Lipid Accumulation by Xylose Metabolism Engineered Mucor circinelloides Strains on Corn Straw Hydrolysate

Production of polyunsaturated fatty acids in pork backfat fermented by Mucor circinelloides

Pork backfat (PB) contains excessive saturated fatty acids (SFAs), but lacks polyunsaturated fatty acids (PUFAs). Excessive SFAs can be used as a substrate for the growth of certain microorganisms that convert them into PUFAs and monounsaturated fatty acids (MUFAs), and the added value of PB can be enhanced. In this study, Mucor circinelloides CBS 277.49 and Lactiplantacillus plantarum CGMCC 24189 were co-cultured for conversion of PB into fermented pork backfat (FPB) with high level of PUFAs. Our results showed that the content of γ-linolenic acid (GLA) and linoleic acid (LA) in the surface of FPB reached 9.04 ± 0.14 mg/g and 107.31 ± 5.16 mg/g for 7-day fermentation, respectively. To convert the internal SFAs of PB, ultrasound combined with papain was used to promote the penetrative growth of M. circinelloides into the internal PB, and the GLA level in the third layer of fat reached 2.58 ± 0.31 mg/g FPB. The internal growth of M. circinelloides in PB was promoted by adjusting the oxygen rate and ventilation rate through the wind velocity sensor. When the oxygen rate is 2 m/s and the ventilation rate is 18 m3/h, the GLA level in the third layer of fat reached 4.13 ± 1.01 mg/g FPB. To further improve the level of PUFAs in PB, FPB was produced by M. circinelloides at 18 °C. The GLA content on the surface of FPB reached 15.73 ± 1.13 mg/g FPB, and the GLA yield in the second and third layers of fat reached 8.68 ± 1.77 mg/g FPB and 6.13 ± 1.28 mg/g FPB, the LA yield in the second and third layers of fat reached 105.45 ± 5.01 mg/g FPB and 98.46 ± 4.14 mg/g FPB, respectively. These results suggested that excessive SFAs in PB can be converted into PUFAs and provided a new technique for improving PUFAs in FPB. KEY POINTS: • This article achieved the conversion of PUFAs in pork backfat by Mucor circinelloides CBS 277.49 and Lactiplantacillus plantarum CGMCC 24189. • This article solved the internal growth of M. circinelloides CBS277.49 in pork backfat by ultrasound combined with papain. • This article proposed an innovative of promoting the internal growth of M. circinelloides and increasing the PUFAs production by oxygen ventilation in pork backfat.

Influence of Borassus flabellifer Endocarps Hydrolysate on Fungal Biomass and Fatty Acids Production by the Marine Fungus Aspergillus sp

Polyunsaturated Fatty Acids (PUFAs) are important nutrients for human health. We aimed to evaluate the efficiency of marine water fungus Aspergillus sp. (Accession no: MZ505709) for lipid biosynthesis. The Yeast Extract Glucose (YEG) medium was supplemented with different concentration of Borassus flabellifer Endocarps Hydrolysate (BFEH; 1-5%) to evaluate the fungal biomass and its lipid accumulation. The combination of glucose and BFEH as carbon source increased the fresh weight (25.43 ± 0.33 g/L), dry weight (21.39 ± 0.77 g/L) and lipid yield (3.14 ± 0.09 g/L) of fungal biomass. The lipid content of dried fungal biomass has shown 91.08 ± 5.07 mg cod liver oil equivalents/g and 125.98 ± 5.96 mg groundnut oil equivalents/g biomass. GC-MS and NMR spectrometry analysis revealed the compounds involved in fatty acid metabolism and lipid signaling pathways along with the presence of linolenic acid. Interestingly, fungus grown in BFEH enriched medium has recorded the maximum amount of lipids with major fatty acid derivatives. Increase in the growth rate of Artemia franciscana was observed, when the extracted fungal lipid was supplemented as a food supplement. Therefore, this study suggests that marine fungal lipid may serve as potential natural compound as nutraceuticals and aquafeeds.

Lipid Accumulation by Snf-β Engineered Mucor circinelloides Strains on Glucose and Xylose

Sucrose non-fermenting 1 (SNF1) protein kinase plays the regulatory roles in the utilization of selective carbon sources and lipid metabolism. Previously, the role of β subunit of SNF1 in lipid accumulation was evaluated by overexpression and knockout of Snf-β in oleaginous fungus M. circinelloides. In the present study, the growth and lipid accumulation of Snf-β overexpression and knockout strains were further analyzed and compared with glucose or xylose as a single or mixed carbon sources. The results showed that the lipid contents in Snf-β knockout strain improved by 23.2% (for glucose), 28.4% (for xylose), and 30.5% (for mixed glucose and xylose) compared with that of the control strain, respectively. The deletion of Snf-β subunit also altered the transcriptional level of acetyl-CoA carboxylase (ACC). The highest transcriptional levels of ACC1 in Snf-β knockout strain at 24 h were increased by 2.4-fold (for glucose), 2.8-fold (for xylose), and 3.1-fold (for mixed glucose and xylose) compared with that of the control strain, respectively. Our results indicated that Snf-β subunit enhanced lipid accumulation through the regulation of ACC1 in response to xylose or mixed sugars of glucose and xylose more significantly than that of response to glucose. This is the first study to explore the effect of Snf-β subunit of M. circinelloides in regulating lipid accumulation responding to different carbon nutrient signals of glucose and xylose. This study provides a foundation for the future application of the Snf-β engineered strains in lipid production from lignocellulose.

Lipid accumulation and SNF1 transcriptional analysis of Mucor circinelloides on xylose under nitrogen limitation

Sucrose non-fermenting 1 (SNF1) plays a crucial role in utilizing non-glucose carbon sources and regulating lipid metabolism. However, the mechanism by which SNF1 regulates lipid accumulation in oleaginous filamentous fungi in response to nutrient signals remains unclear. In the present study, by analysing the growth and lipid accumulation of M. circinelloides on xylose under nitrogen limitation, combined with the transcriptional changes of each subunit of SNF1, the regulation of SNF1 between nutrient signal and lipid accumulation was explored. The results showed that with the increase of carbon/nitrogen (C/N) ratio, the xylose consumption and cell growth of M. circinelloides decreased, and the lipid accumulation increased gradually. The optimal C/N ratio was 160:1, and the maximum lipid yield was 4.1 g/L. Two subunits of SNF1, Snf-α1 and Snf-β, are related to the regulation of lipid metabolism in response to nutrient signals from different external nitrogen sources. This is the first report on the transcriptional analysis of SNF1 subunits on xylose metabolism under nitrogen limitation. This study provides a basis for further understanding of lipid synthesis mechanism on xylose in oleaginous fungi, and it also lays a foundation for the genetic engineering of high-lipid strain.

Effect of Different Carbons on Lipid Production and SNF1 Transcription in Mucor Circinelloides

Sucrose non-fermenting 1 (SNF1) protein kinase plays an important role in the utilization of selective carbon sources and regulation of lipid metabolism. In order to further explore the function of SNF1 in regulating lipid accumulation by responding nutritional signals from non-glucose carbon sources, in the present study, the lipid production and SNF1 transcriptional levels of Mucor circinelloides were analyzed and compared on different carbon sources. The results indicated that M. circinelloides could effectively utilize some secondary metabolic carbon sources of monosaccharides and disaccharides for growth and lipids production, such as fructose, maltose and galactose. Snf-β subunit was associated with the regulation of lipid metabolism in response to nutritional signals from different carbon sources. This is the first report on the transcriptional analysis of SNF1 subunits on different carbons metabolism in oleaginous filamentous fungi. This research has suggested that genetic engineering of SNF1 subunits will alter the lipid production of M. circinelloides from alternative carbon sources.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12088-023-01070-z.

Homologous and Heterologous Expression of Delta(12)-Desaturase in Mucor circinelloides Enhanced the Production of Linolenic Acid

Linolenic acid (LA) is gaining more interest within the scientific community. This is because it has a potential medical role in reducing the risk of inflammation, carcinogenesis, atherosclerosis and diabetes and is a valuable nutraceutical for human health. The oleaginous fungus Mucor circinelloides produces a high lipid content (36%), including valuable polyunsaturated fatty acids (PUFAs). However, the critical step in which oleic acid (OA) is converted into LA is not efficient at supplying enough substrates for PUFA synthesis. Hence, we propose a method to increase LA production based on genetic engineering. The overexpression of the Δ12-desaturase gene from M. circinelloides and Mortierella alpina increased the LA content and improved the lipid accumulation (from 14.9% to 21.6% in the Δ12-desaturase gene of the M. circinelloides overexpressing strain (Mc-D12MC) and from 14.9% to 18.7% in the Δ12-desaturase gene of M. alpina overexpressing strain (Mc-D12MA)). Additionally, the up-regulated expression levels of these genes targeted the genes involved in NADPH production, implying that the elevated Δ12-desaturase gene may function as a critical regulator of NADPH and lipid synthesis in M. circinelloides. This study provides the first evidence to support the design of metabolic engineering related to LA and PUFA production in M. circinelloides for potential industrial applications.

Production, Biosynthesis, and Commercial Applications of Fatty Acids From Oleaginous Fungi

Oleaginous fungi (including fungus-like protists) are attractive in lipid production due to their short growth cycle, large biomass and high yield of lipids. Some typical oleaginous fungi including Galactomyces geotrichum, Thraustochytrids, Mortierella isabellina, and Mucor circinelloides, have been well studied for the ability to accumulate fatty acids with commercial application. Here, we review recent progress toward fermentation, extraction, of fungal fatty acids. To reduce cost of the fatty acids, fatty acid productions from raw materials were also summarized. Then, the synthesis mechanism of fatty acids was introduced. We also review recent studies of the metabolic engineering strategies have been developed as efficient tools in oleaginous fungi to overcome the biochemical limit and to improve production efficiency of the special fatty acids. It also can be predictable that metabolic engineering can further enhance biosynthesis of fatty acids and change the storage mode of fatty acids.

Improved γ-Linolenic Acid Production from Cellulose in Mucor circinelloides via Coexpression of Cellobiohydrolase and Delta-6 Desaturase

The present study was aimed at facilitating the production of γ-linolenic acid (GLA) from the cellulosic substrate with the engineered oleaginous fungus Mucor circinelloides WJ11. Here, the homologous recombination technology was used to overexpress the cellobiohydrolase (CBH2) derived from Trichoderma longibrachiatum and the original delta-6 fatty acid desaturase (D6) in M. circinelloides to construct genetically engineered strains capable of effectively using cellulose to enhance GLA synthesis. When cultivated in modified K&R medium supplemented with microcrystalline cellulose, the CBH2 and D6 coexpressing strains led to increases in the biomass (up to 12.8 g/L) and lipid yield (up to 3.7 g/L) of 87% and 2.4-fold, respectively, compared to that of the control strain. Notably, when CBH2 and D6 were coexpressed in M. circinelloides, the yield of GLA reached 608 mg/L, which was a dramatic increase of 3.9-fold compared to that of the control strain. This is the first report on promoting the GLA production from the cellulosic substrate via coexpression of CBH2 and delta-6 desaturase. This work provides a theoretical basis for efficient transformation from the cellulosic substrate to functional GLA by CBH2 and D6 coexpressing strains, which might play a positive role in promoting the sustainable development of biological industry.

Mucoromycota fungi as powerful cell factories for modern biorefinery

Biorefinery employing fungi can be a strategy for valorizing low-cost rest materials, by-products and wastes into several valuable bioproducts through the fungal fermentation. Mucoromycota fungi are soil fungi with a highly versatile metabolic system that positions them as powerful microbial cell factories for biorefinery applications. Lipids, pigments, chitin/chitosan, polyphosphates, ethanol, organic acids and enzymes are main Mucoromycota products that can be refined from the fermentation process and applied in nutrition, chemical or biofuel industries. In addition, Mucoromycota biomass can be used as it is for specific purposes, such as feed. Mucoromycota fungi can be employed in developing co-production processes, whereby several intra- and extracellular products are simultaneously formed in a single fermentation process, and, thus, economic viability of the process can be improved. This mini review provides a comprehensive overview over the recent advances in the production of valuable metabolites by Mucoromycota fungi and fermentation strategies which could be potentially applied in the industrial biorefinery settings. KEY POINTS: • Biorefineries utilizing Mucoromycota fungi as production cell factories can provide a wide range of bioproducts. • Mucoromycota fungi are able to perform co-production of various metabolites in a single fermentation process. • Versatile metabolism of Mucoromycota allows valorization of a various low-cost substrates such as wastes and rest materials.

Improved glucose and xylose co-utilization by overexpression of xylose isomerase and/or xylulokinase genes in oleaginous fungus Mucor circinelloides

Most of the oleaginous microorganisms cannot assimilate xylose in the presence of glucose, which is the major bottleneck in the bioconversion of lignocellulose to biodiesel. Our present study revealed that overexpression of xylose isomerase (XI) gene xylA or xylulokinase (XK) gene xks1 increased the xylose consumption by 25 to 37% and enhanced the lipid content by 8 to 28% during co-fermentation of glucose and xylose. In xylA overexpressing strain Mc-XI, the activity of XI was 1.8-fold higher and the mRNA level of xylA at 24 h and 48 h was 11- and 13-fold higher than that of the control, respectively. In xks1 overexpressing strain Mc-XK, the mRNA level of xks1 was 4- to 11-fold of that of the control strain and the highest XK activity of 950 nmol min^-1 mg^-1 at 72 h which was 2-fold higher than that of the control. Additionally, expression of a translational fusion of xylA and xks1 further enhanced the xylose utilization rate by 45%. Our results indicated that overexpression of xylA and/or xks1 is a promising strategy to improve the xylose and glucose co-utilization, alleviate the glucose repression, and produce lipid from lignocellulosic biomass in the oleaginous fungus M. circinelloides. KEY POINTS: • Overexpressing xylA or xks1 increased the xylose consumption and the lipid content. • The xylose isomerase activity and the xylA mRNA level were enhanced in strain Mc-XI. • Co-expression of xylA and xks1 further enhanced the xylose utilization rate by 45%.

Enhanced lipid production by addition of malic acid in fermentation of recombinant Mucor circinelloides Mc-MT-2

In our previous work, we reported a novel approach for increasing lipid production in an oleaginous fungus Mucor circinelloides by overexpression of mitochondrial malate transporter protein. This transporter plays a vital role in fatty acid biosynthesis during malate and citrate transport systems in oleaginous fungi. In this study, the controlling metabolic supplementation strategy was used to improve the lipid production by overexpression of malate transporter protein in M. circinelloides strain coded as Mc-MT-2. The effects of different metabolic intermediates on lipid production in batch fermentation by Mc-MT-2 were investigated. The optimal lipid production was obtained at 0.8% malic acid after 24 h of fermentation. Furthermore, in fed-batch bioreactors containing glucose as a carbon source supplemented with malic acid, the highest cell growth, and lipid production were achieved. The resulting strain showed the fungal dry biomass of 16 g/L, a lipid content of 32%, lipid yield of 5.12 g/L in a controlled bench-top bioreactor, with 1.60-, 1.60- and 2.56-fold improvement, respectively, compared with the batch control without supplementation of malic acid. Our findings revealed that the addition of malic acid during fermentation might play an important role in lipid accumulation in the recombinant M. circinelloides Mc-MT-2. This study provides valuable insights for enhanced microbial lipid production through metabolic supplementation strategy in large scale and industrial applications.