Enhanced malic acid production using Aspergillus niger coupled with in situ product recovery

Unveiling malic acid biorefinery: Comprehensive insights into feedstocks, microbial strains, and metabolic pathways

The over-reliance on fossil fuels and resultant environmental issues necessitate sustainable alternatives. Microbial fermentation of biomass for malic acid production offers a viable, eco-friendly solution, enhancing resource efficiency and minimizing ecological damage. This review covers three core aspects of malic acid biorefining: feedstocks, microbial strains, and metabolic pathways. It emphasizes the significance of utilizing biomass sugars, including the co-fermentation of different sugar types to improve feedstock efficiency. The review discusses microbial strains for malic acid fermentation, addressing challenges related to by-products from biomass breakdown and strategies for overcoming them. It delves into the crucial pathways and enzymes for malic acid production, outlining methods to optimize its metabolism, focusing on enzyme regulation, energy balance, and yield enhancement. These insights contribute to advancing the field of consolidated bioprocessing in malic acid biorefining.

Engineering the reductive tricarboxylic acid pathway in Aureobasidium pullulans for enhanced biosynthesis of poly-L-malic acid

Aureobasidium pullulans produced poly-L-malic acid (PMA) as the main metabolite in fermentation but with relatively low productivity and yield limiting its industrial application. In this study, A. pullulans ZX-10 was engineered to overexpress cytosolic malate dehydrogenase (MDH) and pyruvate carboxylase (PYC) and PMA synthetase (PMS) using a high-copy yeast episomal plasmid with the gpdA promoter from Aspergillus nidulans. Overexpressing endogenous PMS and heterologous MDH and PYC from Aspergillus oryzae respectively increased PMA production by 19 % - 37 % (0.64 - 0.74 g/g vs. 0.54 g/g for wild type) in shake-flask fermentations, demonstrating the importance of the reductive tricarboxylic acid (rTCA) pathway in PMA biosynthesis. A. pullulans co-expressing MDH and PYC produced 96.7 g/L PMA at 0.90 g/L∙h and 0.68 g/g glucose in fed-batch fermentation, which were among the highest yield and productivity reported. The engineered A. pullulans with enhanced rTCA pathway is advantageous and promising for PMA production.

Cofactor Metabolic Engineering of Escherichia coli for Aerobic L-Malate Production with Lower CO2 Emissions

Escherichia coli has been engineered for L-malate production via aerobic cultivation. However, the maximum yield obtained through this mode is inferior to that of anaerobic fermentation due to massive amounts of CO2 emissions. Here, we aim to address this issue by reducing CO2 emissions of recombinant E. coli during aerobic L-malate production. Our findings indicated that NADH oxidation and ATP-synthesis-related genes were down-regulated with 2 g/L of YE during aerobic cultivations of E. coli E23, as compared to 5 g/L of YE. Then, E23 was engineered via the knockout of nuoA and the introduction of the nonoxidative glycolysis (NOG) pathway, resulting in a reduction of NAD+ and ATP supplies. The results demonstrate that E23 (ΔnuoA, NOG) exhibited decreased CO2 emissions, and it produced 21.3 g/L of L-malate from glucose aerobically with the improved yield of 0.43 g/g. This study suggests that a restricted NAD+ and ATP supply can prompt E. coli to engage in incomplete oxidization of glucose, leading to the accumulation of metabolites instead of utilizing them in cellular respiration.

Metabolic engineering and fermentation optimization strategies for producing organic acids of the tricarboxylic acid cycle by microbial cell factories

The organic acids of the tricarboxylic acid (TCA) pathway are important platform compounds and are widely used in many areas. The high-productivity strains and high-efficient and low-cost fermentation are required to satisfy a huge market size. The high metabolic flux of the TCA pathway endows microorganisms potential to produce high titers of these organic acids. Coupled with metabolic engineering and fermentation optimization, the titer of the organic acids has been significantly improved in recent years. Herein, we discuss and compare the recent advances in synthetic pathway engineering, cofactor engineering, transporter engineering, and fermentation optimization strategies to maximize the biosynthesis of organic acids. Such engineering strategies were mainly based on the TCA pathway and glyoxylate pathway. Furthermore, organic-acid-secretion enhancement and renewable-substrate-based fermentation are often performed to assist the biosynthesis of organic acids. Further strategies are also discussed to construct high-productivity and acid-resistant strains for industrial large-scale production.

Recent advances in the production of malic acid by native fungi and engineered microbes

Malic acid is mainly produced by chemical methods which lead to various environmental sustainability concerns associated with CO₂ emissions and resulting global warming. Since malic acid is naturally synthesized, microorganisms offer an eco-friendly and cost-effective alternative for its production. An additional advantage of microbial production is the synthesis of pure L-form of malic acid. Due to its numerous applications, biotechnologically- produced L-malic acid is a much sought-after platform chemical. Malic acid can be produced by microbial fermentation via oxidative/reductive TCA and glyoxylate pathways. This article elaborates the potential and limitations of high malic acid producing native fungi belonging to Aspergillus, Penicillium, Ustilago and Aureobasidium spp. The utilization of industrial side streams and low value renewable substrates such as crude glycerol and lignocellulosic biomass is also discussed with a view to develop a competitive bio-based production process. The major impediments present in the form of toxic compounds from lignocellulosic residues or synthesized during fermentation along with their remedial measures are also described. The article also focuses on production of polymalic acid from renewable substrates which opens up a cost-cutting dimension in production of this biodegradable polymer. Finally, the recent strategies being employed for its production in recombinant organisms have also been covered.

Integrated fermentative production and downstream processing of L-malic acid by Aspergillus wentii using cassava peel waste

L-malic acid (L-MA) is an industrially significant chemical with enormous potential. The fungal cell factories could be exploited to harvest it on large scales. In our study, Aspergillus wentii strain (MTCC 1901 ^T) was explored for L-MA production. Initially, the L-MA production was carried out using glucose with optimization of parameters influencing product accumulation (pH and CaCO₃). The fermentation resulted in L-MA titer of 37.9 g/L with 0.39 g/g yield. Then, cassava peel waste (CPW) was used for L-MA production by separate hydrolysis and fermentation. Optimized acidic and enzymatic hydrolysis resulted in glucose release of 0.53 and 0.66 g/g CPW, respectively. The strain accumulated 20.9 g/L and 33.1 g/L L-MA with corresponding yields of 0.25 g/g and 0.34 g/g during batch cultivation using acid and enzyme hydrolysate, respectively. Finally, the produced L-MA was separated using an inexpensive solvent extraction method. Among various solvents used, n-butanol exhibited maximum L-MA extraction efficiency (31%).

Metabolic Engineering of Trichoderma reesei for l-Malic Acid Production

l-Malic acid has various applications in the chemical and food industries. The filamentous fungus Trichoderma reesei is known to be an efficient enzyme producer. Here, through metabolic engineering, T. reesei was constructed for the first time as an excellent cell factory for l-malic acid production. The heterologous overexpression of genes encoding the C4-dicarboxylate transporter from Aspergillus oryzae and Schizosaccharomyces pombe initiated l-malic acid production. The overexpression of pyruvate carboxylase from A. oryzae in the reductive tricarboxylic acid pathway further increased both the titer and yield of l-malic acid, resulting in the highest titer reported in a shake-flask culture. Furthermore, the deletion of malate thiokinase blocked l-malic acid degradation. Finally, the engineered T. reesei strain produced 220.5 g/L of l-malic acid in a 5 L fed-batch culture (productivity of 1.15 g/L/h). A T. reesei cell factory was created for the efficient production of l-malic acid.

Improving the productivity of malic acid by alleviating oxidative stress during Aspergillus niger fermentation

BACKGROUND

As an attractive platform chemical, malic acid has been commonly used in the food, feed and pharmaceutical field. Microbial fermentation of biobased sources to produce malic acid has attracted great attention because it is sustainable and environment-friendly. However, most studies mainly focus on improving yield and ignore shortening fermentation time. A long fermentation period means high cost, and hinders the industrial applications of microbial fermentation. Stresses, especially oxidative stress generated during fermentation, inhibit microbial growth and production, and prolong fermentation period. Previous studies have shown that polypeptides could effectively relieve stresses, but the underlying mechanisms were poorly understood.

RESULTS

In this study, polypeptides (especially elastin peptide) addition improves the productivity of malic acid in A. niger, resulting in shortening of fermentation time from 120 to 108 h. Transcriptome and biochemical analyses demonstrated that both antioxidant enzyme-mediated oxidative stress defense system, such as superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPX), and nonenzymatic antioxidant system, such as glutathione, were enhanced in the presence of elastin peptide, suggesting elastin peptide relieving oxidative stresses is involved in many pathways. In order to further investigate the relationship between oxidative stress defense and malic acid productivity, we overexpressed three enzymes (Sod1, CAT, Tps1) related to oxidation resistance in A. niger, respectively, and these resulting strains display varying degree of improvement in malic acid productivity. Especially, the strain overexpressing the Sod1 gene achieved a malate titer of 91.85 ± 2.58 g/L in 96 h, corresponding to a productivity of 0.96 g/L/h, which performs better than elastin peptide addition.

CONCLUSIONS

Our investigation provides an excellent reference for alleviating the stress of the fungal fermentation process and improving fermentation efficiency.

Recent advances in microbial production of L-malic acid

Over the last few decades, increasing concerns regarding fossil fuel depletion and excessive CO₂ emissions have led to extensive fundamental studies and industrial trials regarding microbial chemical production. As an additive or precursor, L-malic acid has been shown to exhibit distinctive properties in the food, pharmaceutical, and daily chemical industries. L-malic acid is currently mainly fabricated through a fumarate hydratase-based biocatalytic conversion route, wherein petroleum-derived fumaric acid serves as a substrate. In this review, for the first time, we comprehensively describe the methods of malic acid strain transformation, raw material utilization, malic acid separation, etc., especially recent progress and remaining challenges for industrial applications. First, we summarize the various pathways involved in L-malic acid biosynthesis using different microorganisms. We also discuss several strain engineering strategies for improving the titer, yield, and productivity of L-malic acid. We illustrate the currently available alternatives for reducing production costs and the existing strategies for optimizing the fermentation process. Finally, we summarize the present challenges and future perspectives regarding the development of microbial L-malic acid production. KEY POINTS: • A range of wild-type, mutant, laboratory-evolved, and metabolically engineered strains which could produce L-malic acid were comprehensively described. • Alternative raw materials for reducing production costs and the existing strategies for optimizing the fermentation were sufficiently summarized. • The present challenges and future perspectives regarding the development of microbial L-malic acid production were elaboratively discussed.

Techno economic analysis of malic acid production using crude glycerol derived from waste cooking oil

In the present work, Aspergillus niger was employed to produce commercially valuable malic acid from crude glycerol derived from waste cooking oil. Crude glycerol dosage, yeast extract dosage and initial pH were the influencing factors playing a significant role in the malic acid synthesis. The optimal condition for malic acid biosynthesis was studied by using response surface methodology. Further the feasibility analysis for biosynthesis of malic acid from crude glycerol was studied using the laboratory scale optimized data, with this experimentally optimized data, plant was simulated using SuperPro Designer (v10). The cost involved for malic acid synthesis per unit volume was likely expected to be $0.43/kg of malic acid using reactive extraction method. Thus, process optimization combined with techno-economical analysis of malic acid production could be beneficial.

Recent advances in the development of Aspergillus for protein production

Aspergillus had been widely used in the industrial production of recombinant proteins. In addition to the safety and broad substrate utilization spectrum, its efficient post-translational modification and strong protein secretion capacity have significant advantages for developing an excellent protein-producing cell factory in industrial production. However, the difficulties in genetic manipulation of Aspergillus and varying expression levels of different heterologous proteins hampered its further development and application. Recently, the development of CRISPR genome editing and high-throughput screening platforms has facilitated the Aspergillus development of a wide range of modifications and applications. Meanwhile, multi-omics analysis and multiplexed genetic engineering have promoted effective knowledge mining. This paper provides a comprehensive and updated review of these advances, including high-throughput screening, genome editing, protein expression modules, and fermentation optimization. It also highlights and discusses the latest significant progress, aiming to provide a practical guide for implementing Aspergillus as an efficient protein-producing cell factory.

Surfactants in biorefineries: Role, challenges & perspectives

The use of lignocellulosic biomass (LCB) as feedstock has received increasing attention as an alternative to fossil-based refineries. Initial steps such as pretreatment and enzymatic hydrolysis are essential to breakdown the complex structure of LCB to make the sugar molecules available to obtain bioproducts by fermentation. However, these steps increase the cost of the bioproduct and often reduces its competitiveness against synthetic products. Currently, the use of surfactants has shown considerable potential to enhance lignocellulosic biomass processing. This review addresses the main mechanisms and role of surfactants as key molecules in various steps of biorefinery processes, viz., increasing the removal of lignin and hemicellulose during the pretreatments, increasing enzymatic stability and enhancing the accessibility of enzymes to the polymeric fractions, and improving the downstream process during fermentation. Further, technical advances, challenges in application of surfactants, and future perspectives to augment the production of several high value-added bioproducts have been discussed.