Proton Transport in the Outer-Membrane Flavocytochrome Complex Limits the Rate of Extracellular Electron Transport

The microbial transfer of electrons to extracellularly located solid compounds, termed extracellular electron transport (EET), is critical for microbial electrode catalysis. Although the components of the EET pathway in the outer membrane (OM) have been identified, the role of electron/cation coupling in EET kinetics is poorly understood. We studied the dynamics of proton transport associated with EET in an OM flavocytochrome complex in Shewanella oneidensis MR‐1. Using a whole‐cell electrochemical assay, a significant kinetic isotope effect (KIE) was observed following the addition of deuterated water (D2O). The removal of a flavin cofactor or key components of the OM flavocytochrome complex significantly increased the KIE in the presence of D2O to values that were significantly larger than those reported for proton channels and ATP synthase, thus indicating that proton transport by OM flavocytochrome complexes limits the rate of EET

Synthesis of an amorphous Geobacter-manganese oxide biohybrid as an efficient water oxidation catalyst

The development of a low cost and efficient oxygen evolution reaction (OER) catalyst has paramount importance to meet the future sustainable energy demand. Nature's photosynthetic machinery deploy manganese-based complex in the photosystem II to oxidize water. Inspired by nature, herein, we synthesized a high performing manganese-based OER catalyst using an electrochemically active and iron-rich bacterium, Geobacter sulfurreducens. The as-synthesized biohybrid catalyst (amorphous Geobacter-Mn2O3) produced a current density of 10 mA cm−2 at an overpotential of 290 ± 9 mV versus a reversible hydrogen electrode with a low Tafel slope of 59 mV dec−1. The catalyst exhibited remarkable stability, evidenced through a long-term chronopotentiometry experiment. Multiple evidence showed that G. sulfurreducens contributed OER active elements (iron and phosphorus) to the biohybrid catalyst, and the as-synthesized Geobacter-Mn2O3 is amorphous. The amorphous structure of the biohybrid catalyst provided a large electrochemically active surface area and excess catalytic sites for the OER catalysis. In addition, Mn3+ present in the biohybrid catalyst is believed to be the precursor for oxygen evolution. The OER activity of the biohybrid catalyst outperformed commercial-Mn2O3, commercial-IrO2 and most of the benchmark precious OER catalysts, thus supporting its suitability for large-scale applications. The proposed green approach to synthesize a biohybrid catalyst paves a new avenue to develop robust and cost-effective electrocatalysts for energy-related applications

A three-dimensional hybrid electrode with electroactive microbes for efficient electrogenesis and chemical synthesis

Integration of electroactive bacteria into electrodes combines strengths of intracellular biochemistry with electrochemistry for energy conversion and chemical synthesis. However, such biohybrid systems are often plagued with suboptimal electrodes, which limits the incorporation and productivity of the bacterial colony. Here, we show that an inverse opal-indium tin oxide electrode hosts a large population of current-producing Geobacter and attains a current density of 3 mA cm−2 stemming from bacterial respiration. Differential gene expression analysis revealed Geobacter’s transcriptional regulations to express more electron-relaying proteins when interfaced with electrodes. The electrode also allows coculturing with Shewanella for syntrophic electrogenesis, which grants the system additional flexibility in converting electron donors. The biohybrid electrode containing Geobacter can also catalyze the reduction of soluble fumarate and heterogenous graphene oxide, with electrons from an external power source or an irradiated photoanode. This biohybrid electrode represents a platform to employ live cells for sustainable power generation and biosynthesis

Disparity of Cytochrome Utilization in Anodic and Cathodic Extracellular Electron Transfer Pathways of Geobacter sulfurreducens Biofilms.

Extracellular electron transfer (EET) in microorganisms is prevalent in nature and has been utilized in functional bioelectrochemical systems. EET of Geobacter sulfurreducens has been extensively studied and has been revealed to be facilitated through c-type cytochromes, which mediate charge between the electrode and G. sulfurreducens in anodic mode. However, the EET pathway of cathodic conversion of fumarate to succinate is still under debate. Here, we apply a variety of analytical methods, including electrochemistry, UV-vis absorption and resonance Raman spectroscopy, quartz crystal microbalance with dissipation, and electron microscopy, to understand the involvement of cytochromes and other possible electron-mediating species in the switching between anodic and cathodic reaction modes. By switching the applied bias for a G. sulfurreducens biofilm coupled to investigating the quantity and function of cytochromes, as well as the emergence of Fe-containing particles on the cell membrane, we provide evidence of a diminished role of cytochromes in cathodic EET. This work sheds light on the mechanisms of G. sulfurreducens biofilm growth and suggests the possible existence of a nonheme, iron-involving EET process in cathodic mode.N.K. was supported by a Royal Society Newton International Fellowship, NF160054. E.R., X.F. and N.H. acknowledge the European Research Council (ERC) Consolidator Grant “MatEnSAP” (682833). S. K. was supported by a Marie Skłodowska-Curie Fellowship (EMES, 744317). K. H. Ly acknowledges the Open Topic Postdoc Programme of the Technische Universität Dresden and the Marie Sklodowska Curie IF, GAN 701192. The TEM was funded through the EPSRC underpinning multi-user equipment call (EP/P030467/1

Disparity of Cytochrome Utilization in Anodic and Cathodic Extracellular Electron Transfer Pathways of Geobacter sulfurreducens Biofilms.

Extracellular electron transfer (EET) in microorganisms is prevalent in nature and has been utilized in functional bioelectrochemical systems. EET of Geobacter sulfurreducens has been extensively studied and has been revealed to be facilitated through c-type cytochromes, which mediate charge between the electrode and G. sulfurreducens in anodic mode. However, the EET pathway of cathodic conversion of fumarate to succinate is still under debate. Here, we apply a variety of analytical methods, including electrochemistry, UV-vis absorption and resonance Raman spectroscopy, quartz crystal microbalance with dissipation, and electron microscopy, to understand the involvement of cytochromes and other possible electron-mediating species in the switching between anodic and cathodic reaction modes. By switching the applied bias for a G. sulfurreducens biofilm coupled to investigating the quantity and function of cytochromes, as well as the emergence of Fe-containing particles on the cell membrane, we provide evidence of a diminished role of cytochromes in cathodic EET. This work sheds light on the mechanisms of G. sulfurreducens biofilm growth and suggests the possible existence of a nonheme, iron-involving EET process in cathodic mode.N.K. was supported by a Royal Society Newton International Fellowship, NF160054. E.R., X.F. and N.H. acknowledge the European Research Council (ERC) Consolidator Grant “MatEnSAP” (682833). S. K. was supported by a Marie Skłodowska-Curie Fellowship (EMES, 744317). K. H. Ly acknowledges the Open Topic Postdoc Programme of the Technische Universität Dresden and the Marie Sklodowska Curie IF, GAN 701192. The TEM was funded through the EPSRC underpinning multi-user equipment call (EP/P030467/1

Bioinspired Synthesis of Reduced Graphene Oxide-Wrapped Geobacter sulfurreducens as a Hybrid Electrocatalyst for Efficient Oxygen Evolution Reaction

Doping/decorating of graphene or reduced graphene oxide (rGO) with heteroatoms provides a promising route for the development of electrocatalysts which will be useful in many technologies, including water splitting. However, current doping approaches are complicated, not eco-friendly, and not cost-effective. Herein, we report the synthesis of doped/decorated rGO for oxygen evolution reaction (OER) using a simple approach that is cost-effective, sustainable, and easy to scale up. The OER catalyst was derived from the reduction of GO by an exo-electron-transferring bacterium, Geobacter sulfurreducens. Various analytical tools indicate that OER active elements such as Fe, Cu, N, P, and S decorate the rGO flakes. The hybrid catalyst (i.e., Geobacter/rGO) produces a geometric current density of 10 mA cm–2 at an overpotential of 270 mV versus the reversible hydrogen electrode with a Tafel slope of 43 mV dec–1 and possesses high durability, as evidenced through 10 h of stability testing. Electrochemical analyses suggest the importance of Fe and its possible role as an active site for OER. Overall, this work represents a simple approach toward the development of an earth-abundant, eco-friendly, and highly active OER electrocatalyst for various applications such as solar fuel production, rechargeable metal–air batteries, and microbial electrosynthesis

Hollow Palladium Nanoparticles Facilitated Biodegradation of an Azo Dye by Electrically Active Biofilms

Dye wastewater severely threatens the environment due to its hazardous and toxic effects. Although many methods are available to degrade dyes, most of them are far from satisfactory. The proposed research provides a green and sustainable approach to degrade an azo dye, methyl orange, by electrically active biofilms (EABs) in the presence of solid and hollow palladium (Pd) nanoparticles. The EABs acted as the electron generator while nanoparticles functioned as the electron carrier agents to enhance degradation rate of the dye by breaking the kinetic barrier. The hollow Pd nanoparticles showed better performance than the solid Pd nanoparticles on the dye degradation, possibly due to high specific surface area and cage effect. The hollow cavities provided by the nanoparticles acted as the reaction centers for the dye degradation

Semi-biological approaches to solar-to-chemical conversion.

This review presents a comprehensive summary of the recent development in semi-artificial photosynthesis, a biological-material hybrid approach to solar-to-chemical conversion that provides new concepts to shape a sustainable future fuelled by solar energy. We begin with a brief introduction to natural and artificial photosynthesis, followed by a discussion of the motivation and rationale behind semi-artificial photosynthesis. Then, we summarise how various enzymes can be combined with synthetic materials for light-driven water oxidation, H2 evolution, CO2 reduction, and chemical synthesis more broadly. In the following section, we discuss the strategies to incorporate microorganisms in photocatalytic and (photo)electrochemical systems to produce fuels and chemicals with renewable sources. Finally, we outline emerging analytical techniques to study the bio-material hybrid systems and propose unexplored research opportunities in the field of semi-artificial photosynthesis

Microbial fermentation of polyethylene terephthalate (PET) plastic waste for the production of chemicals and electricity

Ideonella sakaiensis (I. sakaiensis) can grow on polyethylene terephthalate (PET) as the sole carbon and energy source. Previous work has shown that conversion of the hydrolysis products terephthalic acid (TPA) and ethylene glycol (EG) under aerobic conditions released carbon dioxide and water while yielding adenosine triphosphate (ATP) through oxidative phosphorylation. This study demonstrates that under anaerobic conditions I. sakaiensis ferments PET to the feedstock chemicals acetate and ethanol while co-producing ATP by substrate-level phosphorylation. In addition to PET, maltose, EG, and ethanol can also serve as fermenting substrates. Co-culturing of I. sakaiensis with electrogenic Geobacter sulfurreducens produced electricity from PET or EG. This newly identified plastic fermentation process by I. sakaiensis provides a novel biosynthetic route to produce high-value chemicals and electricity from plastic waste streams