Strain MR-1 Research Articles

Co-existing siderite alleviates the Fe(II) oxidation-induced inactivation of Fe(III)-reducing bacteria.

Abiotic Fe (II) oxidation widely occurs in the natural subsurface environment and engineered dynamic processes, which possibly impacts the growth of indigenous microbes. As previously discovered, the oxidation of aqueous Fe2+ at neutral pH effectively inactivates iron-reducing bacteria Shewanella oneidensis strain MR-1 (MR-1). Herein, the impacts of co-existing iron mineral on the oxidation of aqueous Fe2+ and the subsequent disinfection activity on MR-1 were investigated with siderite selected as a representative iron mineral in the subsurface environment. The oxidation rate of aqueous Fe2+ and the amount of generated OH radical increased as the content of siderite increased, while the MR-1 inactivation was alleviated. An initial concentration of 2.0 × 106 CFU/mL MR-1 was inactivated by about 2.7 orders of magnitude during oxidation of 0.2 mM FeSO4 alone for 30 min, which was reduced to only about 0.6 orders of magnitude in the presence of 4.3 mM co-existing siderite. ROS scavenging results confirmed that the OH radical generated in the bulk solution was not the leading role for the inactivation of MR-1. Morphological changes of the cells observed by SEM demonstrated that the disruption of the cell membrane was alleviated by siderite, which was further supported by the XRD and FTIR spectra. The underlying mechanism was proposed to be the reduced contact time of Fe2+ and MR-1 cells due to the accelerated oxidation. This work provides new insights into the disinfection behavior of heterogeneous Fe (II) oxidation on iron cycling bacterial in the natural environment.

Genome-wide identification of long non-coding RNAs (lncRNAs) associated with malathion resistance in Bactrocera dorsalis.

Long non-coding RNAs (lncRNAs) play important roles in the regulation of biological processes and have been identified in many species including insects. However, the association between lncRNAs and pesticide resistance in insect species such as Bactrocera dorsalis is unknown. RNA-seq was performed on malathion resistant (MR1) and susceptible (MS) strains of B. dorsalis and a total of 6171 lncRNAs transcripts were identified. These included 3728 lincRNAs, 653 antisense lncRNAs, 1402 intronic lncRNAs, and 388 sense lncRNAs. A total of 40 and 52 upregulated lncRNAs were found in females and males of the MR1 strain compared to 54 and 49 in the same sexes of the MS strain, respectively. Twenty-seven of these lncRNAs showed the same trend of expression in both females and males in the MR1 strain, in which 15 lncRNAs were upregulated and 12 were downregulated. RT-qPCR results indicated that the differentially expressed lncRNAs were associated with malathion resistance. The lnc15010.10 and lnc3774.2 were highly expressed in the cuticle of the MR1 strain, indicating that these two lncRNAs may be related to malathion resistance. RNAi of lnc3774.2 and a bioassay showed that malathion resistance was possibly influenced by changes in the B. dorsalis cuticle. LncRNAs of B. dorsalis potentially related to the malathion resistance were identified. Two lncRNAs appear to influence malathion resistance via modulating the structure, or components, of the cuticle. © 2021 Society of Chemical Industry.

Ameliorative Function of a Probiotic Bacterium, Lactobacillus rhamnosus MR1 on Acute Iron Toxicity in Rats

Overuse of iron supplements can lead to an acute inflammation of the gastrointestinal tract. This study investigates the ameliorative and prophylactic effects of a probiotic bacterium, L. rhamnosus MR1, on acute iron poisoning in rats. In this study, a probiotic strain was isolated from yogurt and characterized for its probiotic properties, including antibiotic-resistant, bile salt (BS) and acid resistance, iron tolerance, cell hydrophobicity of the bacterial cells. The anti-inflammatory effect of strain MR1 was studied on the iron exposed-Caco-2 cell line. In vivo experiments were conducted for the assessment of survival in rats overdosed with treatment. These findings indicate high bacterial tolerance in acidic conditions, high concentrations of bile salts, and iron. The anti-inflammatory effects of strain MR1 were confirmed by decreasing the concentration of pro-inflammatory cytokine IL-8 and increasing anti-inflammatory cytokine IL-4 in treated groups. Prophylactic and acute effects of strain MR1 in rats caused a significant reduction in intestinal iron poisoning by 50 % during 6 h. Prophylactic regimen by L. rhamnosus MR1 increased the viability of about 33% in acutely poisoned rats. Since no report is found in the current literature about the effect of probiotic supplements on iron's acute toxicity, these interesting results can provide a useful background for further studies on dietary supplements.

Outer membrane compositions enhance the rate of extracellular electron transport via cell-surface MtrC protein in Shewanella oneidensis MR-1

While cell membrane composition is critical for the function of membrane proteins, membrane modification has not been used to control the rate of extracellular electron transfer (EET) via the outer membrane protein complexes. Here, the rate of electron flow via the cell-surface redox protein, MtrC, was highly enhanced upon change in the outer membrane composition in Shewanella oneidensis MR-1. The MR-1 strain was pre-cultured at 4 °C and 30 °C to initiate differentiation of membrane composition. The whole-cell difference electrochemical assay of wild-type and mutant strains lacking MtrC suggested that the rate of EET via MtrC increased approximately 18 times at 4 °C than 30 °C. Circular dichroism spectroscopy showed that the molar exciton coupling coefficient for inter-heme interaction in MtrC increased in MR-1 at 4 °C than 30 °C. Results suggest that membrane modification may be a novel strategy for improving the efficiency of EET-based technologies.

Elevated intracellular cyclic-di-GMP level in Shewanella oneidensis increases expression of c-type cytochromes.

Electrochemically active biofilms are capable of exchanging electrons with solid electron acceptors and have many energy and environmental applications such as bioelectricity generation and environmental remediation. The performance of electrochemically active biofilms is usually dependent on c-type cytochromes, while biofilm development is controlled by a signal cascade mediated by the intracellular secondary messenger bis-(3'-5') cyclic dimeric guanosine monophosphate (c-di-GMP). However, it is unclear whether there are any links between the c-di-GMP regulatory system and the expression of c-type cytochromes. In this study, we constructed a S. oneidensis MR-1 strain with a higher cytoplasmic c-di-GMP level by constitutively expressing a c-di-GMP synthase and it exhibited expected c-di-GMP-influenced traits, such as lowered motility and increased biofilm formation. Compared to MR-1 wild-type strain, the high c-di-GMP strain had a higher Fe(III) reduction rate (21.58 vs 11.88pM of Fe(III)/h cell) and greater expression of genes that code for the proteins involved in the Mtr pathway, including CymA, MtrA, MtrB, MtrC and OmcA. Furthermore, single-cell Raman microspectroscopy (SCRM) revealed a great increase of c-type cytochromes in the high c-di-GMP strain as compared to MR-1 wild-type strain. Our results reveal for the first time that the c-di-GMP regulation system indirectly or directly positively regulates the expression of cytochromes involved in the extracellular electron transport (EET) in S.oneidensis, which would help to understand the regulatory mechanism of c-di-GMP on electricity production in bacteria.

Heterologous expression of azoreductase-encoding gene azrS of Bacillus sp. MR-1/2 for enhanced azo dye decolorization and wastewater treatment.

In Pakistan, 55% of textile exports are contributed by textile-units of Faisalabad. The effluents of these textile units, being discharged without any treatment, contain the contamination of a huge amount of synthetic azo dyes. The objective of the current research was to evaluate the contribution of an azoreductase-encoding gene (azrS) from a pre-characterized azo dye decolorizing bacterial strain Bacillus sp. MR-1/2 in a high copy number host system (pUC19-T7-Top-T) of Escherichia colistrain DH5α followed by in-silico prediction of azoreductase enzyme (AzrS) function. The recombinant cells that contained azrS had a significantly higher rate of color removal in congo red and reactive black-5 dyes when compared to wild-type MR-1/2 and E. coli DH5α after 72h of incubation. Moreover, we were able to show that the recombinant strain significantly reduced the values of all tested parameters (pH, EC, turbidity, TSS, and COD) in actual wastewater. In support of our results, it was also predicted through bioinformatics analysis that the deduced azoreductase protein of strain MR-1/2 is linked with the dye decolorization ability of the strain through NAD(P)H-ubiquinone: oxidoreductase activity. Furthermore, we also found that the deduced protein resembled closely related proteins of protein databank in many features, yet some unique features were predicted in the enzyme activity of strain MR-1/2. It was concluded that the recombinant strain could be examined in pilot-scale experiments for textile wastewater treatment.

Interplay between iron species transformation and hydroxyl radicals production in soils and sediments during anoxic-oxic cycles

Iron (Fe) redox transformation in soils and sediments plays an essential role in the biogeochemical cycling of associated nutrients and elements. It is documented recently that hydroxyl radicals (OH) can be produced upon oxygenation of Fe(II)-bearing minerals in soils and sediments. As OH induces serious oxidative stress to microbes, it is likely that OH produced upon Fe(II) oxygenation inactivates the coexisting dissimilatory iron-reducing bacteria and subsequently impacts Fe redox transformation. This hidden interplay between Fe species transformation and OH production in soils and sediments during anoxic-oxic cycles remains unexplored. In this study, a field sediment was incubated with 1 × 109 CFU/mL Shewanella oneidensis strain MR-1 and treated for six anoxic-oxic cycles (117-h anoxic and 6-h oxic periods). Fe species transformation, OH production and MR-1 activity variation were measured throughout the six cycles. Fe in the sediments prior to anoxic-oxic cycles mainly existed in Fe-bearing silicates (60–70%) and Fe (hydr)oxides (30–40%). For the anoxic periods, a portion of Fe(III) in sediments was either reduced to structural Fe(II) in (hydr)oxides and silicates or reductively dissolved to aqueous Fe2+. Total Fe(II) produced from Fe(III) bio-reduction increased from 2.46 to 8.24 mM in the first two anoxic periods, but decreased to 0.10 mM in the following four anoxic periods. For the oxic periods, Fe(II) was oxidized along with a production of OH, and the cumulative OH concentrations were around 31 µM in the first three periods but decreased to 6.3 µM in the subsequent three periods. MR-1 was inactivated by about 0.6 order of magnitude in the first two oxic periods, and the inactivation became insignificant for the later oxic periods. The six cycles of anoxic-oxic treatments promoted the transformation of Fe(III) (hydr)oxides into more amorphous phase. A strong interplay between Fe species transformation and OH production occurred in anoxic-oxic cycles. Fe(III) bio-reduction by MR-1 in anoxic periods determined the content and speciation of Fe(II), which governed OH production in the subsequent oxic periods. The generated OH affected MR-1 activity in oxic periods, which further controlled Fe(III) bio-reduction in anoxic periods. The hidden interplay identified herein improves our understanding of the cycling of Fe and associated elements under redox fluctuating environments.

Promoting bidirectional extracellular electron transfer of Shewanella oneidensis MR-1 for hexavalent chromium reduction via elevating intracellular cAMP level.

The bioreduction capacity of Cr(VI) by Shewanella is mainly governed by its bidirectional extracellular electron transfer (EET). However, the low bidirectional EET efficiency restricts its wider applications in remediation of the environments contaminated by Cr(VI). Cyclic adenosine 3',5'-monophosphate (cAMP) commonly exists in Shewanella strains and cAMP-cyclic adenosine 3',5'-monophosphate receptor protein (CRP) system regulates multiple bidirectional EET-related pathways. This inspires us to strengthen the bidirectional EET through elevating the intracellular cAMP level in Shewanella strains. In this study, an exogenous gene encoding adenylate cyclase from the soil bacterium Beggiatoa sp. PS is functionally expressed in Shewanella oneidensis MR-1 (the strain MR-1/pbPAC) and a MR-1 mutant lacking all endogenous adenylate cyclase encoding genes (the strain Δca/pbPAC). The engineered strains exhibit the enhanced bidirectional EET capacities in microbial electrochemical systems compared with their counterparts. Meanwhile, a three times more rapid reduction rate of Cr(VI) is achieved by the strain MR-1/pbPAC than the control in batch experiments. Furthermore, a higher Cr(VI) reduction efficiency is also achieved by the strain MR-1/pbPAC in the Cr(VI)-reducing biocathode experiments. Such a bidirectional enhancement is attributed to the improved production of cAMP-CRP complex, which upregulates the expression levels of the genes encoding the c-type cytochromes and flavins synthetic pathways. Specially, this strategy could be used as a broad-spectrum approach for the other Shewanella strains. Our results demonstrate that elevating the intracellular cAMP levels could be an efficient strategy to enhance the bidirectional EET of Shewanella strains and improve their pollutant transformation capacity.

Enhanced bioreduction of 2,5-dichlorobenzene by an AHQ/RGO binary nanocomposite through a synergistic effect with outer membrane proteins of Shewanella oneidensis MR-1

The slow anaerobic reduction rate of 2,5-dichloronitrobenzene (DCNB) is a bottleneck during its biotreatment. In this study, the process and mechanism of enhanced DCNB bioreduction by a 2-amino-3-hydroxy-anthraquinone/reduced grapheme oxide (AHQ/RGO) binary nanocomposite in the presence of Shewanella oneidensis MR-1 (MR-1) are reported. AHQ/RGO was prepared using a one-step covalent combination method under mild conditions, and the as-prepared binary nanocomposite was further characterized by SEM, TEM, AFM, EA, UV–Vis, XRD, XPS, FTIR and electrochemical analyses. The results of catalytic assays showed that enhanced DCNB bioreduction to 2,5-dichloroaniline could be achieved in the presence of 30 mg/L AHQ/RGO and the k (h−1) value of the AHQ/RGO-supplemented system is 7.7- and 4.2-fold greater than that of the control and GO-supplemented systems, respectively. Moreover, molecular studies showed that the genes encoding cytochromes and reductases (especially yceJ and azoR) play key roles during the reduction of DCNB by MR-1 cells and multiple electron transfer pathways are involved during AHQ/RGO-mediated DCNB bioreduction. In addition, a synergistic effect between AHQ/RGO and outer membrane proteins of MR-1 was confirmed by mutant assays and is likely responsible for the catalytic performance of AHQ/RGO. This work provides a potential application of strain MR-1 and AHQ/RGO in enhancing the treatment of DCNB-containing wastewater.

Preventing Hydrogen Disposal Increases Electrode Utilization Efficiency by Shewanella oneidensis

Many bacteria use hydrogen anaerobically as both a source and sink for electrons; consuming hydrogen when it is plentiful and producing it when concentrations are low enough to allow proton reduction. While this can increase an organism’s competitiveness, hydrogen uptake or excretion can also make it difficult to control electron flux to a specific product. For example, when Shewanella oneidensis strain MR-1 is used to oxidize organic molecules and recover electrons in microbial electrochemical devices, small changes in ambient hydrogen concentrations could dramatically alter the efficiency of electron capture at the anode due to this organism’s respiratory flexibility. When new three-electrode reactor designs created to minimize oxygen intrusion during anodic growth were tested with lactate-oxidizing S. oneidensis, current production decreased significantly in reactors vented to remove hydrogen produced at the counter electrode, suggesting a role for hydrogen uptake or disposal when cells used electrodes as electron acceptors. A ∆hydA∆hyaB mutant lacking both hydrogenases reversed this trend, and nearly doubled current production rates. This increase was shown to be due to the efficiency of lactate oxidation, as 90% of electrons were recovered as electricity in the ∆hydA∆hyaB mutant compared to only 50% for wild type. Experiments with Fe(III) oxide provided additional evidence that S. oneidensis generates hydrogen reducing equivalents during reduction of insoluble electron acceptors, while experiments with cells incubated with Fe(III) citrate showed increased survival of wild-type compared to ∆hydA∆hyaB in stationary phase. Together these data show how the multiple routes of electron disposal of S. oneidensis, while beneficial under changing conditions, limits the efficiency of electron recovery in electrochemical systems, and demonstrates a simple approach to increasing current production rates in systems where hydrogen is being captured as a product.

Attenuation of Fe(III)-reducing bacteria during table fluctuation of groundwater containing Fe2+

Groundwater table fluctuation during natural and anthropogenic processes can facilitate the interaction between oxygen (O2) from the unsaturated zone and ferrous iron (Fe2+) from the saturated zone. In light of previous findings that Fe(III)-reducing bacteria can be killed by the reactive oxidants produced from Fe2+ oxidation under static oxic conditions, we hypothesize that Fe(III)-reducing bacteria will be attenuated during groundwater table fluctuations. To test this hypothesis, this study explored the variations of cell numbers of Shewanella oneidensis strain MR-1 (MR-1), a typical strain of Fe(III)-reducing bacteria, together with dissolved oxygen (DO) and Fe2+, at different points during controlled groundwater table fluctuations in a sand column. The results showed that, during the rise of the water table, O2 in the pore air was entrapped by the deoxygenated groundwater, and Fe2+ in the groundwater was oxidized by the entrapped O2. In this process, 1.40–2.42 orders of magnitude of viable MR-1 cells were killed at different points in the column. Further investigation proposed that the death of MR-1 is caused by the production of intracellular reactive oxidants, such as O2•− and OH•, from the oxidation of adsorbed/absorbed Fe2+ instead of by bulk reactive oxidants, such as OH• and Fe(IV), produced from the oxidation of aqueous Fe2+. The findings here provide new insights for Fe biogeochemical cycling in the redox-dynamic zone.

Transport and retention of Shewanella oneidensis strain MR1 in water-saturated porous media with different grain-surface properties

Bacterium Shewanella oneidensis strain (MR1), a facultative microorganism that plays critical role in contaminant transformation and degradation, was used as an example to decipher the transport and retention of microorganisms in water-saturated porous media with different grain-surface properties and under different ionic compositions (i.e. Na+, Ca2+, and Mg2+). Dolomite and quartz sands, which contained different surface charge properties, were used as the representative minerals. Dolomite was selected because its surface charges are significantly affected by solution composition. The mobility of MR1 in the dolomite column was lower than that in the quartz column, because the lower energy barrier between MR1 and dolomite than that between MR1 and quartz, resulting in the larger retention of MR1 in the dolomite column. The breakthrough curves were well simulated by the two sites kinetic model with HYDRUS-1D. The maximum concentration of attached bacteria (Smax) were positively correlated to the ionic strength regardless of mineral types. The values of Smax were about 1.1–4.0 times larger in the MR1-dolomite system than that in the MR1-quartz system under different ionic strength conditions. The retention of the MR1 on dolomite surfaces in the presence of divalent cations Ca2+ is significantly higher than that on quartz surfaces primarily due to the larger electrostatic attraction energy between the MR1 and dolomite grains. The findings demonstrate that the porous media with the lower negative charge has the higher capacity for the retention and deposition of MR1, potentially affecting the transport of MR1 and other bacteria in the subsurface.

A new recombineering system for precise genome-editing in Shewanella oneidensis strain MR-1 using single-stranded oligonucleotides

Shewanella oneidensis MR-1 is an invaluable host for the discovery and engineering of pathways important for bioremediation of toxic and radioactive metals and understanding extracellular electron transfer. However, genetic manipulation is challenging due to the lack of genetic tools. Previously, the only reliable method used for introducing DNA into Shewanella spp. at high efficiency was bacterial conjugation, enabling transposon mutagenesis and targeted knockouts using suicide vectors for gene disruptions. Here, we describe development of a robust and simple electroporation method in S. oneidensis that allows an efficiency of ~4.0 x 106 transformants/µg DNA. High transformation efficiency is maintained when cells are frozen for long term storage. In addition, we report a new prophage-mediated genome engineering (recombineering) system using a λ Red Beta homolog from Shewanella sp. W3-18-1. By targeting two different chromosomal alleles, we demonstrate its application for precise genome editing using single strand DNA oligonucleotides and show that an efficiency of ~5% recombinants among total cells can be obtained. This is the first effective and simple strategy for recombination with markerless mutations in S. oneidensis. Continued development of this recombinant technology will advance high-throughput and genome modification efforts to engineer and investigate S. oneidensis and other environmental bacteria.

Enhanced performance of sediment microbial fuel cell by immobilization of Shewanella oneidensis MR-1 on an anode surface

The anodic microbial community especially relative abundance of exoelectrogens is crucial for performance enhancement of sediment microbial fuel cell (SMFC). In this study, Shewanella oneidensis MR-1 was immobilized on an active carbon fiber anode and the SMFC's performance was evaluated. The maximum power density of SMFC with immobilized strain MR-1 (SMFC-ImSW) displayed 61.0 mW/m2, and SMFC with conventional bioaugmentation (SMFC-SW) and no treatment (SMFC-CK) of strain MR-1 obtained 11.1 and 2.4 mW/m2, respectively. The chemical oxygen demand removal efficiency of SMFC-ImSW was 14.2% and 24.2% higher than SMFC-SW and SMFC-CK groups, respectively. Furthermore, the organics removal efficiencies of sediment were also increased in SMFC-Imsw. GFP-tagging and RT-PCR results showed that strain MR-1 was colonized on the anode biofilm of SMFC-ImSW. Cyclic voltammetry and Electrochemical impedance spectroscopy results showed that higher redox potential and charge transfer efficiency presented with immobilized strain MR-1. MiSeq sequencing results showed that Sphaerochaetaceae and Marinilabiaceae were also enriched on the anode biofilm, which are related to power generation. These results demonstrated that immobilized bioaugmentation is an ideal approach to improve SMFC performance.

Bacterial Synthesis of Cadmium and Zinc Sulfide Nanoparticles: Characteristics and Prospects of Application

Nanoparticles of cadmium sulfide (NpCdS) and zinc sulfide (NpZnS) are shown to be formed in the presence of Shewanella oneidensis MR-1 bacteria. These nanoparticles were located in the culture liquid and had a spherical shape. The nanoparticles were characterized by a small range of size distribution; the maximal size of NpCdS and NpZnS was approximately 5–6 nm; over 70% of the nanoparticles had a size of approximately 2–4 nm. The effective diameter of NpCdS and NpZnS was 158.9 nm and 219.3 nm, respectively. The value of the ζ-potential of NpCdS and NpZnS was –22.43 and –31.46 mV, respectively, which characterized the aqueous suspensions of these nanoparticles as metastable. The selectivity of the adsorption of certain proteins from the common pool of the protein molecules, synthesized by S. oneidensis MR-1 strain during cultivation, on the surface of nanoparticles is ascertained. Independently of the chemical nature of the particles (NpCdS and NpZnS), the protein molecules adsorbed on their surface are shown to have similar molecular weights. For the first time, biogenic NpCdS and NpZnS are used to develop a model composite system by the immobilization of amine-containing poly(glycidyl methacrylate) (PGMA) microspheres on the surface.

Enzymatic detoxification of azo dyes by a multifarious Bacillus sp. strain MR-1/2-bearing plant growth-promoting characteristics.

This study was conducted to elucidate the inherent potential of Bacillus sp. MR-1/2, which was isolated from root zone of maize crop grown on a textile wastewater-irrigated soil. The isolated strain was identified through its ribosomal RNA sequence. Under in vitro conditions, the strain demonstrated its tolerance for high concentrations of various heavy metal ions as determined by minimum inhibitory concentration. Moreover, the strain MR-1/2 exhibited many important phytobeneficial traits such as inorganic P solubilization and 1-aminocyclopropane-1-carboxylate (ACC) deaminase ability even under high metal and salt stress. Results showed that the strain proficiently decolorizes various azo dye compounds, e.g., reactive black-5, reactive red-120, and direct blue-1 and congo red, in broth culture. The bioremediation potential of the strain MR-1/2 was further confirmed by analyzing the retrieved azoreductase gene sequence through bioinformatics tools, whereby a subsequent prediction revealed that the azoreductase enzyme activity was involved in decolorization process. When mung bean seeds were grown in pots under various concentrations of decolorized and non-decolorized azo dye, the Bacillus sp. MR-1/2 not only alleviated the azo dye toxicity, but also increased the plant growth parameters. In conclusion, the strain MR-1/2 efficiently decolorized the azo dyes and helped in mung bean plant growth by alleviating azo dye toxicity.

Redox Structures of Humic Acids Derived From Different Sediments and Their Effects on Microbial Reduction Reactions.

Herein, we investigated the chemical, electrochemical, and spectroscopic characteristics of humic acids (HAs) extracted from sediments of different origin [Ling Qiao river, Xi Xi wetland, Qi Zhen lake (QZ), and Hu Zhou pond in Zhejiang province, China], paying particular attention to their role in the enhancement of nitrate and FeOOH reduction. Notably, the highest C/N ratio (16.16), O/C ratio (1.89), and Fe content (11.57 g kg-1 sample) were observed for HAs extracted from QZ sediment. Cyclic voltammetry analyses confirmed that all HAs contained redox-active groups and exhibited redox potentials between -0.36 and -0.28 V vs. the standard hydrogen electrode. All HAs showed similar Fourier transform infrared spectra with variable absorption intensity, the spectra verified the presence of aromatic C=C, C–H, and C=O of quinone ketones group in HAs. Electron spin resonance suggested that quinone moieties within HAs are the redox-active centers. All HAs promoted the microbial reduction of nitrate and amorphous FeOOH by Shewanella oneidensis strain MR-1, achieving high nitrate reduction extents of 79–98.4%, compared to the biotic and abiotic control values of 29.6 and 0.006%, respectively. The corresponding extents of Fe(II) production equaled 43.25–60.5%, exceeding those of biotic and abiotic controls (28.5 and 0.005%, respectively). In addition to the highest C/N, O/C ratio, and Fe content, HA extracted from QZ sediment also exhibited the highest nitrate and FeOOH reduction performances. Although the proportion of organic redox-active carbon is small, the potential electron-mediating ability is not ignorable. HAs are redox active for enhancing microbial reduction of nitrate and amorphous FeOOH regardless of the location or texture of parent sediments, implying their great potential for acting as redox mediator in enhancing multiple microbial reduction, thereby affecting various biogeochemical processes (i.e., iron cycle, nitrogen cycle, etc.) as well as in situ remediation in anaerobic environment.

Impact of Fe(II) oxidation in the presence of iron-reducing bacteria on subsequent Fe(III) bio-reduction

The interplay of Fe(II) oxidation and Fe(III) bio-reduction occurs widely in both natural and engineered redox-dynamic systems. This study aimed to unravel the impact of Fe(II) oxidation by O2 in the presence of iron-reducing bacteria on subsequent Fe(III) bio-reduction. Mixed solutions of Fe2+ (0.1–0.5 mM) and Shewanella oneidensis strain MR-1 (MR-1, 2.0 × 107 CFU/mL) at neutral pH were first exposed to laboratory air for Fe(II) oxidation and bacterial inactivation, and then the resultant Fe(III) suspensions were switched to anoxic conditions for bio-reduction by the surviving bacteria. In the oxidation step, the coexisting MR-1 was inactivated by 0.8–1.71 orders of magnitude within 60 min. In the subsequent bio-reduction step, the resultant Fe(III) was bio-reduced by the surviving MR-1. Bio-reduction of the resultant Fe(III) by the surviving MR-1 was 1.8–2.5 times faster than that of the Fe(III) that was produced from Fe2+ oxidation without MR-1 by fresh MR-1 cells at 2.0 × 107 CFU/mL. Although MR-1 inactivation during Fe(II) oxidation may inhibit Fe(III) bio-reduction, the increase in bio-availability of the resultant Fe(III) and the residual reactivity of dead cells led to net enhancement of bio-reduction under the tested conditions. Lepidocrocite was the sole Fe(III) mineral that was produced from Fe2+ oxidation without MR-1, while 19% ferrihydrite was produced from Fe2+ oxidation in the presence of MR-1. The formation of low-crystallinity ferrihydrite accounts for the increase in bio-availability of the Fe(III) minerals. The findings of this study highlight an important but overlooked impact underlying the interplay of Fe(II) oxidation and Fe(III) bio-reduction.

Deletion of PBP1a/LpoA complex compromises cell envelope integrity in Shewanella oneidensis.

High molecular weight penicillin-binding proteins (PBPs) are responsible for the biosynthesis of peptidoglycan. In Escherichia coli, PBP1a and PBP1b form multienzyme peptidoglycan-synthesizing complexes with outer membrane lipoproteins LpoA and LpoB, respectively. The two complexes appear to be largely redundant, although their distinct physiological roles remain unclear. PBP1a/LpoA and PBP1b/LpoB also exist in Shewanella oneidensis strain MR-1, but effects of the two complexes on aerobic growth and β-lactam resistance are quite different. In this study, the phenotypes of strains lacking a certain complex in S. oneidensis were compared. Deletion of PBP1a/LpoA caused aberrant cell morphology (including branches and bulges), enhanced sensitivity to various envelope stresses and outer membrane permeability. On the contrary, strains lacking PBP1b/LpoB displayed phenotypes similar to the wild type.

Similar structural stabilities of 3-isopropylmalate dehydrogenases from the obligatory piezophilic bacterium Shewanella benthica strain DB21MT-2 and its atmospheric congener S. oneidensis strain MR-1

We previously found that the enzymatic activity of 3-isopropylmalate dehydrogenase from the obligatory piezophilic bacterium Shewanella benthica strain DB21MT-2 (SbIPMDH) was pressure-tolerant up to 100 MPa, but that from its atmospheric congener S. oneidensis strain MR-1 (SoIPMDH) was pressure-sensitive. Such characteristics were determined by only one amino acid residue at position 266, serine (SoIPMDH) or alanine (SbIPMDH) [Y. Hamajima et al. Extremophiles 20: 177, 2016]. In this study, we investigated the structural stability of these enzymes. At pH 7.6, SoIPMDH was slightly more stable against hydrostatic pressure than SbIPMDH, contrary to the physiological pressures of their normal environments. Pressure unfolding of these IPMDHs followed a two-state unfolding model between a native dimer and two unfolded monomers, and the dimer structure was pressure-tolerant up to 200 MPa, employing a midpoint pressure of 245.3 ± 0.1 MPa and a volume change of −225 ± 24 mL mol−1 for the most unstable mutant, SbIPMDH A266S. Thus, their pressure-dependent activity did not originate from structural perturbations such as unfolding or dimer dissociation. Conversely, urea-induced unfolding of these IPMDHs followed a three-state unfolding model, including a dimer intermediate. Interestingly, the first transition was strongly pH-dependent but pressure-independent; however, the second transition showed the opposite pattern. Obtained volume changes due to urea-induced unfolding were almost equal for both IPMDHs, approximately +10 and −30 mL mol−1 for intermediate formation and dimer dissociation, respectively. These results indicated that both IPMDHs have similar structural stability, and a pressure-adaptation mechanism was provided for only the enzymatic activity of SbIPMDH.