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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.

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.

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.

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.

Soil and groundwater pollution by chlorinated aliphatic hydrocarbons (CAHs) poses a threat to environmental and human health. A large number of sites around the world are contaminated with the CAHs carbon tetrachloride (CT) and perchloroethene (PCE). The employment of bacteria to remediate pollution events, termed in situ bioremediation, is currently gaining momentum as an effective and environmentally friendly treatment technology. A class of anaerobic bacteria named organohalide respiring bacteria (ORB) are able to remove PCE directly by utilizing it as an electron acceptor for growth. Unfortunately, no such bacteria are known which can transform CT in the same manner. Furthermore, CT and its partly dechlorinated product chloroform (CF) negatively impact the enzymatic degradation of PCE by ORB, creating major challenges for the bioremediation of sites co-contaminated with CT and PCE. While it cannot be used as a growth substrate, CT can be transformed by certain reactive metabolites of anaerobic bacteria. In particular, sulphide (HS-) and ferrous iron (Fe(II)) generated by sulphate- and iron-reducing bacteria (SRB and IRB) can transform CT. In this work, experimental results are presented which support the exploitation of this CT transformation pathway combined with the activity of organohalide respiring bacteria in view of bioremediating mixed CT and PCE plumes. Firstly, experiments were carried out to investigate the solvent tolerance of anaerobic bacteria in general, and revealed that fermentative strains are more tolerant to CAHs than SRB and IRB. Avenues to take advantage of this finding are proposed. Secondly, IRB and SRB enriched from several environmental sources displayed the ability to survive and produce Fe(II) and HS- respectively in the presence of CT and PCE. Thirdly, the potential of naturally-occuring electron-shuttling compounds was tested for their enhancing effects on CT transformation by Fe(II) and HS-. Phenolic-rich solutions including tea and wine were found to significantly decrease the proportion of CF formed in CT dechlorination and in some cases also increased dechlorination rates. Finally, a proof-of-concept study incorporating HS- production by Desulfovibrio vulgaris and organohalide respiration by ORB pointed to the feasibility of combining these two biogeochemical processes to remediate CT and PCE mixtures.