An insight into the bioelectrochemical photoreduction of CO2 to value-added chemicals

Metabolic Engineering of E. coli for Enhanced Diols Production from Acetate

Effective employment of renewable carbon sources is highly demanded to develop sustainable biobased manufacturing. Here, we developed Escherichia coli strains to produce 2,3-butanediol and acetoin (collectively referred to as diols) using acetate as the sole carbon source by stepwise metabolic engineering. When tested in fed-batch experiments, the strain overexpressing the entire acetate utilization pathway was found to consume acetate at a 15% faster rate (0.78 ± 0.05 g/g/h) and to produce a 35% higher diol titer (1.16 ± 0.01 g/L) than the baseline diols-producing strain. Moreover, singularly overexpressing the genes encoding alternative acetate uptake pathways as well as alternative isoforms of genes in the malate-to-pyruvate pathway unveiled that leveraging ackA-pta and maeA is more effective in enhancing acetate consumption and diols production, compared to acs and maeB. Finally, the increased substrate consumption rate and diol production obtained in flask-based experiments were confirmed in bench-scale bioreactors operated in fed-batch mode. Consequently, the highest titer of 1.56 g/L achieved in this configuration increased by over 30% compared to the only other similar effort carried out so far.

High-rate biohydrogen production in single-chamber microbial electrolysis cell using iron-sulfide modified biocathode

Microbial electrolysis cells (MEC) can produce hydrogen (H2) at a low energy expense, but the H2 production rate is often limited by poor microbe-electrode interaction. This study aimed to enhance the interaction of microbes with a cathode electrode modified with an iron-sulfide (FeS) catalyst in MECs to achieve an efficient H2 evolution reaction (HER) and to investigate performance at different substrate concentrations, ranging from 1 to 3 g/L of glucose. The electrochemical analysis revealed FeS, a highly active catalyst for HER, surpassing the performance of a 10% platinum (Pt-C)-modified cathode. At 2 g/L glucose, MECs with a FeS-modified cathode (MEC-FeS) produced H2 at the highest yield of 7.01 mol H2/mol glucose, and the H2 production rate was 1.96 ± 0.09 m3/m3·d. The control operations of MEC with a pristine cathode and dark fermentation resulted in a reduced H2 yield of 5.83 ± 0.49 mol H2/mol glucose and 2.12 ± 0.12 mol H2/mol glucose, respectively. Moreover, the MEC-FeS achieved a high energy efficiency of 78 ± 5% compared to the MEC without catalyst (60 ± 5%) and the dark fermentation (24 ± 1%). This study suggests that utilizing FeS as a cathode catalyst in MECs can ensure high-rate hydrogen generation with optimal substrate concentration, paving the way for efficient upscaling and field application.

Advancements in Biological Conversion of C1 Feedstocks: Sustainable Bioproduction and Environmental Solutions

The use of one-carbon (C1) feedstocks, including carbon dioxide (CO2), carbon monoxide (CO), formate (HCO2H), methanol (CH3OH), and methane (CH4), presents a significant opportunity for sustainable bioproduction and environmental conservation. This Perspective explores the development of biological methods for converting C1 feedstocks into valuable products, emphasizing major progress from engineering native C1 assimilation pathways to the creation of synthetic autotrophs and methylotrophs that utilize these carbon sources. Additionally, we discuss hybrid approaches that merge biological and electrochemical systems, particularly for the conversion of CO2. This Perspective underscores the importance of C1 bioconversion in promoting sustainable biotechnological strategies for a low-carbon future.

Microbial electrosynthesis technology for CO2 mitigation, biomethane production, and ex-situ biogas upgrading

Currently, global annual CO2 emissions from fossil fuel consumption are extremely high, surpassing tens of billions of tons, yet our capacity to capture and utilize CO2 remains below a small fraction of the amount generated. Microbial electrosynthesis (MES) systems, an integration of microbial metabolism with electrochemistry, have emerged as a highly efficient and promising bio-based carbon-capture-and-utilization technology over other conventional techniques. MES is a unique technology for lowering the atmospheric CO2 as well as CO2 in the biogas, and also simultaneously convert them to renewable bioenergy, such as biomethane. As such, MES techniques could be applied for biogas upgrading to generate high purity biomethane, which has the potential to meet natural gas standards. This article offers a detailed overview and assessment of the latest advancements in MES for biomethane production and biogas upgrading, in terms of selecting optimal methane production pathways and associated electron transfer processes, different electrode materials and types, inoculum sources and microbial communities, ion-exchange membrane, externally applied energy level, operating temperature and pH, mode of operation, CO2 delivery method, selection of inorganic carbon source and its concentration, start-up time, and system pressure. It also highlights the current MES challenges associated with upscaling, design and configuration, long-term stability, energy demand, techno-economics, achieving net negative carbon emission, and other operational issues. Moreover, we provide a summary of current and future opportunities to integrate MES with other unique biosystems, such as methanotrophic bioreactors, and incorporate quorum sensing, 3D printing, and machine learning to further develop MES as a better biomethane-producer and biogas upgrading technique.

Hydrogen production in microbial electrolysis cells with biocathodes

Electroautotrophic microbes at biocathodes in microbial electrolysis cells (MECs) can catalyze the hydrogen evolution reaction with low energy demand, facilitating long-term stable performance through specific and renewable biocatalysts. However, MECs have not yet reached commercialization due to a lack of understanding of the optimal microbial strains and reactor configurations for achieving high performance. Here, we critically analyze the criteria for the inocula selection, with a focus on the effect of hydrogenase activity and microbe-electrode interactions. We also evaluate the impact of the reactor design and key parameters, such as membrane type, composition, and electrode surface area on internal resistance, mass transport, and pH imbalances within MECs. This analysis paves the way for advancements that could propel biocathode-assisted MECs toward scalable hydrogen gas production.

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.

Copper foam supported g-C3N4-metal-organic framework bacteria biohybrid cathode catalyst for CO2 reduction in microbial electrosynthesis

Microbial electrosynthesis (MES) presents a versatile approach for efficiently converting carbon dioxide (CO2) into valuable products. However, poor electron uptake by the microorganisms from the cathode severely limits the performance of MES. In this study, a graphitic carbon nitride (g-C3N4)-metal-organic framework (MOF) i.e. HKUST-1 composite was newly designed and synthesized as the cathode catalyst for MES operations. The physiochemical analysis such as X-ray diffraction, scanning electron microscopy (SEM), and X-ray fluorescence spectroscopy showed the successful synthesis of g-C3N4-HKUST-1, whereas electrochemical assessments revealed its enhanced kinetics for redox reactions. The g-C3N4-HKUST-1 composite displayed excellent biocompatibility to develop electroactive biohybrid catalyst for CO2 reduction. The MES with g-C3N4-HKUST-1 biohybrid demonstrated an excellent current uptake of 1.7 mA/cm2, which was noted higher as compared to the MES using g-C3N4 biohybrid (1.1 mA/cm2). Both the MESs could convert CO2 into acetic and isobutyric acid with a significantly higher yield of 0.46 g/L.d and 0.14 g/L.d respectively in MES with g-C3N4-HKUST-1 biohybrid and 0.27 g/L.d and 0.06 g/L.d, respectively in MES with g-C3N4 biohybrid. The findings of this study suggest that g-C3N4-HKUST-1 is a highly efficient catalytic material for biocathodes in MESs to significantly enhance the CO2 conversion.

Inorganic Carbon Assimilation and Electrosynthesis of Platform Chemicals in Bioelectrochemical Systems (BESs) Inoculated with Clostridium saccharoperbutylacetonicum N1-H4

The need for greener processes to satisfy the demand of platform chemicals together with the possibility of reusing CO2 from human activities has recently encouraged research on the set-up, optimization, and development of bioelectrochemical systems (BESs) for the electrosynthesis of organic compounds from inorganic carbon (CO2, HCO3-). In the present study, we tested the ability of Clostridium saccharoperbutylacetonicum N1-4 (DSMZ 14923) to produce acetate and D-3-hydroxybutyrate from inorganic carbon present in a CO2:N2 gas mix. At the same time, we tested the ability of a Shewanella oneidensis MR1 and Pseudomonas aeruginosa PA1430/CO1 consortium to provide reducing power to sustain carbon assimilation at the cathode. We tested the performance of three different systems with the same layouts, inocula, and media, but with the application of 1.5 V external voltage, of a 1000 Ω external load, and without any connection between the electrodes or external devices (open circuit voltage, OCV). We compared both CO2 assimilation rate and production of metabolites (formate, acetate 3-D-hydroxybutyrate) in our BESs with the values obtained in non-electrogenic control cultures and estimated the energy used by our BESs to assimilate 1 mol of CO2. Our results showed that C. saccharoperbutylacetonicum NT-1 achieved the maximum CO2 assimilation (95.5%) when the microbial fuel cells (MFCs) were connected to the 1000 Ω external resistor, with the Shewanella/Pseudomonas consortium as the only source of electrons. Furthermore, we detected a shift in the metabolism of C. saccharoperbutylacetonicum NT-1 because of its prolonged activity in BESs. Our results open new perspectives for the utilization of BESs in carbon capture and electrosynthesis of platform chemicals.

Novel approaches to energize microbial biocatalysts

An efficient and cheap energization of microbial biocatalysts is essential in current biotechnological processes. A promising alternative to the use of common organic or inorganic electron donors is the semiconductor nanoparticles (SNs) that absorb light and transfer electrons (photoelectrons) behaving as artificial photosynthetic systems (biohybrid systems). Excited photoelectrons generated by illuminated SNs are highly reductive and readily accepted by membrane-bound proteins and electron shuttles to drive specific cell reduction processes and energy generation in microbes. However, the operational mechanisms of these hybrid systems are still poorly understood, especially at the material-microbe interface, and therefore the design and production of efficient biohybrids are challenging. Some major limitations/challenges and future prospects of SNs as microbial energization systems are discussed.

Overview of recent progress in exogenous hydrogen supply biogas upgrading and future perspective

With the rapid development of renewable and sustainable energy, biogas upgrading for producing high-quality biomethane as an alternative to natural gas has attracted worldwide attention. This paper comprehensively reviews the current state of biogas upgrading technologies. The advances in physicochemical, photosynthetic autotrophic, and chemical autotrophic biogas upgrading technologies are briefly described with particular attention to the key challenges. New chemical autotrophic biogas upgrading strategies, such as direct and indirect exogenous hydrogen supply, for overcoming barriers to biogas upgrading and realizing highly efficient bioconversion of carbon dioxide are summarized. For each approach to exogenous hydrogen supply for biogas upgrading, the key findings and technical limitations are summarized and critically analyzed. Finally, future developments are also discussed to provide a reference for the development of biogas upgrading technology that can address the global energy crisis and climate change issues related to the application of biogas.

Microbial electrolysis cells for the production of biohydrogen in dark fermentation - A review

To assess biohydrogen for future green energy, this review revisited dark fermentation and microbial electrolysis cells (MECs). Hydrogen evolution rate in mesophilic dark fermentation is as high as 192 m³ H₂/m³-d, however hydrogen yield is limited. MECs are ideal for improving hydrogen yield from carboxylate accumulated from dark fermentation, whereas hydrogen production rate is too slow in MECs. Hence, improving anode kinetic is very important for realizing MEC biohydrogen. Intracellular electron transfer (IET) and extracellular electron transfer (EET) can limit current density in MECs, which is proportional to hydrogen evolution rate. EET does not limit current density once electrically conductive biofilms are formed on anodes, potentially producing 300 A/m². Hence, IET kinetics mainly govern current density in MECs. Among parameters associated with IET kinetic, population of anode-respiring bacteria in anode biofilms, biofilm density of active microorganisms, biofilm thickness, and alkalinity are critical for current density.

Fundamentals and recent progress in bioelectrochemical system-assisted biohythane production

Biohythane, a balanced mixture of 10%-30% v/v of hydrogen and 70%-90% v/v of methane, could be the backbone of an all-purpose future energy supply. Recently, bioelectrochemical systems (BES) became a new sensation among environmental biotechnology processes with the potential to sustainably generate biohythane. Therefore, to unleash its full potential for scaling up, researchers are consistently improving microbial metabolic pathways, novel reactors, and electrode designs. This review presents a detailed analysis of recently discovered fundamental mechanisms and science and engineering intervention of different strategies to improve the biohythane composition and production rate from BES. However, several milestones are to be achieved, for instance, improving electrode kinetics using efficient catalysts, engineered microbial communities, and improved reactor configurations, for commercializing this sustainable technology. Thus, a future perspective section is included to recommend novel research lines, mainly focusing on the microbial communities and the efficient electrocatalysts, to enhance reactor performance.

Using synthetic biology to improve photosynthesis for sustainable food production

Photosynthesis is responsible for the primary productivity and maintenance of life on Earth, boosting biological activity and contributing to the maintenance of the environment. In the past, traditional crop improvement was considered sufficient to meet food demands, but the growing demand for food coupled with climate change has modified this scenario over the past decades. However, advances in this area have not focused on photosynthesis per se but rather on fixed carbon partitioning. In short, other approaches must be used to meet an increasing agricultural demand. Thus, several paths may be followed, from modifications in leaf shape and canopy architecture, improving metabolic pathways related to CO₂ fixation, the inclusion of metabolic mechanisms from other species, and improvements in energy uptake by plants. Given the recognized importance of photosynthesis, as the basis of the primary productivity on Earth, we here present an overview of the latest advances in attempts to improve plant photosynthetic performance. We focused on points considered key to the enhancement of photosynthesis, including leaf shape development, RuBisCO reengineering, Calvin-Benson cycle optimization, light use efficiency, the introduction of the C₄ cycle in C₃ plants and the inclusion of other CO₂ concentrating mechanisms (CCMs). We further provide compelling evidence that there is still room for further improvements. Finally, we conclude this review by presenting future perspectives and possible new directions on this subject.

Modular configurations of living biomaterials incorporating nano-based artificial mediators and synthetic biology to improve bioelectrocatalytic performance: A review

Currently, the industrial application of bioelectrochemical systems (BESs) that are incubated with natural electrochemically active microbes (EABs) is limited due to inefficient extracellular electron transfer (EET) by natural EABs. Notably, recent studies have identified several novel living biomaterials comprising highly efficient electron transfer systems allowing unparalleled proficiency of energy conversion. Introduction of these biomaterials into BESs could fundamentally increase their utilization for a wide range of applications. This review provides a comprehensive assessment of recent advancements in the design of living biomaterials that can be exploited to enhance bioelectrocatalytic performance. Further, modular configurations of abiotic and biotic components promise a powerful enhancement through integration of nano-based artificial mediators and synthetic biology. Herein, recent advancements in BESs are synthesized and assessed, including heterojunctions between conductive nanomaterials and EABs, in-situ hybrid self-assembly of EABs and nano-sized semiconductors, cytoprotection in biohybrids, synthetic biological modifications of EABs and electroactive biofilms. Since living biomaterials comprise a broad range of disciplines, such as molecular biology, electrochemistry and material sciences, full integration of technological advances applied in an interdisciplinary framework will greatly enhance/advance the utility and novelty of BESs. Overall, emerging fundamental knowledge concerning living biomaterials provides a powerful opportunity to markedly boost EET efficiency and facilitate the industrial application of BESs to meet global sustainability challenges/goals.

Emerging bioelectrochemical technologies for biogas production and upgrading in cascading circular bioenergy systems

Biomethane is suggested as an advanced biofuel for the hard-to-abate sectors such as heavy transport. However, future systems that optimize the resource and production of biomethane have yet to be definitively defined. This paper assesses the opportunity of integrating anaerobic digestion (AD) with three emerging bioelectrochemical technologies in a circular cascading bioeconomy, including for power-to-gas AD (P2G-AD), microbial electrolysis cell AD (MEC-AD), and AD microbial electrosynthesis (AD-MES). The mass and energy flow of the three bioelectrochemical systems are compared with the conventional AD amine scrubber system depending on the availability of renewable electricity. An energy balance assessment indicates that P2G-AD, MEC-AD, and AD-MES circular cascading bioelectrochemical systems gain positive energy outputs by using electricity that would have been curtailed or constrained (equivalent to a primary energy factor of zero). This analysis of technological innovation, aids in the design of future cascading circular biosystems to produce sustainable advanced biofuels.

Biophotoelectrochemistry for renewable energy and environmental applications

Biophotoelectrochemistry (BPEC) is an interdisciplinary research field and combines bioelectrochemistry and photoelectrochemistry through the utilization of the catalytic abilities of biomachineries and light harvesters to accomplish the production of energy or chemicals driven by solar energy. The BPEC process may act as a new approach for sustainable green chemistry and waste minimization. This review provides the state-of-the-art introduction of BPEC basics and systems, with a focus on light harvesters and biocatalysts, configurations, photoelectron transfer mechanisms, and the potential applications in energy and environment. Several examples of BPEC applications are discussed including H2 production, CO2 reduction, chemical synthesis, pollution control, and biogeochemical cycle of elements. The challenges about BPEC systems are identified and potential solutions are proposed. The review aims to encourage further research of BPEC toward development of practical BPEC systems for energy and environmental applications.