Visible light-induced water oxidation has been demonstrated at an Ir oxide nanocluster coupled to a single CrVI site on the pore surface of MCM-41 mesoporous silica. The photocatalytic unit was ...assembled by the reaction of surface CrO groups with Ir(acac)3 precursor followed by calcination at 300 °C and bond formation monitored by FT-Raman and FT-IR spectroscopy. High-resolution Z-contrast electron micrographs of the calcined material combined with energy-dispersive X-ray spot analysis confirmed the occlusion of Ir oxide nanoparticles inside the mesopores. Oxygen evolution of an aqueous suspension of the Ir x O y −CrMCM-41 upon visible light irradiation of the CrVI−O ligand-to-metal charge-transfer absorption was monitored mass-spectrometrically. Comparison of the product yields for samples with low Cr content (Cr/Si ≤ 0.02) and high Cr content (Cr/Si = 0.05) indicates that only isolated Cr centers are capable of extracting electrons from Ir oxide clusters, while di- or polychromate species are not. Water oxidation at a multielectron-transfer catalyst coupled to a single metal center has not been demonstrated before. The ability to drive water oxidation with a single metal center as electron pump offers opportunities for coupling the oxygen-evolving photocatalytic unit to reducing sites in the nanoporous scaffold.
Although members of the genus Shewanella have common features (e.g., the presence of decaheme c-type cytochromes c-cyts), they are widely variable in genetic and physiological features. The present ...study compared the current-generating ability of S. loihica PV-4 in microbial fuel cells (MFCs) with that of well-characterized S. oneidensis MR-1 and examined the roles of c-cyts in extracellular electron transfer. We found that strains PV-4 and MR-1 exhibited notable differences in current-generating mechanisms. While the MR-1 MFCs maintained a constant current density over time, the PV-4 MFCs continued to increase in current density and finally surpassed the MR-1 MFCs. Coulombic efficiencies reached 26% in the PV-4 MFC but 16% in the MR-1 MFCs. Although both organisms produced quinone-like compounds, anode exchange experiments showed that anode-attached cells of PV-4 produced sevenfold more current than planktonic cells in the same chamber, while planktonic cells of MR-1 produced twice the current of the anode-attached cells. Examination of the genome sequence indicated that PV-4 has more c-cyt genes in the metal reductase-containing locus than MR-1. Mutational analysis revealed that PV-4 relied predominantly on a homologue of the decaheme c-cyt MtrC in MR-1 for current generation, even though it also possesses two homologues of the decaheme c-cyt OmcA in MR-1. These results suggest that current generation in a PV-4 MFC is in large part accomplished by anode-attached cells, in which the MtrC homologue constitutes the main path of electrons toward the anode.
Deep‐sea chemistry: The electrochemical analysis of hydrothermal fluids and the ambient seawater with fuel cells installed in deep‐sea hydrothermal vents provided evidence for the possibility of ...generating electricity in the deep sea. The illumination of light‐emitting diodes in the dark environment also confirmed this prospect.
Certain microbes are capable of transporting electrons from the cell interior‐respiratory electron chain to insoluble electron acceptors located outside of the cell, a process referred to as ...extracellular electron transport (EET). Bacteria capable of EET are currently utilized as “living anode catalysts” in microbial fuel cells. Several EET mechanisms have been proposed, yet they lack molecular‐level consistency. Here, we review our recent work, presenting a “bound‐flavin cofactor” model, which we believe provides a suitable explanation for all of the published data to date for the model EET microbes of Shewanella oneidensis and Geobacter sulfurreducens. We discuss the interaction between free flavin and outer‐membrane c‐type cytochromes based on a protein–ligand binding model, the accumulation of cell‐secreted flavins in nanostructured electrodes, and EET through intermittent direct contacts or conductive extracellular appendages.
Extracellular activity: Bacterial interfacial electron‐transport mechanisms are important for maximizing the performance of microbial fuel‐cell anodes. Recent work is reviewed, presenting a “bound‐flavin cofactor” model, which is believed to provide a suitable explanation for the published data to date, in terms of model electrogenic microbes Geobacter and Shewanella.
We report that an intensely iron-corroding microbe, Desulfovibrio ferrophilus strain IS5, is capable of extracting electrons from an indium tin-doped oxide electrode without consuming ...electrochemically generated hydrogen as an electron carrier. When sulfate was presented as a metabolic electron acceptor, significant cathodic current production was observed at an onset potential of −200 mV vs. SHE, which was approximately 750 mV more positive than the onset for hydrogen evolution in our experimental condition. This finding indicates that hydrogen is not required for the cathodic reaction of IS5, suggesting that IS5 accelerates anaerobic iron corrosion through direct electron uptake.
Temperature gradients are an under‐utilized source of energy with which to drive chemical reactions. Here, we review our past efforts to understand how deep‐sea hydrothermal vents may harness thermal ...energy to promote difficult chemical reactions such as CO2 reduction. Strategies to amplify the driving force using temperature will be covered first, followed by a discussion on how spatially separated thermodynamic gradients can be used to regulate reaction selectivity. Although desirable material properties of hydrothermal vent walls have been inferred previously from the bioenergetic membranes of modern cells, strategies based on fundamental laws of physical chemistry allow naturally occurring chimney minerals to circumvent the lack of structural and catalytic optimization. The principles that underlie both the establishment and the utilization of the thermodynamic driving force at hydrothermal vents can be employed in abiotic systems such as the modern chemical industry, yielding insight into carbon fixation reactions important today and possibly at the autotrophic origin of life.
Origin of life: Deep‐sea hydrothermal vents can utilize temperature and pH gradients to promote the thermodynamics and selectivity of CO2 reduction. The strategies concerning the regulation of electrocatalytic reactions yields physicochemical insight on the origin of life, as well as towards understanding a possible blueprint for the chemical industry.
Abstract
A prevailing scenario of the origin of life postulates thioesters as key intermediates in protometabolism, but there is no experimental support for the prebiotic CO
2
fixation routes to ...thioesters. Here we demonstrate that, under a simulated geoelectrochemical condition in primordial ocean hydrothermal systems (–0.6 to –1.0 V versus the standard hydrogen electrode), nickel sulfide (NiS) gradually reduces to Ni
0
, while accumulating surface-bound carbon monoxide (CO) due to CO
2
electroreduction. The resultant partially reduced NiS realizes thioester (S-methyl thioacetate) formation from CO and methanethiol even at room temperature and neutral pH with the yield up to 35% based on CO. This thioester formation is not inhibited, or even improved, by 50:50 coprecipitation of NiS with FeS or CoS (the maximum yields; 27 or 56%, respectively). Such a simple thioester synthesis likely occurred in Hadean deep-sea vent environments, setting a stage for the autotrophic origin of life.
Only connect: Members of genus Shewanella are Gram‐negative bacteria that can utilize solid‐state metal oxide as a terminal electron acceptor for respiration. The direct electrical connection between ...a single cell and a microelectrode is characterized by an optical tweezers technique (see picture; ITO=indium tin oxide).
Aquaculture in coastal environments has an increasingly important role in the world’s food supply; however, the accumulation of organic compounds on seafloors due to overfeeding adversely affects ...benthic ecosystems. To assess the ecological resilience of aquafarms to nutrient influx, we investigated the redox homeostasis of benthic ecosystems using a marine oligochaete as a model benthic organism in aquaculture fields. Real-time monitoring of the redox potential of a model benthic ecosystem constructed in an electrochemical reactor allowed evaluation of the homeostatic response of the system to nutrient addition. Although the detrimental effects of overfeeding were confirmed by irreversible potential changes in the sediment, redox homeostasis was reinforced through a cooperative relationship between oligochaetes and sediment microorganisms. Specifically, the oligochaetes exhibited reversible changes in metabolism and body position in response to dynamic changes in the sediment potential between −300 and 500 mV, thereby promoting the decomposition of organic compounds. The potential-dependent changes in metabolism and body position were reproduced by artificially manipulating the sediment potential in electrochemical reactors. Given the importance of benthic animals in sustaining coastal ecosystems, the electrochemical monitoring and physiologic regulation of marine oligochaetes could offer an intriguing approach toward sustainable aquaculture.
PDF
