Reconfiguration of the reductive TCA cycle enables high-level succinic acid production by Yarrowia lipolytica

Engineering strategies for producing medium-long chain dicarboxylic acids in oleaginous yeasts

Medium-long chain dicarboxylic acids (DCAs, C ≥ 6) are essential chemical raw materials, with wide applications in the chemical, pharmaceutical, material and food industries. However, the traditional chemical synthesis methods cause environmental pollution and are not in line with goals of sustainable development. With the development of synthetic biology, high-value-added DCAs can be biosynthesized from hydrophobic substrates (HSs) using suitable microorganisms. This review first summarizes the biosynthetic pathway of DCAs in oleaginous yeasts and then emphasizes the related engineering strategies for increasing the product yield, including promoter, enzyme, pathway, cell, fermentation, and downstream engineering. In addition, the challenges and development trends in the biosynthesis of DCAs are discussed, in light of the current progress, challenges, and trends in this field. Finally, guidelines for future research are proposed. Overall, this review systematically summarizes recent engineering strategies for DCAs production in oleaginous yeasts and offers valuable insights for future DCAs biosynthesis.

Enhancing Feammox efficiency through riboflavin and humic acid: Nitrogen and iron transformation, energy metabolism, and microbial response

Optimizing electron shuttles and revealing their mediating mechanisms are crucial for enhancing the ammonium (NH4+-N) oxidation coupled with Fe (III) reduction. In this study, anthraquinone-2,6-disulfonate (AQDs), riboflavin (RF), and humic acid (HA) were optimized in batch tests. The optimal dosages of 6, 2, and 8 mg/L for AQDs, RF, and HA resulted in average maximum NH4+-N removal of 80.2 %, 88.5 %, and 99.2 %, with 91.4 %, 88.5 %, and 74.7 % of the removed NH4+-N converted to nitrate, respectively. In addition, an enhanced extracellular electron transfer was also observed, including an enlarged current, diversified REDOX pathway, and reduced resistance. Outperformed AQDs in nitrogen removal and microbial activity, HA and RF were selected for the subsequent 100-day long-term investigation. During this stage, excess influent Fe tended to be stored as insoluble coatings on the sludge surface, while RF and HA facilitated its use to compensate for the reduced influent Fe3+. Meanwhile, they led to an increase in iron-reducing (Comamonas) and NH4+-N oxidizing bacteria (Nitropsira and Planctomycetes), as well as improvements in electrochemical characteristics and microbial activity. Moreover, microbial N and Fe metabolic potential were efficiently enhanced. Consequently, NH4+-N and TN removal rates were obviously increased to approximately 90 % and 40 %, respectively. The addition of electron shuttles led to long-term improvements in extracellular mass transfer and microbial metabolism, which contributed more than bridging the extracellular electron transfer. These results deepened the understanding of the effect of electron shuttles on Feammox.

Expanding the genetic toolkit of Yarrowia lipolytica: Dynamic promoter engineering enables high-titer biosynthesis of 3-hydroxypropionic acid

The oleaginous yeast Yarrowia lipolytica has emerged as a promising microbial chassis for biosynthesis of platform chemicals such as 3-hydroxypropionic acid (3-HP). However, its industrial potential has been limited by the scarcity of precisely regulated genetic tools. To address this gap, we developed a comprehensive promoter toolkit for Y. lipolytica through transcriptome profiling and functional screening. This toolkit includes 82 gradient-strength promoters and 34 growth phase-responsive promoters. Additionally, we identified three strong promoters (PU12, PU13, and PC48) incorporating novel upstream activating sequences (UAS1PC48 and UAS1PU13), which exhibited 0.76-1.00 × higher activity than common promoter pTEFin. By modularly deploying these tools, we optimized 3-HP biosynthesis: gradient promoters balanced expression levels between different functional domains of malonyl-CoA reductase, growth phase-downregulated promoters dynamically attenuated competitive flux of fatty acid synthesis, and strong promoters boosted malonyl-CoA precursor supply. The engineered strain achieved a record-breaking 100.37 g/L 3-HP-the highest titer reported in any yeast system-with a yield of 0.21 g/g glucose and a productivity of 0.48 g/L/h. This work not only significantly expands Y. lipolytica's genetic toolbox but also establishes a blueprint for engineering dynamic microbial cell factories, addressing the urgent demand for sustainable, high-efficiency biomanufacturing platforms.

An in vivo target mutagenesis system for multiple hosts

In vivo target mutagenesis is a powerful approach to accelerate protein evolution. However, current approaches have been primarily developed in conventional organisms, limiting their capacity to evolve proteins with subtle variations across non-conventional host species. Here, we design an in vivo target mutagenesis system for multiple hosts (ITMU) utilizing the broad host-range plasmid RSF1010 replication element. The ITMU, which is based on a deaminase-helicase fusion and a primase error-prone DNA polymerase I fusion, induces all types of mutation in the target plasmid harboring the RSF1010 replicon, at a mutation rate 1.18 × 105-fold higher than that of the host genome. We show that ITMU-based in vivo continuous evolution is effective in Escherichia coli, Pseudomonas putida, Corynebacterium glutamicum, and Yarrowia lipolytica. This demonstrates that the ITMU is applicable to multiple microbial chassis and provides a viable alternative to in vivo continuous evolution systems.

Issatchenkia orientalis as a platform organism for cost-effective production of organic acids

Driven by the urgent need to reduce the reliance on fossil fuels and mitigate environmental impacts, microbial cell factories capable of producing value-added products from renewable resources have gained significant attention over the past few decades. Notably, non-model yeasts with unique physiological characteristics have emerged as promising candidates for industrial applications, particularly for the production of organic acids. Among them, Issatchenkia orientalis stands out for its exceptional natural tolerance to low pH and high osmotic pressure, traits that are critical for overcoming the limitations of conventional microbial organisms. The acid tolerance of I. orientalis enables organic acid production under low pH conditions, bypassing the need for expensive neutral pH control typically required in conventional processes. Organic acids produced by I. orientalis, such as lactic acid, succinic acid, and itaconic acid, are widely used as building blocks for bioplastics, food additives, and pharmaceuticals. This review summarizes the key findings from systems biology studies on I. orientalis over the past two decades, providing insights into its unique metabolic and physiological traits. Advances in genetic tool development for this non-model yeast are also discussed, enabling targeted metabolic engineering to enhance its production capabilities. Additionally, case studies are highlighted to illustrate the potential of I. orientalis as a platform organism. Finally, the remaining challenges and future directions are addressed to further develop I. orientalis into a robust and versatile microbial cell factory for sustainable biomanufacturing.

Adaptively Evolved and Multiplexed Engineered Saccharomyces cerevisiae for Neutralizer-Free Production of l-Lactic Acid

l-Lactic acid is a three-carbon monocarboxylic acid that has extensive applications. However, the bioproduction of l-lactic acid requires the addition of neutralizers, which significantly increases the production costs and can cause environmental pollution. To address this, a Saccharomyces cerevisiae mutant, TMG2, which can tolerate a lactic acid environment (pH 2.60), was obtained through adaptive laboratory evolution. Subsequently, the "push-pull-restrain" strategy was used to improve l-lactic acid production, resulting in a production of 46.8 g/L l-lactic acid. Finally, by overexpressing the transport protein pPfFNT and improving the NADH and acetyl-CoA supply, the l-lactic acid titer of strain TMG27 was improved by 33.8% to 62.6 g/L. Without neutralizers, the l-lactic acid titer reached 76.2 g/L (the fermentation pH was 2.90) with a productivity of 2.1 g/(L h) in a 5-L bioreactor, representing the highest productivity ever reported. Collectively, these results lay the foundation for the environmentally friendly bioproduction of l-lactic acid.

Progress on production of malic acid and succinic acid by industrially-important engineered microorganisms

Organic acids are widely used as additives in the food, pharmaceutical, chemical, and plastic industries. Currently, the industrial production methods of organic acids mainly include plant extraction and chemical synthesis. The latter mainly uses petroleum-based compounds as raw materials to synthesize organic acids through a series of chemical reactions. All of these methods have problems such as environmental pollution, high cost, and unsustainability. By contrast, microbial fermentation can effectively utilize a variety of carbon sources. Due to its low production cost, environmental friendliness, and high product purity, microbial fermentation has received increasing attention in recent years. However, the low yield and long fermentation cycle of microbial fermentation limits its industrial application. With the development of genomics, transcriptomics, and other omics technologies, the metabolic pathways of various strains producing organic acids have gradually been elucidated. Based on this, new technologies such as synthetic biology and high-throughput screening have also been extensively studied. This review summarizes the latest research progress in improving organic acid biosynthesis through metabolic engineering, focusing on L-malic acid (L-MA) and succinic acid (SA). Finally, we also discuss the challenges and future prospects of this field. This review has important reference value in the fields of food, pharmaceuticals, and chemicals, providing a theoretical basis for the study of organic acid biosynthesis.

Recent advances in biological synthesis of food additive succinate

Succinate, a crucial bio-based chemical building block, has already found extensive applications in fields such as food additives, pharmaceutical intermediates, and the chemical materials industry. To efficiently and economically synthesize succinate, substantial endeavors have been executed to optimize fermentation processes and downstream operations. Nonetheless, there is still a need to enhance cost-effectiveness and competitiveness while considering environmental concerns, particularly in light of the escalating demands and challenges posed by global warming. This article primarily focuses on the application of metabolic engineering strategies to strengthen succinate biosynthesis. These strategies encompass fermentation regulation, metabolic regulation, cellular regulation, and model guidance. By leveraging advanced synthetic biology techniques, this review highlights the potential for developing robust microbial cell factories and shaping the future directions for the integration of microbes in industrial applications.

Engineering Yarrowia lipolytica to Enhance the Production of Malonic Acid via Malonyl-CoA Pathway at High Titer

Malonic acid (MA) is a high-value-added chemical with significant applications in the polymers, pharmaceutical, and food industries. Microbial production of MA presents enzyme inefficiencies, competitive metabolic pathways, and dispersive carbon flux, which collectively limit its biosynthesis. Here, the non-conventional oleaginous yeast Yarrowia lipolytica is genetically engineered to enhance MA production. Initially, the malonyl-CoA pathway, comprising a malonyl-CoA hydrolase from Saccharomyces cerevisiae, is confirmed as the most efficient for MA production in Y. lipolytica. To further enhance MA production, two novel malonyl-CoA hydrolases exhibiting higher activity than the hydrolase from S. cerevisiae, are identified from Y. lipolytica and Fusarium oxysporum, respectively. The introduction of the malonyl-CoA hydrolase from F. oxysporum increases the MA titer to 6.3 g L-1. Subsequently, advanced metabolic engineering strategies are performed to ensure a sufficient flux of the precursors acetyl-CoA and malonyl-CoA for MA production, resulting in a production of 13.8 g L-1 MA in shaking-flasks. Finally, by employing the fermentation conditions and feeding strategies, a maximum concentration of 63.6 g L-1 of MA is achieved at 156 h with a productivity of 0.41 g L-1 h-1 in fed-batch fermentation. This study provides a new way for engineering Y. lipolytica to enhance MA production at high titer.

Facilitated Channeling of Fixed Carbon and Energy into Chemicals in Artificial Phototrophic Communities

Light-driven CO2 biovalorization offers a promising route for coupling carbon mitigation with petrochemical replacement. Synthetic phototrophic communities that mimic lichens can reduce the metabolic burden with improved CO2 utilization. However, inefficient channeling of carbon and energy between species seriously hinders the collaborative CO2-to-molecule route. Herein, we report a universal carbon sequestration (UCS) module based on photosynthetic microbes that provides a high-speed tunnel for channeling carbon and energy to heterotrophs. Compared to that of the traditional CO2-to-sucrose module, the UCS module sequestered 30% more carbon into glycerol, a generally available carbon source with high energy density. We demonstrated that the UCS module can be highly compatible with various industrial chassis and genetically recalcitrant microbes, enabling the rapid development of synthetic phototrophic communities without additional genetic manipulation. Notably, the accelerated electron transport and nutrient recycling systems may facilitate carbon and energy communications between cooperative partners. These UCS module-based communities efficiently channeled CO2 into a wide range of chemicals, with a negative carbon footprint of -25.04 to -440.74 kgCO2e/kg of products. This strategy widens the boundaries of artificial photosynthetic communities and may boost carbon-negative biomanufacturing.

Recent advances in genetic engineering and chemical production in yeast species

Yeasts have emerged as well-suited microbial cell factory for the sustainable production of biofuels, organic acids, terpenoids, and specialty chemicals. This ability is bolstered by advances in genetic engineering tools, including CRISPR-Cas systems and modular cloning in both conventional (Saccharomyces cerevisiae) and non-conventional (Yarrowia lipolytica, Rhodotorula toruloides, Candida krusei) yeasts. Additionally, genome-scale metabolic models and machine learning approaches have accelerated efforts to create a broad range of compounds that help reduce dependency on fossil fuels, mitigate climate change, and offer sustainable alternatives to petrochemical-derived counterparts. In this review, we highlight the cutting-edge genetic tools driving yeast metabolic engineering and then explore the diverse applications of yeast-based platforms for producing value-added products. Collectively, this review underscores the pivotal role of yeast biotechnology in efforts to build a sustainable bioeconomy.

Non-conventional yeasts: promising cell factories for organic acid bioproduction

Microbial production of organic acids has been hindered by the poor acid tolerance of microorganisms and the high costs of waste salt reprocessing. The robustness of non-conventional microorganisms in an acidic environment makes it possible to produce organic acids at low pH and greatly simplifies downstream processing. In this review we discuss the environmental adaptability features of non-conventional yeasts, as well as the latest developments in genomic engineering strategies that have facilitated metabolic engineering of these strains. We also use selected examples of three-carbon (C3), C4, and C6 organic acids to illustrate the ongoing efforts and challenges of using non-conventional yeasts for organic acid production. This review provides theoretical guidance for the construction of highly robust organic acid producers.

Advancing Succinic Acid Biomanufacturing Using the Nonconventional Yeast Yarrowia lipolytica

Succinic acid is an essential bulk chemical with wide-ranging applications in materials, food, and pharmaceuticals. With the advancement of biotechnology, there has been a surge in focus on low-carbon sustainable microbial synthesis methods for producing biobased succinic acid. Due to its high intrinsic acid tolerance, Yarrowia lipolytica has gained recognition as a competitive chassis for the industrial manufacture of succinic acid. This review summarizes the research progress on succinic acid biomanufacturing using Y. lipolytica. First, it introduces the major metabolic routes for succinic acid biosynthesis and the pertinent engineering approaches for building efficient cell factories. Subsequently, we offer a review of methods employed for succinic acid synthesis by Y. lipolytica utilizing alternative substrates as well as the relevant optimization strategies for the fermentation process. Finally, future research directions for improving succinic acid biomanufacturing in Y. lipolytica are delineated in light of the recent progress, obstacles, and trends in this area.

Bioleaching of rare earth elements from ores and waste materials: Current status, economic viability and future prospects

Rare earth elements (REEs) are critical components of numerous products widely used in many areas, and the demand for REEs is increasing dramatically in recent years. Physical-chemical leaching is commonly adopted for the recovery of REEs from ores and solid wastes, but concerns over the generation of hazards, operation safety, and environmental pollution have urged the transition to greener and more sustainable leaching methods. Bioleaching is considered an excellent alternative for the recovery of REEs. This review provided an overview on the REEs recovery from primary and secondary resources via different bioleaching strategies. The techno-economics of bioleaching for REEs recovery were highlighted, and key factors affecting the economic viability of bioleaching were identified. Finally, strategies including the utilization of low-cost substrates as feedstocks, non-sterile bioleaching, recycling and reutilization of biolixiviants, and development of robust bioleaching strains were proposed to improve the economic competitiveness of bioleaching. It is expected that this review could serve as a useful guideline on the design of more economically competitive bioleaching processes for the recovery REEs from different resources.

Paradigm of engineering recalcitrant non-model microorganism with dominant metabolic pathway as a biorefinery chassis

The development and implementation of microbial chassis cells have profound impacts on circular economy. Non-model bacterium Zymomonas mobilis is an excellent chassis owing to its extraordinary industrial characteristics. Here, the genome-scale metabolic model iZM516 is improved and updated by integrating enzyme constraints to simulate the dynamics of flux distribution and guide pathway design. We show that the innate dominant ethanol pathway of Z. mobilis restricts the titer and rate of these biochemicals. A dominant-metabolism compromised intermediate-chassis (DMCI) strategy is then developed through introducing low toxicity but cofactor imbalanced 2,3-butanediol pathway, and a recombinant D-lactate producer is constructed to produce more than 140.92 g/L and 104.6 g/L D-lactate (yield > 0.97 g/g) from glucose and corncob residue hydrolysate, respectively. Additionally, techno-economic analysis (TEA) and life cycle assessment (LCA) demonstrate the commercialization feasibility and greenhouse gas reduction capability of lignocellulosic D-lactate. This work thus establishes a paradigm for engineering recalcitrant microorganisms as biorefinery chassis.

Microbial community assemblage altered by coprecipitation of artificial humic substances and ferrihydrite: Implications for carbon fixation pathway transformation

The suppression of soil carbon mineralization has been demonstrated to be effectively facilitated by carbon‑iron interactions, yet the specific mechanisms by which artificial humic substances (A-HS) coupled with ferrihydrite influence this process remain insufficiently explored. This study is to investigate how the A-HS, specifically artificial fulvic acid (A-FA) and artificial humic acid (A-HA), coupled with ferrihydrite, affect carbon mineralization under anaerobic system that simulates paddy flooding conditions. The object is to investigate trends in carbon emissions and to delineate microbial community structure and functional pathways. The findings indicate that A-HA and A-FA substantially reduce CO2 and CH4 emissions, with A-FA having a particularly pronounced effect on carbon fixation, halving CO2 concentrations. The low concentration of Fe(II) observed suggest that A-FA and A-HA impede the dissimilatory iron reduction (DIR) process. Detailed 16S rDNA sequencing and gene prediction analyses reveal changes in microbial community structures and functions, highlighting Methanobacterium as the dominant hydrogenotrophic methanogens. The reductive citric acid cycle, predominantly utilized by Clostridium carboxidivorans, was identified as the principal carbon fixation pathway. This work provides a novel insight into the microbial mechanisms of carbon sequestration and highlights the potential of A-HS in improving soil fertility and contributing to climate change mitigation through enhancing soil carbon storage.

Boosting succinic acid production of Yarrowia lipolytica at low pH through enhancing product tolerance and glucose metabolism

BACKGROUND

Succinic acid (SA) is an important bio-based C4 platform chemical with versatile applications, including the production of 1,4-butanediol, tetrahydrofuran, and γ-butyrolactone. The non-conventional yeast Yarrowia lipolytica has garnered substantial interest as a robust cell factory for SA production at low pH. However, the high concentrations of SA, especially under acidic conditions, can impose significant stress on microbial cells, leading to reduced glucose metabolism viability and compromised production performance. Therefore, it is important to develop Y. lipolytica strains with enhanced SA tolerance for industrial-scale SA production.

RESULTS

An SA-tolerant Y. lipolytica strain E501 with improved SA production was obtained through adaptive laboratory evolution (ALE). In a 5-L bioreactor, the evolved strain E501 produced 89.62 g/L SA, representing a 7.2% increase over the starting strain Hi-SA2. Genome resequencing and transcriptome analysis identified a mutation in the 26S proteasome regulatory subunit Rpn1, as well as genes involved in transmembrane transport, which may be associated with enhanced SA tolerance. By further fine-tuning the glycolytic pathway flux, the highest SA titer of 112.54 g/L to date at low pH was achieved, with a yield of 0.67 g/g glucose and a productivity of 2.08 g/L/h.

CONCLUSION

This study provided a robust engineered Y. lipolytica strain capable of efficiently producing SA at low pH, thereby reducing the cost of industrial SA fermentation.

Microbial Cell Factories in the Bioeconomy Era: From Discovery to Creation

Microbial cell factories (MCFs) are extensively used to produce a wide array of bioproducts, such as bioenergy, biochemical, food, nutrients, and pharmaceuticals, and have been regarded as the "chips" of biomanufacturing that will fuel the emerging bioeconomy era. Biotechnology advances have led to the screening, investigation, and engineering of an increasing number of microorganisms as diverse MCFs, which are the workhorses of biomanufacturing and help develop the bioeconomy. This review briefly summarizes the progress and strategies in the development of robust and efficient MCFs for sustainable and economic biomanufacturing. First, a comprehensive understanding of microbial chassis cells, including accurate genome sequences and corresponding annotations; metabolic and regulatory networks governing substances, energy, physiology, and information; and their similarity and uniqueness compared with those of other microorganisms, is needed. Moreover, the development and application of effective and efficient tools is crucial for engineering both model and nonmodel microbial chassis cells into efficient MCFs, including the identification and characterization of biological parts, as well as the design, synthesis, assembly, editing, and regulation of genes, circuits, and pathways. This review also highlights the necessity of integrating automation and artificial intelligence (AI) with biotechnology to facilitate the development of future customized artificial synthetic MCFs to expedite the industrialization process of biomanufacturing and the bioeconomy.

High-level succinic acid production by overexpressing a magnesium transporter in Mannheimia succiniciproducens

Succinic acid (SA), a dicarboxylic acid of industrial importance, can be efficiently produced by metabolically engineered Mannheimia succiniciproducens. Although the importance of magnesium (Mg2+) ion on SA production has been evident from our previous studies, the role of Mg2+ ion remains largely unexplored. In this study, we investigated the impact of Mg2+ ion on SA production and developed a hyper-SA producing strain of M. succiniciproducens by reconstructing the Mg2+ ion transport system. To achieve this, optimal alkaline neutralizer comprising Mg2+ ion was developed and the physiological effect of Mg2+ ion was analyzed. Subsequently, the Mg2+ ion transport system was reconstructed by introducing an efficient Mg2+ ion transporter from Salmonella enterica. A high-inoculum fed-batch fermentation of the final engineered strain produced 152.23 ± 0.99 g/L of SA, with a maximum productivity of 39.64 ± 0.69 g/L/h. These findings highlight the importance of Mg2+ ions and transportation system optimization in succinic acid production by M. succiniciproducens.

Sustainable succinic acid production from lignocellulosic hydrolysates by engineered strains of Yarrowia lipolytica at low pH

Succinic acid (SA) is a valuable C4 platform chemical with diverse applications. Lignocellulosic biomass represents an abundant and renewable carbon resource for microbial production of SA. However, the presence of toxic compounds in pretreated lignocellulosic hydrolysates poses challenges to cell metabolism, leading to inefficient SA production. Here, engineered Yarrowia lipolytica Hi-SA2 was shown to utilize glucose and xylose from corncob hydrolysate to produce 32.6 g/L SA in shaking flasks. The high concentration of undetoxified hydrolysates significantly inhibited yeast growth and SA biosynthesis, with furfural identified as the key inhibitor. Through overexpressing glutathione synthetase encoding gene YlGsh2, the tolerance of engineered strain to furfural and toxic hydrolysate was significantly improved. In a 5-L bioreactor, Hi-SA2-YlGsh2 strain produced 45.34 g/L SA within 32 h, with a final pH of 3.28. This study provides a sustainable process for bio-based SA production, highlighting the efficient SA synthesis from lignocellulosic biomass through low pH fermentation.

Med3-mediated NADPH generation to help Saccharomyces cerevisiae tolerate hyperosmotic stress

Hyperosmotic stress tolerance is crucial for Saccharomyces cerevisiae in producing value-added products from renewable feedstock. The limited understanding of its tolerance mechanism has impeded the application of these microbial cell factories. Previous studies have shown that Med3 plays a role in hyperosmotic stress in S. cerevisiae. However, the specific function of Med3 in hyperosmotic stress tolerance remains unclear. In this study, we showed that the deletion of the mediator Med3 impairs S. cerevisiae growth under hyperosmotic stress. Phenotypic analyses and yeast two-hybrid assays revealed that Med3 interacts with the transcription factor Stb5 to regulate the expression of the genes gnd1 and ald6, which are involved in NADPH production under hyperosmotic stress conditions. The deletion of med3 resulted in a decrease in intracellular NADPH content, leading to increased oxidative stress and elevated levels of intracellular reactive oxygen species under hyperosmotic stress, thereby impacting bud formation. These findings highlight the significant role of Med3 as a regulator in maintaining NADPH generation and redox homeostasis in S. cerevisiae during hyperosmotic stress.IMPORTANCEHyperosmotic stress tolerance in the host strain is a significant challenge for fermentation performance in industrial production. In this study, we showed that the S. cerevisiae mediator Med3 is essential for yeast growth under hyperosmotic conditions. Med3 interacts with the transcription factor Stb5 to regulate the expression of genes involved in the NADPH-generation system during hyperosmotic stress. Adequate NADPH ensures the timely removal of excess reactive oxygen species and supports bud formation under these conditions. This work highlights the crucial role of Med3 as a regulator in maintaining NADPH generation and redox homeostasis in S. cerevisiae during hyperosmotic stress.

Upcycling CO2 into succinic acid via electrochemical and engineered Escherichia coli

Converting CO2 into value-added chemicals still remains a grand challenge. Succinic acid has long been considered as one of the top building block chemicals. This study reported efficiently upcycling CO2 into succinic acid by combining between electrochemical and engineered Escherichia coli. In this process, the Cu-organic framework catalyst was synthesized for electrocatalytic CO2-to-ethanol conversion with high Faradaic efficiency (FE, 84.7 %) and relative purity (RP, 95 wt%). Subsequently, an engineered E. coli with efficiently assimilating CO2-derived ethanol to produce succinic acid was constructed by combining computational design and metabolic engineering, and the succinic acid titer reached 53.8 mM with the yield of 0.41 mol/mol, which is 82 % of the theoretical yield. This study effort to link the two processes of efficient ethanol synthesis by electrocatalytic CO2 and succinic acid production from CO2-derived ethanol, paving a way for the production of succinic acid and other value-added chemicals by converting CO2 into ethanol.

Multidimensional combinatorial screening for high-level production of erythritol in Yarrowia lipolytica

Yarrowia lipolytica was successfully engineered to synthesize erythritol from crude glycerol, a cheap by-product of biodiesel production, but the yield remained low. Here, a biosensor-guided adaptive evolution screening platform was constructed to obtain mutant strains which could efficiently utilize crude glycerol to produce erythritol. Erythrose reductase D46A (M1) was identified as a key mutant through whole-genome sequencing of the strain G12, which exhibited higher catalytic activity (1.6-fold of the wild-type). M1 was further modified to obtain a combinatorial mutant with 4.1-fold enhancement of catalytic activity. Finally, the metabolic network was reconfigured to redirect carbon fluxes toward erythritol synthesis. The erythritol titer of the engineered strain G31 reached 220.5 g/L with a productivity of 1.8 g/L/h in a 5-L bioreactor. The study provides valuable guidance for biosensor-based ultra-high-throughput screening strategies in Y. lipolytica, as well as presenting a new paradigm for the sustainable valorization of crude glycerol.

Advances in succinic acid production: the enhancement of CO2 fixation for the carbon sequestration benefits

Succinic acid (SA), one of the 12 top platform chemicals produced from biomass, is a precursor of various high value-added derivatives. Specially, 1 mol CO2 is assimilated in 1 mol SA biosynthetic route under anaerobic conditions, which helps to achieve carbon reduction goals. In this review, methods for enhanced CO2 fixation in SA production and utilization of waste biomass for SA production are reviewed. Bioelectrochemical and bioreactor coupling systems constructed with off-gas reutilization to capture CO2 more efficiently were highlighted. In addition, the techno-economic analysis and carbon sequestration benefits for the synthesis of bio-based SA from CO2 and waste biomass are analyzed. Finally, a droplet microfluidics-based high-throughput screening technique applied to the future bioproduction of SA is proposed as a promising approach.