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Bioelectrochemical systems (BESs) are emerging as attractive routes for sustainable energy generation, environmental remediation, bio-based chemical production and beyond. Electron shuttles (ESs) can be reversibly oxidized and reduced among multiple redox reactions, thereby assisting extracellular electron transfer (EET) process in BESs. Here, we explored the effects of 14 ESs on EET in Shewanella oneidensis MR-1, and found that anthraquinone-2-sulfonate (AQS) led to the highest cathodic current density, total charge production and reduction product formation. Subsequently, we showed that the introduction of -OH or -NH2 group into AQS at position one obviously affected redox potentials. The AQS-1-NH2 exhibited a lower redox potential and a higher Coulombic efficiency compared to AQS, revealing that the ESs with a more negative potential are conducive to minimize energy losses and improve the reduction of electron acceptor. Additionally, the cytochromes MtrA and MtrB were required for optimal AQS-mediated EET of S. oneidensis MR-1. This study will provide new clues for rational design of efficient ESs in microbial electrosynthesis.

Electrode materials capable of increasing the electricigens loading and accelerating the extracellular electron transfer (EET) are urgently required for microbial electrocatalytic technologies. In this work, a monolithic electrode (MDC800AM) based on three-dimensional (3D) interconnected N-doped carbon microtubes (NCMTs) derived from nanostructured polyaniline (PANI)-coated cotton fibers is prepared by a facile and scalable method. The PANI-coating layer and subsequent manganese (Mn)-catalyzed graphitization is helpful in doping the surface with nitrogen atoms and improving its graphitization degree, which contributes significantly to the hydrophilicity, biocompatibility and electronic conductivity of the electrode. Because of the hollow structure and large inner diameter of NCMTs, the MDC800AM is able to buffer the electrolyte facilitating well-localized electron shuttles (ES) within the electrode region as well as increase the surface area for electricigens to inhabit. Based on these advantages, the as-prepared MDC800AM exhibits enhanced microbial electrocatalytic activity and stability such as to outperform its counterpart made up of solid carbon fibers. This work not only provides a monolithic electrode manifesting greatly accelerated activity toward microbial electrocatalysis, but also a proof-of-concept demonstration of boosting the microbial fuel cell (MFC) performance via localized high ES concentration.

For renewable and sustainable bioenergy utilization with cost-effectiveness, electron-shuttles (ESs) (or redox mediators (RMs)) act as electrochemical “catalysts” to enhance rates of redox reactions, catalytically accelerating electron transport efficiency for abiotic and biotic electrochemical reactions. ESs are popularly used in cellular respiratory systems, metabolisms in organisms, and widely applied to support global lives. Apparently, they are applicable to increase power-generating capabilities for energy utilization and/or fuel storage (i.e., dye-sensitized solar cell, batteries, and microbial fuel cells (MFCs)). This first-attempt review specifically deciphers the chemical structure association with characteristics of ESs, and discloses redox-mediating potentials of polyphenolics-abundant ESs via MFC modules. Moreover, to effectively convert electron-shuttling capabilities from non-sustainable antioxidant activities, environmental conditions to induce electrochemical mediation apparently play critical roles of great significance for bioenergy stimulation. For example, pH levels would significantly affect electrochemical potentials to be exhibited (e.g., alkaline pHs are electrochemically favorable for expression of such electron-shuttling characteristics). Regarding chemical structure effect, chemicals withortho- andpara-dihydroxyl substituents-bearing aromatics own convertible characteristics of non-renewable antioxidants and electrochemically catalytic ESs; however, ES capabilities ofmeta-dihydroxyl substituents can be evidently repressed due to lack of resonance effect in the structure for intermediate radical(s) during redox reaction. Moreover, this review provides conclusive remarks to elucidate the promising feasibility to identify whether such characteristics are non-renewable antioxidants or reversible ESs from natural polyphenols via cyclic voltammetry and MFC evaluation. Evidently, considering sustainable development, such electrochemically convertible polyphenolic species in plant extracts can be reversibly expressed for bioenergy-stimulating capabilities in MFCs under electrochemically favorable conditions.

Magnetite nanoparticles (nanoFe3O4) have been reported to facilitate direct interspecies electron transfer (DIET) between syntrophic bacteria and methanogens thereby improving syntrophic methanogenesis. However, whether or how nanoFe3O4 affects acetotrophic methanogenesis remain unknown. Herein, we demonstrate the unique role of nanoFe3O4 in accelerating methane production from direct acetotrophic methanogenesis in Methanosarcina-enriched cultrures, which was further confirmed by pure cultures of Methanosarcina barkeri. Compared with other nanomaterials of higher electrical conductivity such as carbon nanotubes (CNTs) and graphite, nanoFe3O4 with mixed valence Fe(II) and Fe(III) had the most significant stimulatory effect on methane production, suggesting its redox activity rather than electrical conductivity led to enhanced methanogenesis by M. barkeri. Cell morphology and spectroscopy analysis revealed that nanoFe3O4 penetrated into the cell membrane and cytoplasm of M. barkeri. These results provide the unprecedented possibility that nanoFe3O4 in the cell membrane of methanogens serve as electron shuttles to facilitate intracellular electron transfer and thus enhance methane production. This work has important implications not only for understanding the mechanisms of mineral-methanogen interaction but also for optimizing engineered methanogenic processes.

Conductive materials facilitate direct interspecies electron transfer between acidogens and methanogens during methane (CH4) production. Soluble microbial products (SMP) and extracellular polymeric substances (EPS) produced by microorganisms might act as the electron shuttle between microorganisms and conductive materials. In this study, effects of conductive ferroferric oxide (Fe3O4) on anaerobic treatment process and the production of SMP and EPS were investigated. The maximum CH4 production rate was enhanced by 23.3% with the dosage of Fe3O4. The concentrations of proteins, polysaccharides, and humic substances in tightly bound EPS (T-EPS) were promoted, suggesting that extracellular metabolisms were induced by conductive materials. Distribution of potential electron shuttles such as quinone-like substances, flavins, aromatic amino acids, and dipeptides in SMP and EPS phases were comprehensively investigated and these electron shuttles were significantly affected by Fe3O4. Dipeptides consisting of phenylalanine were widely detected in T-EPS of the Fe3O4 reactor, indicating a potential different extracellular electron exchange pattern with the addition of conductive materials.

Density functional calculations have been performed on titanium, nickel, molybdenum and niobium-Schiff base complexes and titanium, nickel-porphyrinogen complexes in order to understand the behavior of these systems in redox processes. In titanium and nickel-Schiff base complexes C-C σ- bonds are formed upon reduction, while in titanium and nickel-porphyrinogen complexes C-C σ bonds are formed upon oxidation. In both systems, the formation or the cleavage of C-C bonds avoids a variation in the oxidation state of the metal and these C-C bonds act not only as electron reservoirs, but also as a buffer for the oxidation state of the metal. The possibility of intramolecular electron transfer from C-C bonds to M-M bonds and vice versa has been investigated for molybdenum and niobium-Schiff base complexes.

Dissimilatory metal reducing organisms play key roles in the biogeochemical cycle of metals as well as in the durability of submerged and buried metallic structures. The molecular mechanisms that support electron transfer across the microbe-metal interface in these organisms remain poorly explored. It is known that outer membrane proteins, in particular multiheme cytochromes, are essential for this type of metabolism, being responsible for direct and indirect, via electron shuttles, interaction with the insoluble electron acceptors. Soluble electron shuttles such as flavins, phenazines, and humic acids are known to enhance extracellular electron transfer. In this work, this phenomenon was explored. All known outer membrane decaheme cytochromes from Shewanella oneidensis MR-1 with known metal terminal reductase activity and a undecaheme cytochrome from Shewanella sp. HRCR-6 were expressed and purified. Their interactions with soluble electron shuttles were studied using stopped-flow kinetics, NMR spectroscopy, and molecular simulations. The results show that despite the structural similarities, expected from the available structural data and sequence homology, the detailed characteristics of their interactions with soluble electron shuttles are different. MtrC and OmcA appear to interact with a variety of different electron shuttles in the close vicinity of some of their hemes, and with affinities that are biologically relevant for the concentrations typical found in the medium for this type of compounds. All data support a view of a distant interaction between the hemes of MtrF and the electron shuttles. For UndA a clear structural characterization was achieved for the interaction with AQDS a humic acid analog. These results provide guidance for future work of the manipulation of these proteins toward modulation of their role in metal attachment and reduction.

Power generation at bioanodes of Pseudomonas aeruginosa for glycerol biotransformation was coupled to the carbon dioxide capture in biocathodes of Chlorella vulgaris in hybrid photosynthetic biofuel cells (HPSBC). Biochemical parameters such as microbial growth, substrate consumption, production of bacterial pigments and CO2 capture were studied. Also electrochemical parameters of maxima current densities (Id max), power output (Pd max) and coulombic efficiencies (C E) were studied. Initially, both systems were evaluated in separate against the corresponding Fe3+|Fe2+ redox pair. In bacterial systems, important results in terms of Id max of 42 ± 2.1 µA cm-2, C E of 48 ± 2.4% and Pd max of 350 ± 17.5 mW cm-2 were achieved. Likewise, for isolated algal cathode systems, Id max of 93 ± 4.65 µA cm-2, C E of 56 ± 2.8% and Pd max of 3.2 ± 0.16 mW cm-2, were achieved. In contrast, when both systems were coupled, a lower Id max of 48.5 ± 2.42 µA cm-2 was observed. Finally, bioelectrochemical conditions were improved based on substrate consumption, electrogenic products, cation transport and mediated electron transfer systems. Thus, higher average values for Id max of 80 ± 4.0 µA cm-2, C E of 71.5 ± 3.57% and Pd max of 650 ± 32.5 mW cm-2 were obtained. A geração de potencial em bioânodos de Pseudomonas aeruginosa a partir da bio transformação de glicerol, foi acoplada à captura de dióxido de carbono em biocátodos de Chlorella vulgaris em uma célula a combustível híbrida fotossintética (HPSBC). Parâmetros bioquímicos tais como crescimento microbiano, consumo de substrato, produção de pigmentos bacterianos e a captura de CO2 foram estudados. Igualmente, parâmetros eletroquímicos de densidades de correntes máximas (Id max), geração de densidade potencia máxima (Pd max) e eficiências coulômbicas (C E), foram estudados. Inicialmente, ambos os sistemas foram avaliados em separado contra os correspondentes pares redox Fe3+|Fe2+. No sistema bacteriano, resultados importantes em termos de Id max de 42 ± 2,1 µA cm-2, C E de 48 ± 2,4% e Pd max de 350 ± 17,5 mW cm-2 foram alcançados. Igualmente, para o sistema catódico algal isolado, valores de Id max de 93 ± 4,65 µA cm-2, C E de 56 ± 2,8% e Pd max de 3,2 ± 0,16 mW cm-2, foram atingidos. Em contraste, quando os dois sistemas foram acoplados, um valor menor de Id max de 48,5 ± 2,42 µA cm-2 foi observado. Finalmente, as condições bioeletroquímicas foram melhoradas com base no consumo de substrato, a geração de produtos eletrogênicos, o transporte de cátions e os sistemas para transporte de elétrons empregados. Assim, maiores valores médios para Id max de 80 ± 4,0 µA cm-2, para C E de 71,5 ± 3,57% e para Pd max de 650 ± 32,5 mW cm-2 foram obtidos.