Production of gamma-linolenic acid by Mucor circinelloides and Mucor rouxii with acetic acid as carbon substrate

Biomanufacturing of γ-linolenic acid-enriched galactosyldiacylglycerols: Challenges in microalgae and potential in oleaginous yeasts

γ-Linolenic acid-enriched galactosyldiacylglycerols (GDGs-GLA), as the natural form of γ-linolenic acid in microalgae, have a range of functional activities, including anti-inflammatory, antioxidant, and anti-allergic properties. The low abundance of microalgae and the structural stereoselectivity complexity impede microalgae extraction or chemical synthesis, resulting in a lack of supply of GDGs-GLA with a growing demand. At present, there is a growing interest in engineering oleaginous yeasts for mass production of GDGs-GLA based on their ability to utilize a variety of hydrophobic substrates and a high metabolic flux toward fatty acid and lipid (triacylglycerol, TAG) production. Here, we first introduce the GDGs-GLA biosynthetic pathway in microalgae and challenges in the engineering of the native host. Subsequently, we describe in detail the applications of oleaginous yeasts with Yarrowia lipolytica as the representative for GDGs-GLA biosynthesis, including the development of synthetic biology parts, gene editing tools, and metabolic engineering of lipid biosynthesis. Finally, we discuss the development trend of GDGs-GLA biosynthesis in Y. lipolytica.

Simultaneous overexpression of ∆6-, ∆12- and ∆9-desaturases enhanced the production of γ-linolenic acid in Mucor circinelloides WJ11

Mucor circinelloides WJ11, an oleaginous filamentous fungus, produces 36% lipid of its cell dry weight when cultured in a high C/N ratio medium, however, the yield of γ-linolenic acid (GLA) is insufficient to make it competitive with other plant sources. To increase the GLA content in M. circinelloides WJ11, this fungus was engineered by overexpression of its key genes such as Δ6-, Δ12-, and Δ9-desaturases involved in GLA production. Firstly, we tried to overexpress two Δ6-desaturase isozymes to determine which one played important role in GLA synthesis. Secondly, Δ6-and Δ12-desaturase were co-overexpressed to check whether linoleic acid (LA), the precursor for GLA synthesis, is a limiting factor or not. Moreover, we tried to explore the effects of simultaneous overexpression of Δ6-, Δ12-, and Δ9-desaturases on GLA production. Our results showed that overexpression (1 gene) of DES61 promoted higher GLA content (21% of total fatty acids) while co-overexpressing (2 genes) DES61 and DES12 and simultaneous overexpressing (3 genes) DES61, DES12, and DES91 increased the GLA production of engineered strains by 1.5 folds and 1.9 folds compared to the control strain, respectively. This study provided more insights into GLA biosynthesis in oleaginous fungi and laid a foundation for further increase in GLA production into fungus such as M. circinelloides.

Fungi (Mold)-Based Lipid Production

There is an increasing need for the development of alternative energy sources with a focus on reducing greenhouse gas emissions and striving toward a sustainable economy. Bioethanol and biodiesel are currently the primary choices of alternative transportation fuels. At present, biodiesel is not competitive with conventional fuel due to its high price, and the only way to compete with conventional fuel is to improve the quality, reduce the costs, and coproduce value-added products. With the high demand for lipids in the energy sector and other industrial applications, microbial lipids accumulated from microorganisms, especially oleaginous fungi and yeasts have been the important topic of many recent research studies. This chapter summarizes the current status of knowledge and technology about lipid production by oleaginous fungi and yeasts for biofuel applications and other value-added products. The chapter focuses on several aspects such as the most promising oleaginous strains, strain development, improvement of lipid production, methods and protocols to cultivate oleaginous fungi, substrate utilization, fermentation process design, and downstream processing. The feasibility and challenges during the large-scale commercial production of microbial lipids as fuel sources are also discussed. It provides an overview of microbial lipid production biorefinery and also future development directions.

Optimization of Diverse Carbon Sources and Cultivation Conditions for Enhanced Growth and Lipid and Medium-Chain Fatty Acid (MCFA) Production by Mucor circinelloides

Keywords: Mucor circinelloides; microbial lipids; medium-chain fatty acids; culture optimization

Thermo-chemo-sonic pre-digestion of waste activated sludge for yeast cultivation to extract lipids for biodiesel production

The low cost biosynthesis of microbial lipids are an efficient feedstock to replace plant based oil for biodiesel production. The present study objective is to explore the effect of thermo-chemo-sonic pre-digestion of municipal Waste Activated Sludge (WAS) to cultivate oleaginous L. starkeyi MTCC-1400 as a model organism to produce high yield biomass and lipid. Higher Suspended Solids (SS) reduction (20 and 15.71%) and Chemical Oxygen Demand (COD) solubilization (27.6 and 22.3%) were achieved at a Specific Energy (SE) input of 5569 kJ/kg for WAS digested with NaOH and KOH, respectively. The maximum biomass of 17.52 g L^-1 and lipid 64.3% dwt were attained in NaOH pre-digested sample. The analyzed lipid profile exhibited high content of palmitic acid (45.6%) and oleic acid (38.7%) which are more suitable for biofuel production. Thus, these results strongly motivate the use of pre-digested WAS as an efficient and economical substrate for biodiesel production.

Influencing fatty acid composition of yeasts by lanthanides

The growth of microorganisms is affected by cultivation conditions, concentration of carbon and nitrogen sources and the presence of trace elements. One of the new possibilities of influencing the production of cell mass or lipids is the use of lanthanides. Lanthanides are biologically non-essential elements with wide applications in technology and industry and their concentration as environmental contaminants is therefore increasing. Although non-essential, lanthanides have been proposed (and even used) to produce beneficial effects in plants but their mechanisms of action are unclear. Recently, it was suggested that they may replace essential elements or operate as potent blockers of Ca(2+) channels. We tested the effect of low concentrations of lanthanides on traditional biotechnologically useful yeast species (Kluyveromyces polysporus, Saccharomyces cerevisiae, Torulospora delbrueckii), and species capable of high accumulation of lipids (Rhodotorula glutinis, Trichosporon cutaneum, Candida sp., Yarrowia lipolytica). Low concentrations of lanthanum and monazite were conducive to an increase in cell mass and lipids and also higher production of palmitoleic acid, commonly used in cosmetics and medicine, and ω6-linoleic acid which is a precursor of thromboxanes, prostaglandins and leucotrienes.

Feasibility of filamentous fungi for biofuel production using hydrolysate from dilute sulfuric acid pretreatment of wheat straw

BACKGROUND

Lipids produced from filamentous fungi show great promise for biofuel production, but a major limiting factor is the high production cost attributed to feedstock. Lignocellulosic biomass is a suitable feedstock for biofuel production due to its abundance and low value. However, very limited study has been performed on lipid production by culturing oleaginous fungi with lignocellulosic materials. Thus, identification of filamentous fungal strains capable of utilizing lignocellulosic hydrolysates for lipid accumulation is critical to improve the process and reduce the production cost.

RESULTS

The growth performances of eleven filamentous fungi were investigated when cultured on glucose and xylose. Their dry cell weights, lipid contents and fatty acid profiles were determined. Six fungal strains with high lipid contents were selected to culture with the hydrolysate from dilute sulfuric acid pretreatment of wheat straw. The results showed that all the selected fungal strains were able to grow on both detoxified liquid hydrolysate (DLH) and non-detoxified liquid hydrolysate (NDLH). The highest lipid content of 39.4% was obtained by Mortierella isabellina on NDLH. In addition, NDLH with some precipitate could help M. isabellina form pellets with an average diameter of 0.11 mm.

CONCLUSION

This study demonstrated the possibility of fungal lipid production from lignocellulosic biomass. M. isabellina was the best lipid producer grown on lignocellulosic hydrolysates among the tested filamentous fungi, because it could not only accumulate oils with a high content by directly utilizing NDLH to simplify the fermentation process, but also form proper pellets to benefit the downstream harvesting. Considering the yield and cost, fungal lipids from lignocellulosic biomass are promising alternative sources for biodiesel production.

Bioconversion of volatile fatty acids into lipids by the oleaginous yeast Yarrowia lipolytica

The valorization of volatile fatty acids into microbial lipids by the oleaginous yeast Yarrowia lipolytica was investigated. Therefore, a two-stage fed-batch strategy was designed: the yeast was initially grown on glucose or glycerol as carbon source, then sequential additions of acetic acid under nitrogen limiting conditions were performed after glucose or glycerol exhaustion. The typical values obtained with an initial 40 g/L concentration of glucose were close to 31 g/L biomass, a lipid concentration of 12.4 g/L, which correspond to a lipid content of the biomass close to 40%. This cultivation strategy was also efficient with other volatile fatty acids (butyric and propionic acids) or with a mixture of these three VFAs. The lipids composition was found quite similar to that of vegetable oils. The study demonstrated the feasibility of simultaneous biovalorization of volatile fatty acids and glycerol, two cheap industrial by-products.

Production of oils from acetic acid by the oleaginous yeast Cryptococcus curvatus

The feasibility of the conversion of acetic acid, a metabolite commonly obtained during anaerobic fermentation processes, into oils using the yeast Cryptococcus curvatus was reported. This microorganism exhibited very slow growth rates on acetate as carbon source, which led to design a two-stage cultivation process. The first consisted of cell growth on glucose as carbon source until its complete exhaustion. The second step involved the use of acetate as carbon source under nitrogen limitation in order to induce lipid accumulation. A typical experiment performed in a bioreactor involved a preliminary yeast growth with a glucose initial concentration of 15 g/L glucose. Further additions of acetate and nitrogen source allowed a final lipid accumulation up to 50% (w/w). These promising results demonstrated the suitability of the technique proposed.

A new cultivation method for microbial oil production: cell pelletization and lipid accumulation by Mucor circinelloides

The recent energy crisis has triggered significant attention on the microbial synthesis of lipids, which comprise the raw material for biodiesel production. Microbial oil accumulation with filamentous fungi has great potential because filamentous fungi can form pellets during cell growth, and these pellets are much easier to harvest from cell broth. This paper focuses on the cell pelletization process of the oleaginous Mucor circinelloides. We have studied the effect of various cultural conditions on pelletized cell growth and lipid accumulation. This study is the first to report that pH adjustment during cell growth plays a key role in pellet formation of M. circinelloides and describes a handy method by which to induce cell pelletization in submerged fungal cultivation. Our study reveals that cell growth and lipid production are not significantly affected by pelletization and that lipid accumulation is triggered at stressed conditions, such as a high carbon-to-nitrogen ratio and high temperature.

The effect of volatile fatty acids as a sole carbon source on lipid accumulation by Cryptococcus albidus for biodiesel production

The use of volatile fatty acids (VFAs) for microbial lipid accumulation was investigated in flask cultures of Cryptococcus albidus. The optimum culture temperature and pH were 25°C and pH 6.0, respectively, and the highest lipid content (27.8%) was obtained with ammonia chloride as a nitrogen source. The lipid yield coefficient on VFAs was 0.167 g/g of C. albidus with a VFAs (acetic, propionic, butyric acids) ratio of 8:1:1, which was in good agreement with a theoretically predicted lipid yield coefficient of the VFAs as a carbon source. The major fatty acids of the lipids accumulated by C. albidus were similar to those of soybean oil and jatropha oil. A preliminary cost analysis shows that VFAs-based biodiesel production is competitive with current palm and soybean based biodiesels. Further process development for lower aeration cost and higher lipid yield will make this process more economical.

High-cell-density fed-batch cultivation of the docosahexaenoic acid producing marine alga Crypthecodinium cohnii

The heterotrophic marine alga Crypthecodinium cohnii is known to produce docosahexaenoic acid (DHA), a polyunsaturated fatty acid with food and pharmaceutical applications, during batch cultivation on complex media containing sea salt, yeast extract, and glucose. In the present study, fed-batch cultivation was studied as an alternative fermentation strategy for DHA production. Glucose and acetic acid were compared as carbon sources. For both substrates, the feed rate was adapted to the maximum specific consumption rate of C. cohnii. In glucose-grown cultures, this was done by maintaining a significant glucose concentration (between 5 and 20 g/L) throughout fermentation. In acetic acid-grown cultures, the medium feed was automatically controlled via the culture pH. A feed consisting of acetic acid (50% w/w) resulted in a higher overall volumetric productivity of DHA (r(DHA)) than a feed consisting of 50% (w/v) glucose (38 and 14 mg/L/h, respectively). The r(DHA) was further increased to 48 mg/L/h using a feed consisting of pure acetic acid. The latter fermentation strategy resulted in final concentrations of 109 g/L dry biomass, 61 g/L lipid, and 19 g/L DHA. These are the highest biomass, lipid, and DHA concentrations reported to date for a heterotrophic alga. Vigorous mixing was required to sustain aerobic conditions during high-cell-density cultivation. This was complicated by culture viscosity, which resulted from the production of viscous extracellular polysaccharides. These may present a problem for large-scale industrial production of DHA. Addition of a commercial polysaccharide-hydrolase preparation could decrease the viscosity of the culture and the required stirring.