In Situ Investigation of Cathode and Local Biofilm Microenvironments Reveals Important Roles of OH^– and Oxygen Transport in Microbial Fuel Cells

Mass transport within a cathode, including OH^– transport and oxygen diffusion, is important for the performance of air-cathode microbial fuel cells (MFCs). However, little is known regarding how mass transport profiles are associated with MFC performance and how they are affected by biofilm that inevitably forms on the cathode surface. In this study, the OH^– and oxygen profiles of a cathode biofilm were probed in situ in an MFC using microelectrodes. The pH of the catalyst layer interface increased from 7.0 ± 0.1 to 9.4 ± 0.3 in a buffered MFC with a bare cathode, which demonstrates significant accumulation of OH^– in the cathode region. Furthermore, the pH of the interface increased to 10.0 ± 0.3 in the presence of the local biofilm, which indicates that OH^– transport was severely blocked. As a result of the significant OH^– accumulation, the maximum power density of the MFC decreased from 1.8 ± 0.1 W/m² to 1.5 ± 0.08 W/m². In contrast, oxygen crossover, which was significant under low current flow conditions, was limited by the cathode biofilm. As a result of the blocked oxygen crossover, higher MFC coulombic efficiency (CE) was achieved in the presence of the cathode biofilm. These results indicate that enhanced OH^– transport and decreased oxygen crossover would be beneficial for high-performance MFC development

Disposable Strip Biosensor for Visual Detection of Hg²⁺ Based on Hg²⁺-Triggered Toehold Binding and Exonuclease III-Assisted Signal Amplification

A disposable strip biosensor for the visual detection of Hg²⁺ in aqueous solution has been constructed on the basis of Hg²⁺-triggered toehold binding and exonuclease III (Exo III)-assisted signal amplification. Thymine-thymine (T-T) mismatches in the toehold domains can serve as specific recognition elements for Hg²⁺ binding with the help of T-Hg²⁺-T base pairs to initiate toehold-mediated strand displacement reaction. Exo III-catalyzed target recycling strategy is introduced to improve the sensitivity. Using gold nanoparticles as a tracer, the output signals can be directly observed by the naked eye. The assay is ultrasensitive, enabling the visual detection of trace amounts of Hg²⁺ as low as 1 pM without instrumentation. This sensing system also displays remarkable specificity to Hg²⁺ against other possible competing ions. This sensor is robust and can be applied to the reliable monitoring of spiked Hg²⁺ in environmental water samples with good recovery and accuracy. With the advantages of cost-effectiveness, simplicity, portability, and convenience, the disposable strip biosensor will be a promising candidate for point-of-use monitoring of Hg²⁺ in environmental and biological samples

Additional file 1: Table S1. of Genome sequence of a dissimilatory Fe(III)-reducing bacterium Geobacter soli type strain GSS01T

Associated MIGS record. (PDF 191 kb

Additional file 2: of Genome sequence of a dissimilatory Fe(III)-reducing bacterium Geobacter soli type strain GSS01T

Amino acid sequence alignment of pilin domain protein SE37_07695 from Geobacter soli GSS01 and those from other bacteria. Amino acid sequence alignments were generated using the Clustal X (1.8). (PDF 40 kb

Nanostructured Macroporous Bioanode Based on Polyaniline-Modified Natural Loofah Sponge for High-Performance Microbial Fuel Cells

Microbial fuel cells (MFCs) are a promising technology to recover electrical energy from different types of waste. However, the power density of MFCs for practical applications is limited by the anode performance, mainly resulting from low bacterial loading capacity and low extracellular electron transfer (EET) efficiency. In this study, an open three-dimensional (3D) structured electrode was fabricated using a natural loofah sponge as the precursor material. The loofah sponge was directly converted into a continuous 3D macroporous carbon material via a simple carbonization procedure. The loofah sponge carbon (LSC) was decorated with nitrogen-enriched carbon nanoparticles by cocarbonizing polyaniline-hybridized loofah sponges to improve their microscopic structures. The macroscale porous structure of the LSCs greatly increased the bacterial loading capacity. The microscale coating of carbon nanoparticles favored EET due to the enhanced interaction between the bacteria and the anode. By using a single-chamber MFC equipped with the fabricated anode, a power density of 1090 ± 72 mW m^–2 was achieved, which is much greater than that obtained by similarly sized traditional 3D anodes. This study introduces a promising method for the fabrication of high-performance anodes from low-cost, sustainable natural materials

Rapid Measurement of Microbial Extracellular Respiration Ability Using a High-Throughput Colorimetric Assay

Microbial extracellular respiration (MER) involves the transfer of electrons to extracellular substrates and has significant environmental implications. Conventional methods for MER ability determination are reagent- and time-consuming, have a low throughput, or require noncommercial instruments. In this study, a plate-based colorimetric assay is proposed to measure MER ability. This method utilizes the peroxidase activity of the key components (multi-heme <i>c</i>-type cytochromes) of the extracellular electron-transfer network. The bacterial intrinsic peroxidase-catalyzed oxidation of chromogen (e.g., tetramethylbenzidine) resulted in a measurable color change correlated with the MER ability of the tested microorganisms. The results of the proposed colorimetric assay correspond well with those of traditional methods, such as the dissimilatory Fe­(III) reduction method (Spearman’s ρ of 0.946; <i>P</i> < 0.01) and the electricity generation method (Spearman’s ρ of 0.893; <i>P</i> < 0.01). The proposed method allows researchers to identify extracellular respiring bacteria within several minutes and to measure their MER ability quantitatively by a plate-based assay

TiO₂ Nanoparticle-Induced Nanowire Formation Facilitates Extracellular Electron Transfer

Semiconductor nanoparticles (NPs) have been reported to facilitate extracellular electron transfer (EET) by increasing the electrical conductivity of electroactive biofilms. However, the underlying molecular mechanisms remain unclear. In this study, we demonstrated the unique role of semiconductor titanium dioxide (TiO₂) NPs in facilitating EET from <i>Geobacter</i> cells to an electrode. Compared to other NPs that include conductor carbon, semiconductor α-Fe₂O₃, and insulator SiO₂, TiO₂ NPs improved the bacterial EET capability most significantly, leading to an approximately 5.1-fold increase in microbial current generation. Cell morphology and gene analysis revealed that TiO₂ NPs specifically induced the formation of conductive nanowires by stimulating <i>pilA</i> expression, which primarily contributed to the enhanced EET in biofilms. In addition, TiO₂ NPs might compensate for the lack of a pili-associated <i>c</i>-cytochrome OmcS in the EET function. This finding had important implications not only for optimizing the performance of electroactive biofilms but also for modulating the ecological impact of NPs in the natural environment

Table_3_Transcriptomic, Proteomic, and Bioelectrochemical Characterization of an Exoelectrogen Geobacter soli Grown With Different Electron Acceptors.pdf

<p>The ability of Geobacter species to transfer electrons outside cells enables them to play an important role in biogeochemical and bioenergy processes. Our knowledge of the extracellular electron transfer (EET) process in the genus Geobacter is mainly from the study of G. sulfurreducens, and in order to fully investigate the EET mechanisms in the genus Geobacter, other Geobacter species should also be considered. This study focused on the EET of Geobacter soli GSS01, which exhibited a capability of reducing insoluble Fe(III) oxides and generating electrical current comparable with G. sulfurreducens PCA. Electrochemical characterization, including cyclic voltammetry, differential pulse voltammetry, and electrochemical in situ FTIR spectra, revealed that different redox proteins contributed to the electrochemical behaviors of G. soli and G. sulfurreducens. Based on comparative transcriptomic and proteomic analyses, OmcS was the most upregulated protein in both G. soli and G. sulfurreducens cells grown with insoluble Fe(III) oxides vs. soluble electron acceptor. However, the proteins including OmcE and PilA that were previously reported as being important for EET in G. sulfurreducens were downregulated or unchanged in G. soli cells grown with insoluble electron acceptors vs. soluble electron acceptor, and many proteins that were upregulated in G. soli cells grown with insoluble electron acceptors vs. soluble electron acceptor, such as OmcN, are not important for EET in G. sulfurreducens. We also identified 30 differentially expressed small RNAs (sRNAs) in G. soli cells grown with different acceptors. Taken together, these findings help to understand the versatile EET mechanisms that exist in the genus Geobacter and point to the possibility of sRNA in modulating EET gene expression.</p

Table_1_Transcriptomic, Proteomic, and Bioelectrochemical Characterization of an Exoelectrogen Geobacter soli Grown With Different Electron Acceptors.xlsx

<p>The ability of Geobacter species to transfer electrons outside cells enables them to play an important role in biogeochemical and bioenergy processes. Our knowledge of the extracellular electron transfer (EET) process in the genus Geobacter is mainly from the study of G. sulfurreducens, and in order to fully investigate the EET mechanisms in the genus Geobacter, other Geobacter species should also be considered. This study focused on the EET of Geobacter soli GSS01, which exhibited a capability of reducing insoluble Fe(III) oxides and generating electrical current comparable with G. sulfurreducens PCA. Electrochemical characterization, including cyclic voltammetry, differential pulse voltammetry, and electrochemical in situ FTIR spectra, revealed that different redox proteins contributed to the electrochemical behaviors of G. soli and G. sulfurreducens. Based on comparative transcriptomic and proteomic analyses, OmcS was the most upregulated protein in both G. soli and G. sulfurreducens cells grown with insoluble Fe(III) oxides vs. soluble electron acceptor. However, the proteins including OmcE and PilA that were previously reported as being important for EET in G. sulfurreducens were downregulated or unchanged in G. soli cells grown with insoluble electron acceptors vs. soluble electron acceptor, and many proteins that were upregulated in G. soli cells grown with insoluble electron acceptors vs. soluble electron acceptor, such as OmcN, are not important for EET in G. sulfurreducens. We also identified 30 differentially expressed small RNAs (sRNAs) in G. soli cells grown with different acceptors. Taken together, these findings help to understand the versatile EET mechanisms that exist in the genus Geobacter and point to the possibility of sRNA in modulating EET gene expression.</p

Table_4_Transcriptomic, Proteomic, and Bioelectrochemical Characterization of an Exoelectrogen Geobacter soli Grown With Different Electron Acceptors.xlsx

<p>The ability of Geobacter species to transfer electrons outside cells enables them to play an important role in biogeochemical and bioenergy processes. Our knowledge of the extracellular electron transfer (EET) process in the genus Geobacter is mainly from the study of G. sulfurreducens, and in order to fully investigate the EET mechanisms in the genus Geobacter, other Geobacter species should also be considered. This study focused on the EET of Geobacter soli GSS01, which exhibited a capability of reducing insoluble Fe(III) oxides and generating electrical current comparable with G. sulfurreducens PCA. Electrochemical characterization, including cyclic voltammetry, differential pulse voltammetry, and electrochemical in situ FTIR spectra, revealed that different redox proteins contributed to the electrochemical behaviors of G. soli and G. sulfurreducens. Based on comparative transcriptomic and proteomic analyses, OmcS was the most upregulated protein in both G. soli and G. sulfurreducens cells grown with insoluble Fe(III) oxides vs. soluble electron acceptor. However, the proteins including OmcE and PilA that were previously reported as being important for EET in G. sulfurreducens were downregulated or unchanged in G. soli cells grown with insoluble electron acceptors vs. soluble electron acceptor, and many proteins that were upregulated in G. soli cells grown with insoluble electron acceptors vs. soluble electron acceptor, such as OmcN, are not important for EET in G. sulfurreducens. We also identified 30 differentially expressed small RNAs (sRNAs) in G. soli cells grown with different acceptors. Taken together, these findings help to understand the versatile EET mechanisms that exist in the genus Geobacter and point to the possibility of sRNA in modulating EET gene expression.</p