Overview of electroactive microorganisms and electron transfer mechanisms in microbial electrochemistry

Electric wiring of bacteria using redox polymers and selective measurement of metabolic activity in the presence of surrounding planktonic bacteria

Non-electroactive bacteria (n-EAB), constituting the majority of known bacteria to date, have been underutilized in electrochemical conversion technologies due to their lack of direct electron transfer to electrodes. In this study, we established an electric wiring between n-EAB (gram-positive Bacillus subtilis and gram-negative Escherichia coli) and an extracellular electrode via a ferrocene-polyethyleneimine-based redox polymer (Fc-PEI). Chronoamperometry recordings indicated that Fc-PEI can transfer intracellular electrons to the extracellular electrode regardless of the molecular organization of PEI (linear or branched) and the membrane structure of bacteria (gram-positive or -negative). As fluorescence staining suggested, Fc-PEI improves the permeability of the bacterial cell membrane, enabling electron carriers in the cell to react with Fc. In addition, experiments with Fc-immobilized electrodes without PEI suggested the existence of an alternative electron transfer pathway from B. subtilis to the extracellular Fc adsorbed onto the cell membrane. Furthermore, we proposed for the first time that the bacteria/Fc-linear PEI modified structure enables selective measurement of immobilized bacterial activity by physically blocking contact between the electrode surface and planktonic cells co-existing in the surrounding media. Such electrodes can be a powerful analytical tool for elucidating the metabolic activities of specific bacteria wired to the electrode even within complex bacterial communities.

The electrochemical mechanism of biochar for mediating the product ratio of N2O/(N2O + N2) in the denitrification process

The biochar electrochemical properties and surface functional groups significantly impact N2O production and reduction during denitrification process. However, its effects on N2O emissions during the denitrification process and its electrochemical mechanisms remain unclear. The study examined the impact of pristine and oxidized biochar combined with two types of nitrogen fertilizers on the N2O/(N2O + N2) ratio and N2O emissions in an incubation experiment with seven treatments: (1) CK (no application of chemical fertilizer); (2) N1 (applying (NH4)2SO4); (3) N1B ((NH4)2SO4 + pristine biochar); (4) N1BO ((NH4)2SO4 + oxidized biochar); (5) N2 (applying KNO3); (6) N2B (KNO3 + pristine biochar); (7) N2BO (KNO3 + oxidized biochar). The study found that in comparison with applying nitrogen fertilizer alone, combining pristine biochar decreased soil N2O concentration by 7.1 %-85.8 %, while combining oxidized biochar increased it by 15.7 %-125.6 %. Applying pristine biochar reduced N2O/(N2O + N2) ratio by 10.4 %-86.2 %, whereas applying oxidized biochar increased it by 12.9 %-121.6 %. The application of pristine biochar increased the nosZ gene abundance and decreased the (nirS + nirK)/nosZ ratio, which contributed to reducing N2O to N2. Compared with oxidized biochar, the oxygen-containing functional groups of pristine biochar decreased by 46.6 %, and it possessed a higher specific surface area (23.01 m2 g-1) and electrical conductivity (0.003 mS cm-1). The correlation analysis showed that DOC and inorganic nitrogen were the key environmental factors affecting N2O emissions. Additionally, the electrical conductivity, specific capacitance, and oxygen-containing functional groups of the biochar were identified as the main factors driving N2O emissions. The SEM analysis suggested that the indirect influence of biochar electrochemical properties on N2O emissions was greater than its direct influence. Our work provides fresh perspectives on reducing soil N2O emissions and establishes a theoretical foundation for the subsequent preparation of biochar materials with enhanced N2O reduction capabilities.

Potential and challenge in accelerating high-value conversion of CO2 in microbial electrosynthesis system via data-driven approach

Microbial electrosynthesis for CO2 utilization (MESCU) producing valuable chemicals with high energy density has garnered attention due to its long-term stability and high coulombic efficiency. The data-driven approaches offer a promising avenue by leveraging existing data to uncover the underlying patterns. This comprehensive review firstly uncovered the potentials of utilizing data-driven approaches to enhance high-value conversion of CO2 via MESCU. Firstly, critical challenges of MESCU advancing have been identified, including reactor configuration, cathode design, and microbial analysis. Subsequently, the potential of data-driven approaches to tackle the corresponding challenges, encompassing the identification of pivotal parameters governing reactor setup and cathode design, alongside the decipheration of omics data derived from microbial communities, have been discussed. Correspondingly, the future direction of data-driven approaches in assisting the application of MESCU has been addressed. This review offers guidance and theoretical support for future data-driven applications to accelerate MESCU research and potential industrialization.

Bacterial biofilm-mediated environmental remediation: Navigating strategies to attain Sustainable Development Goals

Bacterial biofilm is a structured bacterial community enclosed within a three-dimensional polymeric matrix, governed by complex signaling pathways, including two-component systems, quorum sensing, and c-di-GMP, which regulate its development and resistance in challenging environments. The genetic configurations within biofilm empower bacteria to exhibit significant pollutant remediation abilities, offering a promising strategy to tackle diverse ecological challenges and expedite progress toward Sustainable Development Goals (SDGs). Biofilm-based technologies offer advantages such as high treatment efficiency, cost-effectiveness, and sustainability compared to conventional methods. They significantly contribute to agricultural improvement, soil fertility, nutrient cycling, and carbon sequestration, thereby supporting SDG 1 (No poverty), SDG 2 (Zero hunger), SDG 13 (Climate action), and SDG 15 (Life on land). In addition, biofilm facilitates the degradation of organic-inorganic pollutants from contaminated environments, aligning with SDG 6 (Clean water and sanitation) and SDG 14 (Life below water). Bacterial biofilm also has potential applications in industrial innovation, aligning SDG 7 (Affordable and clean energy), SDG 8 (Decent work and economic growth), and SDG 9 (Industry, innovation, and infrastructure). Besides, bacterial biofilm prevents several diseases, aligning with SDG 3 (Good health and well-being). Thus, bacterial biofilm-mediated remediation provides advanced opportunities for addressing environmental issues and progressing toward achieving the SDGs. This review explores the potential of bacterial biofilms in addressing soil pollution, wastewater, air quality improvement, and biodiversity conservation, emphasizing their critical role in promoting sustainable development.

Ethanol-type anaerobic digestion enhanced methanogenic performance by stimulating direct interspecies electron transfer and interspecies hydrogen transfer

Ethanol pre-fermentation of food waste effectively alleviates acidification; however, its effects on interspecies electron transfer remain unknown. This study configured the feed according to COD ratios of ethanol: sodium acetate: sodium propionate: sodium butyrate of 5:2:1.5:1.5 (ethanol-type anaerobic digestion) and 0:5:2.5:2.5 (control), and conducted semi-continuous anaerobic digestion (AD) experiments. The results showed that ethanol-type AD increased maximum tolerable organic loading rate (OLR) to 6.0 gCOD/L/d, and increased the methane production by 1.2-14.8 times compared to the control at OLRs of 1.0-5.0 gCOD/L/d. The abundance of the pilA gene, which was associated with direct interspecies electron transfer (DIET), increased by 5.6 times during ethanol-type AD. Hydrogenase genes related to interspecies hydrogen transfer (IHT), including hydA-B, hoxH-Y, hnd, ech, and ehb, were upregulated during ethanol-type AD. Ethanol-type AD improved methanogenic performance and enhanced microbial metabolism by stimulating DIET and IHT.

Electrogenic performance and carbon sequestration potential of biophotovoltaics

Biophotovoltaics (BPV) is a clean and sustainable solar energy generation technology that operates by utilizing photosynthetic autotrophic microorganisms to capture light energy and generate electricity. However, a major challenge faced by BPV systems is the relatively low electron transfer efficiency from the photosystem to the extracellular electrode, which limits its electrical output. Additionally, the transfer mechanisms of photosynthetic microorganism metabolites in the entire system are still not fully clear. In response to this, this article briefly introduces the basic BPV principles, reviews its development history, and summarizes measures to optimize its electrogenic efficiency. Furthermore, recent studies have found that constructing photosynthetic-electrogenic microbial consortia can achieve high power density and stability in BPV systems. Therefore, the article discusses the potential application of constructing photosynthetic-electrogenic microbial aggregates in BPV systems. Since photosynthetic-electrogenic microbial communities can also exist in natural ecosystems, their potential contribution to the carbon cycle is worth further attention.

Carbon chain elongation characterizations of electrode-biofilm microbes in electro-fermentation

The higher efficiency of electro-fermentation in synthesizing medium-chain fatty acids (MCFAs) compared to traditional fermentation has been acknowledged. However, the functional mechanisms of electrode-biofilm enhancing MCFAs synthesis remain research gaps. To address this, this study proposed a continuous flow electrode-biofilm reactor for chain elongation (CE). After 225 days of operation, stable electrode-biofilms formed and notably improved caproate yield by more than 38 %. The electrode-biofilm was enriched with more CE microorganisms and electroactive bacteria compared to the suspended sludge microorganisms, including Caproicibacterium, Oscillibacter and Pseudoramibacter. Besides, the upregulated CE pathways were evaluated by metagenomic analysis, and the results indicated that the pathways such as acetyl-CoA and malonyl-[acp] formation, reverse beta-oxidation, and fatty acid biosynthesis pathway were all markedly enhanced in cathodic biofilm, more than anodic biofilm and suspended microorganisms. Moreover, microbial community regulated processes like bacterial chemotaxis, flagellar assembly and quorum sensing, crucial for electrode-biofilm formation. Electron transfer, energy metabolism, and microbial interactions were found to be prominently upregulated in the cathodic biofilm, surpassing levels observed in anodic biofilm and suspended sludge microorganisms, which further enhanced CE efficiency. In addition, the statistical analyses further highlighted key microbial functions and interactions within the cathodic biofilm. Oscillospiraceae_bacterium was identified to be the most active microbe, alongside pivotal roles played by Caproiciproducens_sp._NJN-50, Clostridiales_bacterium, Prevotella_sp. and Pseudoclavibacter_caeni. Eventually, the proposed microbial collaboration mechanisms of cathodic biofilm were ascertained. Overall, this study uncovered the biological effects of the electrode-biofilm on MCFAs electrosynthesis, thereby advancing biochemicals production and filling the knowledge gaps in CE electroactive biofilm reactors.

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.

Recent advances and prospects of constructed wetlands in cold climates: a review from 2013 to 2023

Constructed wetland (CW), a promising, environmentally responsible, and effective green ecological treatment technology, is actively involved in the treatment of various forms of wastewater. Low temperatures will, however, lead to issues including plant dormancy, decreased microbial activity, and ice formation in CWs, which will influence how well CWs process wastewater. Applying CWs successfully and continuously in cold areas is extremely difficult. Therefore, it is crucial to find solutions for the pressing issue of increasing the CWs' ability to process wastewater at low temperatures. This review focuses on the effect of cold climate on CWs (plants, substrates, microorganisms, removal effect of pollutants). It meticulously outlines current strategies to enhance CWs' performance under low-temperature conditions, including modifications for the improvement and optimization of the internal components (i.e., plant and substrate selection, bio-augmentation) and enhancement of the external operation conditions of CWs (such as process combination, effluent recirculation, aeration, heat preservation, and operation parameter optimization). Finally, future perspectives on potential research directions and technological innovations that could strengthen CWs' performance in cold climates are prospected. This review aims to contribute valuable insights into the operation strategies, widespread implementation, and subsequent study of CWs in colder climate regions.

Anaerobic digestion integrated with microbial electrolysis cell to enhance biogas production and upgrading in situ

Anaerobic digestion (AD) is an effective and applicable technology for treating organic wastes to recover bioenergy, but it is limited by various drawbacks, such as long start-up time for establishing a stable process, the toxicity of accumulated volatile fatty acids and ammonia nitrogen to methanogens resulting in extremely low biogas productivities, and a large amount of impurities in biogas for upgrading thereafter with high cost. Microbial electrolysis cell (MEC) is a device developed for electrosynthesis from organic wastes by electroactive microorganisms, but MEC alone is not practical for production at large scales. When AD is integrated with MEC, not only can biogas production be enhanced substantially, but also upgrading of the biogas product performed in situ. In this critical review, the state-of-the-art progress in developing AD-MEC systems is commented, and fundamentals underlying methanogenesis and bioelectrochemical reactions, technological innovations with electrode materials and configurations, designs and applications of AD-MEC systems, and strategies for their enhancement, such as driving the MEC device by electricity that is generated by burning the biogas to improve their energy efficiencies, are specifically addressed. Moreover, perspectives and challenges for the scale up of AD-MEC systems are highlighted for in-depth studies in the future to further improve their performance.

Study on the performance of biochar prepared from walnut shell and traditional graphene electrode plate in the treatment of domestic sewage in microbial fuel cells

As a new pollutant treatment technology, microbial fuel cell (MFC) has a broad prospect. In this article, the devices assembled using walnut shells are named biochar-microbial fuel cell (B-MFC), and the devices assembled using graphene are named graphene-microbial fuel cell (G-MFC). Under the condition of an external resistance of 1,000 Ω, the B-MFC with biochar as the electrode plate can generate a voltage of up to 75.26 mV. The maximum power density is 76.61 mW/m2, and the total internal resistance is 3,117.09 Ω. The removal efficiency of B-MFC for ammonia nitrogen (NH3-N), chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP) was higher than that of G-MFC. The results of microbial analysis showed that there was more operational taxonomic unit (OTU) on the walnut shell biochar electrode plate. The final analysis of the two electrode materials using BET specific surface area testing method (BET) and scanning electron microscope (SEM) showed that the pore size of walnut shell biochar was smaller, the specific surface area was larger, and the pore distribution was smoother. The results show that using walnut shells to make electrode plates is an optional waste recycling method and an electrode plate with excellent development prospects.

Biogenic sulfur recovery from sulfate-laden antibiotic production wastewater using a single-chamber up-flow bioelectrochemical reactor

The high-concentration sulfate (SO42-) in the antibiotic production wastewater hinders the anerobic methanogenic process and also proposes possible environmental risk. In this study, a novel single-chamber up-flow anaerobic bioelectrochemical reactor (UBER) was designed to realize simultaneous SO42- removal and elemental sulfur (S0) recovery. With the carbon felt, the cathode was installed underneath and the anode above to meet the different biological niches for sulfate reducing bacteria (SRB) and sulfur oxidizing bacteria (SOB). The bio-anode UBER (B-UBER) demonstrated a much higher average SO42- removal rate (SRR) of 113.2 ± 5.7 mg SO42--S L-1 d-1 coupled with a S0 production rate (SPR) of 54.4 ± 5.8 mg S0-S L-1 d-1 at the optimal voltage of 0.8 V than that in the abio-anode UBER (control reactor) (SRR = 86.6 ± 13.4 mg SO42--S L-1 d-1; SPR = 25.5 ± 9.7 mg S0-S L-1 d-1) under long-term operation. A large amount of biogenic S0 (about 72.2 mg g-1 VSS) was recovered in the B-UBER. The bio-anode, dominated by Thiovirga (SOB genus) and Acinetobacter (electrochemically active bacteria genus), exhibited a higher current density, lower overpotential, and lower internal resistance. C-type cytochromes mainly served as the crucial electron transfer mediator for both direct and indirect electron transfer, so that significantly increasing electron transfer capacity and biogenic S0 recovery. The reaction pathways of the sulfur transformation in the B-UBER were hypothesized that SRB utilized acetate as the main electron donor for SO42- reduction in the cathode zone and SOB transferred electrons to the anode or oxygen to produce biogenic S0 in the anode zone. This study proved a new pathway for biogenic S0 recovery and sulfate removal from sulfate-laden antibiotic production wastewater using a well-designed single-chamber bioelectrochemical reactor.

Electron Transfer in the Biogeochemical Sulfur Cycle

Microorganisms are key players in the global biogeochemical sulfur cycle. Among them, some have garnered particular attention due to their electrical activity and ability to perform extracellular electron transfer. A growing body of research has highlighted their extensive phylogenetic and metabolic diversity, revealing their crucial roles in ecological processes. In this review, we delve into the electron transfer process between sulfate-reducing bacteria and anaerobic alkane-oxidizing archaea, which facilitates growth within syntrophic communities. Furthermore, we review the phenomenon of long-distance electron transfer and potential extracellular electron transfer in multicellular filamentous sulfur-oxidizing bacteria. These bacteria, with their vast application prospects and ecological significance, play a pivotal role in various ecological processes. Subsequently, we discuss the important role of the pili/cytochrome for electron transfer and presented cutting-edge approaches for exploring and studying electroactive microorganisms. This review provides a comprehensive overview of electroactive microorganisms participating in the biogeochemical sulfur cycle. By examining their electron transfer mechanisms, and the potential ecological and applied implications, we offer novel insights into microbial sulfur metabolism, thereby advancing applications in the development of sustainable bioelectronics materials and bioremediation technologies.

Advancements in Bio-inspired Self-Powered Wireless Sensors: Materials, Mechanisms, and Biomedical Applications

The rapid maturation of smart city ecosystems is intimately linked to advances in the Internet of Things (IoT) and self-powered sensing technologies. Central to this evolution are battery-less sensors that are critical for applications such as continuous health monitoring through blood metabolites and vital signs, the recognition of human activity for behavioral analysis, and the operational enhancement of humanoid robots. The focus on biosensors that exploit the human body for energy-spanning wearable, attachable, and implantable variants has intensified, driven by their broad applicability in areas from underwater exploration to biomedical assays and earthquake monitoring. The heart of these sensors lies in their diverse energy harvesting mechanisms, including biofuel cells, and piezoelectric, triboelectric, and pyroelectric nanogenerators. Notwithstanding the wealth of research, the literature still lacks a holistic review that integrates the design challenges and implementation intricacies of such sensors. Our review seeks to fill this gap by thoroughly evaluating energy harvesting strategies from both material and structural perspectives and assessing their roles in powering an array of sensors for myriad uses. This exploration offers a comprehensive outlook on the state of self-powered sensing devices, tackling the nuances of their deployment and highlighting their potential to revolutionize data gathering in autonomous systems. The intent of this review is to chart the current landscape and future prospects, providing a pivotal reference point for ongoing research and innovation in self-powered wireless sensing technologies.

From single-chamber to multi-anodic microbial fuel cells: A review

Microbial fuel cells (MFCs) present a promising solution for wastewater treatment with the added benefits of energy generation, less sludge production and less energy consumption. MFCs have demonstrated high efficiency in the degradation of diverse types of wastewater. Nevertheless, the relatively low power density exhibited by MFCs has imposed certain restrictions on their widespread implementation. Consequently, the need for modification of MFC technology led to the development of stack and multi-chambered MFCs. The modified variations exhibit enhanced scalability and demonstrate greater reliability in terms of power output compared to traditional MFCs. In the present review article, different components of MFCs such as anode, cathode, microbial community and membrane have been reviewed and the advancement in design for better scalability of MFCs has been addressed, emphasizing the benefits associated with stacked and multi-anodic MFCs for enhanced performance. Finally, an update of previous large-scale MFC system applications is presented.

Degradation of sulfamethazine by microbial electrolysis cell with nickel-cobalt co-modified biocathode

In this study, nickel-cobalt co-modified stainless steel mesh (Ni-Co@SSM) was prepared and used as the biocathode in microbial electrolysis cell (MEC) for sulfamethazine (SMT) degradation. The optimal electrochemical performance of the Ni-Co@SSM was obtained at the electrodeposition time of 600 s, electrodeposition current density of 20 mA cm-2, and nickel-cobalt molar ratio of 1:2. The removal of SMT in MEC with the Ni-Co@SSM biocathode (MEC-Ni-Co@SSM) was 82%, which increased by 30% compared with the conventional anaerobic reactor. Thirteen intermediates were identified and the potential degradation pathways of SMT were proposed. Proteobacteria, Firmicutes, Patescibacteria, Chloroflexi, Bacteroidetes, and Euryarchaeota are the dominant bacteria at the phylum level in the MEC-Ni-Co@SSM, which are responsible for SMT metabolism. Due to the electrical stimulation, there was an increase in the abundance of the metabolic function and the genetic information processing. This work provides valuable insight into utilizing MECs for effective treatment of antibiotic-containing wastewater.

Sustainable Production of Biofuels and Biochemicals via Electro-Fermentation Technology

The energy crisis and climate change are two of the most concerning issues for human beings nowadays. For that reason, the scientific community is focused on the search for alternative biofuels to conventional fossil fuels as well as the development of sustainable processes to develop a circular economy. Bioelectrochemical processes have been demonstrated to be useful for producing bioenergy and value-added products from several types of waste. Electro-fermentation has gained great attention in the last few years due to its potential contribution to biofuel and biochemical production, e.g., hydrogen, methane, biopolymers, etc. Conventional fermentation processes pose several limitations in terms of their practical and economic feasibility. The introduction of two electrodes in a bioreactor allows the regulation of redox instabilities that occur in conventional fermentation, boosting the overall process towards a high biomass yield and enhanced product formation. In this regard, key parameters such as the type of culture, the nature of the electrodes as well as the operating conditions are crucial in order to maximize the production of biofuels and biochemicals via electro-fermentation technology. This article comprises a critical overview of the benefits and limitations of this emerging bio-electrochemical technology and its contribution to the circular economy.

Tetracycline hydrochloride degradation in polarity inverted microbial fuel cells: Performance, mechanisms and microbiology

Tetracycline antibiotics are widely used in veterinary medicine, human therapy and agriculture, and their presence in natural water raises environmental concerns. In this study, more than 94% of tetracycline hydrochloride (TCH) could be rapidly degraded within 48 h in polarity-inverted microbial fuel cells. The electrochemically active bacteria had the best electrochemical performance at 1 mg/L of TCH with the minimum internal resistance of 77.38 Ω. The electron-rich functional groups of TCH were continuously attacked and finally degradated into small molecules in three possible degradation pathways. Microbial community structure analysis showed that Comamonas and Shinella were enriched at the electrode as polarity-inverted bacteria. Genomic analysis showed that both direct and indirect electron transfer participated in the degradation of TCH in polarity-inverted microbial fuel cell (MFC) and the functional genes related to electrical conductivity in polarity-inverted MFC were more enriched on the electrode surface than non-polarity-inverted MFC. This study can facilitate further investigations about the biodegradation of TCH in polarity-inverted microbial fuel cell.

Electric field as extracellular enzyme activator promotes conversion of lignocellulose to humic acid in composting process

To promote efficient conversion of lignocellulose to humus (HS) during composting, a novel bio-electrochemical technology was applied and explored the effect and mechanism of electrification on carbon conversion during different composting periods. The results showed that supplementary electric field played different roles during composting. In the early stage, organic matter mineralization was significantly accelerated under electric field application, that was embodied in a 29.8% increase of CO2 emission due to the enhanced metabolic activity of microorganisms. However, the electric field functioned as an extracellular enzyme activator during the later stage since the abundance of functional microorganisms related to lignocellulose degradation was increased by 1.5-2.8 fold that effectively promoted the conversion of lignocellulose to HS. The humic acid content of the compost products increased by 23.0-32.9% compared with control. This study elucidated how electric fields affect carbon conversion during composting, which provides a novel strategy for returning agricultural wastes to soil.

A review of microbial responses to biochar addition in anaerobic digestion system: Community, cellular and genetic level findings

The biochar is a well-developed porous material with various excellent properties, that has been proven with excellent ability in anaerobic digestion (AD) efficiency promotion. Current research is usually focused on the macro effects of biochar on AD, while the systematic review about the mechanisms of biochar on microbial behavior are still lacking. This review summarizes the effects and potential mechanisms of biochar on microorganisms in AD systems, and found that biochar addition can provide habitats for microbial colonization, alleviate toxins stress, supply essential nutrients, and accelerate interspecies electron transferring. Moreover, it also improves microbial community diversity, facilitates EPS secretion, enhances functional enzyme activity, promotes functional genes expression, and inhibits the antibiotic resistance genes transformation. Future research directions including biochar targeted design, in-depth microbial mechanisms revelation, and modified model development were suggested, which could promote the widely practical application of of biochar-amended AD technology.

Exploring the biocapacitance in M3C-based biosensors for the assessment of microbial activity and organic matter

Reliable monitoring of microbial and water quality parameters in freshwater ecosystems (either natural or human-made) is of capital importance for improving both the management of water resources and the assessment of microbially-driven bio-geo-chemical processes. In this context, bioelectrochemical systems (BES), such as microbial three-cell electrodes (M3C), are very promising devices for their use as biosensors. However, current experiences on the use of BES-based devices for biosensing purposes are almost exclusively limited to water-saturated environments. This limitation hampers the use of this technology for a wider range of applications where the biosensor may work discontinuously (such as discontinuously saturated ecosystems). Discontinuous operation of M3C-based biosensors creates an electric current peak immediately after the reconnection of the system due to electron accumulation, in a process known as biocapacitance. The present work aimed at quantifying the bioindication potential of biocapacitance for the assessment of key ecosystem parameters such as microbial metabolic activity and biomass, as well as organic matter concentration. Significant linear regression coefficients (R2 > 0.9) were found for all combinations of parameters tested. Moreover, for most of the ecological parameters assessed, an electric charge accumulation of 1-5 min (biocapacitance elapsed time) and discharge of 5 min was enough to get reliable information. In conclusion, we have demonstrated for the first time that biocapacitance in M3C-based biosensors can be used as a proxy parameter for the assessment of microbial activity, microbial biomass and organic matter concentration in a model nature-based ecosystem.

Treatment of phenolic wastewater by anaerobic fluidized bed microbial fuel cell using carbon brush as anode: microbial community analysis and m-cresol degradation mechanism

Anaerobic fluidized bed microbial fuel cell (AFB-MFC) is a technology that combines fluidized bed reactor and microbial fuel cell to treat organic wastewater and generate electricity. The performance and the mechanism of treating m-cresol wastewater in AFB-MFC using carbon brush as biofilm anode were studied. After 48 h of operation, the m-cresol removal efficiency of AFB-MFC, MAR-AFB (fluidized bed bioreactor with acclimated anaerobic sludge), MAR-FB (ordinary fluidized bed reactor with only macroporous adsorptive resin) and AST (traditional anaerobic sludge treatment) were 95.29 ± 0.67%, 85.78 ± 1.81%, 71.24 ± 1.86% and 70.41 ± 0.32% respectively. The maximum output voltage and the maximum power density of AFB-MFC using carbon brush as biofilm anode were 679.7 mV and 166.6 mW/m2 respectively. The results of high-throughput sequencing analysis indicated the relative abundance of dominant electroactive bacteria, such as Trichococcus, Geobacter, and Pseudomonas, on the anode carbon brushes was higher than that of AST, and also identified such superior m-cresol-degrading bacteria as Bdellovibrio, Thermomonas, Hydrogenophaga, etc. Based on the determination of m-cresol metabolites detected by Gas Chromatography-Mass Spectrometry (GC-MS), the possible biodegradation pathway of m-cresol under anaerobic and aerobic conditions in AFB-MFC was speculated. The results showed that m-cresol was decomposed into formic acid-acetic anhydride and 3-methylpropionic acid under the action of electrochemistry, which is a simple degradation pathway without peripheral metabolism in AFB-MFC.

Bioelectricity production from the anodic inoculation of Geobacter sulfurreducens DL-1 bacteria in constructed wetlands-microbial fuel cells

Environmental pollution problems caused by the use of fossil fuels have led to the search for renewable energy sources to mitigate greenhouse gas emissions. In addition, constructed wetlands-microbial fuel cells (CW-MFC) could contribute to sustainable development, considering that this technology focuses on the production of bioelectricity. One of the main challenges of CW-MFCs is to potentiate their bioelectrochemical performance. Therefore, this research used the Geobacter sulfurreducens DL-1 bacterium (biofilm) as a bioelectrocatalyst to increase bioelectricity generation. For this, three bioreactors were built as CW-MFCs, using Juncus effusus root exudates and Philodendron cordatum macrophytes as endogenous substrates. The biofilm was developed in a nutrient broth acetate fumarate and directly inoculated onto the anodes of each CW-MFC. The results of bioelectrochemical analyses showed that the biofilm generated more bioelectricity when it consumed the exudates of the Juncus effusus macrophyte, resulting in a maximum performance of 107 mW/m2 power density, -361 mV anodic potential, 290 mV cathodic potential, and 124 Ω internal resistance, using a concentration of 27.5 mg/L of total organic carbon as an endogenous substrate. The results determined that the quantity of root exudates consumed by the anodic biofilm is directly related to the production of bioelectricity in CW-MFCs.

Metabolic regulation boosts bioelectricity generation in Zymomonas mobilis microbial fuel cell, surpassing ethanol production

Zymomonas mobilis (Z. mobilis), a bacterium known for its ethanol production capabilities, can also generate electricity by transitioning from ethanol production to electron generation. The purpose of this study is to investigate the ability of Z. mobilis to produce bioelectricity when utilized as a biocatalyst in a single-chamber microbial fuel cell (MFC). Given the bacterium's strong inclination towards ethanol production, a metabolic engineering strategy was devised to identify key reactions responsible for redirecting electrons from ethanol towards electricity generation. To evaluate the electroactivity of cultured Z. mobilis and its ethanol production in the presence of regulators, the reduction of soluble Fe(III) was utilized. Among the regulators tested, CuCl2 demonstrated superior effectiveness. Consequently, the MFC was employed to analyze the electrochemical properties of Z. mobilis using both a minimal and modified medium. By modifying the bacterial medium, the maximum current and power density of the MFC fed with Z. mobilis increased by more than 5.8- and sixfold, respectively, compared to the minimal medium. These findings highlight the significant impact of metabolic redirection in enhancing the performance of MFCs. Furthermore, they establish Z. mobilis as an active electrogenesis microorganism capable of power generation in MFCs.

The aggregated biofilm dominated by Delftia tsuruhatensis enhances the removal efficiency of 2,4-dichlorophenol in a bioelectrochemical system

Bioelectrochemical system is a prospective strategy in organic-contaminated groundwater treatment, while few studies clearly distinguish the mechanisms of adsorption or biodegradation in this process, especially when dense biofilm is formed. This study employed a single chamber microbial electrolysis cell (MEC) with two three-dimensional electrodes for removing a typical organic contaminant, 2,4-dichlorophenol (DCP) from groundwater, which inoculated with anaerobic bacteria derived from sewage treatment plant. Compared with the single biodegradation system without electrodes, the three-dimensional electrodes with a high surface enabled an increase of alpha diversity of the microbial community (increased by 52.6% in Shannon index), and provided adaptive ecological niche for more bacteria. The application of weak voltage (0.6 V) furtherly optimized the microbial community structure, and promoted the aggregation of microorganisms with the formation of dense biofilm. Desorption experiment proved that the contaminants were removed from the groundwater mainly via adsorption by the biofilm rather than biodegradation, and compared with the reactor without electricity, the bioelectrochemical system increased the adsorption capacity from 50.0% to 74.5%. The aggregated bacteria on the surface of electrodes were mainly dominated by Delftia tsuruhatensis (85.0%), which could secrete extracellular polymers and has a high adsorption capacity (0.30 mg/g electrode material) for the contaminants. We found that a bioelectrochemical system with a three-dimensional electrode could stimulate the formation of dense biofilm and remove the organic contaminants as well as their possible more toxic degradation intermediates via adsorption. This study provides important guidance for applying bioelectrochemical system in groundwater or wastewater treatment.

Life in biophotovoltaics systems

As the most suitable potential clean energy power generation technology, biophotovoltaics (BPV) not only inherits the advantages of traditional photovoltaics, such as safety, reliability and no noise, but also solves the disadvantages of high pollution and high energy consumption in the manufacturing process, providing new functions of self-repair and natural degradation. The basic idea of BPV is to collect light energy and generate electric energy by using photosynthetic autotrophs or their parts, and the core is how these biological materials can quickly and low-loss transfer electrons to the anode through mediators after absorbing light energy and generating electrons. In this mini-review, we summarized the biological materials widely used in BPV at present, mainly cyanobacteria, green algae, biological combinations (using multiple microorganisms in the same BPV system) and isolated products (purified thylakoids, chloroplasts, photosystem I, photosystem II), introduced how researchers overcome the shortcomings of low photocurrent output of BPV, pointed out the limitations that affected the development of BPV' biological materials, and put forward reasonable assumptions accordingly.

Enhanced 4-bromophenol anaerobic biodegradation in electricity-stimulated anaerobic system: The key role of humic acid in reshaping microbial eco-interrelations and functions

Electricity-stimulated anaerobic system (ESAS) has shown great potential for halogenated organic pollutants removal. Exogenous redox mediators can improve electron transfer efficiency to enhance pollutants removal in ESAS. In this study, humic acid (HA), a low-cost electron mediator, was added into ESAS to enhance the simultaneous reductive debromination and mineralization of 4-bromophenol (4-BP). Results showed that the highest 4-BP removal efficiency at 48 h was 95.43 % with HA dosage of 30 mg/L at - 700 mV, which was 34.67 % higher than that without HA. The addition of HA decreased the requirement for electron donors and enriched Petrimonas and Rhodococcus for humus respiratory. HA addition regulated microbial interactions, and enhanced species cooperation between Petrimonas and dehalogenation species (Thauera and Desulfovibrio), phenol degradation-related species (Rhodococcus) as well as fermentative species (Desulfobulbus). Functional genes related to 4-BP degradation (dhaA/hemE/xylC/chnB/dmpN) and electron transfer (etfB/nuoA/qor/ccoN/coxA) were increased in abundance by HA addition. The enhanced microbial functions, as well as species cooperation and facilitation, all contributed to the improved 4-BP biodegradation in HA-added ESAS. This study provided a deep insight into microbial mechanism driven by HA and offered a promising strategy for improving halogenated organic pollutants removal from wastewater.

Eight Up-Coming Biotech Tools to Combat Climate Crisis

Biotechnology has a high potential to substantially contribute to a low-carbon society. Several green processes are already well established, utilizing the unique capacity of living cells or their instruments. Beyond that, the authors believe that there are new biotechnological procedures in the pipeline which have the momentum to add to this ongoing change in our economy. Eight promising biotechnology tools were selected by the authors as potentially impactful game changers: (i) the Wood-Ljungdahl pathway, (ii) carbonic anhydrase, (iii) cutinase, (iv) methanogens, (v) electro-microbiology, (vi) hydrogenase, (vii) cellulosome and, (viii) nitrogenase. Some of them are fairly new and are explored predominantly in science labs. Others have been around for decades, however, with new scientific groundwork that may rigorously expand their roles. In the current paper, the authors summarize the latest state of research on these eight selected tools and the status of their practical implementation. We bring forward our arguments on why we consider these processes real game changers.

Electroactive microorganism-assisted remediation of groundwater contamination: Advances and challenges

Groundwater contamination has become increasingly prominent, therefore, the development of efficient remediation technology is crucial for improving groundwater quality. Bioremediation is cost-effective and environmentally friendly, while coexisting pollutant stress can affect microbial processes, and the heterogeneous character of groundwater medium can induce bioavailability limitations and electron donor/acceptor imbalances. Electroactive microorganisms (EAMs) are advantageous in contaminated groundwater because of their unique bidirectional electron transfer mechanism, which allows them to use solid electrodes as electron donors/acceptors. However, the relatively low-conductivity groundwater environment is unfavorable for electron transfer, which becomes a bottleneck problem that limits the remediation efficiency of EAMs. Therefore, this study reviews the recent advances and challenges of EAMs applied in the groundwater environment with complex coexisting ions, heterogeneity, and low conductivity and proposes corresponding future directions.

Electrobiochemical skills of Pseudomonas aeruginosa species that produce pyocyanin or pyoverdine for glycerol oxidation in a microbial fuel cell

Pseudomonas aeruginosa can produce pigments, which mediate external electron transfer (EET). Depending on the mediator, this species can be explored in bioelectrosystems to harvest energy or to obtain chemicals from residual organic compounds. This study has compared the performance of microbial fuel cells (MFCs) inoculated with a Pseudomonas aeruginosa isolate, namely EW603 or EW819, which produce pyocyanin and pyoverdine, respectively. The efficiency of these MFCs in glycerol, a typical residue of biodiesel production, were also compared. The MFCs exhibited different performances. The maximum voltage was 411 and 281 mV m², the power density was 40.1 and 21.3 mW m^-2, and the coulombic efficiency was 5.16 and 1.49% for MFC-EW603 and MFC-EW819, respectively. MFC-EW603 and MFC-EW819 achieved maximum current at 560 and 2200 Ω, at 141.2 and 91.3 mA m^-2, respectively. When the system was operated at the respective maximum current output, MFC-EW603 consumed the total glycerol content (11 mmol L^-1), and no products could be detected after 50 h. In turn, acetic and butyric acids were detected at the end of MFC-EW819 operation (75 h). The results suggested that P. aeruginosa metabolism can be steered in the MFC to generate current or microbial products depending on the pigment-producing strain and the conditions applied to the system, such as the external resistance. In addition, gene cluster pathways related to phenazine production (phzA and phzB) and other electrogenic-related genes (mexGHI-opmB) were identified in the strain genomes, supporting the findings. These results open new possibilities for using glycerol in bioelectrochemical systems.

Electroactive Microorganisms in Advanced Energy Technologies

Large-scale production of green and pollution-free materials is crucial for deploying sustainable clean energy. Currently, the fabrication of traditional energy materials involves complex technological conditions and high costs, which significantly limits their broad application in the industry. Microorganisms involved in energy production have the advantages of inexpensive production and safe process and can minimize the problem of chemical reagents in environmental pollution. This paper reviews the mechanisms of electron transport, redox, metabolism, structure, and composition of electroactive microorganisms in synthesizing energy materials. It then discusses and summarizes the applications of microbial energy materials in electrocatalytic systems, sensors, and power generation devices. Lastly, the research progress and existing challenges for electroactive microorganisms in the energy and environment sectors described herein provide a theoretical basis for exploring the future application of electroactive microorganisms in energy materials.

Isolation and Characterization of Bacteria with High Electroactive Potential from Poultry Wastewater

Natural resources are in short supply, and the ecosystem is being damaged as a result of the overuse of fossil fuels. The creation of novel technology is greatly desired for investigating renewable and sustainable energy sources. Microorganisms have received a lot of interest recently for their potential to transform organic waste into sustainable energy and high-value goods. New exoelectrogens that can transmit electrons to electrodes and remove specific wastewater contaminants are expected to be studied. In this study, we examined three distinct samples (as determined by chemical oxygen demand and pH) that can be used as anolytes to generate power in single-chamber and double-chamber microbial fuel cells using graphite electrodes. Wastewater from poultry farms was studied as an exoelectrogenic anolyte for microbial fuel cell power generation. The study examined 10 different bacterial strains, numbered A1 through A10. Due to their highly anticipated capacity to metabolize organic/inorganic chemicals, the diverse range of microorganisms found in poultry wastewater inspired us to investigate the viability of generating electricity using microbial fuel cells. From the investigated bacterial strains, the highest voltage outputs were produced by strains A1 (Lysinibacillus sphaericus) and A2 (Bacillus cereus), respectively, at 402 mV and 350 mV. Among the 10 different bacterial strains, strain A6 generated the least amount of electricity, measuring 35.03 mV. Furthermore, a maximum power density of 16.16 1.02 mW/m² was achieved by the microbial fuel cell using strain A1, significantly outperforming the microbial fuel cell using a sterile medium. The strain A2 showed significant current and power densities of 35 1.12 mA/m² and 12.25 1.05 mW/m², respectively. Moreover, in the two representative strains, chemical oxygen demand removal and Coulombic efficiency were noted. Samples from the effluent anode chamber were taken in order to gauge the effectiveness of chemical oxygen demand removal. Wastewater had an initial chemical oxygen demand content of 350 mg/L on average. Strains A1 and A2 decomposed 94.28% and 91.71%, respectively, of the organic substrate, according to the chemical oxygen demand removal efficiency values after 72 h. Strains A1 and A2 had electron donor oxidation efficiencies for 72 h of 54.1% and 60.67%, respectively. The Coulombic efficiency increased as the chemical oxygen demand decreased, indicating greater microbial electroactivity. With representative strains A1 and A2, Coulombic efficiencies of 10% and 3.5%, respectively, were obtained in the microbial fuel cell. The findings of this study greatly advance the field as a viable source of power technology for alternative energy in the future, which is important given the depletion of natural resources.

Marinobacter: A case study in bioelectrochemical chassis evaluation

The junction of bioelectrochemical systems and synthetic biology opens the door to many potentially groundbreaking technologies. When developing these possibilities, choosing the correct chassis organism can save a great deal of engineering effort and, indeed, can mean the difference between success and failure. Choosing the correct chassis for a specific application requires a knowledge of the metabolic potential of the candidate organisms, as well as a clear delineation of the traits, required in the application. In this review, we will explore the metabolic and electrochemical potential of a single genus, Marinobacter. We will cover its strengths, (salt tolerance, biofilm formation and electrochemical potential) and weaknesses (insufficient characterization of many strains and a less developed toolbox for genetic manipulation) in potential synthetic electromicrobiology applications. In doing so, we will provide a roadmap for choosing a chassis organism for bioelectrochemical systems.

Intermittent energization improves microbial electrolysis cell-assisted thermophilic anaerobic co-digestion of food waste and spent mushroom substance

Microbial electrolysis cell-assisted thermophilic anaerobic digestion (MEC-TAD) is a promising method to improve anaerobic co-digestion efficiency; however, its application is restricted by high energy consumption. To improve the energy use efficiency of MEC-TAD, this study investigated the effect of different intermittent energization strategies on thermophilic co-digestion performance. Results revealed that an 18 h-ON/6h-OFF energization schedule resulted in the fastest electron transfer rate and the highest methane yield (364.3 mL/g VS). Mechanistic analysis revealed that 18 h-ON/6h-OFF resulted in the enrichment of electroactive microorganisms and increased abundance of enzyme-coding genes associated with energy metabolism (ntp, nuo, atp), electron transfer (pilA, nfrA2, ssuE), and the hydrogenotrophic methanogenic pathway. Finally, energy balance analysis revealed that 18 h-ON/6h-OFF had the highest net energy benefit (2.52 kJ) and energy conversion efficiency (110.76 %). Therefore, intermittent energization of MEC-TAD using an 18 h-ON/6h-OFF schedule can provide improved performance and more energy savings.

Investigating the Performance of a Zinc Oxide Impregnated Polyvinyl Alcohol-Based Low-Cost Cation Exchange Membrane in Microbial Fuel Cells

The current study investigated the development and application of lithium (Li)-doped zinc oxide (ZnO)-impregnated polyvinyl alcohol (PVA) proton exchange membrane separator in a single chambered microbial fuel cell (MFC). Physiochemical analysis was performed via FT-IR, XRD, TEM, and AC impedance analysis to characterize thus synthesized Li-doped ZnO. PVA-ZnO-Li with 2.0% Li incorporation showed higher power generation in MFC. Using coulombic efficiency and current density, the impact of oxygen crossing on the membrane cathode assembly (MCA) area was evaluated. Different amounts of Li were incorporated into the membrane to optimize its electrochemical behavior and to increase proton conductivity while reducing biofouling. When acetate wastewater was treated in MFC using a PVA-ZnO-Li-based MCA, the maximum power density of 6.3 W/m³ was achieved. These observations strongly support our hypothesis that PVA-ZnO-Li can be an efficient and affordable separator for MFC.

Application of Low-Cost Plant-Derived Carbon Dots as a Sustainable Anode Catalyst in Microbial Fuel Cells for Improved Wastewater Treatment and Power Output

Microbial fuel cells (MFC) can generate electric energy from wastewater which can be enhanced further by anode catalysts. The recovery of electrons produced by oxidation of organics catalyzed by bacteria in the anode was enhanced when carbon dots(CDs) were added into the MFC. In this present study, a novel strategy for designing anode material and the fabrication of a high-efficient and environmentally friendly anode for energy generation from wastewater was reported. The CDs were synthesized by the pyrolysis of a peanut shell at the temperature of 250 °C for 2 h with a heating rate of 10 °C min−1. Thus synthesized CDs were characterized by transmission electron microscopy (TEM), UV/Vis spectroscopy, and fluorescence spectroscopy. The TEM analysis showed morphology with an average size of 1.62 nm. The UV/Vis absorbance of the CDs shows a wide absorption band without a characteristic peak. The excitation spectrum of CDs recorded at the emission wavelength of 440 nm exhibits a peak around 320 nm. CDs were investigated as an anode material in a MFC utilizing acetate as the organic substrate. The average chemical oxygen demand (COD) removal in closed circuit operation mode was 89%. The maximum power density production (7.2 W/m3) was observed in MFC containing 1 mg/cm2 CD-impregnated anode (CDsIA). The CDsIA provides the ability to promote efficient biofilm formation. These results emphasize the application of CD-based electrodes in MFCs for the simultaneous treatment of wastewater and electricity generation while also providing additional benefits.

Detoxification mechanisms of electroactive microorganisms under toxicity stress: A review

Remediation of environmental toxic pollutants has attracted extensive attention in recent years. Microbial bioremediation has been an important technology for removing toxic pollutants. However, microbial activity is also susceptible to toxicity stress in the process of intracellular detoxification, which significantly reduces microbial activity. Electroactive microorganisms (EAMs) can detoxify toxic pollutants extracellularly to a certain extent, which is related to their unique extracellular electron transfer (EET) function. In this review, the extracellular and intracellular aspects of the EAMs’ detoxification mechanisms are explored separately. Additionally, various strategies for enhancing the effect of extracellular detoxification are discussed. Finally, future research directions are proposed based on the bottlenecks encountered in the current studies. This review can contribute to the development of toxic pollutants remediation technologies based on EAMs, and provide theoretical and technical support for future practical engineering applications.

Bio-electrocatalytic degradation of tetracycline by stainless-steel mesh based molybdenum carbide electrode

In order to treat antibiotic wastewater with high efficiency and low energy consumption, this study proposed the coupling of electrocatalytic degradation and biodegradation, and explored a new modified electrocatalytic material in the coupling system. The stainless-steel mesh based molybdenum carbide (SS-Mo₂C) was prepared by a low-cost impregnation method and showed superior electrocatalytic degradation ability for tetracycline (TC) when used as the anode in the electrocatalytic system. The degradation rate of TC with SS-Mo₂C anode was 17 times higher than that of stainless-steel (SS) anode, and TC removal efficiency was 77% higher than that of SS anode. The electrocatalytic system prior to the biological reactor was proven to be the optimal coupling method. The external coupling system achieved a significantly higher TC removal (87.0%) than that of the internal coupling system (65.3%) and SS-Mo₂C showed an excellent repeatable and stable performance. The fewer and smaller molecular weight intermediates products were observed in bio-electrocatalytic system, especially in the external coupling system. Alpha diversity analysis further confirmed that bio-electrocatalytic system increased the diversity of the microbial community. The stainless-steel mesh based molybdenum carbide (SS-Mo₂C), which was prepared by a simple and low-cost impregnation method, significantly improved the electrocatalytic activity of anode, thus contributing to tetracycline removal in the bio-electrocatalytic system, especially in the external coupling system.

Opportunities for visual techniques to determine characteristics and limitations of electro-active biofilms

Optimization of bio-electrochemical systems (BESs) relies on a better understanding of electro-active biofilms (EABfs). These microbial communities are studied with a range of techniques, including electrochemical, visual and chemical techniques. Even though each of these techniques provides very valuable and wide-ranging information about EABfs, such as performance, morphology and biofilm composition, they are often destructive. Therefore, the information obtained from EABfs development and characterization studies are limited to a single characterization of EABfs and often limited to one time point that determines the end of the experiment. Despite being scarcer and not as commonly reported as destructive techniques, non-destructive visual techniques can be used to supplement EABfs characterization by adding in-situ information of EABfs functioning and its development throughout time. This opens the door to EABfs monitoring studies that can complement the information obtained with destructive techniques. In this review, we provide an overview of visual techniques and discuss the opportunities for combination with the established electrochemical techniques to study EABfs. By providing an overview of suitable visual techniques and discussing practical examples of combination of visual with electrochemical methods, this review aims at serving as a source of inspiration for future studies in the field of BESs.