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1. The electron transport chain of Shewanella oneidensis MR-1 can operate bidirectionally to enable microbial electrosynthesis

   https://doi.org/10.1128/aem.01387-23  

   Ford, Kathryne C. ; TerAvest, Michaela A. ( January 2024 , Applied and Environmental Microbiology)

   Buan, Nicole R. (Ed.)

   Extracellular electron transfer is a process by which bacterial cells can exchange electrons with a redox-active material located outside of the cell. InShewanella oneidensis, this process is natively used to facilitate respiration using extracellular electron acceptors such as Fe(III) or an anode. Previously, it was demonstrated that this process can be used to drive the microbial electrosynthesis (MES) of 2,3-butanediol (2,3-BDO) inS. oneidensisexogenously expressing butanediol dehydrogenase (BDH). Electrons taken into the cell from a cathode are used to generate NADH, which in turn is used to reduce acetoin to 2,3-BDO via BDH. However, generating NADH via electron uptake from a cathode is energetically unfavorable, so NADH dehydrogenases couple the reaction to proton motive force. We therefore need to maintain the proton gradient across the membrane to sustain NADH production. This work explores accomplishing this task by bidirectional electron transfer, where electrons provided by the cathode go to both NADH formation and oxygen (O₂) reduction by oxidases. We show that oxidases use trace dissolved oxygen in a microaerobic bioelectrical chemical system (BES), and the translocation of protons across the membrane during O₂reduction supports 2,3-BDO generation. Interestingly, this process is inhibited by high levels of dissolved oxygen in this system. In an aerated BES, O₂molecules react with the strong reductant (cathode) to form reactive oxygen species, resulting in cell death.

   Microbial electrosynthesis (MES) is increasingly employed for the generation of specialty chemicals, such as biofuels, bioplastics, and cancer therapeutics. For these systems to be viable for industrial scale-up, it is important to understand the energetic requirements of the bacteria to mitigate unnecessary costs. This work demonstrates sustained production of an industrially relevant chemical driven by a cathode. Additionally, it optimizes a previously published system by removing any requirement for phototrophic energy, thereby removing the additional cost of providing a light source. We also demonstrate the severe impact of oxygen intrusion into bioelectrochemical systems, offering insight to future researchers aiming to work in an anaerobic environment. These studies provide insight into both the thermodynamics of electrosynthesis and the importance of the bioelectrochemical systems’ design.

    

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2. Evidence for Quinol Oxidation Activity of ImoA, a Novel NapC/NirT Family Protein from the Neutrophilic Fe(II)-Oxidizing Bacterium Sideroxydans lithotrophicus ES-1

   https://doi.org/10.1128/mbio.02150-22  

   Jain, Abhiney ; Coelho, Anaísa ; Madjarov, Joana ; Paquete, Catarina M. ; Gralnick, Jeffrey A. ( September 2022 , mBio)

   Newman, Dianne K. (Ed.)

   ABSTRACT Sideroxydans species are important chemolithoautotrophic Fe(II)-oxidizing bacteria in freshwater environments and play a role in biogeochemical cycling of multiple elements. Due to difficulties in laboratory cultivation and genetic intractability, the electron transport proteins required for the growth and survival of this organism remain understudied. In Sideroxydans lithotrophicus ES-1, it is proposed that the Mto pathway transfers electrons from extracellular Fe(II) oxidation across the periplasm to an inner membrane NapC/NirT family protein encoded by Slit_2495 to reduce the quinone pool. Based on sequence similarity, Slit_2495 has been putatively called CymA, a NapC/NirT family protein which in Shewanella oneidensis MR-1 oxidizes the quinol pool during anaerobic respiration of a wide range of substrates. However, our phylogenetic analysis using the alignment of different NapC/NirT family proteins shows that Slit_2495 clusters closer to NirT sequences than to CymA. We propose the name ImoA (inner membrane oxidoreductase) for Slit_2495. Our data demonstrate that ImoA can oxidize quinol pools in the inner membrane and is able to functionally replace CymA in S. oneidensis. The ability of ImoA to oxidize quinol in vivo as opposed to its proposed function of reducing quinone raises questions about the directionality and/or reversibility of electron flow through the Mto pathway in S. lithotrophicus. IMPORTANCE Fe(II)-oxidizing bacteria play an important role in biogeochemical cycles. At circumneutral pH, these organisms perform extracellular electron transfer, taking up electrons from Fe(II) outside the cell, potentially through a porin-cytochrome complex in the outer membrane encoded by the Mto pathway. Electrons from Fe(II) oxidation would then be transported to the quinone pool in the inner membrane via periplasmic and inner membrane electron transfer proteins. Directly demonstrating the functionality of genes in neutrophilic iron oxidizers is challenging due to the absence of robust genetic methods. Here, we heterologously expressed a NapC/NirT family tetraheme cytochrome ImoA, encoded by Slit_2495, an inner membrane protein from the Gram-negative Fe(II)-oxidizing bacterium Sideroxydans lithotrophicus ES-1, proposed to be involved in extracellular electron transfer to reduce the quinone pool. ImoA functionally replaced the inner membrane c-type cytochrome CymA in the Fe(III)-reducing bacterium Shewanella oneidensis. We suggest that ImoA may function primarily to oxidize quinol inS. lithotrophicus. 

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3. Tuning the Potential of Electron Extraction from Microbes with Ferrocene‐Containing Conjugated Oligoelectrolytes

   https://doi.org/10.1002/adbi.201800303  

   McCuskey, Samantha R. ; Rengert, Zachary D. ; Zhang, Mengwen ; Helgeson, Matthew E. ; Nguyen, Thuc‐Quyen ; Bazan, Guillermo C. ( December 2018 , Advanced Biosystems)

   Synthetic systems that facilitate electron transport across cellular membranes are of interest in bio‐electrochemical technologies such as bio‐electrosynthesis, waste water remediation, and microbial fuel cells. The design of second generation redox‐active conjugated oligoelectrolytes (COEs) bearing terminal cationic groups and a π‐delocalized core capped by two ferrocene units is reported. The two COEs, DVFBO and F₄‐DVFBO, have similar membrane affinity, but fluorination of the core results in a higher oxidation potential (422 ± 5 mV compared to 365 ± 4 mV vs Ag/AgCl for the neutral precursors in chloroform). Concentration‐dependent aggregation is suggested by zeta potential measurements and confirmed by cryogenic transmission electron microscopy. When the working electrode potential (E_CA) is poised below the oxidation potential of the COEs (E_CA= 200 mV) in three‐electrode electrochemical cells containingShewanella oneidensisMR‐1, addition of DVFBO and F₄‐DVFBO produces negligible biocurrent enhancement over controls. AtE_CA= 365 mV, DVFBO increases steady‐state biocurrent by 67 ± 12% relative to controls, while the increase with F₄‐DVFBO is 30 ± 5%. Cyclic voltammetry supports that DVFBO increases catalytic biocurrent and that F₄‐DVFBO has less impact, consistent with their oxidation potentials. Overall, electron transfer from microbial species is modulated via tailoring of the COE redox properties.

    

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4. Distinct Electron Conductance Regimes in Bacterial Decaheme Cytochromes

   https://doi.org/10.1002/ange.201800294  

   Barrozo, Alexandre ; El‐Naggar, Mohamed Y. ; Krylov, Anna I. ( May 2018 , Angewandte Chemie)

   Shewanella oneidensis MR‐1 gains energy by extracellular electron transfer to solid surfaces. They employ c‐type cytochromes in two Mtr transmembrane complexes, forming a multiheme wire for electron transport across the cellular outer membrane. We investigated electron‐ and hole‐transfer mechanisms in the external terminal of the two complexes, MtrC and MtrF. Comparison of computed redox potentials with previous voltammetry experiments in distinct environments (isolated and electrode‐bound conditions of PFV or in vivo) suggests that these systems function in different regimes depending on the environment. Analysis of redox potential shifts in different regimes indicates strong coupling between the hemes via an interplay between direct Coulomb and indirect interactions through local structural reorganization. The latter results in the screening of Coulomb interactions and explains poor correlation of the strength of the heme‐to‐heme interactions with the distance between the hemes.

    

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5. Magnetite nanoparticle anchored graphene cathode enhances microbial electrosynthesis of polyhydroxybutyrate by Rhodopseudomonas palustris TIE-1

   Karthikeyan R, Ranaivoarisoa TO ( January 2020 , Nanotechnology)

   null (Ed.)

   Microbial electrosynthesis (MES) is an emerging technology that can convert carbon dioxide (CO2) into value-added organic carbon compounds using electrons supplied from a cathode. However, MES is affected by low product formation due to limited extracellular electron uptake by microbes. Herein, a novel cathode was developed from chemically synthesized magnetite nanoparticles and reduced graphene oxide nanocomposite (rGO-MNPs). This nanocomposite was electrochemically deposited on carbon felt (CF/rGO-MNPs), and the modified material was used as a cathode for MES production. The bioplastic, polyhydroxybutyrate (PHB) produced by Rhodopseudomonas palustris TIE-1 (TIE-1), was measured from reactors with modified and unmodified cathodes. Results demonstrate that the magnetite nanoparticle anchored graphene cathode (CF/rGO-MNPs) exhibited higher PHB production (91.31±0.9 mg l−1). This is ∼4.2 times higher than unmodified carbon felt (CF), and 20 times higher than previously reported using graphite. This modified cathode enhanced electron uptake to −11.7±0.1 μA cm−2, ∼5 times higher than CF cathode (−2.3±0.08 μA cm−2). The faradaic efficiency of the modified cathode was ∼2 times higher than the unmodified cathode. Electrochemical analysis and scanning electron microscopy suggest that rGO-MNPs facilitated electron uptake and improved PHB production by TIE-1. Overall, the nanocomposite (rGO-MNPs) cathode modification enhances MES efficiency. 

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