Production Of Organic Acids Research Articles

Recycling rice straw enhances the solubilisation and plant acquisition of soil phosphorus by altering rhizosphere environment of wheat

An in-situ, rapid, and low cost management of the huge amount of rice straw (RS) is a great challenge in south Asia. On the other hand, mobilisation of soil phosphorus (P) from inorganically fixed pools could improve its efficiency sustainably. To solve these problems concurrently this study was conducted to test whether the application of RS+P solubilizing microbes (PSMs) could solubilise a significant amount of the fixed P from soil and they could influence P transformation, silicon (Si) concentration, organic acid (OA) production, and enzyme activity in the rhizosphere to improve the yield of wheat. The specific objective of this study was to identify the sufficient amount of RS for solubilisation of fixed P to meet its demand to crops and develop a fertiliser application strategy accordingly. Application of RS (at 12 Mg ha−1)+PSM could solubilise ∼2.96–3.26 % of inorganically fixed P in soil depending on soil temperature. It reduced the hysteresis effect by 10 % over control. Oxalic acid, citric acid, formic acid, malic acid, and tartaric acid could be identified in the rhizosphere of wheat. Application of RS (at 12 Mg ha−1)+PSM+ 75%P enhanced the activities of dehydrogenase, alkaline phosphatase activity, cellulase, and peroxidase by ∼1.80, 1.79, 2.14, and 1.40 times over control in the wheat rhizosphere, respectively, at maturity stage. It also enhanced the plant available P by 21 % over mineral fertiliser application during wheat growth. It also boosted Si concentration by 61 % over control in rhizosphere. Application of RS (at 12 Mg ha−1)+PSM+ 75%P resulted in a 45 % greater grain yield of wheat than control. It also improved root length, volume, and P uptake by 1.74, 2.40, and 1.61 times respectively over control. Path analysis indicated that P solubilisation by Si and OAs significantly contributed to P availability in the rhizosphere of wheat. Hence, RS (at 12 Mg ha−1)+PSM+ 75%P of mineral P fertiliser could be recommended to the cultivators for saving 25% P fertiliser without affecting yield.

Increasing lysine level improved methanol assimilation toward butyric acid production in Butyribacterium methylotrophicum

BackgroundMethanol, a promising non-food fermentation substrate, has gained increasing interest as an alternative feedstock to sugars for the bio-based production of value-added chemicals. Butyribacterium methylotrophicum, one of methylotrophic-acetogenic bacterium, is a promising host to assimilate methanol coupled with CO2 fixation for the production of organic acids, such as butyric acid. Although the methanol utilization pathway has been identified in B. methylotrophicum, little knowledge was currently known about its regulatory targets, limiting the rational engineering to improve methanol utilization.ResultsIn this study, we found that methanol assimilation of B. methylotrophicum could be significantly improved when using corn steep liquor (CSL) as the co-substrate. The further investigation revealed that high level of lysine was responsible for enhanced methanol utilization. Through the transcriptome analysis, we proposed a potential mechanism by which lysine confers improved methylotrophy via modulating NikABCDE and FhuBCD transporters, both of which are involved in the uptake of cofactors essential for enzymes of methanol assimilation. The improved methylotrophy was also confirmed by overexpressing NikABCDE or FhuBCD operon. Finally, the de novo synthetic pathway of lysine was further engineered and the methanol utilization and butyric acid production of B. methylotrophicum were improved by 63.2% and 79.7%, respectively. After an optimization of cultivation medium, 3.69 g/L of butyric acid was finally achieved from methanol with a yield of 76.3%, the highest level reported to date.ConclusionThis study revealed a novel mechanism to regulate methanol assimilation by lysine in B. methylotrophicum and engineered it to improve methanol bioconversion to butyric acid, culminating in the synthesis of the highest butyric acid titer reported so far in B. methylotrophicum. What’s more, our work represents a further advancement in the engineering of methylotrophic-acetogenic bacterium to improve C1-compound utilization.Graphical

Denitrification efficiency, microbial communities and metabolic mechanisms of corn cob hydrolysate as denitrifying carbon source

In this study, the denitrification efficacy of corn cob hydrolysate (CCH) was compared and analyzed with that of glucose and acetate to determine its feasibility as an additional carbon source, and its metabolic mechanism as a denitrification carbon source was investigated in depth. By constructing a denitrification reactor, it was found that the TN removal rate exceeded 97% and the effluent COD remained below 70 mg/L during the stable operation with CCH as the carbon source, and the denitrification effect was comparable to that of the glucose stage (GS) and the acetate stage (AS). The analysis of the microbial community showed that the dominant phylum was Proteobacteria and Bacteroidota, where the abundance of Bacteroidota in the hydrolysate stage (HS) (24.37%) was significantly higher than that of GS (4.89%) and AS (11.93%). And the analysis at the genus level showed the presence of a large number of genera of organic matter hydrolysis and acid production in HS that were almost absent in other stages, such as Paludibacter (12.83%), Gracilibacteria (4.27%), f__Prolixibacteraceae_Unclassified (2.94%). In addition, the higher fatty acid metabolism and lower sugar metabolism of HS during carbon metabolism were similar to the ratio of AS, suggesting that CCH was mainly fermented to acids and then involved in the tricarboxylic acid (TCA) cycle. During nitrogen metabolism, the high relative abundance of narG, nirS, and nosZ ensured the denitrification process. The results of this study were expected to provide a theoretical basis and data support for promoting denitrification from novel carbon sources.

The responses of organic acid production and microbial community to different carbon source additions during the anaerobic fermentation of Chinese cabbage waste

The effects of glucose, fructose, sucrose and molasses on organic acid levels, protein degradation, nutrient preservation and bacteriome were studied during the anaerobic fermentation of Chinese cabbage waste. The results showed that fructose and molasses additions caused a significant (p < 0.05) increase in lactic acid production (82.16–89.79 %), acetic acid production (175.41–196.93 %), ammonia nitrogen formation (15.93–37.43 %) and reduction of neutral detergent fiber level (8.17–15.87 %). However, few positive effects of glucose and sucrose additions were found on organic acid production. Furthermore, carbon source additions enriched (p < 0.05) the acid-producing bacteria, such as Lactobacillus paralimentarius and Lactobacillus heilongjiangensis, upregulated (p < 0.05) the pathways of carbohydrate and lipid metabolisms and reduced (p < 0.05) the abundances of Lactobacillus buchneri and Escherichia coli and bacteria that were mobile elements-contained and stress-tolerant. Collectively, fructose and molasses additions enhanced the recycling of Chinese cabbage waste by anaerobic fermentation, in which the desired products are organic acids.

Nematophagous Fungi: A Review of Their Phosphorus Solubilization Potential.

Nematophagous fungi (NF) are a group of diverse fungal genera that benefit plants. The aim of this review is to increase comprehension about the importance of nematophagous fungi and their role in phosphorus solubilization to favor its uptake in agricultural ecosystems. They use different mechanisms, such as acidification in the medium, organic acids production, and the secretion of enzymes and metabolites that promote the bioavailability of phosphorus for plants. This study summarizes the processes of solubilization, in addition to the mechanisms of action and use of NF on crops, evidencing the need to include innovative alternatives for the implementation of microbial resources in management plans. In addition, it provides information to help understand the effect of NF to make phosphorus available for plants, showing how these biological means promote phosphorus uptake, thus improving productivity and yield.

Enhancing methane recovery by intermittent substrate feeding and microbial community response in anaerobic digestion of glycerol

Accumulation of intermediate volatile fatty acids, especially propionic acid, is a limiting factor in anaerobic digestion of glycerol. To mitigate this problem, intermittent substrate feeding was investigated and evaluated. The results showed that intermittent substrate feeding could decrease the effects of high concentrations of propionic acid and improve the performance of CH4 production at high substrate loading rates. CH4 production rate with intermittent substrate feeding (1.60 NL/L/day) was higher than single substrate feeding (1.47 NL/L/day) at 5.0 g COD/L/day of glycerol loading rate. NGS and PICRUSt were applied to monitor the changes in the microbial community. The proportion of dominant bacteria changed with alterations in glycerol loading rate and substrate feeding technique. NGS results elucidated three main bacterial groups that degraded and competed for glycerol, namely: Delftia, Trichococcus, and Desulfuromonas. Trichococcus and Desulfuromonas tended to withstand a higher rate of glycerol loading than Delftia. PICRUSt results demonstrated that Trichococcus is paramount in the glycerol degradation. Delftia and Desulfuromonas might compete with methanogens by utilizing acetate and/or H2. This study suggested the role of microbial communities in CH4 production from glycerol. The intermittent feeding, which balanced organic acid production and its consumption, was clarified to improve methane production from glycerol.

Heterogeneous photocatalysis for biomass valorization to organic acids

This review outlines recent progress in the photocatalytic conversion of biomass into organic acids with an emphasis on the mechanisms involved and strategies for the rational design/regulation of photocatalytic systems to enhance performance.

Deciphering microbial community dynamics along the fermentation course of soy sauce under different temperatures using metagenomic analysis.

Fermented soy sauce consists of microorganisms that exert beneficial effects. However, the microbial community dynamics during the fermentation course is poorly characterized. Soy sauce production is classified into the stages of mash fermentation with koji (S0), brine addition (S1), microbial transformation (S2), flavor creation (S3), and fermentation completion (S4). In this study, microbial succession was investigated across stages at different temperatures using metagenomics analyses. During mash fermentation, Aspergillus dominated the fungal microbiota in all stages, while the bacterial composition was dominated by Bacillus at room temperature and by a diverse composition of enriched lactic acid bacteria (LAB) at a controlled temperature. Compared with a stable fungal composition, bacterial dynamics were mostly attributable to fluctuations of LAB, which break down carbohydrates into lactic acid. After adding brine, increased levels of Enterococcus and decreased levels of Lactococcus from S1 to S4 may reflect differences in salinity tolerance. Staphylococcus, as a fermentation starter at S0, stayed predominant throughout fermentation and hydrolyzed soybean proteins. Meanwhile, Rhizopus and Penicillium may improve the flavor. The acidification of soy sauce was likely attributable to production of organic acids by Bacillus and LAB under room temperature and controlled temperature conditions, respectively. Metagenomic analysis revealed that microbial succession was associated with the fermentation efficiency and flavor enhancement. Controlled temperature nurture more LAB than uncontrolled temperatures and may ensure the production of lactic acid for the development of soy sauce flavor.

Unveiling the Antibiotic Susceptibility and Antimicrobial Potential of Bacteria from Human Breast Milk of Pakistani Women: An Exploratory Study.

Human life quality and expectancy have increased dramatically over the past 5 decades because of improvements in nutrition and antibiotic's usage fighting against infectious diseases. Yet, it was soon revealed that the microbes adapted to develop resistance to any of the drugs that were used. Recently, there is great concern that commensal bacteria from food and the gastrointestinal tract of humans and animals could act as a reservoir for antibiotic resistance genes. Methodology. This study was intended for evaluating the phenotypic antibiotic resistance/sensitivity profiles of probiotic bacteria from human breast milk and evaluating the inhibitory effect of the probiotic bacteria against both Gram-negative and Gram-positive bacteria. The results point out that some of the isolated bacteria were resistant to diverse antibiotics including gentamycin, imipenem, trimethoprim sulfamethoxazole, and nalidixic acid. Susceptibility profile to certain antibiotics like vancomycin, tetracycline, ofloxacin, chloramphenicol, streptomycin, rifampicin, and bacitracin was also observed. The antimicrobial qualities of cell-free supernatants of some probiotic bacteria inhibited the growth of indicator bacteria. Also, antimicrobial properties of the probiotic bacteria from the present study attributed to the production of organic acid, bacterial adhesion to hydrocarbons (BATH), salt aggregation, coaggregation with pathogens, and bacteriocin production. Some isolated bacteria from human milk displayed higher hydrophobicity in addition to intrinsic probiotic properties like Gram-positive classification, catalase-negative activity, resistance to gastric juice (pH 2), and bile salt (0.3%) concentration. This study has added to the data of the antibiotic and antimicrobial activity of some probiotic bacteria from some samples of Pakistani women breast milk. Probiotic bacteria are usually considered to decrease gastrointestinal tract diseases by adhering to the gut epithelial and reducing population of pathogens and in the case of Streptococcus lactarius MB622 and Streptococcus salivarius MB620 in terms of hydrophobicity and exclusion of indicator pathogenic strains.

A sustainable solar-driven electrochemical process for reforming lignocellulosic biomass effluent into high value-added products: green hydrogen, carboxylic and vanillic acids

Energizing the future with lignocellulosic biomass by electrochemical production of organic acids and green H2 production.

Case Study of Continuous Itaconic Acid Fermentation by Aspergillus Terreus in a Bench-Scale Bioreactor

Itaconic acid (IA) is a highly important bioproduct. The traditional biotechnological production of organic acids generates large quantities of biomass and water streams as waste. Bipolar membrane electrodialysis is a suitable technique for directly recovering IA from clarified fermentation broth whereby itaconic acid is separated without chemical loading. As a result, the remaining diluate stream can be reintroduced into the fermentation process. This membrane operation can be integrated into a continuous fermentation process in order to carry out the process using a high glucose concentration in the effluent, opening up new possibilities for the implementation of continuous itaconic acid fermentation. In this study, a possible highly successful implementation of this technique is presented. The applied gentle stirring technique as well as the low dilution rate of 0.007 s-1 is necessary to be complemented by highly efficient oxygenation, thereby promoting itaconic acid generation. With an inlet supplying pure oxygen gas, the acid accumulated and an average of 30.1 g·L-1 titer of IA was achieved in the effluent. The volumetric oxygen uptake rate was monitored during the fermentation which fluctuated from 40-88% so further investigation of this may be worthwhile in the future.

Phosphate Solubilizing Bacteria (PSB) Isolation from Tomato Rhizospheres at Koka District, Ethiopia

Phosphorous is an essential macronutrient for plant growth and development. Most of the soil is deficient in plant available phosphorous and due to economic limitations majority of Ethiopian farmers applied inadequate fertilizers. It is essential to stabilize a mechanism to access P for plants with an efficient, cheap, and eco-friendly approach for enhanced crop growth and production. The main objective of this study was to screen efficient phosphate solubilizing bacteria from tomato rhizosphere soil. Using halo zone formation on PVK agar medium, more than 400 PSB isolates were isolated from 13 rhizosphere soil samples. By evaluating SI, texture in the culture-re-culturing process, liquid medium pH change efficiency and growth rate, upmost three promising PSB isolates (K-1-29, K-10-27, and K-10-41) were selected. Incubation of the isolates in PVK broth for five days showed significant pH reduction. For instance, isolate K-10-41 showed significant pH change (4.02) which indicates the organic acid production. Isolation and evaluation of efficient phosphate solubilizing bacteria maintain soil fertility, promote plant growth, induce plant response to pathogens, reduce agrochemical consumption and promote sustainable agriculture. Therefore, these selected PSB isolates need further detailed study for taxonomic identification, plant growth promotion, host range, and phytopathogen response. Local isolation improves environmental adaptation and indigenous competition.

An efficient marker-free genome editing method for Aspergillus niger

Aspergillus niger is an important industrial strain which has been widely used for production of enzymes and organic acids. Genome modification of A. niger is required to further improve its potential for industrial production. CRISPR/Cas9 is a widely used genome editing technique for A. niger, but its application in industrial strains modification is hampered by the need for integration of a selection marker into the genome or low gene editing efficiency. Here we report a highly efficient marker-free genome editing method for A. niger based on CRISPR/Cas9 technique. Firstly, we constructed a co-expression plasmid of sgRNA and Cas9 with a replication initiation region fragment AMA1 (autonomously maintained in Aspergillus) by using 5S rRNA promoter which improved sgRNA expression. Meanwhile, a strain deficient in non-homologous end-joining (NHEJ) was developed by knocking out the kusA gene. Finally, we took advantage of the instability of plasmid containing AMA1 fragment to cure the co-expression plasmid containing sgRNA and Cas9 through passaging on non-selective plate. With this method, the efficiency of gene editing reached 100% when using maker-free donor DNA with a short homologous arm of 20 bp. This method may facilitate investigation of gene functions and construction of cell factories for A. niger.

Fungal biodiversity and interaction complexity were the important drivers of multifunctionality for flavor production in a spontaneously fermented vinegar

Zhejiang rosy vinegar (ZRV) is produced by spontaneous fermentation. Compared with the microbial community of the vinegar produced by solid-state fermentation, fungi play more important roles in the fermentation of ZRV. In this study, the composition of the fungal community was determined. Aspergillus (average relative abundance: 33.1%), Thermoascus (18.1%), and Candida (15.3%) dominated the fermentation process. Acetic acid and ethanol drove fungal community succession during 40–90 d, while temperature and lactic acid were the drivers during 70–90 d. Keeping low pH and temperature at the initial stage of fermentation was beneficial to improve the quality of ZRV. Multiple members of Rhizomucor, Pichia, and Wickerhamomyces were network hubs during 40–90 d, and they cooperated to ferment starch into ethanol. Moreover, fungal biodiversity and interaction complexity fueled the multifunctionality of organic acid production. These results guided the targeted control of ZRV fermentation to improve the quality of ZRV. Industrial relevanceThese combined findings represent a significant milestone in the understanding of the dynamic profiles of the fungal community, which can add valuable insights to the ZRV manufacturer. Our findings guide the control of fungal community succession during ZRV fermentation to improve the quality.

Increasing the Bioavailability of Phosphate by Using Microorganisms

Phosphorous (P) is a nonrenewable and one of the most important macronutrients for all living organisms. The formation of complexes with cations such as Al, Fe, and Ca reduces the solubility of P leading to limiting the absorption of P by plants. Therefore, we need to apply excessive amounts of P through conventional fertilizers. However, plants can use only a small portion of P of these added fertilizers whenever those become unavailable. Therefore, utilizing excess amounts of phosphate as fertilizers can lead to various environmental issues like eutrophication. Phosphate-solubilizing microorganisms (PSM) have the ability to solubilize soil phosphate through the production of organic acids, inorganic acids, enzymes, protons, siderophores, and exopolysaccharides resulting in the absorption of P by plants. The application of PSM has the potential to be used as an efficient, eco-friendly, and sustainable approach that can replace traditional fertilizers. This review aimed to give an overview of the diversity of PSM, methods of P solubilization, current trends, and technological advances that can assist in using PSM to achieve Sustainable Development Goals (SDGs).

Removal of orange G dye by Aspergillus niger and its effect on organic acid production

Azo dyes have been found in wastewater from textile industries. These compounds continuously persist in the environment for long periods of time and may be toxic for living beings. An alternative treatment for dye removal that has proven to be effective is aerobic treatment with fungi. In this study, Aspergillus niger was investigated as a mechanism to remove orange G (OG). Removal of 200 mg/L of OG by A. niger biomass was carried out in solid and liquid medium, which showed a positive correlation between A. niger growth and dye removal. In liquid media what was proved is that the efficiency of OG removal by A. niger depends on its concentration; at 200 mg/L of OG remove by degradation and at 400 mg/L by processes as sorption and degradation. During OG removal, the generation of organic acids by A. niger was modified compared to constitutive generation, one of the modifications was the increase of gluconic acid production and the decrease of acids involved in the Krebs cycle, as well as the null detection of oxalic acid. The monitoring of organic acids by high-performance liquid chromatography (HPLC) was important because some of them have been linked to dye removal.

Diurnal Variation of Epiphytic Microbiota: an Unignorable Factor Affecting the Anaerobic Fermentation Characteristics of Sorghum-Sudangrass Hybrid Silage.

Forage epiphytic microbiota exhibits pronounced changes in composition and function throughout the day. However, the effects of these changes on silage fermentation are rarely explored. Here, we transplanted the epiphytic microbiota of sorghum-sudangrass hybrid (SSG) harvested at 7:00h (AM), 12:00h (M), and 17:00h (PM) to sterilized SSG to evaluate the effects of diurnal variation of epiphytic microbiota on fermentation characteristics. During fermentation, remarkable differences in bacterial community successions were observed between silages inoculated with AM and M microbiota. Compared to AM microbiota, M microbiota inoculation increased the proportions of Pantoea dispersa, Leuconostoc lactis, Enterobacter, and Klebsiella variicola, whereas it decreased the proportions of Weissella cibaria and Lactobacillus plantarum during fermentation. This led to the most rapid pH declines and organic acid production in AM silage and the slowest in M silage. Both M and PM microbiota affected the bacterial cooccurrence patterns, indicated by decreased complexity and stability in the community structures of M and PM silages compared to that of AM silage. The predicted functions indicated that some key carbohydrate metabolism pathways related to lactic acid synthesis were downregulated, while some competing pathways (ascorbate and aldarate metabolism and C5-branched dibasic acid metabolism) were upregulated in M silage compared to AM silage after 3days of fermentation. Correlation analysis revealed positive correlations between competing pathways and enterobacterial species. The current study highlights the importance of diurnal variation of epiphytic microbiota in affecting the silage bacterial community, potentially providing an effective strategy to improve silage quality by optimizing harvest time. IMPORTANCE Ensiling is a way to preserve wet biomass for animal and bioenergy production worldwide. The fermentation quality of silage is largely dependent on the epiphytic microbiota of the material. Plant epiphytic microbiota exhibit diurnal changes in composition and function. However, the effects of these changes on silage fermentation are rarely explored. The results presented here demonstrated that diurnal variation of epiphytic microbiota could affect the fermentation characteristics and bacterial community during SSG fermentation. Marked bacterial community differences were observed between AM and M silages during the initial 3days of fermentation. The dominance rate of Lactobacillus plantarum was highest in AM silage, whereas enterobacterial species were more abundant in M silage. The predicted function revealed downregulated lactic acid synthesis pathways and upregulated competing pathways in M silage compared to those in AM silage. This study provides clues for technological-parameter optimization of the fermentation process by the selection of harvest time.

Modeling of Growth and Organic Acid Kinetics and Evolution of the Protein Profile and Amino Acid Content during Lactiplantibacillus plantarum ITM21B Fermentation in Liquid Sourdough.

The application of mathematical modeling to study and characterize lactic acid bacterial strains with pro-technological and functional features has gained attention in recent years to solve the problems relevant to the variabilities of the fermentation processes of sourdough. Since the key factors contributing to the sourdough quality are relevant to the starter strain growth and its metabolic activity, in this study, the cardinal growth parameters for pH, temperature (T), water activity (aw), and undissociated lactic acid of the sourdough strain Lactiplantibacillus plantarum ITM21B, were determined. The strain growth, pH, organic acids (lactic, acetic, phenyllactic, and hydroxy-phenyllactic), total free amino acids, and proteins were monitored during fermentation of a liquid sourdough based on wheat flour and gluten (Bio21B) after changing the starting T, pH, and inoculum load. Results demonstrated that the different fermentation conditions affected the strain growth and metabolite pattern. The organic acid production and growth performance were modeled in Bio21B, and the resulting predictive model allowed us to simulate in silico the strain performances in liquid sourdough under different scenarios. This mathematical predictive approach can be useful to optimize the fermentation conditions needed to obtain the suitable nutritional and technological characteristics of the L. plantarum ITM21B liquid sourdough.

Galactitol Transport Factor GatA Relieves ATP Supply Restriction to Enhance Acid Tolerance of Escherichia coli in the Two-Stage Fermentation Production of D-Lactate

Escherichia coli is a major contributor to the industrial production of organic acids, but its production capacity and cost are limited by its acid sensitivity. Enhancing acid resistance in E. coli is essential for improving cell performance and production value. Here, we propose a feasible strategy for improving cellular acid tolerance by reducing ATP supply restriction. Transcriptome assays of acid-tolerant evolved strains revealed that the galactitol phosphotransferase system transporter protein GatA is an acid-tolerance factor that assists E. coli in improving its resistance to a variety of organic acids. Enhanced GatA expression increased cell survival under conditions of lethal stress due to D-lactic acid, itaconic acid and succinic acid by 101.8-fold, 29.4-fold and 41.6-fold, respectively. In addition, fermentation patterns for aerobic growth and oxygen-limited production of D-lactic acid were identified, and suitable transition and induction stages were evaluated. GatA effectively compensated for the lack of cellular energy during oxygen limitation and enabled the D-lactic acid producing strain to exhibit more sustainable productivity in acidic fermentation environments with a 55.7% increase in D-lactic acid titer from 9.5 g·L−1 to 14.8 g·L−1 and reduced generation of by-product. Thus, this study developed a method to improve the acid resistance of E. coli cells by compensating for the energy gap without affecting normal cell metabolism while reducing the cost of organic acid production.

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

BackgroundMalic 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.ResultsIn 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.ConclusionsThis 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.