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Hexabromocyclododecane (HBCD) is a widely used brominated flame retardant; however, it is a persistent organic pollutant as well as affects the human thyroid hormones and causes cancer. However, the degradation of HBCD has received little attention from researchers. Due to its bioaccumulative and hazardous properties, an appropriate strategy for its remediation is required. In this study, we investigated the biodegradation of HBCD using Shewanella oneidensis MR-1 under optimized conditions. The Box-Behnken design (BBD) was implemented for the optimization of the physical degradation parameters of HBCD. S. oneidensis MR-1 showed the best degradation performance at a temperature of 30 °C, pH 7, and agitation speed of 115 rpm, with an HBCD concentration of 1125 μg/L in mineral salt medium (MSM). The strain tolerated up to 2000 μg/L HBCD. Gas chromatography-mass spectrometry analysis identified three intermediates, including 2-bromo dodecane, 2,7,10-trimethyldodecane, and 4-methyl-1-decene. The results provide an insightful understanding of the biodegradation of HBCD by S. oneidensis MR-1 under optimized conditions and could pave the way for further eco-friendly applications. Key points • HBCD biodegradation by Shewanella oneidensis • Optimization of HBCD biodegradation by the Box-Behnken analysis • Identification of useful metabolites from HBCD degradation

Background The microbial fuel cell (MFC) is a green and sustainable technology for electricity energy harvest from biomass, in which exoelectrogens use metabolism and extracellular electron transfer pathways for the conversion of chemical energy into electricity. However, Shewanella oneidensis MR-1, one of the most well-known exoelectrogens, could not use xylose (a key pentose derived from hydrolysis of lignocellulosic biomass) for cell growth and power generation, which limited greatly its practical applications. Results Herein, to enable S. oneidensis to directly utilize xylose as the sole carbon source for bioelectricity production in MFCs, we used synthetic biology strategies to successfully construct four genetically engineered S. oneidensis (namely XE, GE, XS, and GS) by assembling one of the xylose transporters (from Candida intermedia and Clostridium acetobutylicum) with one of intracellular xylose metabolic pathways (the isomerase pathway from Escherichia coli and the oxidoreductase pathway from Scheffersomyces stipites), respectively. We found that among these engineered S. oneidensis strains, the strain GS (i.e. harbouring Gxf1 gene encoding the xylose facilitator from C. intermedi, and XYL1, XYL2, and XKS1 genes encoding the xylose oxidoreductase pathway from S. stipites) was able to generate the highest power density, enabling a maximum electricity power density of 2.1 ± 0.1 mW/m2. Conclusion To the best of our knowledge, this was the first report on the rationally designed Shewanella that could use xylose as the sole carbon source and electron donor to produce electricity. The synthetic biology strategies developed in this study could be further extended to rationally engineer other exoelectrogens for lignocellulosic biomass utilization to generate electricity power.

Shewanella oneidensis has demonstrated excellent potential for azo dye decolorization and degradation. However, in anaerobic environments, S. oneidensis has a narrow carbon source spectrum, which requires additional electron donors, such as sodium lactate. This increases the practical application costs for wastewater treatment. Here, we aimed to expand the carbon source utilization range of S. oneidensis FJAT-2478 by co-culturing it with Lactobacillus plantarum FJAT-7926, leveraging their commensalism relationship to develop a metabolic chain. Results showed that a 1:2 initial ratio of L. plantarum FJAT-7926 to S. oneidensis FJAT-2478 achieved a 97.16% decolorization rate of methyl orange when glucose served as the sole carbon source. This co-culture system achieved a decolorization rate comparable to that obtained using sodium lactate as an electron donor and was significantly higher than that achieved by L. plantarum FJAT-7926 (7.88%) or S. oneidensis FJAT-2478 (6.89%) alone. After undergoing five cycles, the co-culture system continued to exhibit effective decolorization. It was demonstrated that the co-culture system could use common and inexpensive carbon sources, such as starch, molasses, sucrose, and maltose, to decolorize azo dyes. For instance, 100 mg/L methyl orange could be degraded by over 98.05% within 24 h. The results indicated that the degradation rates of methyl orange were higher when L. plantarum was inoculated first, followed by a subsequent inoculation of S. oneidensis after 2 h. The co-culturing of L. plantarum FJAT-7926 and S. oneidensis FJAT-2478 proved to be an effective strategy in treating azo dye wastewater, expanding the potential practical applications of S. oneidensis.

Biochar-mediated microbial degradation is receiving increased attention for remediating organic pollutants. However, the mechanism of biochar-facilitated bioremediation is poorly understood. This study demonstrated that wheat straw biochar could significantly enhance roxarsone biodegradation by Shewanella oneidensis MR-1. The average rate constants were calculated to be 0.0410, 0.0477, 0.0575, and 0.0441 h−1 for treatments with 0.05, 0.1, 0.5, and 1 g/L biochar, respectively, which were all higher than that in treatment with only MR-1 (0.0363 h−1). The impact of biochar dosage on roxarsone degradation followed as 0.5 g/L > 0.1 g/L > 1.0 g/L > 0.05 g/L biochar. The high specific surface area of biochar provided more opportunity for contact with MR-1, which was certified by the scanning electron microscope images. The extracellular polymeric substances were affected by the addition of biochar. The concentrations of protein and polysaccharide were lower with various concentrations of biochar to the system with only MR-1. The cell proliferation was promoted by the addition of biochar and the optical density value of MR-1 followed 0.5 g/L biochar group > 0.1 g/L biochar group > 1 g/L biochar group > 0.05 g/L biochar group > only MR-1. These results improved our understanding of biochar-mediated biodegradation and showed that an optimized dosage of wheat straw-derived biochar could be a microbial growth activator, accelerating the roxarsone conversion rate.

Carbon-based catalysts for activating persulfate to drive advanced oxidation processes (AOPs) are widely used in wastewater treatment. In this study, Shewanella oneidensis MR-1, a typical ferric reducing electroactive microorganism, was utilized as the raw material of biochar (BC) to prepare a novel green catalyst (MBC). The effect of MBC on activating persulfate (PS) to degrade rhodamine B (RhB) was evaluated. Experimental results showed that MBC could effectively activate PS to degrade RhB to reach 91.70% within 270 min, which was 47.4% higher than that of pure strain MR-1. The increasing dosage of PS and MBC could improve the removal of RhB. Meanwhile, MBC/PS can well perform in a wide pH range, and MBC showed good stability, achieving 72.07% removal of RhB with MBC/PS after 5 cycles. Furthermore, the free radical quenching test and EPR experiments confirmed the presence of both free radical and non-free radical mechanisms in the MBC/PS system, with •OH, SO 4 •− and 1 O 2 contributing to the effective degradation of RhB. This study successfully provided a new application for bacteria to be used in the biochar field.

Peroxymonosulfate (PMS) advanced oxidation is gaining recognition as a promising method for tackling persistent soil pollutants. However, developing an efficient PMS activator remains a formidable task. This study harnessed Shewanella oneidensis MR-1, a model dissimilatory metal-reducing bacterium (DMRB), to synthesize Mn2O3 nanoparticles by oxidizing Mn(II). These nanoparticles were employed to activate PMS for phenanthrene degradation in soil. Remarkably, biogenic Mn2O3 outperformed chemically synthesized Mn2O3, removing 77.4% of phenanthrene compared to 55.7%. This superior performance is attributed to biogenic Mn2O3's faster electron transfer rate and higher Mn(III) ratio, facilitating electron donation to PMS. Additionally, we assessed the feasibility of PMS advanced oxidation for soil remediation by examining microbial community diversity. Given manganese's prevalence in natural soil and groundwater, in-situ biogenic Mn2O3 synthesis emerges as an innovative soil remediation strategy.

Bioreduction of Cr(VI) is cost-effective and environmentally friendly, however, the slow bioreduction rate limits its application. In this study, the potential synergistic enhancement of Cr(VI) bioreduction by shewanella oneidensis MR-1 (S. oneidensis) with goethite and riboflavin (RF) was investigated. The results showed that the S. oneidensis reaction system reduce 29.2% of 20 mg/L Cr(VI) after 42 h reaction, while the S. oneidensis/goethite/RF reaction system increased the Cr(VI) reduction rate to 87.74%. RF as an efficient electron shuttle and Fe(II) from goethite bioreduction were identified as the crucial components in Cr(VI) reduction. XPS analysis showed that the final precipitates of Cr(VI) reduction were Cr(CH3C(O)CHC(O)CH3)3 and Cr2O3 and adhered to the bacterial cell surface. In this process, the microbial surface functional groups such as hydroxyl and carboxyl groups participated in the adsorption and reduction of Cr(VI). Meanwhile, an increase in cytochrome c led to an increase in electron transfer system activity (ETSA), causing a significant enhancement in extracellular electron transfer efficiency. This study provides insight into the mechanism of Cr(VI) reduction in a complex environment where microorganisms, iron minerals and RF coexist, and the synergistic treatment method of Fe(III) minerals and RF has great potential application for Cr(VI) detoxification in aqueous environment.

Removal of Acid Red 88 (AR88) as an azo dye from the synthetic type of wastewater was studied in a laboratory-made constructed wetland microbial fuel cell (CW-MFC) inoculated with Shewanella oneidensis MR-1 (SOMR-1). Plant cultivation was implemented using a typical CW plant known as Cyperus alternifolius . The complexity of the SOMR-1 cell membrane having different carriers of electrons and H + ions gives the microbe special enzymatic ability to participate in the AR88 oxidation link to the O 2 reduction. Nernst equation developed based on analyzing the involved redox potential values in these electron exchanges is describable quantitatively in terms of the spontaneity of the catalyzed reaction. Power density (PD) at 100 mg/L of the AR88 under closed-circuit conditions in the presence of the plant was 11.83 mW/m 2 . Reduction of internal resistance positively affected the PD value. In determining degradation kinetics, two approaches were undertaken: chemically in terms of first- and second-order reactions and biochemically in terms of the mathematical equations for rate determination developed on the basis of substrate inhibitory concept. The first-order rate constant was 0.263 h −1 without plant cultivation and 0.324 h −1 with plant cultivation. The Haldane kinetic model revealed low k s and k i values indicating effective degradation of the AR88. Moreover, the importance of acclimatization in terms of the crucial role of lactate was discussed. These findings suggest that integrating the SOMR-1 electrochemical role with CW-MFC could be a promising approach for the efficient degradation of azo dyes in wastewater treatment.

Lipophilic electron shuttles (ESs), such as phenazine and phenoxazine, can penetrate the outer membrane and enter the periplasmic space, mediating extracellular electron transfer reactions. This study investigates how lipophilic ESs (resazurin, a phenoxazine) regulate carbon metabolic pathways in bioelectrochemical systems using Shewanella oneidensis MR‐1 as a model organism. Through the analysis of acetate yield, CO2 production, coulombic efficiency, and other parameters, it is found that resazurin increases coulombic efficiency (26% vs 17% for anthraquinone‐2,6‐disulfonic acid [AQDS]) and reduces acetate yield (82% vs 90% for AQDS) while slightly increasing CO2 production (13.1% vs 11.8% for AQDS), indicating a shift in carbon metabolism. Transcriptome analysis reveals significant upregulation of genes involved in the NADH‐dependent metabolic pathway (e.g., nuoHIJKLMN) and ATP synthesis (atpABDEFGH) under resazurin conditions. Mutant strains lacking key genes in oxidative phosphorylation (Δatp) or substrate‐level phosphorylation (Δack&pta) further confirm the regulatory role of lipophilic shuttles. The study proposes that lipophilic ESs penetrate the periplasm, altering the redox state of inner‐membrane quinones and activating the NADH‐dependent metabolic pathway via the Arc system. This mechanism enhances TCA cycle activity and overall lactate metabolic efficiency. The findings provide insights into microbial carbon metabolic regulation and offer strategies for optimizing bioelectrochemical systems for bioremediation. This study clarifies the mechanism by which lipophilic shuttles regulate carbon metabolism in Shewanella. Lipophilic electron shuttles, such as resazurin, permeate into the periplasm to mediate extracellular electron transfer, thereby altering the redox potential sensed by inner‐membrane quinones and consequently activating the NADH‐dependent metabolic pathway via the Arc system.

Substitution of aluminum under natural environmental conditions has been proven to inhibit the transformation of weakly crystalline iron (oxyhydr)-oxides towards well crystalline iron oxides, thereby enhancing their long-term stability. However, exploration on the role of aluminum substitution in bacteria-mediated iron oxides transformation is relatively lacking, especially in the anaerobic underground condition where iron (oxyhydr)-oxides are easy to reduced. In this study, we selected four different levels of substitution aluminum prevalent in iron oxides under natural conditions, which are 0 mol%, 10 mol%, 20 mol%, and 30 mol% (mol Al/mol (Al + Fe)) respectively. With the presence of Shewanella oneidensis MR-1, we conducted a 15-day anaerobic microcosm experiment in simulated groundwater conditions. The experiment data suggested that aluminum substitution result in a decrease in bio-reduction rate constants of ferrihydrite from 0.24 in 0 mol% Al to 0.17 in 30 mol% Al. Besides, when containing substituted aluminum, secondary minerals produced by biological reduction of ferrihydrite changed from magnetite to akaganeite. These results were attributed to the surface coverage of Al during the reduction process, which affects the contact between S. oneidensis MR-1 and the unexposed Fe(III), thus inhibiting the further reduction of ferrihydrite. Since iron (oxyhydr)-oxides exhibit a strong affinity on multiple kinds of pollutants, results in this study may contribute to predicting the migration and preservation of contaminants in groundwater systems. Graphical Abstract