Engineered cytochrome fused extracellular matrix enabled efficient extracellular electron transfer and improved performance of microbial fuel cell

Microbial Biofilms: Features of Formation and Potential for Use in Bioelectrochemical Devices

Microbial biofilms present one of the most widespread forms of life on Earth. The formation of microbial communities on various surfaces presents a major challenge in a variety of fields, including medicine, the food industry, shipping, etc. At the same time, this process can also be used for the benefit of humans-in bioremediation, wastewater treatment, and various biotechnological processes. The main direction of using electroactive microbial biofilms is their incorporation into the composition of biosensor and biofuel cells This review examines the fundamental knowledge acquired about the structure and formation of biofilms, the properties they have when used in bioelectrochemical devices, and the characteristics of the formation of these structures on different surfaces. Special attention is given to the potential of applying the latest advances in genetic engineering in order to improve the performance of microbial biofilm-based devices and to regulate the processes that take place within them. Finally, we highlight possible ways of dealing with the drawbacks of using biofilms in the creation of highly efficient biosensors and biofuel cells.

Hemicyanine-Based Highly Water-Soluble Probe for Extracellular Nitroreductase

Nitroreductase (NTR) has long been a target of interest for its important role involved in the nitro compounds metabolism. Various probes have been reported for NTR analysis, but rarely able to distinguish the extracellular NTR from intracellular ones. Herein we reported a new NTR sensor, HCyS-NO2, which was a hemicyanine molecule with one nitro and two sulfo groups attached. The nitro group acted as the reporting group to respond NTR reduction. Direct linkage of nitro group into the hemicyanine π conjugate system facilitated the intramolecular electron transfer (IET) process and thus quenched the fluorescence of hemicyanine core. Upon reduction with NTR, the nitro group was rapidly converted into the hydroxylamino and then the amino group, eliminating IET process and thus restoring the fluorescence. The sulfo groups installed significantly increased the hydrophilicity of the molecule, and introduced negative charges at physiological pH, preventing the diffusion into bacteria. Both gram-negative and gram-positive bacteria were able to turn on the fluorescence of HCyS-NO2, without detectable diffusion into cells, providing a useful tool to probe the extracellular reduction process.

DIET-like and MIET-like mutualism of S. oneidensis MR-1 and metal-reducing function microflora boosts Cr(VI) reduction

Microbial treatment of Cr(VI) is an environmentally friendly and low-cost approach. However, the mechanism of mutualism and the role of interspecies electron transfer in Cr(VI) reducing microflora are unclear. Herein, we constructed an intersymbiotic microbial association flora to augment interspecies electron transfer via functionalizing electroactive Shewanella oneidensis MR-1 with metal-reducing microflora, and thus the efficiency of Cr(VI) reduction. The findings suggest that the metal-reducing active microflora could converts glucose into lactic acid and riboflavin for S. oneidensis MR-1 to act as a carbon source and electron mediator. Thus, when adding initial 25 mg/L Cr (VI), this microflora exhibited an outstanding Cr (VI) removal efficiency (100%) at 12 h and elevated Cr (III) immobilization efficiency (80%) at 60 h with the assistance of 25 mg/L Cu(II). A series of electrochemical experiments proved this remarkable removal efficiency were ascribed to the improved interspecies electron transfer efficiency through direct interspecies electron transfer and riboflavin through mediated interspecies electron transfer. Furthermore, the metagenomic analysis revealed the expression level of the electron transport pathway was promoted. Intriguing high abundance of genes participating in the bio-reduction and biotransformation of Cr(VI) was also observed in functional microflora. These outcomes give a novel strategy for enhancing the reduction and fixation of harmful heavy metals by coculturing function microflora with electrogenic microorganisms.

Facilely Prepared Carbon Dots as Effective Anode Modifier for Enhanced Performance of Microbial Fuel Cells

Microbial fuel cells (MFCs) are a promising technology for obtaining energy in wastewater. Effective extracellular electron transfer is one of the key factors for its practical application. In this work, carbon dots (CDs) enriched with oxygen-containing groups on the surface were synthesized as an efficient anode modifier using a simple hydrothermal method and common reactants. The experimental findings indicated that anodes modified with CDs exhibited increased electrical conductivity and greater hydrophilicity. These modifications facilitated increased microorganism loading and contributed to enhancing electrochemical processes within the anode biofilm. The CD-modified MFCs exhibited higher maximum power density (661.1 ± 42.6 mW·m-2) and open-circuit voltage (534.50 ± 6.4 mV), which were significantly better than those of the blank group MFCs (484.1 ± 14.1 mW·m-2 and 447.50 ± 12.1 mV). The use of simple carbon materials to improve the microbial loading on the MFCs anode and the electron transfer between the microbial-electrode may provide a new idea for the design of efficient MFCs.

Engineering extracellular electron transfer pathways of electroactive microorganisms by synthetic biology for energy and chemicals production

The excessive consumption of fossil fuels causes massive emission of CO2, leading to climate deterioration and environmental pollution. The development of substitutes and sustainable energy sources to replace fossil fuels has become a worldwide priority. Bio-electrochemical systems (BESs), employing redox reactions of electroactive microorganisms (EAMs) on electrodes to achieve a meritorious combination of biocatalysis and electrocatalysis, provide a green and sustainable alternative approach for bioremediation, CO2 fixation, and energy and chemicals production. EAMs, including exoelectrogens and electrotrophs, perform extracellular electron transfer (EET) (i.e., outward and inward EET), respectively, to exchange energy with the environment, whose rate determines the efficiency and performance of BESs. Therefore, we review the synthetic biology strategies developed in the last decade for engineering EAMs to enhance the EET rate in cell-electrode interfaces for facilitating the production of electricity energy and value-added chemicals, which include (1) progress in genetic manipulation and editing tools to achieve the efficient regulation of gene expression, knockout, and knockdown of EAMs; (2) synthetic biological engineering strategies to enhance the outward EET of exoelectrogens to anodes for electricity power production and anodic electro-fermentation (AEF) for chemicals production, including (i) broadening and strengthening substrate utilization, (ii) increasing the intracellular releasable reducing equivalents, (iii) optimizing c-type cytochrome (c-Cyts) expression and maturation, (iv) enhancing conductive nanowire biosynthesis and modification, (v) promoting electron shuttle biosynthesis, secretion, and immobilization, (vi) engineering global regulators to promote EET rate, (vii) facilitating biofilm formation, and (viii) constructing cell-material hybrids; (3) the mechanisms of inward EET, CO2 fixation pathway, and engineering strategies for improving the inward EET of electrotrophic cells for CO2 reduction and chemical production, including (i) programming metabolic pathways of electrotrophs, (ii) rewiring bioelectrical circuits for enhancing inward EET, and (iii) constructing microbial (photo)electrosynthesis by cell-material hybridization; (4) perspectives on future challenges and opportunities for engineering EET to develop highly efficient BESs for sustainable energy and chemical production. We expect that this review will provide a theoretical basis for the future development of BESs in energy harvesting, CO2 fixation, and chemical synthesis.

Living electronics: A catalogue of engineered living electronic components

Biology leverages a range of electrical phenomena to extract and store energy, control molecular reactions and enable multicellular communication. Microbes, in particular, have evolved genetically encoded machinery enabling them to utilize the abundant redox-active molecules and minerals available on Earth, which in turn drive global-scale biogeochemical cycles. Recently, the microbial machinery enabling these redox reactions have been leveraged for interfacing cells and biomolecules with electrical circuits for biotechnological applications. Synthetic biology is allowing for the use of these machinery as components of engineered living materials with tuneable electrical properties. Herein, we review the state of such living electronic components including wires, capacitors, transistors, diodes, optoelectronic components, spin filters, sensors, logic processors, bioactuators, information storage media and methods for assembling these components into living electronic circuits.

Modular Engineering Strategy to Redirect Electron Flux into the Electron-Transfer Chain for Enhancing Extracellular Electron Transfer in Shewanella oneidensis

Efficient extracellular electron transfer (EET) of exoelectrogens is critical for practical applications of various bioelectrochemical systems. However, the low efficiency of electron transfer remains a major bottleneck. In this study, a modular engineering strategy, including broadening the sources of the intracellular electron pool, enhancing intracellular nicotinamide adenine dinucleotide (NADH) regeneration, and promoting electron release from electron pools, was developed to redirect electron flux into the electron transfer chain in Shewanella oneidensis MR-1. Among them, four genes include gene SO1522 encoding a lactate transporter for broadening the sources of the intracellular electron pool, gene gapA encoding a glyceraldehyde-3-phosphate dehydrogenase and gene mdh encoding a malate dehydrogenase in the central carbon metabolism for enhancing intracellular NADH regeneration, and gene ndh encoding NADH dehydrogenase on the inner membrane for releasing electrons from intracellular electron pools into the electron-transport chain. Upon assembly of the four genes, electron flux was directly redirected from the electron donor to the electron-transfer chain, achieving 62% increase in intracellular NADH levels, which resulted in a 3.5-fold enhancement in the power density from 59.5 ± 3.2 mW/m² (wild type) to 270.0 ± 12.7 mW/m² (recombinant strain). This study confirmed that redirecting electron flux from the electron donor to the electron-transfer chain is a viable approach to enhance the EET rate of S. oneidensis.