A flavin-based extracellular electron transfer mechanism in diverse Gram-positive bacteria

Enhanced electroactive bacteria enrichment and facilitated extracellular electron transfer in microbial fuel cells via polydopamine coated graphene aerogel anode

The structure and surface physicochemical properties of anode play a crucial role in microbial fuel cells (MFCs). To enhance the enrichment of exoelectrogen and facilitate extracellular electron transfer (EET), a three-dimensional macroporous graphene aerogel with polydopamine coating was successfully introduced to modify carbon brush (PGA/CB). The three-dimensional graphene aerogel (GA) with micrometer pores improved the space utilization efficiency of microorganisms. Polydopamine (PDA) coating enhanced the physicochemical properties of the electrode surface by introducing abundant functional groups and nitrogen-containing active sites. MFCs equipped with PGA/CB anodes (PGA/CB-MFCs) demonstrated superior power generation compared to GA/CB-MFCs and CB-MFCs (MFCs with GA/CB and CB anodes respectively), including a 23.0 % and 30.1 % reduction in start-up time, and an increase in maximum power density by 2.43 and 1.24 times respectively. The higher bioelectrochemical activity exhibited by the biofilm of PGA/CB anode and the promoted riboflavin secretion by PGA modification imply the enhanced EET efficiency. 16S rRNA high-throughput sequence analysis of the biofilms revealed successful enrichment of Geobacter on PGA/CB anodes. These findings not only validate the positive impact of the synergistic effects between GA and PDA in promoting EET and improving MFC performance but also provide valuable insights for electrode design in other bioelectrochemical systems.

Exogenous paths regulate electron transfer enhancing sediment phosphorus immobilization

The lack of electron acceptors in anaerobic sediments leads to endogenous phosphorus release and low removal efficiency of organic pollutants. This study introduced electrodes and iron oxides into sediments to construct electron network transport chains to supplement electron acceptors. The sediment total organic carbon (TOC) removal efficiencies of closed-circuit (CC) and closed-circuit with Fe addition (CC-Fe) were estimated to be 1.4 and 1.7 times of the control. Unlike the fluctuation of phosphorus in the overlying water of the controls, the CC-Fe was stabled at 0.04-0.08 mg/L during the 84-d operation. The phosphorus in interstitial water of CC-Fe was 30 % less than in control, whereas in sediment, the redox sensitive phosphorus was increased by 14 %, indicating phosphorus was preferred to fix into sediments rather than interstitial water. This is important to reduce the risk of endogenous phosphorus returning to the overlying water. Microbial community analysis showed that the multiplication of Fonticella in CC-Fe (20 %) was 1.8-fold of control (11 %) which improved the TOC removal efficiency. While electroactive microorganisms accumulated near the electrode reduced the abundance of Fe-reducing bacteria, such as Desulfitobacterium (2.4 %), leading to better phosphorus fixation. These findings suggest a strategy for the efficient bioremediation of endogenous pollution in water, with broader implications for regulating electron transport paths and element cycles in aquatic environments.

Biogeochemical cycles of iron: Processes, mechanisms, and environmental implications

The iron (Fe) biogeochemical cycle is critical for abiotic and biological environmental processes that overlap spatially and may compete with each other. The development of modern molecular biology technologies promoted the understanding of the electron transport mechanisms of Fe-cycling-related microorganisms. Recent studies have revealed a novel pathway for microaerophilic ferrous iron (Fe(II))-oxidizers in extracellular Fe(II) oxidation. In addition, OmcS, OmcZ, and OmcE nanowires on the cell surface have been shown to promote electron transfer between microorganisms and their environment. These processes affect the fate of pollutants in directly or indirectly ways, such as greenhouse gas emissions. In this review, these advances and the environmental implications of the Fe cycle process were discussed, with a particular focus on the mechanisms of intracellular or extracellular electron transport in microorganisms.

The convergence of lactic acid microbiomes and metabolites in long-term electrofermentation

Regulating electron transfer in predominantly fermentative microbiomes has broad implications in environmental, chemical, food, and medical fields. Here we demonstrate electrochemical control in fermenting food waste, digestate, and wastewater to improve lactic acid production. We hypothesize that applying anodic potential will expedite and direct fermentation towards lactic acid. Continued operation that introduced epi/endophytic communities (Lactococcus, Lactobacillus, Weissella) to pure culture Lactiplantibacillus plantarum reactors with static electrodes was associated with the loss of anode-induced process intensification despite 80% L. plantarum retention. Employing fluidized electrodes discouraged biofilm formation and extended electrode influence to planktonic gram-positive fermenters using mediated extracellular electron transfer. While short-term experiments differentially enriched Lactococcus and Klebsiella spp., longer-term operations indicated convergent microbiomes and product spectra. These results highlight a functional resilience of environmental fermentative microbiomes to perturbations in redox potential, underscoring the need to better understand electrode induced polymicrobial interactions and physiological impacts to engineer tunable open-culture or synthetic consortia.

Desalination of the power plant salty wastewater by use of an algae-based photosynthetic microbial desalination cell

In this research the possibility of using a photosynthetic microbial desalination cell (PhMDC) is investigated for desalination of power plant salty wastewater (PPSWW), along with power generation and organic load removal. The PhMDC was operated with anaerobic sludge in the anode chamber, microalgae in the cathode chamber, and different conductivities of PPSWW (10, 20, 40, and 55 mS cm-1) in the desalination chamber under different illumination modes (continuous light mode and light/dark mode). The highest power density (285.5 mW m-2), desalination efficiency (60.9%), and COD removal (74.8%) was achieved at conductivity of 55 mS cm-1 under continuous light mode. The highest algal growth (900 mg. L-1) was also observed at this conductivity. The applied system demonstrated effective removal of different presented cations and anions in PPSWW with removal efficiencies of more than 58%. Dynamic shift of microbial community in the anode chamber showed notable increase in sulfate-reducing bacteria and some specific genera such as Desulfovibrio, Pseudomonas, Rhodobacter, Rhodopseudomonas, and Desulfuromonas due to their potential for electricity generation and adaptation to saline and acidic conditions.

Role of molybdenum compounds in enhancing denitrification: structure-activity relationship and the regulatory mechanisms

The effect and regulatory mechanisms of molybdenum compounds (MoO2, MoS2, MoSe2 and MoSi2) on denitrification were investigated by structure-activity relationships, electrochemical characteristics, microbial metabolism analysis and bacterial community distribution. All the assessed molybdenum compounds exhibited the enhancement effect on denitrification, in the order of MoS2 > MoSi2 > MoSe2 > MoO2, with MoS2 increasing 7.08-fold in 12 h. Analysis of structure-activity relationships suggested that the molybdenum compounds with lower negative redox potential and higher redox reversibility were favorable for promoting denitrification. According to the morphology observation, the interactions between Mo compounds and denitrifying bacteria may be beneficial to extracellular electron transfer. Molybdenum compounds with electron transfer capability facilitated an increase in electron capacitance from 835.1 to 1011.3 μF, promoting the electron exchange rate during denitrification. In the denitrification electron transport chain, the molybdenum compounds upregulated nicotinamide adenine dinucleotide and denitrifying enzyme activity, as well as facilitated the abundance of quinone pools, ATP translocation, and cytochrome c related proteins. Moreover, Mo compounds enriched functional bacteria such as electroactive bacteria and denitrifying functional bacteria. Notably, Mo ions in molybdenum compounds may provide active sites for nitrate reductase, optimizing the electron distribution of the denitrification process and thus improved the partial denitrification efficiency. This work aimed to further understand the regulatory mechanisms of molybdenum on denitrification electron transfer in the compound state and to anticipate the catalytic role of Mo compounds for sustainable water treatment.

Engineering Shewanella oneidensis-Carbon Felt Biohybrid Electrode Decorated with Bacterial Cellulose Aerogel-Electropolymerized Anthraquinone to Boost Energy and Chemicals Production

Interfacial electron transfer between electroactive microorganisms (EAMs) and electrodes underlies a wide range of bio-electrochemical systems with diverse applications. However, the electron transfer rate at the biotic-electrode interface remains low due to high transmembrane and cell-electrode interfacial electron transfer resistance. Herein, a modular engineering strategy is adopted to construct a Shewanella oneidensis-carbon felt biohybrid electrode decorated with bacterial cellulose aerogel-electropolymerized anthraquinone to boost cell-electrode interfacial electron transfer. First, a heterologous riboflavin synthesis and secretion pathway is constructed to increase flavin-mediated transmembrane electron transfer. Second, outer membrane c-Cyts OmcF is screened and optimized via protein engineering strategy to accelerate contacted-based transmembrane electron transfer. Third, a S. oneidensis-carbon felt biohybrid electrode decorated with bacterial cellulose aerogel and electropolymerized anthraquinone is constructed to boost the interfacial electron transfer. As a result, the internal resistance decreased to 42 Ω, 480.8-fold lower than that of the wild-type (WT) S. oneidensis MR-1. The maximum power density reached 4286.6 ± 202.1 mW m-2, 72.8-fold higher than that of WT. Lastly, the engineered biohybrid electrode exhibited superior abilities for bioelectricity harvest, Cr6+ reduction, and CO2 reduction. This study showed that enhancing transmembrane and cell-electrode interfacial electron transfer is a promising way to increase the extracellular electron transfer of EAMs.

Microbial approach towards anode biofilm engineering enhances extracellular electron transfer for bioenergy production

Applying microbial electrolysis cells (MEC) is a biological approach to enhance the growth of high amounts of electroactive biofilm for extracellular electron transfer. The electroactive biofilm degrades the organics by oxidizing them at the anode and producing electrical energy. Addition of waste-activated sludge (WAS) with fat grease oil (FOG) produces an optimal reactor environment for microbial growth to enhance the exchange of electrons between cells via microbial electrolysis. The present work aimed to investigate the microbial approach to increase the extracellular electron transfer (EET) in microbial electrolysis cells. Results revealed that metabolites in electroactive microbes (EAM) grow viable cells that initiate high EET at anode sites. At optimum WAS with FOG addition, volatile fatty acid and current generation yield production was 2.94 ± 0.19 g/L and 17.91 ± 7.23 mA, accompanied by COD removal efficiency of 89.5 ± 14.4%, respectively. This study introduces a novel approach to anode biofilm engineering that significantly enhances extracellular electron transfer, offering a fresh perspective on bioenergy production. Our approach, which demonstrates that anodic biofilm enhances intercellular electron transfer, increases NADH-NAD ratio, and increases metabolite yield-fluxes, has the potential to revolutionize bio-electricity production. Results indicated that the electrolysis highlights MEC performance in power generation of 788 mV with 200 mL of anode volume of active viable cells by utilizing WAS with 11% FOG. The achievements of this study provide critical parameters for the anode biofilm engineering, demonstrating how growth cell volume, intercellular electron transfer, and increases in NADH-NAD ratio are evidence of an increase in the EET, compelling evidence for the resilience treatment and efficient current production. These findings are significant in advancing our understanding of bioenergy production.

High power density redox-mediated Shewanella microbial flow fuel cells

Microbial fuel cells utilize exoelectrogenic microorganisms to directly convert organic matter into electricity, offering a compelling approach for simultaneous power generation and wastewater treatment. However, conventional microbial fuel cells typically require thick biofilms for sufficient metabolic electron production rate, which inevitably compromises mass and charge transport, posing a fundamental tradeoff that limits the achievable power density (<1 mW cm-2). Herein, we report a concept for redox-mediated microbial flow fuel cells that utilizes artificial redox mediators in a flowing medium to efficiently transfer metabolic electrons from planktonic bacteria to electrodes. This approach effectively overcomes mass and charge transport limitations, substantially reducing internal resistance. The biofilm-free microbial flow fuel cell thus breaks the inherent tradeoff in dense biofilms, resulting in a maximum current density surpassing 40 mA cm-2 and a highest power density exceeding 10 mW cm-2, approximately one order of magnitude higher than those of state-of-the-art microbial fuel cells.

Bioelectrochemical technologies for soil and sediment remediation: Recent advances and future perspectives

Soil and sediment serve as the ultimate repositories of pollutants, presenting a significant environmental concern on a global scale. However, there is no effective measure due to the low mobility, high resistance and high cost of contaminated soil or sediment. The bioelectrochemical systems (BESs) combine microbial and electrochemical technology to achieve efficient and rapid degradation of pollutants by enriching electroactive microbial membranes with electrodes. Specifically, BESs offer an ideal solution for in-situ remediation, eliminating the secondary pollution and high energy consumption issues associated with traditional technologies. However, in soil or sediment bioelectrochemical systems (SBESs), further summarization and improvement are required to address the influencing factors during the process of pollutant remediation, given the fragility of complex geographical and natural environments. This paper provides a comprehensive overview and analysis of the removal mechanisms of organic pollutants, heavy metals and emerging contaminants within contaminated soil or sediment, elucidating the influential factors and strategies aimed at enhancing pollutant removal processes within SBESs. The current emerging problems and limitations of microbial electrochemical remediation technology are summarized, and it is suggested that future development should focus on microorganisms, reactors and practical applications.

The corncobs-loaded iron nanoparticles enhanced mechanism of denitrification performance in microalgal-bacterial aggregates system when treating low COD/TN wastewater

To improve denitrification efficiency of microalgal-bacterial aggregates (MABAs) when treating low carbon to nitrogen (C/N) ratio wastewater, CK (the biological control), C1 (untreated corncobs), C2 (alkali-treated corncobs), CFe1 (C1 loaded iron nanoparticles) and CFe2 (C2 loaded iron nanoparticles) five groups of experiments were installed under artificial light (1600 lm). After 36 h of experiment, NO3--N was almost completely converted in CFe1 following by CFe2 when the initial concentration was 60.1 mg/L, whose NO3--N conversion rates were 6.2 and 3.4 times faster than the CK group, respectively. The result showed that the corncobs-loaded iron nanoparticles (CFe1, CFe2) had the potential to promote denitrification process and the CFe1 was more effective. Meanwhile, the CFe1 and CFe2 resulted in a decreased content in extracellular polymeric substances (EPS) secretion because iron nanoparticles (Fes) promoted electron transport and alleviated the nitrate stress. Moreover, the electrochemical analysis of EPS showed that the corncobs and corncobs-loaded iron nanoparticles improved the electron transport rate and redox active substances production. The increase in electron transport activity (ETSA), adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide (NADH) also indicated that the CFe1 and CFe2 promoted microbial metabolic activity and the electron transport rate in MABAs. In addition, the CFe1 group enhanced the enrichment of Proteobacteria, Patescibacteria, Chlorophyta and Ignavibacteriae, which was contributed to the nitrogen removal performance of MABAs. In summary, the enhancement mechanism of corncobs-loaded iron nanoparticles on denitrification process of MABAs was depicted through EPS secretion, electrochemical characteristics, microbial metabolic activity and microbial community. The article provides a viable program for enhancing the denitrification performance of MABAs when treating low C/N wastewater.

Magnetite-mediated shifts in denitrifying consortia in bioelectrochemical system: Insights into species selection and metabolic dynamics

Conductive materials, such as magnetite, are recognized for their ability to enhance electron transfer and stimulate microbial metabolic activities. This study aimed to elucidate the metabolic potential and species interactions of dominant microbial species within complex communities influenced by magnetite. It indicated that the optimal dosage of magnetite at 4.5 mg/cm², would significantly improve denitrification efficiency and then reduce the time for removing 50 mg/L nitrate by 24.33 %. This enhancement was attributed to the reduced charge transfer resistance and the promoted formation of extracellular polymeric substances (EPS) facilitated by magnetite. Metagenomic analysis revealed that magnetite addition mitigated the competition among truncated denitrifiers for downstream nitrogen species, diminished the contribution of bacteria with complete nitrogen metabolism pathways to denitrification, and fostered a transition towards co-denitrification through interspecies cooperation, consequently leading to decreased nitrite accumulation and increased tolerance to nitrate shock loads. Furthermore, an in-depth study on a key species, Geobacter anodireducens JN93 within the bioelectrochemical system revealed that while magnetite with varying Fe(II) and Fe(III) ratios improved denitrification performance, the metabolic potential of Geobacter sp. varied for different nitrogen metabolism pathways. Collectively, this research provides insights into the microecological effects of magnetite on denitrifying consortia by shifting interspecific interactions via enhanced electron transfer.

Regulatory roles of extracellular polymeric substances in uranium reduction via extracellular electron transfer by Desulfovibrio vulgaris UR1

The pathway of reducing U(VI) to insoluble U(IV) using electroactive bacteria has become an effective and promising approach to address uranium-contaminated water caused by human activities. However, knowledge regarding the roles of extracellular polymeric substances (EPS) in the uranium reduction process involving in extracellular electron transfer (EET) mechanisms is limited. Here, this study isolated a novel U(VI)-reducing strain, Desulfovibrio vulgaris UR1, with a high uranium removal capacity of 2.75 mM/(g dry cell). Based on a reliable EPS extraction method (45 °C heating), manipulation of EPS in D. vulgaris UR1 suspensions (removal or addition of EPS) highlighted its critical role in facilitating uranium reduction efficiency. On the second day, U(VI) removal rates varied significantly across systems with different EPS contents: 60.8% in the EPS-added system, 48.5% in the pristine system, and 22.2% in the EPS-removed system. Characterization of biogenic solids confirmed the reduction of U(VI) by D. vulgaris UR1, and the main products were uraninite and UO2 (2.88-4.32 nm in diameter). As EPS formed a permeable barrier, these nanoparticles were primarily immobilized within the EPS in EPS-retained/EPS-added cells, and within the periplasm in EPS-removed cells. Multiple electroactive substances, such as tyrosine/tryptophan aromatic compounds, flavins, and quinone-like substances, were identified in EPS, which might be the reason for enhancement of uranium reduction via providing more electron shuttles. Furthermore, proteomics revealed that a large number of proteins in EPS were enriched in the subcategories of catalytic activity and electron transfer activity. Among these, iron-sulfur proteins, such as hydroxylamine reductase (P31101), pyruvate: ferredoxin oxidoreductase (A0A0H3A501), and sulfite reductase (P45574), played the most critical role in regulating EET in D. vulgaris UR1. This work highlighted the importance of EPS in the uranium reduction by D. vulgaris UR1, indicating that EPS functioned as both a reducing agent and a permeation barrier for access to heavy metal uranium.

The role of Listeria monocytogenes PstA in β-lactam resistance requires the cytochrome bd oxidase activity

c-di-AMP is an essential second messenger that binds and regulates several proteins of different functions within bacterial cells. Among those, PstA is a structurally conserved c-di-AMP-binding protein, but its function is largely unknown. PstA is structurally similar to PII signal transduction proteins, although it specifically binds c-di-AMP rather than other PII ligands such as ATP and α-ketoglutarate. In Listeria monocytogenes, we found that PstA increases β-lactam susceptibility at normal and low c-di-AMP levels, but increases β-lactam resistance upon c-di-AMP accumulation. Examining a PstA mutant defective for c-di-AMP binding, we found the apo form of PstA to be toxic for β-lactam resistance, and the c-di-AMP-bound form to be beneficial. Intriguingly, a role for PstA in β-lactam resistance is only prominent in aerobic cultures, and largely diminished under hypoxic conditions, suggesting that PstA function is linked to aerobic metabolism. However, PstA does not control aerobic growth rate, and has a modest influence on the tricarboxylic acid cycle and membrane potential-an indicator of cellular respiration. The regulatory role of PstA in β-lactam resistance is unrelated to reactive oxygen species or oxidative stress. Interestingly, during aerobic growth, PstA function requires the cytochrome bd oxidase (CydAB), a component of the respiratory electron transport chain. The requirement for CydAB might be related to its function in maintaining a membrane potential, or redox stress response activities. Altogether, we propose a model in which apo-PstA diminishes β-lactam resistance by interacting with an effector protein, and this activity can be countered by c-di-AMP binding or a by-product of redox stress.

IMPORTANCE

PstA is a structurally conserved c-di-AMP-binding protein that is broadly present among Firmicutes bacteria. Furthermore, PstA binds c-di-AMP at high affinity and specificity, indicating an important role in the c-di-AMP signaling network. However, the molecular function of PstA remains elusive. Our findings reveal contrasting roles of PstA in β-lactam resistance depending on c-di-AMP-binding status. We also define physiological conditions for PstA function during aerobic growth. Future efforts can exploit these conditions to identify PstA interaction partners under β-lactam stress.

Isolation and genomic analysis of "Metallumcola ferriviriculae" MK1, a Gram-positive, Fe(III)-reducing bacterium from the Soudan Underground Mine, an iron-rich Martian analog site

The Soudan Underground Mine State Park, found in the Vermilion Iron Range in northern Minnesota, provides access to a ~ 2.7 billion-year-old banded iron formation. Exploratory boreholes drilled between 1958 and 1962 on the 27th level (713 m underground) of the mine intersect calcium and iron-rich brines that have recently been subject to metagenomic analysis and microbial enrichments. Using concentrated brine samples pumped from a borehole depth of up to 55 m, a novel Gram-positive bacterium was enriched under anaerobic, acetate-oxidizing, and Fe(III) citrate-reducing conditions. The isolated bacterium, designated strain MK1, is non-motile, rod-shaped, spore-forming, anaerobic, and mesophilic, with a growth range between 24°C and 30°C. The complete circular MK1 genome was found to be 3,720,236 bp and encodes 25 putative multiheme cytochromes, including homologs to inner membrane cytochromes in the Gram-negative bacterium Geobacter sulfurreducens and cytoplasmic membrane and periplasmic cytochromes in the Gram-positive bacterium Thermincola potens. However, MK1 does not encode homologs of the peptidoglycan (CwcA) and cell surface-associated (OcwA) multiheme cytochromes proposed to be required by T. potens to perform extracellular electron transfer. The 16S rRNA gene sequence of MK1 indicates that its closest related isolate is Desulfitibacter alkalitolerans strain sk.kt5 (91% sequence identity), which places MK1 in a novel genus within the Desulfitibacteraceae family and Moorellales order. Within the Moorellales order, only Calderihabitans maritimus strain KKC1 has been reported to reduce Fe(III), and only D. alkalitolerans can also grow in temperatures below 40°C. Thus, MK1 represents a novel species within a novel genus, for which we propose the name "Metallumcola ferriviriculae" strain MK1, and provides a unique opportunity to study a cytochrome-rich, mesophilic, Gram-positive, spore-forming Fe(III)-reducing bacterium.IMPORTANCEThe Soudan Underground Mine State Park gives access to understudied regions of the deep terrestrial subsurface that potentially predate the Great Oxidation Event. Studying organisms that have been relatively unperturbed by surface conditions for as long as 2.7 billion years may give us a window into ancient life before oxygen dominated the planet. Additionally, studying microbes from anoxic and iron-rich environments can help us better understand the requirements of life in analogous environments, such as on Mars. The isolation and characterization of "Metallumcola ferriviriculae" strain MK1 give us insights into a novel genus and species that is distinct both from its closest related isolates and from iron reducers characterized to date. "M. ferriviriculae" strain MK1 may also act as a model organism to study how the processes of sporulation and germination are affected by insoluble extracellular acceptors, as well as the impact of spores in the deep terrestrial biosphere.

Interaction of living cable bacteria with carbon electrodes in bioelectrochemical systems

Cable bacteria are filamentous bacteria that couple the oxidation of sulfide in sediments to the reduction of oxygen via long-distance electron transport over centimeter distances through periplasmic wires. However, the capability of cable bacteria to perform extracellular electron transfer to acceptors, such as electrodes, has remained elusive. In this study, we demonstrate that living cable bacteria actively move toward electrodes in different bioelectrochemical systems. Carbon felt and carbon fiber electrodes poised at +200 mV attracted live cable bacteria from the sediment. When the applied potential was switched off, cable bacteria retracted from the electrode. qPCR and scanning electron microscopy corroborated this finding and revealed cable bacteria in higher abundance present on the electrode surface compared with unpoised controls. These experiments raise new possibilities to study metabolism of cable bacteria and cultivate them in bioelectrochemical devices for bioelectronic applications, such as biosensing and bioremediation.

IMPORTANCE

Extracellular electron transfer is a metabolic function associated with electroactive bacteria wherein electrons are exchanged with external electron acceptors or donors. This feature has enabled the development of several applications, such as biosensing, carbon capture, and energy recovery. Cable bacteria are a unique class of long, filamentous microbes that perform long-distance electron transport in freshwater and marine sediments. In this study, we demonstrate the attraction of cable bacteria toward carbon electrodes and demonstrate their potential electroactivity. This finding enables electronic control and monitoring of the metabolism of cable bacteria and may, in turn, aid in the development of bioelectronic applications.

Versatile roles of protein flavinylation in bacterial extracyotosolic electron transfer

Bacteria perform diverse redox chemistries in the periplasm, cell wall, and extracellular space. Electron transfer for these extracytosolic activities is frequently mediated by proteins with covalently bound flavins, which are attached through post-translational flavinylation by the enzyme ApbE. Despite the significance of protein flavinylation to bacterial physiology, the basis and function of this modification remain unresolved. Here we apply genomic context analyses, computational structural biology, and biochemical studies to address the role of ApbE flavinylation throughout bacterial life. We identify ApbE flavinylation sites within structurally diverse protein domains and show that multi-flavinylated proteins, which may mediate longer distance electron transfer via multiple flavinylation sites, exhibit substantial structural heterogeneity. We identify two novel classes of flavinylation substrates that are related to characterized proteins with non-covalently bound flavins, providing evidence that protein flavinylation can evolve from a non-covalent flavoprotein precursor. We further find a group of structurally related flavinylation-associated cytochromes, including those with the domain of unknown function DUF4405, that presumably mediate electron transfer in the cytoplasmic membrane. DUF4405 homologs are widespread in bacteria and related to ferrosome iron storage organelle proteins that may facilitate iron redox cycling within ferrosomes. These studies reveal a complex basis for flavinylated electron transfer and highlight the discovery power of coupling comparative genomic analyses with high-quality structural models.

IMPORTANCE

This study explores the mechanisms bacteria use to transfer electrons outside the cytosol, a fundamental process involved in energy metabolism and environmental interactions. Central to this process is a phenomenon known as flavinylation, where a flavin molecule-a compound related to vitamin B2-is covalently attached to proteins, to enable electron transfer. We employed advanced genomic analysis and computational modeling to explore how this modification occurs across different bacterial species. Our findings uncover new types of proteins that undergo this modification and highlight the diversity and complexity of bacterial electron transfer mechanisms. This research broadens our understanding of bacterial physiology and informs potential biotechnological applications that rely on microbial electron transfer, including bioenergy production and bioremediation.

Single-walled Carbon Nanotubes Wrapped with Charged Polysaccharides Enhance Extracellular Electron Transfer

Microbial electrochemical systems (MESs) rely on the microbes' ability to transfer charges from their anaerobic respiratory processes to electrodes through extracellular electron transfer (EET). To increase the generally low output signal in devices, advanced bioelectrical interfaces tend to augment this problem by attaching conducting nanoparticles, such as positively charged multiwalled carbon nanotubes (CNTs), to the base carbon electrode to electrostatically attract the negatively charged bacterial cell membrane. On the other hand, some reports point to the importance of the magnitude of the surface charge of functionalized single-walled CNTs (SWCNTs) as well as the size of functional groups for interaction with the cell membrane, rather than their polarity. To shed light on these phenomena, in this study, we prepared and characterized well-solubilized aqueous dispersions of SWCNTs functionalized by either positively or negatively charged cellulose-derivative polymers, as well as with positively charged or neutral small molecular surfactants, and tested the electrochemical performance of Shewanella oneidensis MR-1 in MESs in the presence of these functionalized SWCNTs. By simple injection into the MESs, the positively charged polymeric SWCNTs attached to the base carbon felt (CF) electrode, and as fluorescence microscopy revealed, allowed bacteria to attach to these structures. As a result, EET currents continuously increased over several days of monitoring, without bacterial growth in the electrolyte. Negatively charged polymeric SWCNTs also resulted in continuously increasing EET currents and a large number of bacteria on CF, although SWCNTs did not attach to CF. In contrast, SWCNTs functionalized by small-sized surfactants led to a decrease in both currents and the amount of bacteria in the solution, presumably due to the detachment of surfactants from SWCNTs and their detrimental interaction with cells. We expect our results will help researchers in designing materials for smart bioelectrical interfaces for low-scale microbial energy harvesting, sensing, and energy conversion applications.

Bioimaging and prospects of night pearls-based persistence phosphors in cancer diagnostics

Inorganic persistent phosphors feature great potential for cancer diagnosis due to the long luminescence lifetime, low background scattering, and minimal autofluorescence. With the prominent advantages of near-infrared light, such as deep penetration, high resolution, low autofluorescence, and tissue absorption, persistent phosphors can be used for deep bioimaging. We focus on highlighting inorganic persistent phosphors, emphasizing the synthesis methods and applications in cancer diagnostics. Typical synthetic methods such as the high-temperature solid state, thermal decomposition, hydrothermal/solvothermal, and template methods are proposed to obtain small-size phosphors for biological organisms. The luminescence mechanisms of inorganic persistent phosphors with different excitation are discussed and effective matrixes including galliumate, germanium, aluminate, and fluoride are explored. Finally, the current directions where inorganic persistent phosphors can continue to be optimized and how to further overcome the challenges in cancer diagnosis are summarized.

Exploring the influence of deposit mineral composition on biofilm communities in oil and gas systems

Introduction

Inside oil and gas pipelines, native microbial communities and different solid compounds typically coexist and form mixed deposits. However, interactions between these deposits (primarily consisting of mineral phases) and microorganisms in oil and gas systems remain poorly understood. Here, we investigated the influence of magnetite (Fe3O4), troilite (FeS), and silica (SiO2) on the microbial diversity, cell viability, biofilm formation, and EPS composition of an oil-recovered multispecies consortium.

Methods

An oilfield-recovered microbial consortium was grown for 2 weeks in separate bioreactors, each containing 10 g of commercially available magnetite (Fe3O4), troilite (FeS), or silica (SiO2) at 40°C ± 1°C under a gas atmosphere of 20% CO2/80% N2.

Results

The microbial population formed in troilite significantly differed from those in silica and magnetite, which exhibited significant similarities. The dominant taxa in troilite was the Dethiosulfovibrio genus, whereas Sulfurospirillum dominated in magnetite and silica. Nevertheless, biofilm formation was lowest on troilite and highest on silica, correlating with the observed cell viability.

Discussion

The dissolution of troilite followed by the liberation of HS- (H2S) and Fe2+ into the test solution, along with its larger particle size compared to silica, likely contributed to the observed results. Confocal laser scanning microscopy revealed that the EPS of the biofilm formed in silica was dominated by eDNA, while those in troilite and magnetite primarily contained polysaccharides. Although the mechanisms of this phenomenon could not be determined, these findings are anticipated to be particularly valuable for enhancing MIC mitigation strategies currently used in oil and gas systems.

Enhanced Stability and Detection Range of Microbial Electrochemical Biotoxicity Sensor by Polydopamine Encapsulation

With the rapid development of modern industry, it is urgently needed to measure the biotoxicity of complex chemicals. Microbial electrochemical biotoxicity sensors are an attractive technology; however, their application is usually limited by their stability and reusability after measurements. Here, we improve their performance by encapsulating the electroactive biofilm with polydopamine (PDA), and we evaluate the improvement by different concentrations of heavy metal ions (Cu2+, Ag+, and Fe3+) in terms of inhibition ratio (IR) and durability. Results indicate that the PDA-encapsulated sensor exhibits a more significant detection concentration than the control group, with a 3-fold increase for Cu2+ and a 1.5-fold increase for Ag+. Moreover, it achieves 15 more continuous toxicity tests than the control group, maintaining high electrochemical activity even after continuous toxicity impacts. Images from a confocal laser scanning microscope reveal that the PDA encapsulation protects the activity of the electroactive biofilm. The study, thus, demonstrates that PDA encapsulation is efficacious in improving the performance of microbial electrochemical biotoxicity sensors, which can extend its application to more complex media.

Mixed Conducting Polymers Alter Electron Transfer Thermodynamics to Boost Current Generation from Electroactive Microbes

Electroactive microbes that can release or take up electrons are essential components of nearly every ecological niche and are powerful tools for the development of alternative energy technologies. Small-molecule mediators are critical for this electron transfer but remain difficult to study and engineer because they perform concerted two-electron transfer in native systems but only individual, one-electron transfers in electrochemical studies. Here, we report that electrode modification with ion- and electron-conductive polymers yields biosimilar, concerted two-electron transfer from Shewanella oneidensis via flavin mediators. S. oneidensis biofilms on these polymers show significantly improved per-microbe current generation and morphologies that more closely resemble native systems, setting a new paradigm for the study and optimization of these electron transfer processes. The unprecedented concerted electron transfer was found to be due to altered mediator electron transfer thermodynamics, enabling biologically relevant studies of electroactive biofilms in the lab for the first time. These important findings pave the way for a complete understanding of the ecological role of electroactive microbes and their broad application in sustainable technologies.

Unlocking Wearable Microbial Fuel Cells for Advanced Wound Infection Treatment

Better infection control will accelerate wound healing and alleviate associated healthcare burdens. Traditional antibacterial dressings often inadequately control infections, inadvertently promoting antibacterial resistance. Our research unveils a novel, dual-functional living dressing that autonomously generates antibacterial agents and delivers electrical stimulation, harnessing the power of spore-forming Bacillus subtilis. This dressing is built on an innovative wearable microbial fuel cell (MFC) framework, using B. subtilis endospores as a powerful, dormant biocatalyst. The endospores are resilient, reactivating in nutrient-rich wound exudate to produce electricity and antibacterial compounds. The combination allows B. subtilis to outcompete pathogens for food and other resources, thus fighting infections. The strategy is enhanced by the extracellular synthesis of tin oxide and copper oxide nanoparticles on the endospore surface, boosting antibacterial action, and electrical stimulation. Moreover, the MFC framework introduces a pioneering dressing design featuring a conductive hydrogel embedded within a paper-based substrate. The arrangement ensures cell stability and sustains a healing-friendly moist environment. Our approach has proven very effective against three key pathogens in biofilms: Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus demonstrating exceptional capabilities in both in vitro and ex vivo models. Our innovation marks a significant leap forward in wearable MFC-based wound care, offering a potent solution for treating infected wounds.

Engineering of bespoke photosensitiser-microbe interfaces for enhanced semi-artificial photosynthesis

Biohybrid systems for solar fuel production integrate artificial light-harvesting materials with biological catalysts such as microbes. In this perspective, we discuss the rational design of the abiotic-biotic interface in biohybrid systems by reviewing microbes and synthetic light-harvesting materials, as well as presenting various approaches to coupling these two components together. To maximise performance and scalability of such semi-artificial systems, we emphasise that the interfacial design requires consideration of two important aspects: attachment and electron transfer. It is our perspective that rational design of this photosensitiser-microbe interface is required for scalable solar fuel production. The design and assembly of a biohybrid with a well-defined electron transfer pathway allows mechanistic characterisation and optimisation for maximum efficiency. Introduction of additional catalysts to the system can close the redox cycle, omitting the need for sacrificial electron donors. Studies that electronically couple light-harvesters to well-defined biological entities, such as emerging photosensitiser-enzyme hybrids, provide valuable knowledge for the strategic design of whole-cell biohybrids. Exploring the interactions between light-harvesters and redox proteins can guide coupling strategies when translated into larger, more complex microbial systems.

Heme utilization by the enterococci

Heme consists of a tetrapyrrole ring ligating an iron ion and has important roles in biological systems. While well-known as the oxygen-binding molecule within hemoglobin of mammals, heme is also cofactor for several enzymes and a major iron source for bacteria within the host. The enterococci are a diverse group of Gram-positive bacteria that exist primarily within the gastrointestinal tract of animals. However, some species within this genus can transform into formidable opportunistic pathogens, largely owing to their extraordinary adaptability to hostile environments. Although enterococci cannot synthesize heme nor depend on heme to grow, several species within the genus encode proteins that utilize heme as a cofactor, which appears to increase their fitness and ability to thrive in challenging environments. This includes more efficient energy generation via aerobic respiration and protection from reactive oxygen species. Here, we review the significance of heme to enterococci, primarily the major human pathogen Enterococcus faecalis, use bioinformatics to assess the prevalence of hemoproteins throughout the genus, and highlight recent studies that underscore the central role of the heme-E. faecalis relationship in host-pathogen dynamics and interspecies bacterial interactions.

Antioxidants are ineffective at quenching reactive oxygen species inside bacteria and should not be used to diagnose oxidative stress

A wide variety of stresses have been proposed to exert killing effects upon bacteria by stimulating the intracellular formation of reactive oxygen species (ROS). A key part of the supporting evidence has often been the ability of antioxidant compounds to protect the cells. In this study, some of the most-used antioxidants-thiourea, glutathione, N-acetylcysteine, and ascorbate-have been examined. Their ability to quench superoxide and hydrogen peroxide was verified in vitro, but the rate constants were orders of magnitude too slow for them to have an impact upon superoxide and peroxide concentrations in vivo, where these species are already scavenged by highly active enzymes. Indeed, the antioxidants were unable to protect the growth and ROS-sensitive enzymes of E. coli strains experiencing authentic oxidative stress. Similar logic posits that antioxidants cannot substantially quench hydroxyl radicals inside cells, which contain abundant biomolecules that react with them at diffusion-limited rates. Indeed, antioxidants were able to protect cells from DNA damage only if they were applied at concentrations that slow metabolism and growth. This protective effect was apparent even under anoxic conditions, when ROS could not possibly be involved, and it was replicated when growth was similarly slowed by other means. Experimenters should discard the use of antioxidants as a way of detecting intracellular oxidative stress and should revisit conclusions that have been based upon such experiments. The notable exception is that these compounds can effectively degrade hydrogen peroxide from environmental sources before it enters cells.

Single-Cell Phenotyping of Extracellular Electron Transfer via Microdroplet Encapsulation

Electroactive organisms contribute to metal cycling, pollutant removal, and other redox-driven environmental processes. Studying this phenomenon in high-throughput is challenging since extracellular reduction cannot easily be traced back to its cell of origin within a mixed population. Here, we describe the development of a microdroplet emulsion system to enrich EET-capable organisms. We validated our system using the model electroactive organism S. oneidensis and describe the tooling of a benchtop microfluidic system for oxygen-limited processes. We demonstrated enrichment of EET-capable phenotypes from a mixed wild-type and EET-knockout population. As a proof-of-concept application, bacteria were collected from iron sedimentation from Town Lake (Austin, TX) and subjected to microdroplet enrichment. We observed an increase in EET-capable organisms in the sorted population that was distinct when compared to a population enriched in a bulk culture more closely akin to traditional techniques for discovering EET-capable bacteria. Finally, two bacterial species, C. sakazakii and V. fessus not previously shown to be electroactive, were further cultured and characterized for their ability to reduce channel conductance in an organic electrochemical transistor (OECT) and to reduce soluble Fe(III). We characterized two bacterial species not previously shown to exhibit electrogenic behavior. Our results demonstrate the utility of a microdroplet emulsions for identifying putative EET-capable bacteria and how this technology can be leveraged in tandem with existing methods.

Nutrient acquisition strategies by gut microbes

The composition and function of the gut microbiota are intimately tied to nutrient acquisition strategies and metabolism, with significant implications for host health. Both dietary and host-intrinsic factors influence community structure and the basic modes of bacterial energy metabolism. The intestinal tract is rich in carbon and nitrogen sources; however, limited access to oxygen restricts energy-generating reactions to fermentation. By contrast, increased availability of electron acceptors during episodes of intestinal inflammation results in phylum-level changes in gut microbiota composition, suggesting that bacterial energy metabolism is a key driver of gut microbiota function. In this review article, we will illustrate diverse examples of microbial nutrient acquisition strategies in the context of habitat filters and anatomical location and the central role of energy metabolism in shaping metabolic strategies to support bacterial growth in the mammalian gut.

Potential coupling of microbial methane, nitrogen, and sulphur cycling in the Okinawa Trough cold seep sediments

The Okinawa Trough (OT) is a back-arc basin with a wide distribution of active cold seep systems. However, our understanding of the metabolic function of microbial communities in the cold seep sediments of the OT remains limited. In this study, we investigated the vertical profiles of functional genes involved in methane, nitrogen, and sulphur cycling in the cold seep sediments of the OT. Furthermore, we explored the possible coupling mechanisms between these biogeochemical cycles. The study revealed that the majority of genes associated with the nitrogen and sulphur cycles were most abundant in the surface sediment layers. However, only the key genes responsible for sulphur disproportionation (sor), nitrogen fixation (nifDKH), and methane metabolism (mcrABG) were more prevalent within sulfate-methane transition zone (SMTZ). Significant positive correlations (P < 0.05) were observed between functional genes involved in sulphur oxidation, thiosulphate disproportionation with denitrification, and dissimilatory nitrate reduction to ammonium (DNRA), as well as between AOM/methanogenesis and nitrogen fixation, and between sulphur disproportionation and AOM. A genome of Filomicrobium (class Alphaproteobacteria) has demonstrated potential in chemoautotrophic activities, particularly in coupling DNRA and denitrification with sulphur oxidation. Additionally, the characterized sulfate reducers such as Syntrophobacterales have been found to be capable of utilizing nitrate as an electron acceptor. The predominant methanogenic/methanotrophic groups in the OT sediments were identified as H2-dependent methylotrophic methanogens (Methanomassiliicoccales and Methanofastidiosales) and ANME-1a. This study offered a thorough understanding of microbial ecosystems in the OT cold seep sediments, emphasizing their contribution to nutrient cycling.IMPORTANCEThe Okinawa Trough (OT) is a back-arc basin formed by extension within the continental lithosphere behind the Ryukyu Trench arc system. Cold seeps are widespread in the OT. While some studies have explored microbial communities in OT cold seep sediments, their metabolic potential remains largely unknown. In this study, we used metagenomic analysis to enhance comprehension of the microbial community's role in nutrient cycling and proposed hypotheses on the coupling process and mechanisms involved in biogeochemical cycles. It was revealed that multiple metabolic pathways can be performed by a single organism or microbes that interact with each other to carry out various biogeochemical cycling. This data set provided a genomic road map on microbial nutrient cycling in OT sediment microbial communities.

In Situ Biosynthesis of FeS Nanoparticles Boosts Current Generation in Bioelectrochemical Systems Through Efficient Electron Transfer

The utility of electrochemical active biofilm in bioelectrochemical systems has received considerable attention for harvesting energy and chemical products. However, the slow electron transfer between biofilms and electrodes hinders the enhancement of performance and still remains challenging. Here, using Fe3O4 /L-Cys nanoparticles as precursors to induce biomineralization, a facile strategy for the construction of an effective electron transfer pathway through biofilm and biological/inorganic interface is proposed, and the underlying mechanisms are elucidated. Taking advantage of an on-chip interdigitated microelectrode array (IDA), the conductive current of biofilm that is related to the electron transfer process within biofilm is characterized, and a 2.10-fold increase in current output is detected. The modification of Fe3O4/L-Cys on the electrode surface facilitates the electron transfer between the biofilm and the electrode, as the bio/inorganic interface electron transfer resistance is only 16% compared to the control. The in-situ biosynthetic Fe-containing nanoparticles (e.g., FeS) enhance the transmembrane EET and the EET within biofilm, and the peak conductivity increases 3.4-fold compared to the control. The in-situ biosynthesis method upregulates the genes involved in energy metabolism and electron transfer from the transcriptome analysis. This study enriches the insights of biosynthetic nanoparticles on electron transfer process, holding promise in bioenergy conversion.

Scrutinizing a Lactococcus lactis mutant with enhanced capacity for extracellular electron transfer reveals a unique role for a novel type-II NADH dehydrogenase

Lactococcus lactis, a lactic acid bacterium used in food fermentations and commonly found in the human gut, is known to possess a fermentative metabolism. L. lactis, however, has been demonstrated to transfer metabolically generated electrons to external electron acceptors, a process termed extracellular electron transfer (EET). Here, we investigated an L. lactis mutant with an unusually high capacity for EET that was obtained in an adaptive laboratory evolution (ALE) experiment. First, we investigated how global gene expression had changed, and found that amino acid metabolism and nucleotide metabolism had been affected significantly. One of the most significantly upregulated genes encoded the NADH dehydrogenase NoxB. We found that this upregulation was due to a mutation in the promoter region of NoxB, which abolished carbon catabolite repression. A unique role of NoxB in EET could be attributed and it was directly verified, for the first time, that NoxB could support respiration in L. lactis. NoxB, was shown to be a novel type-II NADH dehydrogenase that is widely distributed among gut microorganisms. This work expands our understanding of EET in Gram-positive electroactive microorganisms and the special significance of a novel type-II NADH dehydrogenase in EET.IMPORTANCEElectroactive microorganisms with extracellular electron transfer (EET) ability play important roles in biotechnology and ecosystems. To date, there have been many investigations aiming at elucidating the mechanisms behind EET, and determining the relevance of EET for microorganisms in different niches. However, how EET can be enhanced and harnessed for biotechnological applications has been less explored. Here, we compare the transcriptomes of an EET-enhanced L. lactis mutant with its parent and elucidate the underlying reason for its superior performance. We find that one of the most significantly upregulated genes is the gene encoding the NADH dehydrogenase NoxB, and that upregulation is due to a mutation in the catabolite-responsive element that abolishes carbon catabolite repression. We demonstrate that NoxB has a special role in EET, and furthermore show that it supports respiration to oxygen, which has never been done previously. In addition, a search reveals that this novel NoxB-type NADH dehydrogenase is widely distributed among gut microorganisms.

Influence of support materials on the electroactive behavior, structure and gene expression of wild type and GSU1771-deficient mutant of Geobacter sulfurreducens biofilms

Geobacter sulfurreducens DL1 is a metal-reducing dissimilatory bacterium frequently used to produce electricity in bioelectrochemical systems (BES). The biofilm formed on electrodes is one of the most important factors for efficient electron transfer; this is possible due to the production of type IV pili and c-type cytochromes that allow it to carry out extracellular electron transfer (EET) to final acceptors. In this study, we analyzed the biofilm formed on different support materials (glass, hematite (Fe2O3) on glass, fluorine-doped tin oxide (FTO) semiconductor glass, Fe2O3 on FTO, graphite, and stainless steel) by G. sulfurreducens DL1 (WT) and GSU1771-deficient strain mutant (Δgsu1771). GSU1771 is a transcriptional regulator that controls the expression of several genes involved in electron transfer. Different approaches and experimental tests were carried out with the biofilms grown on the different support materials including structure analysis by confocal laser scanning microscopy (CLSM), characterization of electrochemical activity, and quantification of relative gene expression by RT-qPCR. The gene expression of selected genes involved in EET was analyzed, observing an overexpression of pgcA, omcS, omcM, and omcF from Δgsu1771 biofilms compared to those from WT, also the overexpression of the epsH gene, which is involved in exopolysaccharide synthesis. Although we observed that for the Δgsu1771 mutant strain, the associated redox processes are similar to the WT strain, and more current is produced, we think that this could be associated with a higher relative expression of certain genes involved in EET and in the production of exopolysaccharides despite the chemical environment where the biofilm develops. This study supports that G. sulfurreducens is capable of adapting to the electrochemical environment where it grows.

Anaerobic oxidation of methane in landfill and adjacent groundwater environments: Occurrence, mechanisms, and potential applications

Landfills remain the predominant means of solid waste management worldwide. Widespread distribution and significant stockpiles of waste in landfills make them a significant source of methane emissions, exacerbating climate change. Anaerobic oxidation of methane (AOM) has been shown to play a critical role in mitigating methane emissions on a global scale. The rich methane and electron acceptor environment in landfills provide the necessary reaction conditions for AOM, making it a potentially low-cost and effective strategy for reducing methane emissions in landfills. However, compared to other anaerobic habitats, research on AOM in landfill environments is scarce, and there is a lack of analysis on the potential application of AOM in different zones of landfills. Therefore, this review summarizes the existing knowledge on AOM and its occurrence in landfills, analyzes the possibility of AOM occurrence in different zones of landfills, discusses its potential applications, and explores the challenges and future research directions for AOM in landfill management. The identification of research gaps and future directions outlined in this review encourages further investigation and advancement in the field of AOM, paving the way for more effective waste stabilization, greenhouse gas reduction, and pollutant mitigation strategies in landfills.

Roles of flavoprotein oxidase and the exogenous heme- and quinone-dependent respiratory chain in lactic acid bacteria

Lactic acid bacteria (LAB) are a type of bacteria that convert carbohydrates into lactate through fermentation metabolism. While LAB mainly acquire energy through this anaerobic process, they also have oxygen-consuming systems, one of which is flavoprotein oxidase and the other is exogenous heme- or heme- and quinone-dependent respiratory metabolism. Over the past two decades, research has contributed to the understanding of the roles of these oxidase machineries, confirming their suspected roles and uncovering novel functions. This review presents the roles of these oxidase machineries, which are anticipated to be critical for the future applications of LAB in industry and comprehending the virulence of pathogenic streptococci.

Accelerated corrosion of 316L stainless steel in a simulated oral environment via extracellular electron transfer and acid metabolites of subgingival microbiota

316L stainless steel (SS) is widely applied as microimplant anchorage (MIA) due to its excellent mechanical properties. However, the risk that the oral microorganisms can corrode 316L SS is fully neglected. Microbiologically influenced corrosion (MIC) of 316L SS is essential to the health and safety of all patients because the accelerated corrosion caused by the oral microbiota can trigger the release of Cr and Ni ions. This study investigated the corrosion behavior and mechanism of subgingival microbiota on 316L SS by 16S rRNA and metagenome sequencing, electrochemical measurements, and surface characterization techniques. Multispecies biofilms were formed by the oral subgingival microbiota in the simulated oral anaerobic environment on 316L SS surfaces, significantly accelerating the corrosion in the form of pitting. The microbiota samples collected from the subjects differed in biofilm compositions, corrosion behaviors, and mechanisms. The oral subgingival microbiota contributed to the accelerated corrosion of 316L SS via acidic metabolites and extracellular electron transfer. Our findings provide a new insight into the underlying mechanisms of oral microbial corrosion and guide the design of oral microbial corrosion-resistant materials.

Enhancing biohydrogen production from xylose through natural FeS2 ore: Mechanistic insights

In this study, we investigated the advantages of utilizing natural FeS2 ore in the context of dark fermentative hydrogen production within a fermentation system employing heat-treated anaerobic granular sludge with xylose as the carbon source. The results demonstrated a significant improvement in both hydrogen production and the maximum rate, with increases of 2.58 and 4.2 times, respectively. Moreover, the presence of FeS2 ore led to a reduction in lag time by more than 2-3 h. The enhanced biohydrogen production performance was attributed to factors such as the intracellular NADH/NAD+ ratio, redox-active components of extracellular polymeric substances, secreted flavins, as well as the presence of hydrogenase and nitrogenase. Furthermore, the FeS2 ore served as a direct electron donor and acceptor during biohydrogen production. This study shed light on the underlying mechanisms contributing to the improved performance of biohydrogen production from xylose during dark fermentation through the supplementation of natural FeS2 ore.

Advanced Electroanalysis for Electrosynthesis

Electrosynthesis is a popular, environmentally friendly substitute for conventional organic methods. It involves using charge transfer to stimulate chemical reactions through the application of a potential or current between two electrodes. In addition to electrode materials and the type of reactor employed, the strategies for controlling potential and current have an impact on the yields, product distribution, and reaction mechanism. In this Review, recent advances related to electroanalysis applied in electrosynthesis were discussed. The first part of this study acts as a guide that emphasizes the foundations of electrosynthesis. These essentials include instrumentation, electrode selection, cell design, and electrosynthesis methodologies. Then, advances in electroanalytical techniques applied in organic, enzymatic, and microbial electrosynthesis are illustrated with specific cases studied in recent literature. To conclude, a discussion of future possibilities that intend to advance the academic and industrial areas is presented.

In-situ reactivation and reuse of micronsized sulfidated zero-valent iron using SRB-enriched culture: A sustainable PRB technology

Sulfidated zero-valent iron (S-ZVI) is an attractive material of permeable reactive barriers (PRBs) for the remediation of contaminated groundwater. However, S-ZVI is prone to be passivated due to the oxidation of reactive and conductive iron sulfide (FeSx) shell and the formation of inactive and non-conductive ferric (hydr)oxides, which serve as electron transfer barriers to hinder the electron flow from Fe° core to contaminants. This study thus proposed a novel approach for in-situ reactivation and reuse of micronsized S-ZVI (S-mZVI) in PRB using sulfate-reducing bacteria (SRB) enriched culture to realize long-lasting remediation of Cr(VI)-contaminated groundwater. S-mZVI were passivated after reactions with Cr(VI) due to the formation of electron transfer barriers (mainly inactive and non-conductive Fe(III) (hyd)oxides, which increased the polarization resistance from 16.38 to 27.38 kΩ cm2 and hindered the electron transfer from the Fe° core. Interestingly, the passivated S-mZVI was efficiently reactivated by providing the SRB-enriched culture and organic carbon within 12 h, and the Cr(VI) removal capacity of S-mZVI in the three use cycles increased to 37.4 mg Cr/g, which was 2.1 times higher than that of the virgin S-mZVI. After biological reactivation, the Rp of reactivated S-mZVI decreased to 12.30 kΩ cm2. SRB-mediated reactivation removed the electron transfer barriers via biotic and abiotic reduction of Fe(III) (hyd)oxides. Especially, the microbial Fe(III) reduction mediated by FmnA-dmkA-fmnB-pplA-ndh2-eetAB-dmkB protein family enhanced the Fe2+ release from the surface and the subsequent re-formation of reactive and conductive FeSx shell. A long-term PRB column test further demonstrated the feasibility of in-situ biological reactivation and reuse of S-mZVI for enhanced Cr(VI)-contaminated groundwater remediation. Within 64 days, the Cr(VI) removal capacity of S-mZVI in the four use cycles increased by 3.2 times, compared to the virgin one. The bio-reactivation using the SRB-enriched culture and sulfate locally-available in groundwater will reduce the chemical and maintenance costs associated with the frequent replacement of reactive ZVI-based materials. The PRB technology based on the bio-renewable S-mZVI can be a sustainable alternative to the conventional PRBs for the long-lasting and low-cost remediation of groundwater contaminated by oxidative pollutants.

The effect of redox bacteria on the programmed cell death-1 cancer immunotherapy

Background and purpose

Extracellular electron transferring (EET) or redox bacteria employ a shuttle of flavins to transfer electrons to the oxygen in the intestinal mucosa. Although clinical studies suggest that the gut microbiome modulates the efficiency of immune checkpoint therapy in patients with cancer, the modulation mechanisms have not been well-characterized yet.

Experimental approach

In the present study, the oral gavage administration of Shewanella oneidensis MR-1 as a prototypic EET bacteria was assayed in a mouse model of lung cancer to determine the effect of EET bacterium on the efficacy of the programmed cell death protein 1 (PD1)-immune checkpoint therapy.

Findings/Results

It was indicated that in vitro EET from S. oneidensis was mediated by riboflavins that were supplied through extrinsic sources. Co-administration of S. oneidensis and anti-PD 1 antibodies represent better tumor remission compared to the single-administration of each one; however, no statistically significant change was observed in the tumor volume.

Conclusion and implications

More detailed studies are needed to definitively confirm the therapeutic effects of electrogenic bacteria in patients with cancer. Given the findings of the present study, increasing flavin compounds or EET bacteria in the intestine may provide novel strategies for modulating cancer immunotherapy.

Effect of soil organic matter-mediated electron transfer on heavy metal remediation: Current status and perspectives

Soil contamination by heavy metals poses major risks to human health and the environment. Given the current status of heavy metal pollution, many remediation techniques have been tested at laboratory and contaminated sites. The effects of soil organic matter-mediated electron transfer on heavy metal remediation have not been adequately studied, and the key mechanisms underlying this process have not yet been elucidated. In this review, microbial extracellular electron transfer pathways, organic matter electron transfer for heavy metal reduction, and the factors affecting these processes were discussed to enhance our understanding of heavy metal pollution. It was found that microbial extracellular electrons delivered by electron shuttles have the longest distance among the three electron transfer pathways, and the application of exogenous electron shuttles lays the foundation for efficient and persistent remediation of heavy metals. The organic matter-mediated electron transfer process, wherein organic matter acts as an electron shuttle, promotes the conversion of high valence state metal ions, such as Cr(VI), Hg(II), and U(VI), into less toxic and morphologically stable forms, which inhibits their mobility and bioavailability. Soil type, organic matter structural and content, heavy metal concentrations, and environmental factors (e.g., pH, redox potential, oxygen conditions, and temperature) all influence organic matter-mediated electron transfer processes and bioremediation of heavy metals. Organic matter can more effectively mediate electron transfer for heavy metal remediation under anaerobic conditions, as well as when the heavy metal content is low and the redox potential is suitable under fluvo-aquic/paddy soil conditions. Organic matter with high aromaticity, quinone groups, and phenol groups has a stronger electron transfer ability. This review provides new insights into the control and management of soil contamination and heavy metal remediation technologies.

Bibliometric insights into the evolution of uranium contamination reduction research topics: Focus on microbial reduction of uranium

Confronting the threat of environment uranium pollution, decades of research have yielded advanced and significant findings in uranium bioremediation, resulting in the accumulation of tremendous amount of high-quality literature. In this study, we analyzed over 10,000 uranium reduction-related papers published from 1990 to the present in the Web of Science based on bibliometrics, and revealed some critical information on knowledge structure, thematic evolution and additional attention. Methods including contribution comparison, co-occurrence and temporal evolution analysis are applied. The results of the distribution and impact analysis of authors, sources, and journals indicated that the United States is a leader in this field of research and China is on the rise. The top keywords remained stable, primarily focused on chemicals (uranium, iron, plutonium, nitrat, carbon), characters (divers, surfac, speciat), and microbiology (microbial commun, cytochrome, extracellular polymeric subst). Keywords related to new strains, reduction mechanisms and product characteristics demonstrated the strongest uptrend, while some keywords related to mechanism and performance were clearly emerging in the past 5 years. Furthermore, the evolution of the thematic progression can be categorized into three stages, commencing with the discovery of the enzymatic reduction of hexavalent uranium to tetravalent uranium, developing in the groundwater remediation process at uranium-contaminated sites, and delving into the research on microbial reduction mechanisms of uranium. For future research, enhancing the understanding of mechanisms, improving uranium removal performance, and exploring practical applications can be considered. This study provides unique insights into microbial uranium reduction research, providing valuable references for related studies in this field.

Versatile roles of protein flavinylation in bacterial extracyotosolic electron transfer

Bacteria perform diverse redox chemistries in the periplasm, cell wall, and extracellular space. Electron transfer for these extracytosolic activities is frequently mediated by proteins with covalently bound flavins, which are attached through post-translational flavinylation by the enzyme ApbE. Despite the significance of protein flavinylation to bacterial physiology, the basis and function of this modification remains unresolved. Here we apply genomic context analyses, computational structural biology, and biochemical studies to address the role of ApbE flavinylation throughout bacterial life. We find that ApbE flavinylation sites exhibit substantial structural heterogeneity. We identify two novel classes of flavinylation substrates that are related to characterized proteins with non-covalently bound flavins, providing evidence that protein flavinylation can evolve from a non-covalent flavoprotein precursor. We further find a group of structurally related flavinylation-associated cytochromes, including those with the domain of unknown function DUF4405, that presumably mediate electron transfer in the cytoplasmic membrane. DUF4405 homologs are widespread in bacteria and related to ferrosome iron storage organelle proteins that may facilitate iron redox cycling within ferrosomes. These studies reveal a complex basis for flavinylated electron transfer and highlight the discovery power of coupling comparative genomic analyses with high-quality structural models.

MAR4 Streptomyces: A Unique Resource for Natural Product Discovery

Marine-derived Streptomyces have long been recognized as a source of novel, pharmaceutically relevant natural products. Among these bacteria, the MAR4 clade within the genus Streptomyces has been identified as metabolically rich, yielding over 93 different compounds to date. MAR4 strains are particularly noteworthy for the production of halogenated hybrid isoprenoid natural products, a relatively rare class of bacterial metabolites that possess a wide range of biological activities. MAR4 genomes are enriched in vanadium haloperoxidase and prenyltransferase genes, thus accounting for the production of these compounds. Functional characterization of the enzymes encoded in MAR4 genomes has advanced our understanding of halogenated, hybrid isoprenoid biosynthesis. Despite the exceptional biosynthetic capabilities of MAR4 bacteria, the large body of research they have stimulated has yet to be compiled. Here we review 35 years of natural product research on MAR4 strains and update the molecular diversity of this unique group of bacteria.

A comprehensive review of microbial fuel cells considering materials, methods, structures, and microorganisms

Microbial fuel cells (MFCs) are promising for generating renewable energy from organic matter and efficient wastewater treatment. Ensuring their practical viability requires meticulous optimization and precise design. Among the critical components of MFCs, the membrane separator plays a pivotal role in segregating the anode and cathode chambers. Recent investigations have shed light on the potential benefits of membrane-less MFCs in enhancing power generation. However, it is crucial to recognize that such configurations can adversely impact the electrocatalytic activity of anode microorganisms due to increased substrate and oxygen penetration, leading to decreased coulombic efficiency. Therefore, when selecting a membrane for MFCs, it is essential to consider key factors such as internal resistance, substrate loss, biofouling, and oxygen diffusion. Addressing these considerations carefully allows researchers to advance the performance and efficiency of MFCs, facilitating their practical application in sustainable energy production and wastewater treatment. Accelerated substrate penetration could also lead to cathode clogging and bacterial inactivation, reducing the MFC's efficiency. Overall, the design and optimization of MFCs, including the selection and use of membranes, are vital for their practical application in renewable energy generation and wastewater treatment. Further research is necessary to overcome the challenges of MFCs without a membrane and to develop improved membrane materials for MFCs. This review article aims to compile comprehensive information about all constituents of the microbial fuel cell, providing practical insights for researchers examining various variables in microbial fuel cell research.

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.

Mechanism of Cr(VI) bioreduction by Clostridium sp. LQ25 under Fe(III) reducing conditions

The Cr(VI) bioreduction has attracted widespread attention in the field of Cr(VI) pollution remediation due to its environmental friendliness. Further in-depth research on the reduction mechanisms is necessary to enhance the efficiency of Cr(VI) bioreduction. However, the limited research on Cr(VI) bioreduction mechanisms remains a bottleneck for the practical application of Cr(VI) reduction. In this study, The Cr(VI) reduction of strain LQ25 was significantly improved when Fe(III) was used as an electron acceptor, which increased by 1.6-fold maximum within the set Cr(VI) concentration range. Based on this, the electron transfer process of Cr(VI) reduction was analyzed using strain LQ25. Based on genomic data, flavin proteins were found to interact closely with electron transfer-related proteins using protein-protein interaction (PPi) analysis. Transcriptome analysis revealed that flavin synthesis genes (ribE, ribBA, and ribH) and electron transfer flavoprotein genes (fixA, etfA, fixB, and etfB) were significantly upregulated when Fe(III) was used as the electron acceptor. These results indicate that the fermentative dissimilatory Fe(III)-reducing bacterial strain LQ25 mainly uses flavin as an electron shuttle for electron transfer, which differs from the common use of cytochrome c in respiratory bacteria. These findings on the mechanism of Cr(VI) bioreduction provide technical support for improving the efficiency of Cr(VI) reduction which promote the practical application of Cr(VI) bioreduction in the field of Cr(VI) pollution remediation.

Electrochemical Characterization of Two Gut Microbial Strains Cooperatively Promoting Multiple Sclerosis Pathogenesis

In this study, we explored the extracellular electron transfer (EET) capabilities of two bacterial strains, OTU0001 and OTU0002, which are demonstrated in biofilm formation in mouse gut and the induction of autoimmune diseases like multiple sclerosis. OTU0002 displayed significant electrogenic behaviour, producing microbial current on an indium tin-doped oxide electrode surface, particularly in the presence of glucose, with a current density of 60 nA/cm2. The presence of cell-surface redox substrate potentially mediating EET was revealed by the redox-based staining method and electrochemical voltammetry assay. However, medium swapping analyses and the addition of flavins, a model redox mediator, suggest that the current production is dominated by soluble endogenous redox substrates in OTU0002. Given redox substrates were detected at the cell surface, the secreted redox molecule may interact with the cellular surface of OTU0002. In contrast to OTU0002, OTU0001 did not exhibit notable electrochemical activity, lacking cell-surface redox molecules. Further, the mixture of the two strains did not increase the current production from OTU0001, suggesting that OTU0001 does not support the EET mechanism of OTU0002. The present work revealed the coexistence of EET and non-EET capable pathogens in multi-species biofilm.

Organic Semiconducting Polymers for Augmenting Biosynthesis and Bioconversion

Solar-driven biosynthesis and bioconversion are essential for achieving sustainable resources and renewable energy. These processes harness solar energy to produce biomass, chemicals, and fuels. While they offer promising avenues, some challenges and limitations should be investigated and addressed for their improvement and widespread adoption. These include the low utilization of light energy, the inadequate selectivity of products, and the limited utilization of inorganic carbon/nitrogen sources. Organic semiconducting polymers offer a promising solution to these challenges by collaborating with natural microorganisms and developing artificial photosynthetic biohybrid systems. In this Perspective, we highlight the latest advancements in the use of appropriate organic semiconducting polymers to construct artificial photosynthetic biohybrid systems. We focus on how these systems can enhance the natural photosynthetic efficiency of photosynthetic organisms, create artificial photosynthesis capability of nonphotosynthetic organisms, and customize the value-added chemicals of photosynthetic synthesis. By examining the structure-activity relationships and emphasizing the mechanism of electron transfer based on organic semiconducting polymers in artificial photosynthetic biohybrid systems, we aim to shed light on the potential of this novel strategy for artificial photosynthetic biohybrid systems. Notably, these coupling strategies between organic semiconducting polymers and organisms during artificial photosynthetic biohybrid systems will pave the way for a more sustainable future with solar fuels and chemicals.

Electron transport bifurcation in bioanode with the metabolic shift to nitrate reduction

Electron transport bifurcation in bioanode determines the performance of microbial electrochemical technologies with the presence of an alternative electron acceptor. Here, the bioanode responses including electron transfer efficiency, microbial community, and microbial structure are investigated with the metabolic shift from current production to denitrification. Electrochemical measurements including cyclic voltammetry and electrochemical impedance spectra are performed to identify the change of electron transfer pathways in bioanode. Electron transfer efficiency for electrode reduction decreases ∼17 % with nitrate reduction. Biofilm resistance and charge transfer resistance increase from 23.3 Ω and 22.5 Ω to 36.6 Ω and 61.4 Ω with the metabolic shift, respectively. These results are mainly due to the loss of exoelectrogens inhabited in bioanode. Confocal imaging results indicate the elevated proportion of inactive cells in bioanode as the denitrification. Our results propose a possible mechanism for electron transfer bifurcation in bioanode with the metabolic shift from electrode reduction to soluble electron acceptor reduction.

Dual role of birnessite on the modulation of acid production and reinforcement of interspecific electron transfer in anaerobic digestion

Achieving efficient anaerobic digestion of highly loaded substrates is one of the most challenging problems in the field of waste resourcing. Here, the addition of birnessite (2.0 g/L) to kitchen wastewater increased the acetate and final methane yields by 40.53 and 99.18 %, respectively, while reducing the yields of propionate and butyrate by 38.17 and 48.86 %, respectively. There were two main pathways for birnessite to enhance anaerobic digestion, one of which is to act as an electron acceptor, by inducing an alteration in the ratio of reduced-state coenzyme I in the microorganism, allowing the acid production process to proceed towards deeper oxidation. Another pathway enhances the interspecific electron transfer between bacteria and archaea and improves methane yield by optimizing the metabolic relationship. The Kyoto Encyclopedia of Genes and Genomes (KEGG) functional predictions suggest that the extracellular electron transport pathway of the microorganism is enhanced with the addition of birnessite and that its intracellular metabolic pathway is biased towards the nicotinamide adenine dinucleotide (NADH) generation pathway. This work demonstrated that anaerobic digestion facilitation by metallic minerals was not monolithic; that is, different properties of the minerals were employed to intensify the different stages of anaerobic digestion and obtain an amplification cascade.