Improvement of arachidonic acid production by mutants with lower n-3 desaturation activity derived from Mortierella alpina

Bioprospecting lipid-producing microorganisms: From metagenomic-assisted isolation techniques to industrial application and innovations

Traditionally, lipid-producing microorganisms have been obtained via conventional bioprospecting based on isolation and screening techniques, demanding time and effort. Thus, high-throughput sequencing combined with conventional microbiological approaches has emerged as an advanced and rapid strategy for recovering novel oleaginous microorganisms from target environments. This review highlights recent developments in lipid-producing microorganism bioprospecting, following (i) from traditional cultivation techniques to state-of-the-art metagenomics approaches; (ii) related topics on workflow, next-generation sequencing platforms, and knowledge bioinformatics; and (iii) biotechnological potential of the production of docosahexaenoic acid (DHA) by Aurantiochytrium limacinum, arachidonic acid (ARA) by Mortierella alpina and biodiesel by Rhodosporidium toruloides. These three species have been shown to be highly promising and studied in research articles, patents and commercialized products. Trends, innovations and future perspectives of these microorganisms are also addressed. Thus, these microbial lipids allow the development of food, feed and biofuels as alternative solutions to animal and vegetable oils.

Genome-wide identification of the fatty acid desaturases gene family in four Aspergillus species and their expression profile in Aspergillus oryzae

Fatty acid desaturases play a key role in producing polyunsaturated fatty acids by converting single bonds to double bonds. In the present study, a total of 13, 12, 8 and 8 candidate fatty acid desaturases genes were identified in the Aspergillus oryzae, Aspergillus flavus, Aspergillus fumigatus and Aspergillus nidulans genomes through database searches, which were classified into five different subfamilies based on phylogenetic analysis. Furthermore, a comprehensive analysis was performed to characterize conserved motifs and gene structures, which could provide an intuitive comprehension to learn the relationship between structure and functions of the fatty acid desaturases genes in different Aspergillus species. In addition, the expression pattern of 13 fatty acid desaturases genes of A. oryzae was tested in different growth stages and under salt stress treatment. The results revealed that the fatty acid desaturases genes in A. oryzae were highly expressed in adaptive phase growth and up-regulated under salt stress treatment. This study provided a better understanding of the evolution and functions of the fatty acid desaturases gene family in the four Aspergillus species, and would be useful for seeking methods to improve the production of unsaturated fatty acids and enhance efforts for the genetic improvement of strains to adapt to the complex surrounding environment.

Arachidonic acid production by the oleaginous fungus Mortierella alpina 1S-4: A review

The filamentous fungus Mortierella alpina 1S-4 is capable of accumulating a large amount of triacylglycerol containing C20 polyunsaturated fatty acids (PUFAs). Indeed, triacylglycerol production by M. alpina 1S-4 can reach 20 g/L of culture broth, and the critical cellular signaling and structural PUFA arachidonic acid (ARA) comprises 30%–70% of the total fatty acid. The demonstrated health benefits of functional PUFAs have in turn encouraged the search for rich sources of these compounds, including fungal strains showing enhanced production of specific PUFAs. Screening for mutants and targeted gene manipulation of M. alpina 1S-4 have elucidated the functions of various enzymes involved in PUFA biosynthesis and established lines with improved PUFA productivity. In some cases, these strains have been used for indistrial-scale production of PUFAs, including ARA. In this review, we described practical ARA production through mutant breeding, functional analyses of genes encoding enzymes involved in PUFA biosynthesis, and recent advances in the production of specific PUFAs through molecular breeding of M. alpina 1S-4.

Improving arachidonic acid fermentation by Mortierella alpina through multistage temperature and aeration rate control in bioreactor

Effective production of arachidonic acid (ARA) using Mortierella alpina was conducted in a 30-L airlift bioreactor. Varying the aeration rate and temperature significantly influenced cell morphology, cell growth, and ARA production, while the optimal aeration rate and temperature for cell growth and product formation were quite different. As a result, a two-stage aeration rate control strategy was constructed based on monitoring of cell morphology and ARA production under various aeration rate control levels (0.6-1.8 vvm). Using this strategy, ARA yield reached 4.7 g/L, an increase of 38.2% compared with the control (constant aeration rate control at 1.0 vvm). Dynamic temperature-control strategy was implemented based on the fermentation performance at various temperatures (13-28°C), with ARA level in total cellular lipid increased by 37.1% comparing to a constant-temperature control (25°C). On that basis, the combinatorial fermentation strategy of two-stage aeration rate control and dynamic temperature control was applied and ARA production achieved the highest level of 5.8 g/L.

Fatty Acid Desaturases, Polyunsaturated Fatty Acid Regulation, and Biotechnological Advances

Polyunsaturated fatty acids (PUFAs) are considered to be critical nutrients to regulate human health and development, and numerous fatty acid desaturases play key roles in synthesizing PUFAs. Given the lack of delta-12 and -15 desaturases and the low levels of conversion to PUFAs, humans must consume some omega-3 and omega-6 fatty acids in their diet. Many studies on fatty acid desaturases as well as PUFAs have shown that fatty acid desaturase genes are closely related to different human physiological conditions. Since the first front-end desaturases from cyanobacteria were cloned, numerous desaturase genes have been identified and animals and plants have been genetically engineered to produce PUFAs such as eicosapentaenoic acid and docosahexaenoic acid. Recently, a biotechnological approach has been used to develop clinical treatments for human physiological conditions, including cancers and neurogenetic disorders. Thus, understanding the functions and regulation of PUFAs associated with human health and development by using biotechnology may facilitate the engineering of more advanced PUFA production and provide new insights into the complexity of fatty acid metabolism.

Reconstruction and analysis of a genome-scale metabolic model of the oleaginous fungus Mortierella alpina

Background

Mortierella alpina is an oleaginous fungus used in the industrial scale production of arachidonic acid (ARA). In order to investigate the metabolic characteristics at a systems level and to explore potential strategies for enhanced lipid production, a genome-scale metabolic model of M. alpina was reconstructed.

Results

This model included 1106 genes, 1854 reactions and 1732 metabolites. On minimal growth medium, 86 genes were identified as essential, whereas 49 essential genes were identified on yeast extract medium. A series of sequential desaturase and elongase catalysed steps are involved in the synthesis of polyunsaturated fatty acids (PUFAs) from acetyl-CoA precursors, with concomitant NADPH consumption, and these steps were investigated in this study. Oxygen is known to affect the degree of unsaturation of PUFAs, and robustness analysis determined that an oxygen uptake rate of 2.0 mmol gDW⁻¹ h⁻¹ was optimal for ARA accumulation. The flux of 53 reactions involving NADPH was significantly altered at different ARA levels. Of these, malic enzyme (ME) was confirmed as a key component in ARA production and NADPH generation. When using minimization of metabolic adjustment, a knock-out of ME led to a 38.28% decrease in ARA production.

Conclusions

The simulation results confirmed the model as a useful tool for future research on the metabolism of PUFAs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-014-0137-8) contains supplementary material, which is available to authorized users.

ω3 fatty acid desaturases from microorganisms: structure, function, evolution, and biotechnological use

The biosynthesis of very-long-chain polyunsaturated fatty acids involves an alternating process of fatty acid desaturation and elongation catalyzed by complex series of enzymes. ω3 desaturase plays an important role in converting ω6 fatty acids into ω3 fatty acids. Genes for this desaturase have been identified and characterized in a wide range of microorganisms, including cyanobacteria, yeasts, molds, and microalgae. Like all fatty acid desaturases, ω3 desaturase is structurally characterized by the presence of three highly conserved histidine-rich motifs; however, unlike some desaturases, it lacks a cytochrome b5-like domain. Understanding the structure, function, and evolution of ω3 desaturases, particularly their substrate specificities in the biosynthesis of very-long-chain polyunsaturated fatty acids, lays the foundation for potential production of various ω3 fatty acids in transgenic microorganisms.

Fungal arachidonic acid-rich oil: research, development and industrialization

Fungal arachidonic acid (ARA)-rich oil is an important microbial oil that affects diverse physiological processes that impact normal health and chronic disease. In this article, the historic developments and technological achievements in fungal ARA-rich oil production in the past several years are reviewed. The biochemistry of ARA, ARA-rich oil synthesis and the accumulation mechanism are first introduced. Subsequently, the fermentation and downstream technologies are summarized. Furthermore, progress in the industrial production of ARA-rich oil is discussed. Finally, guidelines for future studies of fungal ARA-rich oil production are proposed in light of the current progress, challenges and trends in the field.

Enzymatic hydrolysis and extraction of arachidonic acid rich lipids from Mortierella alpina

A novel method for efficient arachidonic acid rich lipids extraction was investigated. Six different enzymes (papain, pectinase, snailase, neutrase, alcalase and cellulase) were used to extract lipids from Mortierella alpina. The effects of enzyme concentration, temperature and hydrolysis time on oil recovery were evaluated using factorial experimental design and polynomial regression for each enzyme. Hydrolysis time is found to be the most important parameter for all enzymes. The ratios of enzyme mixtures were also studied. It showed that the mixtures of pectinase and papain (5:3, v/v), pectinase and alcalase (5:1, v/v) were better combined effects on oil yields. The effects of hydrolysis time and temperature were then analyzed by response surface methodology, and oil recoveries were satisfactory (104.6% for pectinase and papain and 101.3% for pectinase and alcalase). In the whole process, the lipid composition was not affected by the enzyme treatments according to fatty acid profile.

Identification of mutation sites on omega3 desaturase genes from Mortierella alpina 1S-4 mutants

The mutation sites on omega3 desaturase genes in two omega3 desaturase-defective mutants derived from arachidonic acid-producing Mortierella alpina 1S-4 were identified. The mutations each resulted in an amino acid replacement (W232Stop or W386Stop) which caused a lack of omega3 desaturase activity in these mutants.

Single cell oil production by Mortierella alpina

A filamentous fungus, Mortierella alpina 1S-4, was obtained, through extensive screening, as an potential producer of triacylglycerol containing C20 polyunsaturated fatty acids (PUFAs) such as arachidonic acid. With this discovery as a starting point, we conducted employing methods from metabolic engineering and molecular biology for controlling cultures and breeding mutant strains. These parental and mutant strains are now used for large-scale production of a variety of PUFAs.

High-level production of arachidonic acid by fed-batch culture of Mortierella alpina using NH4OH as a nitrogen source and pH control

Mortierella alpina was grown in a fed-batch culture using a 12-l jar fermenter with an initial 8-l working volume containing 20 g glucose l-1 and 10 g corn-steep powder l-1. Glucose was intermittently fed to give 32 g l-1 at each time. The pH of culture was maintained using 14% (v/v) NH4OH, which also acted as a nitrogen source. A final cell density of 72.5 g l-1 was reached after 12.5 days with a content of arachidonic acid (ARA) at 18.8 g l-1. These values were 4 and 1.8 times higher than the respective values in batch culture. Our results suggest that the combined feeding of glucose and NH4+ to the growth of M. alpina could be applied for the industrial scale production of ARA.

A novel fungal omega3-desaturase with wide substrate specificity from arachidonic acid-producing Mortierella alpina 1S-4

A filamentous fungus, Mortierella alpina 1S-4, is capable of producing not only arachidonic acid (AA; 20:4n-6) but also eicosapentaenoic acid (EPA; 20:5n-3) below a cultural temperature of 20 degrees C. Here, we describe the isolation and characterization of a gene (maw3) that encodes a novel omega3-desaturase from M. alpina 1S-4. Based on the conserved sequence information for M. alpina 1S-4 Delta12-desaturase and Saccharomyces kluyveri omega3-desaturase, the omega3-desaturase gene from M. alpina 1S-4 was cloned. Homology analysis of protein databases revealed that the amino acid sequence showed 51% identity, at the highest, with M. alpina 1S-4 Delta12-desaturase, whereas it exhibited 36% identity with Sac. kluyveri omega3-desaturase. The cloned cDNA was confirmed to encode the omega3-desaturase by its expression in the yeast Sac. cerevisiae. Analysis of the fatty acid composition of the yeast transformant demonstrated that 18-carbon and 20-carbon n-3 polyunsaturated fatty acids (PUFAs) were accumulated through conversion of exogenous 18-carbon and 20-carbon n-6 PUFAs. The substrate specificity of the M. alpina 1S-4 omega3-desaturase differs from those of the known fungal omega3-desaturases from Sac. kluyveri and Saprolegnia diclina. Plant, cyanobacterial and Sac. kluyveri omega3-desaturases desaturate 18-carbon n-6 PUFAs, Spr. diclina omega3-desaturase desaturates 20-carbon n-6 PUFAs and Caenorhabditis elegans omega3-desaturase prefers 18-carbon n-6 PUFAs as substrates rather than 20-carbon n-6 PUFAs. The substrate specificity of M. alpina 1S-4 omega3-desaturase is rather similar to that of C. elegans omega3-desaturase, but the M. alpina omega3-desaturase can more effectively convert AA into EPA when expressed in yeast. The M. alpina 1S-4 omega3-desaturase is the first known fungal desaturase that uses both 18-carbon and 20-carbon n-6 PUFAs as substrates.