Extracellular Electron Uptake: Among Autotrophs and Mediated by Surfaces

Using a non-precious metal catalyst for long-term enhancement of methane production in a zero-gap microbial electrosynthesis cell

Microbial electrosynthesis (MES) cells exploit the ability of microbes to convert CO2 into valuable chemical products such as methane and acetate, but high rates of chemical production may need to be mediated by hydrogen and thus require a catalyst for the hydrogen evolution reaction (HER). To avoid the usage of precious metal catalysts and examine the impact of the catalyst on the rate of methane generation by microbes on the electrode, we used a carbon felt cathode coated with NiMo/C and compared performance to a bare carbon felt or a Pt/C-deposited cathode. A zero-gap configuration containing a cation exchange membrane was developed to produce a low internal resistance, limit pH changes, and enhance direct transport of H2 to microorganisms on the biocathode. At a fixed cathode potential of -1 V vs Ag/AgCl, the NiMo/C biocathode enabled a current density of 23 ± 4 A/m2 and a high methane production rate of 4.7 ± 1.0 L/L-d. This performance was comparable to that using a precious metal catalyst (Pt/C, 23 ± 6 A/m2, 5.4 ± 2.8 L/L-d), and 3-5 times higher than plain carbon cathodes (8 ± 3 A/m2, 1.0 ± 0.4 L/L-d). The NiMo/C biocathode was operated for over 120 days without observable decay or severe cathode catalyst leaching, reaching an average columbic efficiency of 53 ± 9 % based on methane production under steady state conditions. Analysis of microbial community on the biocathode revealed the dominance of the hydrogenotrophic genus Methanobacterium (∼40 %), with no significant difference found for biocathodes with different materials. These results demonstrated that HER catalysts improved rates of methane generation through facilitating hydrogen gas evolution to an attached biofilm, and that the long-term enhancement of methane production in MES was feasible using a non-precious metal catalyst and a zero-gap cell design.

Integrated Proteomics and Metabolomics Reveal Altered Metabolic Regulation of Xanthobacter autotrophicus under Electrochemical Water-Splitting Conditions

Biological-inorganic hybrid systems are a growing class of technologies that combine microorganisms with materials for a variety of purposes, including chemical synthesis, environmental remediation, and energy generation. These systems typically consider microorganisms as simple catalysts for the reaction of interest; however, other metabolic activity is likely to have a large influence on the system performance. The investigation of biological responses to the hybrid environment is thus critical to the future development and optimization. The present study investigates this phenomenon in a recently reported hybrid system that uses electrochemical water splitting to provide reducing equivalents to the nitrogen-fixing bacteria Xanthobacter autotrophicus for efficient reduction of N2 to biomass that may be used as fertilizer. Using integrated proteomic and metabolomic methods, we find a pattern of differentiated metabolic regulation under electrochemical water-splitting (hybrid) conditions with an increase in carbon fixation products glycerate-3-phosphate and acetyl-CoA that suggests a high energy availability. We further report an increased expression of proteins of interest, namely, those responsible for nitrogen fixation and assimilation, which indicate increased rates of nitrogen fixation and support previous observations of faster biomass accumulation in the hybrid system compared to typical planktonic growth conditions. This work complicates the inert catalyst view of biological-inorganic hybrids while demonstrating the power of multiomics analysis as a tool for deeper understanding of those systems.

Proposed mechanisms of electron uptake in metal-corroding methanogens and their potential for CO2 bioconversion applications

Some methanogens are electrotrophic bio-corroding microbes that can acquire electrons from solid surfaces including metals. In the laboratory, pure cultures of methanogenic cells oxidize iron-based materials including carbon steel, stainless steel, and Fe0. For buried or immersed pipelines or other metallic structures, methanogens are often major components of corroding biofilms with complex interspecies relationships. Models explaining how these microbes acquire electrons from solid donors are multifaceted and include electron transfer via redox mediators such as H2 or by direct contact through membrane proteins. Understanding the electron uptake (EU) routes employed by corroding methanogens is essential to develop efficient strategies for corrosion prevention. It is also beneficial for the development of bioenergy applications relying on methanogenic EU from solid donors such as bioelectromethanogenesis, hybrid photosynthesis, and the acceleration of anaerobic digestion with electroconductive particles. Many methanogenic species carrying out biocorrosion are the same ones forming the extensive abiotic-biological interfaces at the core of these bio-applications. This review will discuss the interactions between corrosive methanogens and metals and how the EU capability of these microbes can be harnessed for different sustainable biotechnologies.

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.

Genetic tools for the electrotroph Sporomusa ovata and autotrophic biosynthesis

Sporomusa ovata is a Gram-negative acetogen of the Sporomusaceae family with a unique physiology. This anerobic bacterium is a core microbial catalyst for advanced CO2-based biotechnologies including gas fermentation, microbial electrosynthesis, and hybrid photosystem. Until now, no genetic tools exist for S. ovata, which is a critical obstacle to its optimization as an autotrophic chassis and the acquisition of knowledge about its metabolic capacities. Here, we developed an electroporation protocol for S. ovata. With this procedure, it became possible to introduce replicative plasmids such as pJIR751 and its derivatives into the acetogen. This system was then employed to demonstrate the feasibility of heterologous expression by introducing a functional β-glucuronidase enzyme under the promoters of different strengths in S. ovata. Next, a recombinant S. ovata strain producing the non-native product acetone both from an organic carbon substrate and from CO2 was constructed. Finally, a replicative plasmid capable of integrating itself on the chromosome of the acetogen was developed as a tool for genome editing, and gene deletion was demonstrated. These results indicate that S. ovata can be engineered and provides a first-generation genetic toolbox for the optimization of this biotechnological workhorse.IMPORTANCES. ovata harbors unique features that make it outperform most microbes for autotrophic biotechnologies such as a capacity to acquire electrons from different solid donors, a low H2 threshold, and efficient energy conservation mechanisms. The development of the first-generation genetic instruments described in this study is a key step toward understanding the molecular mechanisms involved in these outstanding metabolic and physiological characteristics. In addition, these tools enable the construction of recombinant S. ovata strains that can synthesize a wider range of products in an efficient manner.

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.

Carbon dioxide reduction to high-value chemicals in microbial electrosynthesis system: Biological conversion and regulation strategies

Large emissions of atmospheric carbon dioxide (CO2) are causing climatic and environmental problems. It is crucial to capture and utilize the excess CO2 through diverse methods, among which the microbial electrosynthesis (MES) system has become an attractive and promising technology to mitigate greenhouse effects while reducing CO2 to high-value chemicals. However, the biological conversion and metabolic pathways through microbial catalysis have not been clearly elucidated. This review first introduces the main acetogenic bacteria for CO2 reduction and extracellular electron transfer mechanisms in MES. It then intensively analyzes the CO2 bioconversion pathways and carbon chain elongation processes in MES, together with energy supply and utilization. The factors affecting MES performance, including physical, chemical, and biological aspects, are summarized, and the strategies to promote and regulate bioconversion in MES are explored. Finally, challenges and perspectives concerning microbial electrochemical carbon sequestration are proposed, and suggestions for future research are also provided. This review provides theoretical foundation and technical support for further development and industrial application of MES for CO2 reduction.

Extracellular electron uptake from a cathode by the lactic acid bacterium Lactiplantibacillus plantarum

A subset of microorganisms that perform respiration can endogenously utilize insoluble electron donors, such as Fe(II) or a cathode, in a process called extracellular electron transfer (EET). However, it is unknown whether similar endogenous EET can be performed by primarily fermentative species like lactic acid bacteria. We report for the first time electron uptake from a cathode by Lactiplantibacillus plantarum, a primarily fermentative bacteria found in the gut of mammals and in fermented foods. L. plantarum consumed electrons from a cathode and coupled this oxidation to the reduction of both an endogenous organic (pyruvate) and an exogenous inorganic electron acceptor (nitrate). This electron uptake from a cathode reroutes glucose fermentation toward lactate degradation and provides cells with a higher viability upon sugar exhaustion. Moreover, the associated genes and cofactors indicate that this activity is mechanistically different from that one employed by lactic acid bacteria to reduce an anode and to perform respiration. Our results expand our knowledge of the diversity of electroactive species and of the metabolic and bioenergetic strategies used by lactic acid bacteria.

Microbially mediated metal corrosion

A wide diversity of microorganisms, typically growing as biofilms, has been implicated in corrosion, a multi-trillion dollar a year problem. Aerobic microorganisms establish conditions that promote metal corrosion, but most corrosion has been attributed to anaerobes. Microbially produced organic acids, sulfide and extracellular hydrogenases can accelerate metallic iron (Fe0) oxidation coupled to hydrogen (H2) production, as can respiratory anaerobes consuming H2 as an electron donor. Some bacteria and archaea directly accept electrons from Fe0 to support anaerobic respiration, often with c-type cytochromes as the apparent outer-surface electrical contact with the metal. Functional genetic studies are beginning to define corrosion mechanisms more rigorously. Omics studies are revealing which microorganisms are associated with corrosion, but new strategies for recovering corrosive microorganisms in culture are required to evaluate corrosive capabilities and mechanisms. Interdisciplinary studies of the interactions among microorganisms and between microorganisms and metals in corrosive biofilms show promise for developing new technologies to detect and prevent corrosion. In this Review, we explore the role of microorganisms in metal corrosion and discuss potential ways to mitigate it.

Design and Construction of Artificial Biological Systems for One-Carbon Utilization

The third-generation (3G) biorefinery aims to use microbial cell factories or enzymatic systems to synthesize value-added chemicals from one-carbon (C1) sources, such as CO2, formate, and methanol, fueled by renewable energies like light and electricity. This promising technology represents an important step toward sustainable development, which can help address some of the most pressing environmental challenges faced by modern society. However, to establish processes competitive with the petroleum industry, it is crucial to determine the most viable pathways for C1 utilization and productivity and yield of the target products. In this review, we discuss the progresses that have been made in constructing artificial biological systems for 3G biorefineries in the last 10 years. Specifically, we highlight the representative works on the engineering of artificial autotrophic microorganisms, tandem enzymatic systems, and chemo-bio hybrid systems for C1 utilization. We also prospect the revolutionary impact of these developments on biotechnology. By harnessing the power of 3G biorefinery, scientists are establishing a new frontier that could potentially revolutionize our approach to industrial production and pave the way for a more sustainable future.

Synergetic effects of catalyst-surface dual-electric centers and microbes for efficient removal of ciprofloxacin in water

Antibiotics and antibiotic resistance genes (ARGs) are still a problem in biological treatment. Herein, we propose a synergetic strategy between microbes and dual-electric centers catalysts (CCN/Cu-Al2O3/ceramsite) for Ciprofloxacin (CIP)-contained (5 mg/L) water treatment in an up-flow biological filter. CIP was cleaved into small molecules by the catalyst, bringing a 57.6% removal and reducing 10.5% ARG. The characterization results verified that a Cu-π electrostatic force occurs on the catalyst surface, forming electron-rich areas around Cu and electron-poor areas at the carbon-doped g-C3N4 (CCN) aromatic ring. Thus, the electrons of adsorbed CIP were delocalized and then captured by the adsorbed extracellular polymeric substance at the electron-rich areas. Therefore, the synergetic process weakened the stress of CIP on bacteria and reduced ARG accumulation. It also enriched more electro-active bacteria on the surface of CCN/Cu-Al2O3/ceramsite, promoting the expression of extracellular electron transfer-related genes and reconstructing the energy metabolism mode. This result provides an opportunity for refractory antibiotic treatment in the biological process.

A photocatalytic-microbial coupling system for simultaneous removal of harmful algae and enhanced denitrification: Construction, performance and mechanism of action

Recently, harmful algal blooms (HABs) have become occurred with increasingly frequency worldwide. High nitrate content is one of the primary causes of eutrophication. Research has shown that photocatalytic materials enhance the effectiveness of microbial denitrification while removing other contaminants, despite some shortcomings. Based on this, we loaded TiO2/C3N4 heterojunctions onto weaveable, flexible carbon fibers and established a novel photocatalytically enhanced microbial denitrification system for the simultaneous removal of harmful algae and Microcystin-LR. We found that 99.35% of Microcystis aeruginosa and 95.34% of MC-LR were simultaneously and effectively removed. Compared to existing denitrification systems, the nitrate removal capacity improved by 72.33%. The denitrifying enzyme activity and electron transport system activity of microorganisms were enhanced by 3.54-3.86 times. Furthermore, the microbial community structure was optimized by the regulation of photogenerated electrons, and the relative abundance of main denitrifying bacteria increased from 50.72% to 66.45%, including Proteobacteria and Bacteroidetes. More importantly, we found that the increased secretion of extracellular polymeric substances by microorganisms may be responsible for the persistence of the reinforcing effect caused by photogenerated electrons in darkness. The higher removal of Microcystis aeruginosa and Microcystin-LR (MC-LR) achieved by the proposed system would reduce the frequency of HAB outbreaks and prevent the associated secondary pollution.

Living electrochemical biosensing: Engineered electroactive bacteria for biosensor development and the emerging trends

Bioelectrical interfaces made of living electroactive bacteria (EAB) provide a unique opportunity to bridge biotic and abiotic systems, enabling the reprogramming of electrochemical biosensing. To develop these biosensors, principles from synthetic biology and electrode materials are being combined to engineer EAB as dynamic and responsive transducers with emerging, programmable functionalities. This review discusses the bioengineering of EAB to design active sensing parts and electrically connective interfaces on electrodes, which can be applied to construct smart electrochemical biosensors. In detail, by revisiting the electron transfer mechanism of electroactive microorganisms, engineering strategies of EAB cells for biotargets recognition, sensing circuit construction, and electrical signal routing, engineered EAB have demonstrated impressive capabilities in designing active sensing elements and developing electrically conductive interfaces on electrodes. Thus, integration of engineered EAB into electrochemical biosensors presents a promising avenue for advancing bioelectronics research. These hybridized systems equipped with engineered EAB can promote the field of electrochemical biosensing, with applications in environmental monitoring, health monitoring, green manufacturing, and other analytical fields. Finally, this review considers the prospects and challenges of the development of EAB-based electrochemical biosensors, identifying potential future applications.

Retracted: The bidirectional extracellular electron transfer process aids iron cycling by Geoalkalibacter halelectricus in a highly saline-alkaline condition

Bidirectional extracellular electron transfer (EET) is crucial to upholding microbial metabolism with insoluble electron acceptors or donors in anoxic environments. Investigating bidirectional EET-capable microorganisms is desired to understand the cell-cell and microbe-mineral interactions and their role in mineral cycling besides leveraging their energy generation and conversion, biosensing, and bio-battery applications. Here, we report on iron cycling by haloalkaliphilic Geoalkalibacter halelectricus via bidirectional EET under haloalkaline conditions. It efficiently reduces Fe3+ oxide (Fe2O3) to Fe0 at a 0.75 ± 0.08 mM/mgprotein/d rate linked to acetate oxidation via outward EET and oxidizes Fe0 to Fe3+ at a 0.24 ± 0.03 mM/mgprotein/d rate via inward EET to reduce fumarate. Bioelectrochemical cultivation confirmed its outward and inward EET capabilities. It produced 895 ± 23 µA/cm2 current by linking acetate oxidation to anode reduction via outward EET and reduced fumarate by drawing electrons from the cathode (‒2.5 ± 0.3 µA/cm2) via inward EET. The cyclic voltammograms of G. halelectricus biofilms revealed redox moieties with different formal potentials, suggesting the involvement of different membrane components in bidirectional EET. The cyclic voltammetry and GC-MS analysis of the cell-free spent medium revealed the lack of soluble redox mediators, suggesting direct electron transfer by G. halelecctricus in achieving bidirectional EET. By reporting on the first haloalkaliphilic bacterium capable of oxidizing and reducing insoluble Fe0 and Fe3+ oxide, respectively, this study advances the limited understanding of the metabolic capabilities of extremophiles to respire on insoluble electron acceptors or donors via bidirectional EET and invokes the possible role of G. halelectricus in iron cycling in barely studied haloalkaline environments. IMPORTANCE Bidirectional extracellular electron transfer (EET) appears to be a key microbial metabolic process in anoxic environments that are depleted in soluble electron donor and acceptor molecules. Though it is an ecologically important and applied microbial phenomenon, it has been reported with a few microorganisms, mostly from nonextreme environments. Moreover, direct electron transfer-based bidirectional EET is studied for very few microorganisms with electrodes in engineered systems and barely with the natural insoluble electron acceptor and donor molecules in anoxic conditions. This study advances the understanding of extremophilic microbial taxa capable of bidirectional EET and its role in barely investigated Fe cycling in highly saline-alkaline environments. It also offers research opportunities for understanding the membrane components involved in the bidirectional EET of G. halelectricus. The high rate of Fe3+ oxide reduction activity by G. halelectricus suggests its possible use as a biocatalyst in the anaerobic iron bioleaching process under neutral-alkaline pH conditions.

Sunlight Significantly Enhances Soil Denitrification via an Interfacial Biophotoelectrochemical Pathway

Denitrification is an essential step of the nitrogen cycle in soil. However, although sunlight is an important environmental factor for soil, the investigation of the influence of sunlight on soil denitrification is limited to plant photosynthesis-mediated processes. Herein, a new pathway, denoted as a biophotoelectrochemical process, which is induced by the direct photoexcitation of soil, was found to greatly enhance soil denitrification. Using red soil as the research object, the soil with irradiation showed nitrate reduction that was 2.6-4.7 times faster than that without irradiation. The irradiation of soil accelerated the reduction of nitrite and enhanced the conversion of nitrous oxide to nitrogen, indicating that more electron sources were generated. This resulted from the photoinduced generation of ferrous substrates and photoelectrons. The contribution of irradiation to soil denitrification was almost half (45.4%), of which 30.9% was from photoinduced ferrous substrates and 14.5% was from photoelectrons. Moreover, a designed biophotoelectrochemical cell provided solid evidence for direct photoelectron transfer from soil photosensitive substrates to microorganisms. Irradiation promoted the enrichment of Alicyclobacillus, which participates in iron oxidation and electroautotrophy. This finding reveals a role of sunlight in soil denitrification that has been thus seriously overlooked and provides solid evidence for the natural occurrence of photoelectrotrophic effects.

Bioelectrocatalysis for CO2 reduction: recent advances and challenges to develop a sustainable system for CO2 utilization

Activation and turning CO₂ into value added products is a promising orientation to address environmental issues caused by CO₂ emission. Currently, electrocatalysis has a potent well-established role for CO₂ reduction with fast electron transfer rate; but it is challenged by the poor selectivity and low faradic efficiency. On the other side, biocatalysis, including enzymes and microbes, has been also employed for CO₂ conversion to target C_n products with remarkably high selectivity; however, low solubility of CO₂ in the liquid reaction phase seriously affects the catalytic efficiency. Therefore, a new synergistic role in bioelectrocatalysis for CO₂ reduction is emerging thanks to its outstanding selectivity, high faradic efficiency, and desirable valuable C_n products under mild condition that are surveyed in this review. Herein, we comprehensively discuss the results already obtained for the integration craft of enzymatic-electrocatalysis and microbial-electrocatalysis technologies. In addition, the intrinsic nature of the combination is highly dependent on the electron transfer. Thus, both direct electron transfer and mediated electron transfer routes are modeled and concluded. We also explore the biocompatibility and synergistic effects of electrode materials, which emerge in combination with tuned enzymes and microbes to improve catalytic performance. The system by integrating solar energy driven photo-electrochemical technics with bio-catalysis is further discussed. We finally highlight the significant findings and perspectives that have provided strong foundations for the remarkable development of green and sustainable bioelectrocatalysis for CO₂ reduction, and that offer a blueprint for C_n valuable products originate from CO₂ under efficient and mild conditions.

Extracellular electron transfer and the conductivity in microbial aggregates during biochemical wastewater treatment: A bottom-up analysis of existing knowledge

Microbial extracellular electron transfer (EET) plays a crucial role in bioenergy production and resource recovery from wastewater. Interdisciplinary efforts have been made to unveil EET processes at various spatial scales, from nanowires to microbial aggregates. Electrical conductivity has been frequently measured as an indicator of EET efficiency. In this review, the conductivity of nanowires, biofilms, and granular sludge was summarized, and factors including subjects, measurement methods, and conducting conditions that affect the conductivity difference were discussed in detail. The high conductivity of nanowires does not necessarily result in efficient EET in microbial aggregates due to the existence of non-conductive substances and contact resistance. Improving the conductivity measurement of microbial aggregates is important because it enables the calculation of an EET flux from conductivity and a comparison of the flux with mass transfer coefficients. This review provides new insight into the significance, characterization, and optimization of EET in microbial aggregates during a wastewater treatment process.

Bioconversion of Glycerol to 1,3-Propanediol Using Klebsiella pneumoniae L17 with the Microbially Influenced Corrosion of Zero-Valent Iron

The bacterial redox state is essential for controlling the titer and yield of the final metabolites in most bioconversion processes. Glycerol conversion to 1,3-propanediol (PDO) requires a large amount of reducing equivalent and the expression of reductive pathways. Zero-valent iron (ZVI) was used in the glycerol bioconversion of Klebsiella pneumoniae L17. The level of 1,3-PDO production increased with the oxidation of ZVI (31.8 ± 1.2 vs. 25.7 ± 0.5, ZVI vs. no ZVI) while the cellular NADH/NAD+ level increased (0.6 vs. 0.3, ZVI vs. no ZVI). X-ray diffraction showed that the iron oxide (Fe2O3) was formed during glycerol fermentation. L17 obtained electrons from ZVI and dissolved the iron continuously to form cracks on the surface, suggesting microbially influenced corrosion (MIC) was involved on the surface of ZVI. The ZVI-implemented fermentation shifted bioconversion to a more glycerol-reductive pathway. The qPCR-presented glycerol dehydratase (DhaB) with ZVI implementation was strongly expressed compared to the control. These results suggest that ZVI can contribute to the biotransformation of PDO by inducing intracellular metabolic shifts. This study could also suggest a novel microbial fermentation strategy with the application of MIC.

Recent Update on anaerobic digestion of paddy straw for biogas production: Advancement, limitation and recommendations

At present, development and production of advanced green energy sources are highly demanded, and this may offer a clean and sustainable environment to our modern society. In this reference, biogas is emerging as a promising green energy source and seems to have high potential to replace fossil-fuel based energy sources in the coming future. Further, lignocellulosic biomass (LCB) based biogas production technology has been found to be highly promising owing to several advantages associated therewith. Rich inorganic content, renewable nature, huge availability and low-cost are the key beneficial factors of LCB-based feedstock l to produce biogas. Among the varieties of LCB, paddy straw is one of the most demanding feedstocks and is highly rich in organic compounds that are imperative to producing biogas. Nevertheless, it is noticed that paddy straw as a waste material is usually disposed-off by direct burning, whereas it exhibits low natural digestibility due to the presence of high lignin and silica content which causes severe environmental pollution. On the other hand, paddy straw can be a potential feedstock to produce biogas through anaerobic digestion. Therefore, based on the current ongoing research studies worldwide, this review evaluates the advancements made in the AD process. Meanwhile, existing limitations and future recommendations to improve the yield and productivity of the biogas using paddy straw have been discussed. The emphasis has also been given to various operational parameters developments, related shortcomings, and strategies to improve biogas production at pilot scale.

Enabling Ethanologenesis in Moorella thermoacetica through Construction of a Replicating Shuttle Vector

Replicating plasmid shuttle vectors are key tools for efficient genetic and metabolic engineering applications, allowing the development of sustainable bioprocesses using non-model organisms with unique metabolic capabilities. To date, very limited genetic manipulation has been achieved in the thermophilic acetogen, Moorella thermoacetica, partly due to the lack of suitable shuttle vectors. However, M. thermoacetica has considerable potential as an industrial chassis organism, which can only be unlocked if reliable and effective genetic tools are in place. This study reports the construction of a replicating shuttle vector for M. thermoacetica through the identification and implementation of a compatible Gram-positive replicon to allow plasmid maintenance within the host. Although characterisation of plasmid behaviour proved difficult, the designed shuttle vector was subsequently applied for ethanologenesis, i.e., ethanol production in this organism. The non-native ethanologenesis in M. thermoacetica was achieved via plasmid-borne overexpression of the native aldh gene and heterologous expression of Clostridium autoethanogenum adhE1 gene. This result demonstrates the importance of the developed replicating plasmid vector for genetic and metabolic engineering efforts in industrially important M. thermoacetica.

Promoted microbial denitrification and carbon dioxide fixation via photogenerated electrons stored in novel core/shell memory photocatalysts in darkness

Excess nitrogen in water and greenhouse gases, especially atmospheric carbon dioxide (CO₂) from the rapid development of modern society have become an acute threat to the environment. Herein, novel core/shell structured g-C₃N₄@WO₃ memory photocatalyst was fabricated by coating g-C₃N₄ on the surface of WO₃ nanoparticles and applied in the simultaneous coupling of memory photocatalysts and microbial communities (SCMPMC) for the synergistic removal of microbial nitrate and CO₂ fixation in darkness. The results showed that ∼98.6% of nitrate was removed and ∼17.7% of CO₂ was fixed in darkness by microorganisms in the presence of g-C₃N₄@WO₃ memory photocatalyst within 48 h. Besides, the investigation of the mechanism evidenced that g-C₃N₄@WO₃ memory photocatalyst can promote electron transfer in the SCMPMC system. Moreover, key enzyme activities (i.e., NAR, NIR, CAT, and ETSA) were accelerated, indicating that the activities of enzymes within microorganisms could be remarkably enhanced by the continuous release of stored electrons by the g-C₃N₄@WO₃ memory photocatalyst in the dark. Furthermore, microbial community analysis revealed that the g-C₃N₄@WO₃ memory photocatalyst increased the relative abundance of denitrifiers (i.e., Acidobacterota, Actinobacteria, Chloroflexi, and Proteobacteria) and CO₂-assimilating microorganisms (i.e., Pseudomonas), in the treated communities compared with the original community in river sediment, demonstrating the positive effects of g-C₃N₄@WO₃ memory photocatalyst on river sediment microbial communities. The results in this study could shed new light on the establishment of promising synergistic microbial nitrate removal and CO₂ fixation methods and mechanisms in darkness.

Enhanced CO2 reduction and acetate synthesis in autotrophic biocathode by N-Hexanoyl-L-homoserine lactone (C6HSL)-based quorum-sensing regulation

The aim of this study was to investigate the ecological role of quorum-sensing signaling molecule on the autotrophic biocathode for CO₂ reduction and acetate synthesis. As a typical quorum-sensing signaling molecule, N-Hexanoyl-L-homoserine lactone (C6HSL) was used to regulate the construction of cathode biofilm. Results showed that the maximum acetate production from CO₂ reduction improved by 94.8%, and the maximum Faraday efficiency of the microbial electrosynthesis system enhanced by 71.7%, with the regulation of C6HSL. Electrochemical analyses indicated that higher electrochemical activity and lower charge resistance of biocathode were obtained with C6HSL than without C6HSL. Confocal laser scanning microscopy and electron inhibitor experiment suggested that exogenous C6HSL increased living biomass in the biofilm and facilitated the electron transfer pathway related to NADH dehydrogenase-CoQ and proton motive force. With the C6HSL regulation, the relative abundance of hydrogen producers (e.g., Desulfovibrio and Desulfomicrobium) increased, contributing to the improved performance of autotrophic biocathode.

Fumarate disproportionation by Geobacter sulfurreducens and its involvement in biocorrosion and interspecies electron transfer

The model electroactive bacterium Geobacter sulfurreducens can acquire electrons directly from solid donors including metals and other species. Reports on this physiology concluding that solid donors are the only electron sources were conducted with fumarate believed to serve exclusively as the terminal electron acceptor (TEA). Here, G. sulfurreducens was repeatedly transferred for adaptation within a growth medium containing only fumarate and no other solid or soluble substrate. The resulting evolved strain grew efficiently with either the C4-dicarboxylate fumarate or malate acting simultaneously as electron donor, carbon source, and electron acceptor via disproportionation. Whole-genome sequencing identified 38 mutations including one in the regulator PilR known to repress the expression of the C4-dicarboxylate antiporter DcuB essential to G. sulfurreducens when growing with fumarate. Futhermore, the PilR mutation was identical to the sole mutation previously reported in an evolved G. sulfurreducens grown in a co-culture assumed to derive energy solely from direct interspecies electron transfer, but cultivated with fumarate as the TEA. When cultivating the fumarate-adapted strain in the presence of stainless steel and fumarate, biocorrosion was observed and bacterial growth was accelerated 2.3 times. These results suggest that G. sulfurreducens can conserve energy concomitantly from C4-dicarboxylate disproportionation and the oxidation of a solid electron donor. This co-metabolic capacity confers an advantage to Geobacter for survival and colonization and explains in part why these microbes are omnipresent in different anaerobic ecosystems.

The integration of bio-catalysis and electrocatalysis to produce fuels and chemicals from carbon dioxide

The dependence on fossil fuels has caused excessive emissions of greenhouse gases (GHGs), leading to climate changes and global warming. Even though the expansion of electricity generation will enable a wider use of electric vehicles, biotechnology represents an attractive route for producing high-density liquid transportation fuels that can reduce GHG emissions from jets, long-haul trucks and ships. Furthermore, to achieve immediate alleviation of the current environmental situation, besides reducing carbon footprint it is urgent to develop technologies that transform atmospheric CO₂ into fossil fuel replacements. The integration of bio-catalysis and electrocatalysis (bio-electrocatalysis) provides such a promising avenue to convert CO₂ into fuels and chemicals with high-chain lengths. Following an overview of different mechanisms that can be used for CO₂ fixation, we will discuss crucial factors for electrocatalysis with a special highlight on the improvement of electron-transfer kinetics, multi-dimensional electrocatalysts and their hybrids, electrolyser configurations, and the integration of electrocatalysis and bio-catalysis. Finally, we prospect key advantages and challenges of bio-electrocatalysis, and end with a discussion of future research directions.

A novel simultaneous coupling of memory photocatalysts and microbial communities for alternate removal of dimethyl phthalate and nitrate in water under light/dark cycles

Efficient and sustainable removal of both organic and inorganic pollutants from contaminated water is an important but difficult task. Here, a novel chemical-biological coupling concept, namely simultaneous coupling of memory photocatalysts and microbial communities (SCMPMC), is proposed for the first time that alternates the removal of organic and inorganic pollutants under successive light/dark cycles. We established this novel coupling system with WO₃/g-C₃N₄ memory photocatalysts and river sediment microbial communities, and applied it to alternately remove dimethyl phthalate (DMP) and nitrate under light/dark cycles. The performance of SCMPMC under the light/dark cycles (12/12 h) showed that ~84.90% of the DMP was removed mainly via robust photocatalytic oxidation during the light phase, and ~86.80% of the nitrate was removed via microbial reduction enhanced by photogenerated electrons stored in the WO₃/g-C₃N₄ memory photocatalysts during the dark phase within one cycle. The microbial communities were positively affected by adding WO₃/g-C₃N₄, as evidenced by increased enzyme activities, cellular antigen metabolism, and relative abundance of typical denitrifiers, including Proteobacteria and Bacteroidetes. These results will contribute to the development of promising decontamination methods and mechanisms to control water pollution driven by the natural day/night cycle.

Enhanced biological nitrate removal by gC3N4/TiO2 composite and role of extracellular polymeric substances

The biological denitrification in the presence of gC₃N₄ doped TiO₂ composite was investigated through series of batch experiment. gC₃N₄ doped TiO₂ was synthesized and characterized by FT-IR, XRD, SEM-EDAX and the prepared composite used as electron donor for the enhancement biological denitrification. The role of extracellular polymeric substances in the biological nitrate reduction and electron transfer process has been elucidated. The XRD result confirms that TiO₂ nanoparticle has 80% anatase and 20% rutile phase. The gC₃N₄ shows the diffraction peaks at 27.57°, corresponds to the diffraction planes of (002) the hexagonal graphitic carbon nitride. The SEM image of modified gC₃N₄/TiO₂ nanocomposites showed agglomerated small spherical TiO₂ nanoparticles attached on the surface of gC3N4. The highest level of nitrate removal was 90% (from 100 mg/L to 10 mg/L nitrate) in gC3N4/TiO2 nanocomposite in the 15% wt TiO₂ doped gC3N4. The nitrate reduction in the biofilm with gC₃N₄ doped TiO₂ composite have significantly enhanced the nitrate reduction than the control. Photoexcited electrons were generated from gC₃N₄ doped TiO₂ photocatalyst act as excellent electron donor to the microbial communities. Extracellular polymeric substances acted as a passing media for microbial extracellular electron transfer and protective barrier for microbes. The electroactive microbes were harvested electrons from the gC₃N₄ doped TiO₂ composite under irradiation and enhancing the biological nitrate reduction. Overall, the present study suggests that insight into the mechanism of photoexcited electron facilitated biological nitrate reduction and role of extracellular polymeric substances. The successful integration of gC₃N₄ doped TiO₂ photocatalyst and biofilm is a promising technology for nitrate removal.

Cathodic biofilms - A prerequisite for microbial electrosynthesis

Cathodic biofilms have an important role in CO₂ bio-reduction to carboxylic acids and biofuels in microbial electrosynthesis (MES) cells. However, robust and resilient electroactive biofilms for an efficient CO₂ conversion are difficult to achieve. In this review, the fundamentals of cathodic biofilm formation, including energy conservation, electron transfer and development of catalytic biofilms, are presented. In addition, strategies for improving cathodic biofilm formation, such as the selection of electrode and carrier materials, cell design and operational conditions, are described. The knowledge gaps are individuated, and possible solutions are proposed to achieve stable and productive biofilms in MES cathodes.

Improved polyhydroxybutyrate production by Cupriavidus necator and the photocatalyst graphitic carbon nitride from fructose under low light intensity

The photocatalyst graphitic carbon nitride (g-C₃N₄) is known to photostimulate the production of the bioplastic polyhydroxybutyrate (PHB) by Cupriavidus necator. In previous studies, the combination of C. necator and g-C₃N₄ increased PHB yield from either an organic or inorganic carbon substrate under a light intensity of 4200 lx. Here, different parameters including light intensity, pH, temperature, nitrogen and carbon concentrations, aeration, and inoculum size were explored to maximize PHB production by hybrid photosynthesis from fructose and visible light. A g-C₃N₄/C. necator culture grown with a lower light intensity of 2100 lx, an inoculum size of 128.30 × 10⁶ CFU ml^-1, and constant aeration produced 7.16 g l^-1 d^-1 PHB with a product yield from fructose of 60.94%. Furthermore, the ratio of incident photons harvested by g-C₃N₄ converted into NADPH+H⁺ by C. necator for PHB production was improved to 19.74% after the process optimization. In comparison, the PHB production rate of a non-optimized g-C₃N₄/C. necator system exposed to 4200 lx was only 2.94 g l^-1 d^-1 with a product yield from fructose of 33.29%. These results demonstrate that hybrid photosynthesis productivity can be significantly augmented by decreasing light intensity and adjusting other parameters, which is promising for future bioproduction applications.

Modulation of Antioxidant Activity Enhances Photoautotrophic Cell Growth of Rhodobacter sphaeroides in Microbial Electrosynthesis

Global warming is currently accelerating due to an increase in greenhouse gas emissions by industrialization. Microbial electrosynthesis (MES) using electroactive autotrophic microorganisms has recently been reported as a method to reduce carbon dioxide, the main culprit of greenhouse gas. However, there are still few cases of application of MES, and the molecular mechanisms are largely unknown. To investigate the growth characteristics in MES, we carried out growth tests according to reducing power sources in Rhodobacter sphaeroides. The growth rate was significantly lower when electrons were directly supplied to cells, compared to when hydrogen was supplied. Through a transcriptome analysis, we found that the expression of reactive oxygen species (ROS)-related genes was meaningfully higher in MES than in normal photoautotrophic conditions. Similarly, endogenous contents of H2O2 were higher and peroxidase activities were lower in MES. The exogenous application of ascorbic acid, a representative biological antioxidant, promotes cell growth by decreasing ROS levels, confirming the inhibitory effects of ROS on MES. Taken together, our observations suggest that reduction of ROS by increasing antioxidant activities is important for enhancing the cell growth and production of CO2-converting substances such as carotenoids in MES in R. sphaeroides

Effects of anode/cathode electroactive microorganisms on arsenic removal with organic/inorganic carbon supplied

This study reports the effects of an external voltage (0 V, 0.4 V and 0.9 V) on soil arsenic (As) release and sequestration when amended with organic carbon (NaAc) and inorganic carbon (NaHCO₃), respectively, in a soil bioelectrochemistry system (BES). The results demonstrated that although an external voltage had no effect on the As removal capacity in an oligotrophic environment fueled with NaHCO_3, 93.6% of As(III) in the supernatant was removed at 0.9 V with an NaAc amendment. Interestingly, the content of As detected on the electrodes was higher than that removed from the supernatant, implying a continuous release of soil As under external voltages and rapid adsorption onto the electrodes, especially the cathode. In addition, the species of As on the cathode were similar to those in the supernatant (the As(III)/As(V) ratio was approximately 3:1), indicating that the removal capacity was independent of preoxidation. From the viewpoint of electroactive microorganisms (EABs), the relative abundances of the arrA gene and Geobacter genus were specifically enriched at the anode, thus signifying stimulation of the reduction and release of soil As in the anode region. By comparison, Bacillus was particularly abundant at the cathode, which could contribute to the oxidation and sequestration of As in the cathode region. Additionally, specific extracellular polymeric substances (EPSs) secreted by EABs could combine with As, which was followed by electrostatic attraction to the cathode under the effect of an electric field. Furthermore, the formation of secondary minerals and coprecipitation in the presence of iron (Fe) may have also contributed to As removal from solution. The insights from this study will enable us to further understand the biogeochemical cycle of soil As and to explore the feasibility of in situ As bioremediation techniques, combining the aspects of microbial and physicochemical processes in soil bioelectrochemical systems.

Energetics, electron uptake mechanisms and limitations of electroautotrophs growing on biocathodes - A review

Electroautotrophs are microorganisms that can take the electrons needed for energy generation, CO₂ fixation and other metabolic reactions from a polarized electrode. They have been the focus of intense research for its application in wastewater treatment, bioelectrosynthetic processes and hydrogen generation. As a general trend, current densities produced by the electron uptake of these microorganisms are low, limiting their applicability at large scale. In this work, the electron uptake mechanisms that may operate in electroautotrophs are reviewed, aiming at finding possible causes for this low performance. Biomass yields, growth rates and electron uptake rates observed when these microorganisms use chemical electron donors are compared with those typically obtained with electrodes, to explore limitations and advantages inherent to the electroautotrophic metabolism. Also, the factors affecting biofilm development are analysed to show how interfacial interactions condition bacterial adhesion, biofilm growth and electrons uptake. Finally, possible strategies to overcome these limitations are described.

Surface Modification of a Graphite Felt Cathode with Amide-Coupling Enhances the Electron Uptake of Rhodobacter sphaeroides

Microbial electrosynthesis (MES) is a promising technology platform for the production of chemicals and fuels from CO2 and external conducting materials (i.e., electrodes). In this system, electroactive microorganisms, called electrotrophs, serve as biocatalysts for cathodic reaction. While several CO2-fixing microorganisms can reduce CO2 to a variety of organic compounds by utilizing electricity as reducing energy, direct extracellular electron uptake is indispensable to achieve highly energy-efficient reaction. In the work reported here, Rhodobacter sphaeroides, a CO2-fixing chemoautotroph and a potential electroactive bacterium, was adopted to perform a cathodic CO2 reduction reaction via MES. To promote direct electron uptake, the graphite felt cathode was modified with a combination of chitosan and carbodiimide compound. Robust biofilm formation promoted by amide functionality between R. sphaeroides and a graphite felt cathode showed significantly higher faradaic efficiency (98.0%) for coulomb to biomass and succinic acid production than those of the bare (34%) and chitosan-modified graphite cathode (77.8%), respectively. The results suggest that cathode modification using a chitosan/carbodiimide composite may facilitate electron utilization by improving direct contact between an electrode and R. sphaeroides.

Delivery and effects of proton pump inhibitor on anaerobic digestion of food and kitchen waste under ammonia stress

Ammonia stress changes microbial metabolism of anaerobic digestion and decreases methane yield, where proton pump overactivated by free ammonia suggested to be the centre of the metabolism changes in anaerobic digestion under ammonia stress. The work demonstrated that proton pump inhibitor (PPI) could alleviate the overactivated proton pump and mitigate ammonia inhibition. Its impacts on iron transporter, substrates uptake, and energy conservation were investigated in anaerobic digestion treating food and kitchen waste. The PPI formed a stimuli-responsive drug delivery system driven by pH for the more inhibited microbe (p < 0.01), confirmed by FE-SEM/EDS and high throughput sequencing, implying the PPI was activated at inhibited microbe more than mixed liquor. Consistent microbial population increase observed in syntrophs and methanogens, who utilized the substrates for high yielding pathway and facilitated the energy sharing by direct interspecies electron transfer. These results demonstrated PPI could recovery methane production and could mitigate fatty-acid accumulation under high ammonia stress by delivery and activation in acetoclastic methanogen.

Enrichment of salt-tolerant CO2-fixing communities in microbial electrosynthesis systems using porous ceramic hollow tube wrapped with carbon cloth as cathode and for CO2 supply

Microbial inocula from marine origins are less explored for CO₂ reduction in microbial electrosynthesis (MES) system, although effective CO₂-fixing communities in marine environments are well-documented. We explored natural saline habitats, mainly salt marsh (SM) and mangrove (M) sediments, as potential inoculum sources for enriching salt-tolerant CO₂ reducing community using two enrichment strategies: H₂:CO₂ (80:20) enrichment in serum vials and enrichment in cathode chamber of MES reactors operated at -1.0 V vs. Ag/AgCl. Porous ceramic hollow tube wrapped with carbon cloth was used as cathode and for direct CO₂ delivery to CO₂ reducing communities growing on the cathode surface. Methanogenesis was dominant in both the M- and SM-seeded MES and the methanogenic Archaea Methanococcus was the most dominant genus. Methane production was slightly higher in the SM-seeded MES (4.9 ± 1.7 mmol) compared to the M-seeded MES (3.8 ± 1.1 mmol). In contrast, acetate production was almost two times higher in the M-seeded MES (3.1 ± 0.9 mmol) than SM-seeded MES (1.5 ± 1.3 mmol). The high relative abundance of the genus Acetobacterium in the M-seeded serum vials correlates with the high acetate production obtained. The different enrichment strategies affected the community composition, though the communities in MES reactors and serum vials were performing similar functions (methanogenesis and acetogenesis). Despite similar operating conditions, the microbial community composition of M-seeded serum vials and MES reactors differed from the SM-seeded serum vials and MES reactors, supporting the importance of inoculum source in the evolution of CO₂-reducing microbial communities.

Synthetic Biology Approaches To Enhance Microalgal Productivity

The major bottleneck in commercializing biofuels and other commodities produced by microalgae is the high cost associated with phototrophic cultivation. Improving microalgal productivities could be a solution to this problem. Synthetic biology methods have recently been used to engineer the downstream production pathways in several microalgal strains. However, engineering upstream photosynthetic and carbon fixation metabolism to enhance growth, productivity, and yield has barely been explored in microalgae. We describe strategies to improve the generation of reducing power from light, as well as to improve the assimilation of CO₂ by either the native Calvin cycle or synthetic alternatives. Overall, we are optimistic that recent technological advances will prompt long-awaited breakthroughs in microalgal research.

Improved robustness of microbial electrosynthesis by adaptation of a strict anaerobic microbial catalyst to molecular oxygen

Microbial electrosynthesis (MES) and other bioprocesses such as syngas fermentation developed for energy storage and the conversion of carbon dioxide into valuable chemicals often employs acetogens as microbial catalysts. Acetogens are sensitive to molecular oxygen, which means that bioproduction reactors must be maintained under strict anaerobic conditions. This requirement increases cost and does not eliminate the possibility of O₂ leakage. For MES, the risk is even greater since the system generates O₂ when water splitting is the anodic reaction. Here, we show that O₂ from the anode of a MES reactor diffuses into the cathode chamber where strict anaerobes reduce CO₂. To overcome this drawback, a stepwise adaptive laboratory evolution (ALE) strategy is used to develop the O₂ tolerance of the acetogen Sporomusa ovata. Two heavily-mutated S. ovata strains growing well autotrophically in the presence of 0.5 to 5% O₂ were obtained. The adapted strains were more performant in the MES system than the wild type converting electrical energy and CO₂ into acetate 1.5 fold faster. This study shows that the O₂ tolerance of acetogens can be increased, which leads to improvement of the performance and robustness of energy-storage bioprocesses such as MES where O₂ is an inhibitor.

Metabolic Engineering of Cupriavidus necator H16 for Sustainable Biofuels from CO2

Decelerating global warming is one of the predominant challenges of our time and will require conversion of CO₂ to usable products and commodity chemicals. Of particular interest is the production of fuels, because the transportation sector is a major source of CO₂ emissions. Here, we review recent technological advances in metabolic engineering of the hydrogen-oxidizing bacterium Cupriavidus necator H16, a chemolithotroph that naturally consumes CO₂ to generate biomass. We discuss recent successes in biofuel production using this organism, and the implementation of electrolysis/artificial photosynthesis approaches that enable growth of C. necator using renewable electricity and CO₂. Last, we discuss prospects of improving the nonoptimal growth of C. necator in ambient concentrations of CO₂.

Extracellular electron transfer across bio-nano interfaces for CO2 electroreduction

Acetogenic bacteria represent a class of organisms capable of converting reducing equivalents and carbon dioxide into products with carbon-carbon bonds. Materials-based bio-electrochemical approaches are attractive for supplying biological organisms directly with grid-supplied electrons to convert carbon dioxide to value-added chemicals. Carbon nanotube-modified biocathodes have emerged as promising candidates for microbial electrosynthesis with high yields of carbon product formation, but a fundamental understanding of extracellular charge transfer at this electrode-biofilm interface is still lacking. Here, we utilize solid-state interfaces between semiconducting single-walled carbon nanotubes (s-SWCNT) and a model acetogenic bacterium for mechanistic studies of electro-catalytic CO2 conversion to acetate. Studies of bacteria/s-SWCNT interactions in a transistor-based device suggest direct extracellular electron transfer (EET) at the bio-nano interface. Deuterium isotope labeling experiments confirmed that the availability of electrochemically produced H2 as a redox mediator does not limit the efficiency of EET and CO2 electro-reduction for C. ljungdahlii biofilms, suggesting the primary reducing equivalents are the electrons delivered across the electrode/bacterium interface or involvement of biological redox mediators. Additional isotope labeling studies demonstrate high Faradaic efficiency for CO2 electro-reduction at the SWCNT/bacterium interface. These results provide important information about EET across the bacterium/material interface in a model biocathode.

Photo-augmented PHB production from CO2 or fructose by Cupriavidus necator and shape-optimized CdS nanorods

Particulate photocatalysts developed for the solar energy-driven reduction of the greenhouse gas CO₂ have a small product range and low specificity. Hybrid photosynthesis expands the number of products with photocatalysts harvesting sunlight and transferring charges to microbes harboring versatile metabolisms for bioproduction. Besides CO₂, abiotic photocatalysts have been employed to increase microbial production yields of reduced compounds from organic carbon substrates. Most single-reactor hybrid photosynthesis systems comprise CdS assembled in situ by microbial activity. This approach limits optimization of the morphology, crystal structure, and crystallinity of CdS for higher performance, which is usually done via synthesis methods incompatible with life. Here, shape and activity optimized CdS nanorods were hydrothermally produced and subsequently applied to Cupriavidus necator for the heterotrophic and autotrophic production of the bioplastic polyhydroxybutyrate (PHB). C. necator with CdS NR under light produced 1.5 times more PHB when compared to the same bacterium with suboptimal commercially-available CdS. Illuminated C. necator with CdS NR synthesized 1.41 g PHB from fructose over 120 h and 28 mg PHB from CO₂ over 48 h. Interestingly, the beneficial effect of CdS NR was specific to C. necator as the metabolism of other microbes often employed for bioproduction including yeast and bacteria was negatively impacted. These results demonstrate that hybrid photosynthesis is more productive when the photocatalyst characteristics are optimized via a separated synthesis process prior to being coupled with microbes. Furthermore, bioproduction improvement by CdS-based photocatalyst requires specific microbial species highlighting the importance of screening efforts for the development of performant hybrid photosynthesis.

Effect of proton pump inhibitor on microbial community, function, and kinetics in anaerobic digestion with ammonia stress

The proton pump is a convincing mechanism for ammonia inhibition in anaerobic digestion, which explained how the ammonia accumulated intercellularly due to diffusion of free ammonia. Proton pump inhibitor (PPI) was dosed for mitigating the accumulation in anaerobic digestion with ammonia stress, with respect to kinetics. Results show PPI inhibited β-oxidation of fatty acids by targeting ATPase in anaerobic digestion with ammonia stress. Alternatively, PPI stimulated syntrophic acetate oxidization. Random forest located key genera as syntrophic consortia. Methane increased 18.72 ± 7.39% with 20 mg/L PPI at the first peak, consistent with microbial results. The deterministic Gompertz kinetics and stochastic Gaussian processes contributed 97.63 ± 8.93% and 2.37 ± 8.93% in accumulated methane production, respectively. Thus, the use of PPI for anaerobic digestion allowed mitigate ammonia inhibition based on the mechanism of proton pump, facilitate intercellularly ammonia accumulation, stimulate syntrophic consortia, and eliminate uncertainty of process failure, which resulted in efficient methane production under ammonia stress.

Recent developments and key barriers to microbial CO2 electrobiorefinery

The electrochemical conversion of CO₂ can include renewable surplus electricity storage and CO₂ utilisation. This review focuses on the microbial CO₂ electrobiorefinery based on microbial electrosynthesis (MES) which merges electrochemical and microbial conversion to produce biofuels and higher-value chemicals. In this review, recent developments are discussed about bioelectrochemical conversion of CO₂ into biofuels and chemicals in MES via microbial CO₂-fixation and electricity utilisation reactions. In addition, this review examines technical approaches to overcome the current limitations of MES including the following: engineering of the biocathode, application of electron mediators, and reactor optimisation, among others. An in-depth discussion of strategies for the CO₂ electrobiorefinery is presented, including the integration of the biocathode with inorganic catalysts, screening of novel electroactive microorganisms, and metabolic engineering to improve target productivity from CO₂.

Improving the Cathodic Biofilm Growth Capabilities of Kyrpidia spormannii EA-1 by Undirected Mutagenesis

The biotechnological usage of carbon dioxide has become a relevant aim for future processes. Microbial electrosynthesis is a rather new technique to energize biological CO₂ fixation with the advantage to establish a continuous process based on a cathodic biofilm that is supplied with renewable electrical energy as electron and energy source. In this study, the recently characterized cathodic biofilm forming microorganism Kyrpidia spormannii strain EA-1 was used in an adaptive laboratory evolution experiment to enhance its cathodic biofilm growth capabilities. At the end of the experiment, the adapted cathodic population exhibited an up to fourfold higher biofilm accumulation rate, as well as faster substratum coverage and a more uniform biofilm morphology compared to the progenitor strain. Genomic variant analysis revealed a genomically heterogeneous population with genetic variations occurring to various extends throughout the community. Via the conducted analysis we identified possible targets for future genetic engineering with the aim to further optimize cathodic growth. Moreover, the results assist in elucidating the underlying processes that enable cathodic biofilm formation.

Graphene: An Antibacterial Agent or a Promoter of Bacterial Proliferation?

Graphene materials (GMs) are being investigated for multiple microbiological applications because of their unique physicochemical characteristics including high electrical conductivity, large specific surface area, and robust mechanical strength. In the last decade, studies on the interaction of GMs with bacterial cells appear conflicting. On one side, GMs have been developed to promote the proliferation of electroactive bacteria on the surface of electrodes in bioelectrochemical systems or to accelerate interspecies electron transfer during anaerobic digestion. On the other side, GMs with antibacterial properties have been synthesized to prevent biofilm formation on membranes for water treatment, on medical equipment, and on tissue engineering scaffolds. In this review, we discuss the mechanisms and factors determining the positive or negative impact of GMs on bacteria. Furthermore, we examine the bacterial growth-promoting and antibacterial applications of GMs and debate their practicability.

Microbial electrosynthesis: Towards sustainable biorefineries for production of green chemicals from CO2 emissions

Decarbonisation of the economy has become a priority at the global level, and the resulting legislative pressure is pushing the chemical and energy industries away from fossil fuels. Microbial electrosynthesis (MES) has emerged as a promising technology to promote this transition, which will further benefit from the decreasing cost of renewable energy. However, several technological challenges need to be addressed before the MES technology can reach its maturity. The aim of this review is to critically discuss the bottlenecks hampering the industrial adoption of MES, considering the whole production process (from the CO₂ source to the marketable products), and indicate future directions. A flexible stack design, with flat or tubular MES modules and direct CO₂ supply, is required for site-specific decentralised applications. The experience gained for scaling-up electrochemical cells (e.g. electrolysers) can serve as a guideline for realising pilot MES stacks to be technologically and economically evaluated in industrially relevant conditions. Maximising CO₂ abatement rate by targeting high-rate production of acetate can promote adoption of MES technology in the short term. However, the development of a replicable and robust strategy for production and in-line extraction of higher-value products (e.g. caproic acid and hexanol) at the cathode, and meaningful exploitation of the currently overlooked anodic reactions, can further boost MES cost-effectiveness. Furthermore, the use of energy storage and smart electronics can alleviate the fluctuations of renewable energy supply. Despite the unresolved challenges, the flexible MES technology can be applied to decarbonise flue gas from different sources, to upgrade industrial and wastewater treatment plants, and to produce a wide array of green and sustainable chemicals. The combination of these benefits can support the industrial adoption of MES over competing technologies.