect increased with increasing sorbent concentration and sorption capacity (goethite < boehmite < ferrihydrite). The model consistently underestimated the concentrations of dissolved As(V) and the rates of microbial As(V) reduction after addition of S. putrefaciens (∼5 × 10(9) cells mL(-1)), suggesting that attachment of S. putrefaciens cells to oxide mineral surfaces promoted As(V) desorption and thereby facilitated As(V) reduction. The interplay between As(V) sorption to mineral surfaces and bacterially induced desorption may thus be critical in controlling the kinetics of As reduction and release in reducing soils and sediments.