Enhancing L-malate production of Aspergillus oryzae FMME218-37 by improving inorganic nitrogen utilization

Multilevel systemic engineering of Bacillus licheniformis for efficient production of acetoin from lignocellulosic hydrolysates

Bio-refining lignocellulosic resource offers a renewable and sustainable approach for producing biofuels and biochemicals. However, the conversion efficiency of lignocellulosic resource is still challenging due to the intrinsic inefficiency in co-utilization of xylose and glucose. In this study, the industrial bacterium Bacillus licheniformis was engineered for biorefining lignocellulosic resource to produce acetoin. First, adaptive evolution was conducted to improve acetoin tolerance, leading to a 19.6 % increase in acetoin production. Then, ARTP mutagenesis and 60Co-γ irradiation was carried out to enhance the production of acetoin, obtaining 73.0 g/L acetoin from glucose. Further, xylose uptake and xylose utilization pathway were rewired to facilitate the co-utilization of xylose and glucose, enabling the production of 60.6 g/L acetoin from glucose and xylose mixtures. Finally, this efficient cell factory was utilized for acetoin production from lignocellulosic hydrolysates with the highest titer of 68.3 g/L in fed-batch fermentation. This strategy described here holds great applied potential in the biorefinery of lignocellulose for the efficient synthesis of high-value chemicals.

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.

Molecular regulation of fungal secondary metabolism

Many bioactive secondary metabolites synthesized by fungi have important applications in many fields, such as agriculture, food, medical and others. The biosynthesis of secondary metabolites is a complex process involving a variety of enzymes and transcription factors, which are regulated at different levels. In this review, we describe our current understanding on molecular regulation of fungal secondary metabolite biosynthesis, such as environmental signal regulation, transcriptional regulation and epigenetic regulation. The effects of transcription factors on the secondary metabolites produced by fungi were mainly introduced. It was also discussed that new secondary metabolites could be found in fungi and the production of secondary metabolites could be improved. We also highlight the importance of understanding the molecular regulation mechanisms to activate silent secondary metabolites and uncover their physiological and ecological functions. By comprehensively understanding the regulatory mechanisms involved in secondary metabolite biosynthesis, we can develop strategies to improve the production of these compounds and maximize their potential benefits.

Engineering growth phenotypes of Aspergillus oryzae for L-malate production

Improving the growth status of Aspergillus oryzae is an efficient way to enhance L-malate production. However, the growth mechanism of filamentous fungi is relatively complex, which limits A. oryzae as a cell factory to produce L-malate industrially. This study determined the relationship between growth status and L-malate production. The optimal ranges of colony diameter, percentage of vegetative mycelia, and pellet number of A. oryzae were determined to be 26-30 mm, 35-40%, and 220-240/mL, respectively. To achieve this optimum range, adaptive evolution was used to obtain the evolved strain Z07 with 132.54 g/L L-malate and a productivity of 1.1 g/L/h. Finally, a combination of transcriptome analysis and morphological characterization was used to identify the relevant pathway genes that affect the growth mechanism of A. oryzae. The strategies used in this study and the growth mechanism provide a good basis for efficient L-malate production by filamentous fungi.

Recent advances in producing food additive L-malate: Chassis, substrate, pathway, fermentation regulation and application

In addition to being an important intermediate in the TCA cycle, L-malate is also widely used in the chemical and beverage industries. Due to the resulting high demand, numerous studies investigated chemical methods to synthesize L-malate from petrochemical resources, but such approaches are hampered by complex downstream processing and environmental pollution. Accordingly, there is an urgent need to develop microbial methods for environmentally-friendly and economical L-malate biosynthesis. The rapid progress and understanding of DNA manipulation, cell physiology, and cell metabolism can improve industrial L-malate biosynthesis by applying intelligent biochemical strategies and advanced synthetic biology tools. In this paper, we mainly focused on biotechnological approaches for enhancing L-malate synthesis, encompassing the microbial chassis, substrate utilization, synthesis pathway, fermentation regulation, and industrial application. This review emphasizes the application of novel metabolic engineering strategies and synthetic biology tools combined with a deep understanding of microbial physiology to improve industrial L-malate biosynthesis in the future.

Engineering Microorganisms to Produce Bio-Based Monomers: Progress and Challenges

Bioplastics are polymers made from sustainable bio-based feedstocks. While the potential of producing bio-based monomers in microbes has been investigated for decades, their economic feasibility is still unsatisfactory compared with petroleum-derived methods. To improve the overall synthetic efficiency of microbial cell factories, three main strategies were summarized in this review: firstly, implementing approaches to improve the microbial utilization ability of cheap and abundant substrates; secondly, developing methods at enzymes, pathway, and cellular levels to enhance microbial production performance; thirdly, building technologies to enhance microbial pH, osmotic, and metabolites stress tolerance. Moreover, the challenges of, and some perspectives on, exploiting microorganisms as efficient cell factories for producing bio-based monomers are also discussed.

Microbial cell factories based on filamentous bacteria, yeasts, and fungi

BACKGROUND

Advanced DNA synthesis, biosensor assembly, and genetic circuit development in synthetic biology and metabolic engineering have reinforced the application of filamentous bacteria, yeasts, and fungi as promising chassis cells for chemical production, but their industrial application remains a major challenge that needs to be solved.

RESULTS

As important chassis strains, filamentous microorganisms can synthesize important enzymes, chemicals, and niche pharmaceutical products through microbial fermentation. With the aid of metabolic engineering and synthetic biology, filamentous bacteria, yeasts, and fungi can be developed into efficient microbial cell factories through genome engineering, pathway engineering, tolerance engineering, and microbial engineering. Mutant screening and metabolic engineering can be used in filamentous bacteria, filamentous yeasts (Candida glabrata, Candida utilis), and filamentous fungi (Aspergillus sp., Rhizopus sp.) to greatly increase their capacity for chemical production. This review highlights the potential of using biotechnology to further develop filamentous bacteria, yeasts, and fungi as alternative chassis strains.

CONCLUSIONS

In this review, we recapitulate the recent progress in the application of filamentous bacteria, yeasts, and fungi as microbial cell factories. Furthermore, emphasis on metabolic engineering strategies involved in cellular tolerance, metabolic engineering, and screening are discussed. Finally, we offer an outlook on advanced techniques for the engineering of filamentous bacteria, yeasts, and fungi.

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.

Optimization of l-malic acid production from acetate with Aspergillus oryzae DSM 1863 using a pH-coupled feeding strategy

Background

Malic acid, a dicarboxylic acid mainly used in the food industry, is currently produced from fossil resources. The utilization of low-cost substrates derived from biomass could render microbial processes economic. Such feedstocks, like lignocellulosic hydrolysates or condensates of fast pyrolysis, can contain high concentrations of acetic acid. Acetate is a suitable substrate for l-malic acid production with the filamentous fungus Aspergillus oryzae DSM 1863, but concentrations obtained so far are low. An advantage of this carbon source is that it can be used for pH control and simultaneous substrate supply in the form of acetic acid. In this study, we therefore aimed to enhance l-malate production from acetate with A. oryzae by applying a pH-coupled feeding strategy.

Results

In 2.5-L bioreactor fermentations, several feeding strategies were evaluated. Using a pH-coupled feed consisting of 10 M acetic acid, the malic acid concentration was increased about 5.3-fold compared to the batch process without pH control, resulting in a maximum titer of 29.53 ± 1.82 g/L after 264 h. However, it was not possible to keep both the pH and the substrate concentration constant during this fermentation. By using 10 M acetic acid set to a pH of 4.5, or with the repeated addition of NaOH, the substrate concentration could be maintained within a constant range, but these strategies did not prove beneficial as lower maximum titers and yields were obtained. Since cessation of malic acid production was observed in later fermentation stages despite carbon availability, a possible product inhibition was evaluated in shake flask cultivations. In these experiments, malate and succinate, which is a major by-product during malic acid production, were added at concentrations of up to 50 g/L, and it was found that A. oryzae is capable of organic acid production even at high product concentrations.

Conclusions

This study demonstrates that a suitable feeding strategy is necessary for efficient malic acid production from acetate. It illustrates the potential of acetate as carbon source for microbial production of the organic acid and provides useful insights which can serve as basis for further optimization.

Effect of process mode, nitrogen source and temperature on L-malic acid production with Aspergillus oryzae DSM 1863 using acetate as carbon source

Malic acid, mainly used as acidulant and taste enhancer in the food industry, is currently produced from fossil resources. In this study, microbial L-malate production with the filamentous fungus A. oryzae using the carbon source acetate was evaluated. Acetate is for example contained in biomass-derived substrates such as lignocellulosic hydrolysates and condensates of fast pyrolysis, thus avoiding competition with food production. Since research on malic acid synthesis from acetate is limited and reported productivities and yields are low, this work aimed to improve the process. First, different cultivation temperatures were tested. This parameter was found to affect the ratio between malic and succinic acid, which is the major by-product of organic acid production with A. oryzae. At 32°C, the malate share was highest (53.7 ± 1.6%), while it was lowest at 38°C (43.3 ± 1.1%) whereas succinate represented the main product (51.5 ± 1.0%). Besides the temperature, the type of nitrogen source was also found to affect malate synthesis as well as biomass production. In the pre-culture, the biomass concentration was increased by a factor of 3.4–3.9, and germination started earlier with the complex nitrogen sources yeast extract, casein hydrolysate and peptone compared to the defined nitrogen source (NH₄)₂SO₄. Especially with yeast extract, malate synthesis in the main culture was accelerated and the titer obtained after 48 h was about 2.6 times higher than that quantified with (NH₄)₂SO₄. To reduce substrate inhibition in acetate medium, fed-batch and repeated-batch processes were evaluated using (NH₄)₂SO₄ or yeast extract as nitrogen source. In the fed-batch process, the period of malate production was extended, and the maximum product concentration was increased to 11.49 ± 1.84 g/L with (NH₄)₂SO₄ and 12.08 ± 1.25 g/L with yeast extract. In the repeated-batch process, the total acid production was highest within the first 240 h of fermentation, but optimization is required to maintain high production rates in later cycles. The lessons learned in this study will help in the development of further process strategies to maximize malate production using acetate as alternative substrate to the commonly used glucose.

Effect of an inorganic nitrogen source (NH4)2SO4 on the production of welan gum from Sphingomonas sp. mutant obtained through UV-ARTP compound mutagenesis

As one of the most expensive extracellular polysaccharides, welan gum is widely used in biomedicine, food products, and petroleum because of its unique structure and excellent rheological properties. To reduce the cost of welan gum fermentation, together with (NH₄)₂SO₄, which served as the sole nitrogen source, a high-welan-gum-producing mutant, B-8, screened through UV-ARTP compound mutagenesis was used. Under optimum conditions (C:N ratio 25:1, sucrose 50 g/L, (NH₄)₂SO₄ 4 g/L, and adding 8 mM NaCl at 32 h fermentation), the yield of welan gum and sucrose conversion were 18.86 g/L and 0.38 g/g, respectively, which were 98.95% and 137.50% higher than those achieved with the parent strain FM01, respectively. After the same treatment process, IN-welan (obtained with (NH₄)₂SO₄) consumed less 95% ethanol, had higher molecular weight, and exhibited better rheological properties than ON-welan (obtained with beef extract). Transcriptome analysis revealed that (NH₄)₂SO₄ could affect the synthetic pathway and monosaccharide content of welan gum by increasing bacterial chemotaxis and the availability of key intermediates. The fermentation performance of Sphingomonas sp. mutants could further be improved by providing several target genes to the mutants through metabolic engineering.

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.

Efficient production of GlcNAc in an aqueous-organic system with a Chitinolyticbacter meiyuanensis SYBC-H1 mutant

OBJECTIVES

Shellfish waste is a primary source for making N-acetyl-D-glucosamine. Thus, establishing a high-efficiency and low-cost bioconversion method to produce N-acetyl-D-glucosamine directly from shellfish waste was promising.

RESULTS

A mutant C81 was obtained from Chitinolyticbacter meiyuanensis SYBC-H1 via ⁶⁰Co-γ irradiation. This mutant C81 showed the highest chitinase activity of 9.8 U/mL that was 85% higher than the parent strain. The mutant C81 exhibted improved antioxidant activities, including total antioxidant capacity, superoxide radical ability, and hydroxyl radical scavenging ability, compared to that of the parent strain. Four out of nine organic solvents increased the chitinase activity by 1.9%, 6.8%, 11.7%, and 15.8%, corresponding to methylbenzene, n-heptane, petroleum ether, and n-hexane, respectively. The biphase system composed of aqueous and hexane presented a five-fold reduction of cell viability compared to the control. Using a continuous fermentation bioconversion process, 4.2 g/L GlcNAc was produced from crayfish shell powder with a yield of 80% of the chitin content.

CONCLUSIONS

This study demonstrated that the mutant C81 is suitable for converting crayfish shell powder into GlcNAc in an aqueous-organic system.

Enhanced l-Malic Acid Production by Aspergillus oryzae DSM 1863 Using Repeated-Batch Cultivation

l-Malic acid is a C4-dicarboxylic acid and a potential key building block for a bio-based economy. At present, malic acid is synthesized petrochemically and its major market is the food and beverages industry. In future, malic acid might also serve as a building block for biopolymers or even replace the commodity chemical maleic anhydride. For a sustainable production of l-malic acid from renewable resources, the microbial synthesis by the mold Aspergillus oryzae is one possible route. As CO₂ fixation is involved in the biosynthesis, high yields are possible, and at the same time greenhouse gases can be reduced. In order to enhance the production potential of the wild-type strain Aspergillus oryzae DSM 1863, process characteristics were studied in shake flasks, comparing batch, fed-batch, and repeated-batch cultivations. In the batch process, a prolonged cultivation time led to malic acid consumption. Keeping carbon source concentration on a high level by pulsed feeding could prolong cell viability and cultivation time, however, did not result in significant higher product levels. In contrast, continuous malic acid production could be achieved over six exchange cycles and a total fermentation time of 19 days in repeated-batch cultivations. Up to 178 g/L l-malic acid was produced. The maximum productivity (0.90 ± 0.05 g/L/h) achieved in the repeated-batch cultivation had more than doubled than that achieved in the batch process and also the average productivity (0.42 ± 0.03 g/L/h for five exchange cycles and 16 days) was increased considerably. Further repeated-batch experiments confirmed a positive effect of regular calcium carbonate additions on pH stability and malic acid synthesis. Besides calcium carbonate, nitrogen supplementation proved to be essential for the prolonged malic acid production in repeated-batch. As prolonged malic acid production was only observed in cultivations with product removal, product inhibition seems to be the major limiting factor for malic acid production by the wild-type strain. This study provides a systematic comparison of different process strategies under consideration of major influencing factors and thereby delivers important insights into natural l-malic acid production.

Microbial Biosynthesis of L-Malic Acid and Related Metabolic Engineering Strategies: Advances and Prospects

Malic acid, a four-carbon dicarboxylic acid, is widely used in the food, chemical and medical industries. As an intermediate of the TCA cycle, malic acid is one of the most promising building block chemicals that can be produced from renewable sources. To date, chemical synthesis or enzymatic conversion of petrochemical feedstocks are still the dominant mode for malic acid production. However, with increasing concerns surrounding environmental issues in recent years, microbial fermentation for the production of L-malic acid was extensively explored as an eco-friendly production process. The rapid development of genetic engineering has resulted in some promising strains suitable for large-scale bio-based production of malic acid. This review offers a comprehensive overview of the most recent developments, including a spectrum of wild-type, mutant, laboratory-evolved and metabolically engineered microorganisms for malic acid production. The technological progress in the fermentative production of malic acid is presented. Metabolic engineering strategies for malic acid production in various microorganisms are particularly reviewed. Biosynthetic pathways, transport of malic acid, elimination of byproducts and enhancement of metabolic fluxes are discussed and compared as strategies for improving malic acid production, thus providing insights into the current state of malic acid production, as well as further research directions for more efficient and economical microbial malic acid production.

Enhancing L-malate production of Aspergillus oryzae by nitrogen regulation strategy

Regulating morphology engineering and fermentation of Aspergillus oryzae makes it possible to increase the titer of L-malate. However, the existing L-malate-producing strain has limited L-malate production capacity and the fermentation process is insufficiently mature, which cannot meet the needs of industrial L-malate production. To further increase the L-malate production capacity of A. oryzae, we screened out a mutant strain (FMME-S-38) that produced 79.8 g/L L-malate in 250-mL shake flasks, using a newly developed screening system based on colony morphology on the plate. We further compared the extracellular nitrogen (N1) and intracellular nitrogen (N2) contents of the control and mutant strain (FMME-S-38) to determine the relationship between the curve of nitrogen content (N1 and N2) and the L-malate titer. This correlation was then used to optimize the conditions for developing a novel nitrogen supply strategy (initial tryptone concentration of 6.5 g/L and feeding with 3 g/L tryptone at 24 h). Fermentation in a 7.5-L fermentor under the optimized conditions further increased the titer and productivity of L-malate to 143.3 g/L and 1.19 g/L/h, respectively, corresponding to 164.9 g/L and 1.14 g/L/h in a 30-L fermentor. This nitrogen regulation-based strategy cannot only enhance industrial-scale L-malate production but also has generalizability and the potential to increase the production of similar metabolites.Key Points• Construction of a new screening system based on colony morphology on the plate.• A novel nitrogen regulation strategy used to regulate the production of L-malate.• A nitrogen supply strategy used to maximize the production of L-malate.

Acetate as substrate for L-malic acid production with Aspergillus oryzae DSM 1863

BACKGROUND

Microbial malic acid production is currently not able to compete economically with well-established chemical processes using fossil resources. The utilization of inexpensive biomass-based substrates containing acetate could decrease production costs and promote the development of microbial processes. Acetate is a by-product in lignocellulosic hydrolysates and fast pyrolysis products or can be synthesized by acetogens during syngas fermentation. For the fermentation of these substrates, a robust microorganism with a high tolerance for biomass-derived inhibitors is required. Aspergillus oryzae is a suitable candidate due to its high tolerance and broad substrate spectrum. To pave the path towards microbial malic acid production, the potential of acetate as a carbon source for A. oryzae is evaluated in this study.

RESULTS

A broad acetate concentration range was tested both for growth and malic acid production with A. oryzae. Dry biomass concentration was highest for acetic acid concentrations of 40-55 g/L reaching values of about 1.1 g/L within 48 h. Morphological changes were observed depending on the acetate concentration, yielding a pellet-like morphology with low and a filamentous structure with high substrate concentrations. For malic acid production, 45 g/L acetic acid was ideal, resulting in a product concentration of 8.44 ± 0.42 g/L after 192 h. The addition of 5-15 g/L glucose to acetate medium proved beneficial by lowering the time point of maximum productivity and increasing malic acid yield. The side product spectrum of cultures with acetate, glucose, and cultures containing both substrates was compared, showing differences especially in the amount of oxalic, succinic, and citric acid produced. Furthermore, the presence of CaCO3, a pH regulator used for malate production with glucose, was found to be crucial also for malic acid production with acetate.

CONCLUSIONS

This study evaluates relevant aspects of malic acid production with A. oryzae using acetate as carbon source and demonstrates that it is a suitable substrate for biomass formation and acid synthesis. The insights provided here will be useful to further microbial malic acid production using renewable substrates.

Enhancement of Acid Protease Activity of Aspergillus oryzae Using Atmospheric and Room Temperature Plasma

Atmospheric and room temperature plasma (ARTP) system is a novel and efficient mutagenesis protocol for microbial breeding. In this study, ARTP was employed to treat spores of Aspergillus oryzae strain 3.042 for selection of high acid protease producers. With an irradiation time of 150 s at the lethal rate of 90%, 19 mutants with higher acid protease activity were initially selected based on different mutant colony morphology and ratio of the clarification halo of protease activity to the colony diameter. Measurements of the acid protease activity revealed that mutant strain B-2 is characterized by a steady hereditary stability with increased acid protease, neutral protease and total protease activities of 54.7, 17.3, and 8.5%, respectively, and decreased alkaline protease activity of 8.1%. In summary, the identified mutant strain B-2 exhibits great potential for the enhancement of the insufficient acid protease activity during the middle and later stages of soy sauce fermentation.