Goethite and riboflavin synergistically enhance Cr(VI) reduction by Shewanella oneidensis MR-1

Enhanced dissimilatory nitrate reduction to ammonium and electron transfer mechanisms in bidirectional electron transfer biofilm constructed by iron phthalocyanine

Bidirectional electron transfer biofilms (BETB) could efficiently reduce nitrate without accumulating nitrite, representing a promising biological electrochemical denitrification technology. This study utilized iron phthalocyanine modified carbon felt (FePc-CF) to enrich electroactive bacteria, constructing a long-term stable FePc-BETB. Its nitrate removal rate reached 91%, far exceeding the traditional nitrate-reducing biocathode (45%) and Con-BETB (46%). The dissimilatory nitrate reduction to ammonium (DNRA) dominated nitrate reduction in FePc-BETB, consuming 35% of the total electrons. Additionally, FePc-BETB effectively reduced the accumulation of NO2--N and N2O. Electrochemical analysis demonstrated FePc-BETB exhibited stronger electrochemical activity and electron transfer capability. Mediated electron transfer (MET) enhanced by increased extracellular humic acid in FePc-BETB favored the electron supplement for nitrate removal. The relative abundance of nrfA, marker of the DNRA, increased significantly. This study provided new insights into regulating denitrification and DNRA pathways and treating nitrate wastewater lacking electron donors.

Enhanced bio-reduction of Cr(VI) using Shewanella putrefaciens CN32 mediated by Fe(III) minerals and riboflavin synergistically

Iron minerals and the coupling of electron shuttle media can effectively overcome the problem of the insolubility of iron minerals and the higher cross-medium resistance consequently to enhance the bio-reduction rate of Cr(VI) by dissimilatory metal-reducing bacteria (DMRB). This study explored the potential synergistic enhancement of Cr(VI) bio-reduction by Shewanella putrefaciens CN32 in combination with three iron minerals (ferrihydrite, goethite and hematite) and riboflavin (RF). The addition of RF accelerates the transfer of electrons from bacterial cells to Fe minerals, which in turn promotes the production of large amounts of Fe(II). The results indicated that compared to the control group, the Cr(VI) reduction rates in the CN32/RF/hematite, goethite, ferrihydrite systems increased to 93.03%, 91.07%, and 86.83%, hematite was capable of generating 2.24 mM Fe(II) due to its stable structure and efficient synergy with riboflavin. Enhancement factor(EF) was used to quantify the synergistic effect of RF and iron minerals on the bio-reduction of Cr(VI). At all three reaction times, the FEF (KCN32+RF+Fe/KCN32) of three Fe(III) minerals were all greater than 1. XPS analysis revealed that the primary reduction products of Cr(VI) were identified as Cr(CH3C(O)CHC(O)CH3)3, Cr2O3 and Fe(II)-Cr(III) hydroxide, were predominantly deposited on both bacterial and mineral surfaces, thereby influencing their synergistic interactions. This study unveiled the dynamic synergistic mechanism changes of Cr(VI) reduction in different iron minerals environment,which offers new ideas for the remediation of Cr(VI) pollution.

Identification of key chromium resistance genes in Cellulomonas using transcriptomics

Cellulomonas fimi Clb-11 can reduce high toxic Cr(VI) to less toxic Cr(III), and transcriptomics was used to reveal the key Cr(VI) uptake and reduction genes of C. fimi Clb-11 in this study. The results showed that under 0.5 mM Cr(VI) stress, 654 genes were upregulated. Among the upregulated genes, phosphate transport protein encoding genes phoU and TC.PIT, and molybdate transport protein encoding genes modA, modB, and modC were involved in the cell uptake of Cr(VI). Cytochrome c subunits encoding genes qcrA and qcrC were involved in the intracellular reduction of Cr(VI), and cytochrome c oxidase subunits encoding genes coxB and coxC were involved in extracellular electron secretion. Additionally, several unreported genes were found to be upregulated in C. fimi Clb-11 under Cr(VI) stress. The upregulated manganese transport protein encoding gene mntH may also assist Cr(VI) uptake in C. fimi Clb-11. Proton pump subunit encoding genes atpA, atpB, atpE, atpF, and atpH, as well as sodium-hydrogen antiporter subunit encoding genes mnhA and mnhC were upregulated, it may be involved in the extracellular proton secretion to reduce Cr(VI). Iron complex transport system substrate-binding protein encoding gene ABC.FEV.S, vacuolar iron transporter encoding gene VIT, FMN reductase gene encoding gene ssuE, and quinone oxidoreductase encoding genes qor and qorB were upregulated to reduce Cr(VI) in the intracellular. Our study showed theoretical importance by revealing the Cr(VI) reduction mechanism using chrome-resistant microorganisms.

Vanadium (V) reduction and the performance of electroactive biofilms in microbial fuel cells with Shewanella putrefaciens

The electron transfer ability of biofilms significantly influences the electrochemical activity of microbial fuel cells (MFCs). However, there is limited understanding of pentavalent vanadium (V(V)) bioreduction and microbial response characteristics in MFCs. In this study, the effect of gradient concentrations of V(V) on the performance of EABs with Shewanella putrefaciens in MFCs was investigated. The results showed that as V(V) concentration increased (0-100 mg/L), the voltage output, power densities, polarization, and electrode potential decreased. V(V) was found to act as an electron acceptor and was reduced during MFCs operation, with a yield of 83.16% being observed at 25 mg/L V(V). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) indicated declining electrochemical performance of the MFCs with escalating V(V) concentration. The content of protein and polysaccharide from extracellular polymeric substances (EPS) in anodic biofilms increased to 66.75 and 49.15 mg/L at 75 mg/L V(V), respectively. Three-dimensional fluorescence spectroscopy confirmed increased humic substances in EPS extraction with V(V) exposure. The functional genes narG, nirK, and gor involved in V(V) reduction were upregulated with rising V(V) concentration through quantitative polymerase chain reaction (qPCR) analysis. Additionally, riboflavin, cytochrome c, nicotinamide adenine dinucleotide (NADH), and electron transport system activity (ETSA), key indicators for assessing electron transfer behavior, exhibited a negative correlation with various V(V) concentrations, decreasing by 31.81%, 57.14%, 67.39%, and 51.41%, respectively, at a concentration of 100 mg/L V(V) compared to the blank control. These findings contribute valuable insights into the response of EABs to V(V) exposure, presenting potential strategies for enhancing their effectiveness in the treatment of vanadium-contaminated wastewater.

Ferric citrate enhanced bioreduction of Cr(VI) by Bacillus cereus RCr in aqueous solutions: reduction performance and mechanisms

The bioreduction characteristics and mechanisms of Cr(VI) onto Bacillus cereus RCr enhanced by ferric citrate were investigated. The optimum conditions were initial pH 9, temperature 40 °C, inoculation amount 4%, and glucose 3 g/L, respectively. The addition of 1.5 g/L ferric citrate increased the average reduction rate from 120.43 to 220.61 mg/(L∙h) compared with the control (without ferric citrate). The binding capacity of Cr(III) on the cell surface increased to 21%, in which the precipitates were mainly CrO(OH), Cr2O3, and FeCr2O4. Cell membrane was the main site of reduction, related important functional groups: - COOH, C-H, - NH2, C = C, and P-O. Fe(III) increased the yield of NADH and cytochrome c by approximately 48.51% and 68.63%, which significantly facilitated the electron generation and electron transfer, thus increasing the amount of electrons in the bioreduction of heavy metals by an average of 110%. Among the electrons obtained by Cr(VI), the proportion of indirect reduction mediated by Fe(III)/Fe(II) shuttle was 62% on average, whereas direct reduction mediated by reductase was 38%. These results may provide insights into the bioreduction process by bacteria enhanced by Fe(III) for detoxification of heavy metals with multiple valences, as an important step towards improving microbial remediation.

Biotransformation of Chlorpyrifos Shewanella oneidensis MR-1 in the Presence of Goethite: Experimental Optimization and Degradation Products

In this study, the degradation system of Shewanella oneidensis MR-1 and goethite was constructed with chlorpyrifos as the target contaminant. The effects of initial pH, contaminant concentration, and temperature on the removal rate of chlorpyrifos during the degradation process were investigated. The experimental conditions were optimized by response surface methodology with a Box-Behnken design (BBD). The results show that the removal rate of chlorpyrifos is 75.71% at pH = 6.86, an initial concentration of 19.18 mg·L-1, and a temperature of 30.71 °C. LC-MS/MS analyses showed that the degradation products were C4H11O3PS, C7H7Cl3NO4P, C9H11Cl2NO3PS, C7H7Cl3NO3PS, C9H11Cl3NO4P, C4H11O2PS, and C5H2Cl3NO. Presumably, the degradation pathways involved are: enzymatic degradation, hydrolysis, dealkylation, desulfur hydrolysis, and dechlorination. The findings of this study demonstrate the efficacy of the goethite/S. oneidensis MR-1 complex system in the removal of chlorpyrifos from water. Consequently, this research contributes to the establishment of a theoretical framework for the microbial remediation of organophosphorus pesticides in aqueous environments.

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.

Mechanism of Cr(VI) bioreduction by Clostridium sp. LQ25 under Fe(III) reducing conditions

The Cr(VI) bioreduction has attracted widespread attention in the field of Cr(VI) pollution remediation due to its environmental friendliness. Further in-depth research on the reduction mechanisms is necessary to enhance the efficiency of Cr(VI) bioreduction. However, the limited research on Cr(VI) bioreduction mechanisms remains a bottleneck for the practical application of Cr(VI) reduction. In this study, The Cr(VI) reduction of strain LQ25 was significantly improved when Fe(III) was used as an electron acceptor, which increased by 1.6-fold maximum within the set Cr(VI) concentration range. Based on this, the electron transfer process of Cr(VI) reduction was analyzed using strain LQ25. Based on genomic data, flavin proteins were found to interact closely with electron transfer-related proteins using protein-protein interaction (PPi) analysis. Transcriptome analysis revealed that flavin synthesis genes (ribE, ribBA, and ribH) and electron transfer flavoprotein genes (fixA, etfA, fixB, and etfB) were significantly upregulated when Fe(III) was used as the electron acceptor. These results indicate that the fermentative dissimilatory Fe(III)-reducing bacterial strain LQ25 mainly uses flavin as an electron shuttle for electron transfer, which differs from the common use of cytochrome c in respiratory bacteria. These findings on the mechanism of Cr(VI) bioreduction provide technical support for improving the efficiency of Cr(VI) reduction which promote the practical application of Cr(VI) bioreduction in the field of Cr(VI) pollution remediation.