Recent advances in the role of biocatalyst in biofuel cells and its application: An overview

Biofuel cells have recently gained popularity as a green and renewable energy source. Biofuel cells are unique devices of energy and are capable of converting the stored chemical energy from waste materials such as pollutants, organics and wastewater into reliable, renewable, pollution-free energy sources through the action of biocatalysts such as various microorganisms and enzymes. It is a promising technological device to treat waste to compensate for global warming and the energy crisis through the green energy production process. Due to their unique properties, various potential biocatalysts are attracting researchers to apply them to various microbial biofuel cells for improving electricity and power. Recent research in biofuel cells is focusing on the exploitation of different biocatalysts and how they are enhancing power generation for various applications in the field of environmental technology, and biomedical fields such as implantable devices, testing kits, and biosensors. This review focusing the importance of microbial fuel cells (MFCs) and enzymatic fuel cells (ECFs) and role of different types of biocatalysts and their mechanisms for improving biofuel cell efficiency gathered from recent reports. Finally, its multifaceted applications with special emphasis on environmental technology and biomedical field will be described, along with future perspectives.

Electricity generation by Pseudomonas putida B6-2 in microbial fuel cells using carboxylates and carbohydrate as substrates

Microbial fuel cells (MFCs) employing Pseudomonas putida B6-2 (ATCC BAA-2545) as an exoelectrogen have been developed to harness energy from various conventional substrates, such as acetate, lactate, glucose, and fructose. Owing to its metabolic versatility, P. putida B6-2 demonstrates adaptable growth rates on diverse, cost-effective carbon sources within MFCs, exhibiting distinct energy production characteristics. Notably, the anode chamber's pH rises with carboxylates' (acetate and lactate) consumption and decreases with carbohydrates' (glucose and fructose) utilization. The MFC utilizing fructose as a substrate achieved the highest power density at 411 mW m-2. Initial analysis revealed that P. putida B6-2 forms biofilms covered with nanowires, contributing to bioelectricity generation. These microbial nanowires are likely key players in direct extracellular electron transport through physical contact. This study established a robust foundation for producing valuable compounds and bioenergy from common substrates in bioelectrochemical systems (BESs) utilizing P. putida as an exoelectrogen.

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.

Enhanced depolluting capabilities of microbial bioelectrochemical systems by synthetic biology

Microbial bioelectrochemical system (BES) is a promising sustainable technology for the electrical energy recovery and the treatment of recalcitrant and toxic pollutants. In microbial BESs, the conversion of harmful pollutants into harmless products can be catalyzed by microorganisms at the anode (Type I BES), chemical catalysts at the cathode (Type II BES) or microorganisms at the cathode (Type III BES). The application of synthetic biology in microbial BES can improve its pollutant removing capability. Synthetic biology techniques can promote EET kinetics, which is helpful for microbial anodic electro-respiration, expediting pollutant removing not only at the anode but also at the cathode. They offer tools to promote biofilm development on the electrode, enabling more microorganisms residing on the electrode for subsequent catalytic reactions, and to overexpress the pollutant removing-related genes directly in microorganisms, contributing to the pollutant decomposition. In this work, based on the summarized aspects mentioned above, we describe the major synthetic biology strategies in designing and improving the pollutant removing capabilities of microbial BES. Lastly, we discuss challenges and perspectives for future studies in the area.

Sustainable strategy on microbial fuel cell to treat the wastewater for the production of green energy

Microbial fuel cell (MFC) is one of the promising alternative energy systems where the catalytic conversion of chemical energy into electrical energy takes places with the help of microorganisms. The basic configuration of MFC consists of three major components such as electrodes (anode and cathode), catalyst (microorganism) and proton transport/exchange membrane (PEM). MFC classified into four types based on the substrate utilized for the catalytic energy conversion process such as Liquid-phase MFC, Solid-phase MFC, Plant-MFC and Algae-MFC. The core performance of MFC is organic substrate oxidation and electron transfer. Microorganisms and electrodes are the key factors that decide the efficiency of MFC system for electricity generation. Microorganism catalysis degradation of organic matters and assist the electron transfer to anode surface, the conductivity of anode material decides the rate of electron transport to cathode through external circuit where electrons are reduced with hydrogen and form water with oxygen. Not limited to electricity generation, MFC also has diverse applications in different sectors including wastewater treatment, biofuel (biohydrogen) production and used as biosensor for detection of biological oxygen demand (BOD) of wastewater and different contaminants concentration in water. This review explains different types of MFC systems and their core performance towards energy conversion and waste management. Also provides an insight on different factors that significantly affect the MFC performance and different aspects of application of MFC systems in various sectors. The challenges of MFC system design, operations and implementation in pilot scale level and the direction for future research are also described in the present review.

Synergistic improvement of Shewanella loihica PV-4 extracellular electron transfer using a TiO2@TiN nanocomposite

Extracellular electron transfer (EET) allows microorganisms to perform anaerobic respiration using insoluble electron acceptors, including minerals and electrodes. EET-based applications require efficient electron transfer between living and non-living systems. To improve EET efficiency, the TiO₂@TiN nanocomposite was used to form hybrid biofilms with Shewanella loihica PV-4 (PV-4). Chronoamperometry showed that peak current was increased 4.6-fold via the addition of the TiO₂@TiN nanocomposite. Different biofilms were further tested in a dual-chamber microbial fuel cell. The PV-4 biofilm resulted a maximum power density of 33.4 mW/m², while the hybrid biofilm of the TiO₂@TiN nanocomposite with PV-4 yielded a 92.8% increase of power density. Electrochemical impedance spectroscopy analyses showed a lower electron-transfer resistance in the hybrid biofilm. Biological measurements revealed that both flavin secretion and cytochrome c expression were increased when the TiO₂@TiN nanocomposite presented. These results demonstrated that the TiO₂@TiN nanocomposite could synergistically enhance the EET of PV-4 through altering its metabolism. Our findings provide a new strategy for optimizing biotic-abiotic interactions in bioelectrochemical systems.

Strategies for improving the electroactivity and specific metabolic functionality of microorganisms for various microbial electrochemical technologies

Electroactive microorganisms, which possess extracellular electron transfer (EET) capabilities, are the basis of microbial electrochemical technologies (METs) such as microbial fuel and electrolysis cells. These are considered for several applications ranging from the energy-efficient treatment of waste streams to the production of value-added chemicals and fuels, bioremediation, and biosensing. Various aspects related to the microorganisms, electrodes, separators, reactor design, and operational or process parameters influence the overall functioning of METs. The most fundamental and critical performance-determining factor is, however, the microorganism-electrode interactions. Modification of the electrode surfaces and microorganisms for optimizing their interactions has therefore been the major MET research focus area over the last decade. In the case of microorganisms, primarily their EET mechanisms and efficiencies along with the biofilm formation capabilities, collectively considered as microbial electroactivity, affect their interactions with the electrodes. In addition to electroactivity, the specific metabolic or biochemical functionality of microorganisms is equally crucial to the target MET application. In this article, we present the major strategies that are used to enhance the electroactivity and specific functionality of microorganisms pertaining to both anodic and cathodic processes of METs. These include simple physical methods based on the use of heat and magnetic field along with chemical, electrochemical, and growth media amendment approaches to the complex procedure-based microbial bioaugmentation, co-culture, and cell immobilization or entrapment, and advanced toolkit-based biofilm engineering, genetic modifications, and synthetic biology strategies. We further discuss the applicability and limitations of these strategies and possible future research directions for advancing the highly promising microbial electrochemistry-driven biotechnology.

A novel biosensor for zinc detection based on microbial fuel cell system

Microbial fuel cell (MFC) biosensors are self-sustainable device for monitoring of various substrates; however, for heavy metals detection are still scarce. In this study, E. coli BL21 was engineered to express the zntR, ribB, and oprF genes with P_zntA promoter, which could sense zinc (Zn²⁺) for riboflavin and porin production. The engineered strain produced high levels of riboflavin (2.4-3.6 μM) and improved cell membrane permeability, with a positive correlation of Zn²⁺ (0-400 μM). The strain was then employed in MFC biosensor under the following operational parameters: external resistance 1000 Ω, pH 9, and temperature 37 °C for Zn²⁺ sensing. The maximum voltages (160, 183, 260, 292, and 342 mV) of the constructed MFC biosensor have a linear relationship with Zn²⁺ concentrations (0, 100, 200, 300, and 400 μM, respectively) (R² = 0.9777). An Android App was developed for the biosensor system that could sense Zn²⁺ in real-time and in situ. The biosensor was applied to wastewater with different Zn²⁺ concentrations and the results showed that the detection range for Zn²⁺ was 20-100 μM, which covers common Zn²⁺ safety standards. The results obtained with developed MFC biosensor were comparable to conventional methods such as colorimetric, flame atomic absorption spectroscopy (FAAS), and inductively coupled plasma optical emission spectroscopy (ICP-OES). In summary, MFC biosensor with biosynthetic strain is an efficient and affordable system for real-time monitoring and sensing of heavy metals.

Microbial Electrosynthesis I: Pure and Defined Mixed Culture Engineering

In the past 6 years, microbial bioelectrochemistry has strongly increased in attraction and audience when expanding from mainly environmental technology applications to biotechnology. In particular, the promise to combine electrosynthesis with microbial catalysis opens attractive approaches for new sustainable redox-cofactor recycling, redox-balancing, or even biosynthesis processes. Much of this promise is still not fulfilled, but it has opened and fueled entirely new research areas in this discipline. Activities in designing, tailoring, and applying specific microbial catalysts as pure or defined co-cultures for defined target bioproductions are greatly accelerating. This chapter gives an overview of the current progress as well as the emerging trends in molecular and ecological engineering of defined microbial biocatalysts to prepare them for evolving microbial electrosynthesis processes. In addition, the multitude of microbial electrosynthetic processes with complex undefined mixed cultures is covered by ter Heijne et al. (Adv Biochem Eng Biotechnol. https://doi.org/10.1007/10_2017_15 , 2017). Graphical Abstract.

Shewanella putrefaciens CN32 outer membrane cytochromes MtrC and UndA reduce electron shuttles to produce electricity in microbial fuel cells

The extracellular electron transfer (EET) process of Shewanella species is believed to be indispensable for their anaerobic respiration with an electrode. However, the function of outer membrane c-type cytochromes (OM c-Cyts, the primary components of the EET pathway) is still controversial. In this study, we investigated the effect of two OM c-Cyts (MtrC and UndA) of Shewanella putrefaciens CN32 with respect to electricity production and anodic EET efficiency. Deletion of the mtrC gene severely prolonged the microbial fuel cell (MFC) start-up time and decreased electricity production due to depressed flavin-mediated electron transfer, whereas deletion of the undA gene did not have a significant impact. Strikingly, the depression of EET by the deletion of mtrC could be partially relieved by acclimation, which might be due to an increase in the transmembrane transport of electron shuttles and/or the activation of other redox proteins. These results suggested that MtrC may be the primary reductase of flavins to ensure fast indirect EET, which plays a crucial role in MFC electricity generation.

Improving electron trans-inner membrane movements in microbial electrocatalysts

A novel nondestructive strategy of improving electron trans-inner membrane movements in bioelectrocatalysts is realized by overexpressing NADH dehydrogenase II in the inner membrane. A microbial fuel cell loaded with these improved bioelectrocatalysts shows significantly enhanced performance based on promoting the utilization of intracellular primary electron donors in bioelectrocatalysts.