Production of secondary metabolites through glycerol fermentation under carbon-excess conditions by the yeasts Yarrowia lipolytica and Rhodosporidium toruloides

Growth Response of Non-Conventional Yeasts on Sugar-Rich Media: Part 2: Citric Acid Production and Circular-Oriented Valorization of Glucose-Enriched Olive Mill Wastewaters Using Novel Yarrowia lipolytica Strains

The global market for citric acid (CA) is one of the biggest and fastest expanding markets in the food industry. The CA production employing microbial bioprocessing with efficient GRAS strains and renewable waste streams is in line with the European Union binding targets for resource efficiency, sustainable consumption-production, and low-carbon technologies. In this work, the potential of three novel wild-type Yarrowia lipolytica strains (namely LMBF Y-46, LMBF Y-47 and ACA-YC 5033) regarding the production of CA and other valuable metabolites was tested on glucose-based media, and the most promising amongst the screened strains (viz. the strain ACA-YC 5033) was cultured on glucose-based media, in which part of the fermentation water had been replaced by olive-mill wastewaters (OMWs) in a novel approach of simultaneous OMW valorization and bioremediation. In the first part of this study, the mentioned strains were cultured under nitrogen-limited conditions with commercial (low-cost) glucose employed as a sole carbon source in shake-flask cultures at an initial concentration (S0) ≈ of 50 g/L. Variable quantities of secreted citric acid (CA) and intra-cellular compounds (viz. polysaccharides and lipids) were produced. All strains did not accumulate significantly high lipid quantities (i.e., maximum lipid in dry cell weight [DCW] values ≈30% w/w were noted) but produced variable CA quantities. The most promising strain, namely ACA-YC 5033, produced CA up to c. 24 g/L, with a yield of CA produced on glucose consumed (YCA/S) ≈ 0.45 g/g. This strain in stirred tank bioreactor experiments, at remarkably higher S0 concentrations (≈110 g/L) and the same initial nitrogen quantity added into the medium, produced notably higher CA quantities, up to 57 g/L (YCA/S ≈ 0.52 g/g). The potential of the same strain (ACA-YC 5033) to bioremediate OMWs and to produce value-added compounds, i.e., yeast cells, CA, and intra-cellular metabolites, was also assessed; under nitrogen-limited conditions in which OMWs had partially replaced tap water and significant glucose concentrations had been added (S0 ≈ 100 g/L, simultaneous molar ratio C/N ≈ 285 g/g, initial phenolic compounds [Phen0] adjusted to ≈1.0 g/L; these media were similar to the OMWs generated from the traditional press extraction systems) the notable CA quantity of 60.2 g/L with simultaneous YCA/S = 0.66 g/g, was obtained in shake flasks, together with satisfactory phenolic compounds removal (up to 19.5% w/w) and waste decolorization (up to 47.0%). Carbon-limited conditions with Phen0 ≈ 1.0 g/L favored the production of yeast DCW (up to 25.3 g/L), with equally simultaneous interesting phenolic compounds and color removal. The fatty acid profile showed that cellular lipids were highly unsaturated with oleic, linoleic and palmitoleic acids, accounting for more than 80% w/w. This study proposed an interesting approach that could efficiently address the biotreatment of toxic effluents and further convert them into circular-oriented bioproducts.

Growth Response of Non-Conventional Yeasts on Sugar-Rich Media: Part 1: High Production of Lipid by Lipomyces starkeyi and Citric Acid by Yarrowia lipolytica

Sugar-rich waste streams, generated in very high quantities worldwide, constitute an important source of environmental pollution. Their eco-friendly conversions into a plethora of added-value compounds through the use of microbial fermentations is currently a very "hot" scientific topic. The aim of this study, was to assess the potential of single cell oil (SCO), microbial mass and citric acid (CA) production by non-conventional yeast strains growing on expired ("waste") glucose. Six yeast strains (viz. Rhodosporidium toruloides DSM 4444, Rhodotorula glutinis NRRL YB-252, R. toruloides NRRL Y-27012, Yarrowia lipolytica LFMB Y-20, Y. lipolytica ACA-DC 50109 and Lipomyces starkeyi DSM 70296) were initially grown in shake flasks with expired glucose used as substrate under nitrogen limitation, in order to "boost" the cellular metabolism towards the synthesis of SCO and CA, and their growth response was quantitatively evaluated. Initial glucose concentration (Glc₀) was adjusted at c. 50 g/L. Besides Y. lipolytica, all other yeast strains produced noticeable SCO quantities [lipid in dry cell weight (DCW) ranging from 25.3% w/w to 55.1% w/w]. Lipids of all yeasts contained significant quantities of oleic acid, being perfect candidates for the synthesis of 2nd generation biodiesel. The highest DCW production (=13.6 g/L) was obtained by L. starkeyi DSM 70296, while both Y. lipolytica strains did not accumulate noticeable lipid quantities, but produced non-negligible CA amounts. The most promising CA-producing strain, namely Y. lipolytica ACA-DC 50109 was further studied in stirred-tank bioreactor systems, while the very promising DCW- and SCO-producing L. starkeyi DSM 70296 was further studied in shake flasks. Both strains were grown on media presenting higher Glc₀ concentrations and the same initial nitrogen quantity as previously. Indeed, L. starkeyi grown at Glc₀ = 85 g/L, produced DCW_max = 34.0 g/L, that contained lipid =34.1% w/w (thus SCO was =11.6 g/L). The strain ACA-DC 50109 in stirred tank bioreactor with Glc₀ ≈ 105 g/L produced CA up to 46 g/L (yield of CA produced on glucose consumed; Y_CA/Glc ≈ 0.45 g/g). Finally, in fed-batch bioreactor experiment, the significant CA quantity of 82.0 g/L (Y_CA/Glc = 0.50 g/g) was recorded. Concluding, "waste" glucose proved to be a suitable substrate for a number of non-conventional yeast strains. Y. lipolytica ACA-DC 50109 produced significant quantities of CA while L. starkeyi DSM 70296 was a very interesting DCW- and SCO-producing candidate. These strains can be used as potential cell factories amenable to convert glucose-based residues into the mentioned metabolic compounds, that present high importance for food, chemical and biofuel facilities.

Upgrading Major Waste Streams Derived from the Biodiesel Industry and Olive Mills via Microbial Bioprocessing with Non-Conventional Yarrowia lipolytica Strains

This study reports the development of a bioprocess involving the valorization of biodiesel-derived glycerol as the main carbon source for cell proliferation of Yarrowia lipolytica strains and production of metabolic compounds, i.e., citric acid (Cit), polyols, and other bio-metabolites, the substitution of process tap water with olive mill wastewater (OMW) in batch fermentations, and partial detoxification of OMW (up to 31.1% decolorization). Increasing initial phenolics (Phen) of OMW-glycerol blends led to substantial Cit secretion. Maximum Cit values, varying between 64.1–65.1 g/L, combined with high yield (YCit/S = 0.682–0.690 g Cit/g carbon sources) and productivity (0.335–0.344 g/L/h) were achieved in the presence of Phen = 3 g/L. The notable accumulation of endopolysaccharides (EPs) on the produced biomass was determined when Y. lipolytica LMBF Y-46 (51.9%) and ACA-YC 5033 (61.5%) were cultivated on glycerol-based media. Blending with various amounts of OMW negatively affected EPs and polyols biosynthesis. The ratio of mannitol:arabitol:erythritol was significantly affected (p < 0.05) by the fermentation media. Erythritol was the major polyol in the absence of OMW (53.5–62.32%), while blends of OMW-glycerol (with Phen = 1–3 g/L) promoted mannitol production (54.5–76.6%). Nitrogen-limited conditions did not favor the production of cellular lipids (up to 16.6%). This study addressed sustainable management and resource efficiency enabling the bioconversion of high-organic-load and toxic waste streams into valuable products within a circular bioeconomy approach.

Microbial lipid production from soybean hulls using Lipomyces starkeyi LPB53 in a circular economy

Soybean hulls are lignocellulosic residuesgeneratedinthe industrial processing of soybean, representing about 5 % of the mass of the whole bean. This by-product isan importantsource of polymers suchas cellulose(34 %) and hemicellulose (11 %),which could bevalorizedvia biotechnology to improvethe economic returnof the oilseed chain. In the present work,soybean hulls were evaluated as a carbon sourcefor biolipid productionbyLipomycesstarkeyi LPB 53. Initially the hulls were treated physicochemically and enzymatically to obtain fermentable sugars. Subsequently, biomass growth was evaluated using different nitrogen sources andthe lipid production was optimized, reaching a maximum cell biomass concentration of 26.5 g/L with 42.5 % of lipids. Around 65 % of the xylose content was consumed.The obtained oil wasmajorlycomposed of oleic, palmitic, palmitoleic, linoleic and stearic fatty acids in a proportion of 54 %, 32 %, 4 %, 3 % and 2 %, respectively.

Development and Perspective of Rhodotorula toruloides as an Efficient Cell Factory

Rhodotorula toruloides is receiving significant attention as a novel cell factory because of its high production of lipids and carotenoids, fast growth and high cell density, as well as the ability to utilize a wide variety of substrates. These attractive traits of R. toruloides make it possible to become a low-cost producer that can be engineered for the production of various fuels and chemicals. However, the lack of understanding and genetic engineering tools impedes its metabolic engineering applications. A number of research efforts have been devoted to filling these gaps. This review focuses on recent developments in genetic engineering tools, advances in systems biology for improved understandings, and emerging engineered strains for metabolic engineering applications. Finally, future trends and barriers in developing R. toruloides as a cell factory are also discussed.

Use of Pressurized and Airlift Bioreactors for Citric Acid Production by Yarrowia lipolytica from Crude Glycerol

Citric acid production is generally carried out in an aqueous medium in stirred tank reactors (STR), where the solubility of oxygen is low and the oxygen demand of microbial cultures is high. Thus, for this bioprocess, providing adequate oxygen mass transfer rate (OTR) from the gas phase into the aqueous culture medium is the main challenge of bioreactor selection and operation. In this study, citric acid production by Yarrowia lipolytica W29 from crude glycerol, in batch cultures, was performed in two non-conventional bioreactors normally associated with high mass transfer efficiency: a pressurized STR and an airlift bioreactor. Increased OTR was obtained by raising the total air pressure in the pressurized STR and by increasing the aeration rate in the airlift bioreactor. An improvement of 40% in maximum citric acid titer was obtained by raising the air pressure from 1 bar to 2 bar, whereas, in the airlift bioreactor, a 30% improvement was attained by increasing the aeration rate from 1 vvm to 1.5 vvm. Both bioreactor types can be successfully applied for the citric acid production process using alternative ways of improving OTR than increasing mechanical stirring power input, thus leading to important operating saving costs.

Enhancing Red Yeast Biomass Yield and Lipid Biosynthesis by Using Waste Nitrogen Source by Glucose Fed-Batch at Low Temperature

This work reports the effect of simple feeding strategies and temperature to obtain high-cell-density cultures of Rhodotorula glutinis var. rubescens LOCKR13 maximizing the de novo lipid productivity using deproteinated potato wastewater (DPW) as a basic medium. Feeding DPW with glucose enables a high yield of Rhodotorula glutinis var. rubescens LOCKR13 biomass (52 g d.w. L−1) to be obtained. The highest values of lipid accumulation (34.15%, w/w), production (14.68 g L−1) and yield coefficients (YL/S: 0.242 g g−1), and volumetric productivity (PL: 0.1 g L−1 h−1) were reached by the strain in the two-stage fed-batch process at 20 °C. The lipid of yeast biomass was rich in oleic acid (Δ9C18:1) and palmitic acid (C16:0), and the lower temperature of incubation significantly increased the MUFA (especially oleic acid) content. For the first time, a unique set of thermal analyses of the microbial oil was performed. The isotherms of the oxidation kinetics (PDSC) showed that lipids extracted from the biomass of red yeast had high oxidative stability. This feature of the yeast oil can be useful for long-shelf-life food products and can be promising for the production of biodiesel.

Yarrowia lipolytica as an Alternative and Valuable Source of Nutritional and Bioactive Compounds for Humans

Yarrowia lipolytica, an oleagineous species of yeast, is a carrier of various important nutrients. The biomass of this yeast is an extensive source of protein, exogenous amino acids, bioavailable essenctial trace minerals, and lipid compounds as mainly unsaturated fatty acids. The biomass also contains B vitamins, including vitamin B12, and many other bioactive components. Therefore, Y. lipolytica biomass can be used in food supplements for humans as safe and nutritional additives for maintaining the homeostasis of the organism, including for vegans and vegetarians, athletes, people after recovery, and people at risk of B vitamin deficiencies.

Bioconversions of Biodiesel-Derived Glycerol into Sugar Alcohols by Newly Isolated Wild-Type Yarrowia lipolytica Strains

The utilization of crude glycerol, generated as a by-product from the biodiesel production process, for the production of high value-added products represents an opportunity to overcome the negative impact of low glycerol prices in the biodiesel industry. In this study, the biochemical behavior of Yarrowia lipolytica strains FMCC Y-74 and FMCC Y-75 was investigated using glycerol as a carbon source. Initially, the effect of pH value (3.0–7.0) was examined to produce polyols, intracellular lipids, and polysaccharides. At low pH values (initial pH 3.0–5.0), significant mannitol production was recorded. The highest mannitol production (19.64 g L−1) was obtained by Y. lipolytica FMCC Y-74 at pH = 3.0. At pH values ranging between 5.0 and 6.0, intracellular polysaccharides synthesis was favored, while polyols production was suppressed. Subsequently, the effect of crude glycerol and its concentration on polyols production was studied. Y. lipolytica FMCC Y-74 showed high tolerance to impurities of crude glycerol. Initial substrate concentrations influence polyols production and distribution with a metabolic shift toward erythritol production being observed when the initial glycerol concentration (Gly0) increased. The highest total polyols production (=56.64 g L−1) was obtained at Gly0 adjusted to ≈120 g L−1. The highest polyols conversion yield (0.59 g g−1) and productivity (4.36 g L−1 d−1) were reached at Gly0 = 80 g L−1. In fed-batch intermittent fermentation with glycerol concentration remaining ≤60 g L−1, the metabolism was shifted toward mannitol biosynthesis, which was the main polyol produced in significant quantities (=36.84 g L−1) with a corresponding conversion yield of 0.51 g g−1.

High value-added products derived from crude glycerol via microbial fermentation using Yarrowia clade yeast

BACKGROUND

Contemporary biotechnology focuses on many problems related to the functioning of developed societies. Many of these problems are related to health, especially with the rapidly rising numbers of people suffering from civilization diseases, such as obesity or diabetes. One factor contributing to the development of these diseases is the high consumption of sucrose. A very promising substitute for this sugar has emerged: the polyhydroxy alcohols, characterized by low caloric value and sufficient sweetness to replace table sugar in food production.

RESULTS

In the current study, yeast belonging to the Yarrowia clade were tested for erythritol, mannitol and arabitol production using crude glycerol from the biodiesel and soap industries as carbon sources. Out of the 13 tested species, Yarrowia divulgata and Candida oslonensis turned out to be particularly efficient polyol producers. Both species produced large amounts of these compounds from both soap-derived glycerol (59.8-62.7 g dm-3) and biodiesel-derived glycerol (76.8-79.5 g dm-3). However, it is equally important that the protein and lipid content of the biomass (around 30% protein and 12% lipid) obtained after the processes is high enough to use this yeast in the production of animal feed.

CONCLUSIONS

The use of waste glycerol for the production of polyols as well as utilization of the biomass obtained after the process for the production of feed are part of the development of modern waste-free technologies.

Advances in improving the biotechnological application of oleaginous fungus Mortierella alpina

Mortierella alpina is an oleaginous filamentous fungus with considerable lipid productivity, and it has been widely used for industrial production of arachidonic acid. The fermentation process of M. alpina is complicated and can be affected by various factors; therefore, a comprehensive knowledge of its metabolic characteristics and key factors governing lipid biosynthesis is required to further improve its industrial performance. In this review, we discuss the metabolic features and extracellular factors that affect lipid biosynthesis in M. alpina. The current progress in fermentation optimisation and metabolic engineering to improve lipid yield are also summarised. Moreover, we review the applications of M. alpina in the food industry and propose fermentation strategies for better utilisation of this genus in the future. In our opinion, the economic performance of M. alpina should be enhanced from multiple levels, including strains with ideal traits, efficient fermentation strategies, controllable fermentation costs, and competitive products of both high value and productivity. By reviewing the peculiarities of M. alpina and current progress to improve its suitability for biotechnological production, we wish to provide more efficient strategies for future development of M. alpina as a high-value lipid cell factory. KEY POINTS: • Understanding M. alpina metabolism is helpful for rational design of its fermentation processes. • Nitrogen source is a key point that affects PUFA's component and fermentation cost in M. alpina. • Dynamic fermentation strategy combined with breeding is needed to increase lipid yield in M. alpina.

Yarrowia lipolytica Strains and Their Biotechnological Applications: How Natural Biodiversity and Metabolic Engineering Could Contribute to Cell Factories Improvement

Among non-conventional yeasts of industrial interest, the dimorphic oleaginous yeast Yarrowia lipolytica appears as one of the most attractive for a large range of white biotechnology applications, from heterologous proteins secretion to cell factories process development. The past, present and potential applications of wild-type, traditionally improved or genetically modified Yarrowia lipolytica strains will be resumed, together with the wide array of molecular tools now available to genetically engineer and metabolically remodel this yeast. The present review will also provide a detailed description of Yarrowia lipolytica strains and highlight the natural biodiversity of this yeast, a subject little touched upon in most previous reviews. This work intends to fill this gap by retracing the genealogy of the main Yarrowia lipolytica strains of industrial interest, by illustrating the search for new genetic backgrounds and by providing data about the main publicly available strains in yeast collections worldwide. At last, it will focus on exemplifying how advances in engineering tools can leverage a better biotechnological exploitation of the natural biodiversity of Yarrowia lipolytica and of other yeasts from the Yarrowia clade.

Lipid metabolism research in oleaginous fungus Mortierella alpina: Current progress and future prospects

The oleaginous fungus Mortierella alpina has distinct advantages in long-chain PUFAs production, and it is the only source for dietary arachidonic acid (ARA) certificated by FDA and European Commission. This review provides an overall introduction to M. alpina, including its major research methods, key factors governing lipid biosynthesis, metabolic engineering and omics studies. Currently, the research interests in M. alpina focus on improving lipid yield and fatty acid desaturation degree by enhancing fatty acid precursors and the reducing power NADPH, and genetic manipulation on PUFAs synthetic pathways is carried to optimise fatty acid composition. Besides, multi-omics studies have been applied to elucidate the global regulatory mechanism of lipogenesis in M. alpina. However, research challenges towards achieving a lipid cell factory lie in strain breeding and cost control due to the coenocytic mycelium, long fermentation period and insufficient conversion rate from carbon to lipid. We also proposed future research goals based on a multilevel regulating strategy: obtaining ideal chassis by directional evolution and high-throughput screening; rewiring central carbon metabolism and inhibiting competitive pathways by multi-gene manipulation system to enhance carbon to lipid conversion rate; optimisation of protein function based on post-translational modification; application of dynamic fermentation strategies suitable for different fermentation phases. By reviewing the comprehensive research progress of this oleaginous fungus, we aim to further comprehend the fungal lipid metabolism and provide reference information and guidelines for the exploration of microbial oils from the perspectives of fundamental research to industrial application.

Yarrowia lipolytica as a biorefinery platform for effluents and solid wastes valorization - challenges and opportunities

Due to its physiological and enzymatic features, Yarrowia lipolytica produces several valuable compounds from a wide range of substrates. Appointed by some authors as an industrial workhorse, Y. lipolytica has an extraordinary ability to use unrefined and complex low-cost substrates as carbon and nitrogen sources, aiding to reduce the waste surplus and to produce added-value compounds in a cost-effective way. Dozens of review papers regarding Y. lipolytica have been published till now, proving the interest that this yeast arouses in the scientific community. However, most of them are focused on metabolic pathways involved in substrates assimilation and product formation, or the development of synthetic biology tools in order to obtain engineered strains for biotechnological applications. This paper provides an exhaustive and up-to-date revision on the application of Y. lipolytica to valorize liquid effluents and solid wastes and its role in developing cleaner biotechnological approaches, aiming to boost the circular economy. Firstly, a general overview about Y. lipolytica is introduced, describing its intrinsic features and biotechnological applications. Then, an extensive survey of the literature regarding the assimilation of oily wastes (waste cooking oils, oil cakes and olive mill wastewaters), animal fat wastes, hydrocarbons-rich effluents, crude glycerol and agro-food wastes by Y. lipolytica strains will be discussed. This is the first article that brings together the environmental issue of all such residues and their valorization as feedstock for valuable compounds production by Y. lipolytica. Finally, it will demonstrate the potential of this non-conventional yeast to be used as a biorefinery platform.

Differences in the Fatty Acid Profile, Morphology, and Tetraacetylphytosphingosine-Forming Capability Between Wild-Type and Mutant Wickerhamomyces ciferrii

One tetraacetylphytosphingosine (TAPS)-producing Wickerhamomyces ciferrii mutant was obtained by exposing wild-type W. ciferrii to γ-ray irradiation. The mutant named 736 produced up to 9.1 g/L of TAPS (218.7 mg-TAPS/g-DCW) during batch fermentation in comparison with 1.7 g/L of TAPS (52.2 mg-TAPS/g-DCW) for the wild type. The highest production, 17.7 g/L of TAPS (259.6 mg-TAPS/g-DCW), was obtained during fed-batch fermentation by mutant 736. Fatty acid (FA) analysis revealed an altered cellular FA profile of mutant 736: decrease in C16:0 and C16:1 FA levels, and increase in C18:1 and C18:2 FA levels. Although a significant change in the cellular FA profile was observed, scanning electron micrographs showed that morphology of wild-type and mutant 736 cells was similar. Genetic alteration analysis of eight TAPS biosynthesis-related genes revealed that there are no mutations in these genes in mutant 736; however, mRNA expression analysis indicated 30% higher mRNA expression of TCS10 among the eight genes in mutant 736 than that in the wild-type. Collectively, these results imply that the enhancement of TAPS biosynthesis in mutant 736 may be a consequence of system-level genetic and physiological alterations of a complicated metabolic network. Reverse metabolic engineering based on system-level omics analysis of mutant 736 can make the mutant more suitable for commercial production of TAPS.

Functional characterization and overexpression of Δ12-desaturase in the oleaginous yeast Rhodotorula toruloides for production of linoleic acid-rich lipids

Linoleic acid (LA) has garnered much attention due to its potential applications in the oleochemical and nutraceutical industries. The oleaginous yeast Rhodotorula toruloides has outstanding lipogenecity, and is considered a potential alternative to the current plant-based platforms for LA production. Δ12-fatty acid desaturases (Δ12-Fads) are involved in LA synthesis in various fungi and yeasts, but their functions in R. toruloides remain poorly understood. To achieve the production of LA-rich lipids in R. toruloides, we investigated the function of the native Δ12-FAD (RtFAD2). First, the overexpression of RtFAD2 and its co-overexpression with RtFAD1 (encoding R. toruloides Δ9-Fad) and their effects on LA production in R. toruloides were investigated. The function of RtFad2 was confirmed by heterologous expression in Saccharomyces cerevisiae. Overexpression of RtFAD2 significantly elevated the LA contents and titers in the wild-type strain R. toruloides DMKU3-TK16 (TK16) and in a thermotolerant derivative of TK16 (L1-1). Additionally, overexpression of RtFAD2 in R. toruloides strains also increased the lipid titer and content. Overexpression of RtFAD1 was down-regulated in the RtFAD1 and RtFAD2 co-overexpressing strains, suggesting that the elevated LA content may function as a key regulator of RtFAD1 expression to control C18 fatty-acid synthesis in R. toruloides. We characterized the function of RtFAD2 and showed that its overexpression in R. toruloides increased the lipid and LA production. These findings may assist in the rational design of metabolic engineering related to LA or polyunsaturated fatty acid production in R. toruloides.

Lignocellulosic Biomass as a Substrate for Oleaginous Microorganisms: A Review

Microorganisms capable of accumulating lipids in high percentages, known as oleaginous microorganisms, have been widely studied as an alternative for producing oleochemicals and biofuels. Microbial lipid, so-called Single Cell Oil (SCO), production depends on several growth parameters, including the nature of the carbon substrate, which must be efficiently taken up and converted into storage lipid. On the other hand, substrates considered for large scale applications must be abundant and of low acquisition cost. Among others, lignocellulosic biomass is a promising renewable substrate containing high percentages of assimilable sugars (hexoses and pentoses). However, it is also highly recalcitrant, and therefore it requires specific pretreatments in order to release its assimilable components. The main drawback of lignocellulose pretreatment is the generation of several by-products that can inhibit the microbial metabolism. In this review, we discuss the main aspects related to the cultivation of oleaginous microorganisms using lignocellulosic biomass as substrate, hoping to contribute to the development of a sustainable process for SCO production in the near future.

Physiological Characterization of a Novel Wild-Type Yarrowia lipolytica Strain Grown on Glycerol: Effects of Cultivation Conditions and Mode on Polyols and Citric Acid Production

A new yeast wild-type Yarrowia lipolytica isolate presented efficient growth on glycerol. During flask cultures, nitrogen limitation led to the secretion of sugar-alcohols as the major metabolites of the process (mannitol, arabitol and erythritol), whereas insignificant quantities of citrate were synthesized. Although in some instances high initial glycerol concentrations were employed (≈150 g/L), remarkable glycerol assimilation and polyol secretion was observed. Total polyols ≈ 52 g/L (conversion yield on glycerol consumed = 0.43 g/g) was recorded in the flask experiments. The sugar-alcohol production bioprocess was successfully simulated with the aid of a modified Velhlust–Aggelis model that fitted very well with the experimental data, while optimized parameter values seemed to be quite consistent. In bioreactor trials, a noticeable metabolic shift towards citric acid production was observed, while simultaneously insignificant polyol quantities were produced. In fed-batch bioreactor experiments, a total citric acid quantity ≈ 102 g/L was recorded—one of the highest in the literature for wild-type Y. lipolytica strains. This metabolic transition was due to higher oxygen saturation into the medium that occurred in the bioreactor experiments compared with the flasks. Cellular lipids produced in the bioreactor trial contained higher concentrations of unsaturated fatty acids compared with those produced in flasks.

Lipids by Yarrowia lipolytica Strains Cultivated on Glucose in Batch Cultures

Oleaginous microorganisms, such as Yarrowia lipolytica, accumulate lipids that can have interesting applications in food biotechnology or the synthesis of biodiesel. Y. lipolytica yeast can have many advantages such as wide substrate range usage and robustness to extreme conditions, while under several culture conditions it can produce high lipid productivity. Based on this assumption, in this study, 12 different Yarrowia lipolytica strains were used to investigate microbial lipid production using a glucose-based medium under nitrogen-limited conditions in shake-flask cultivations. Twelve wild-type or mutant strains of Yarrowia lipolytica which were newly isolated or belonged to official culture collections were tested, and moderate lipid quantities (up to 1.30 g/L) were produced; in many instances, nitrogen limitation led to citric acid production in the medium. Lipids were mainly composed of C16 and C18 fatty acids. Most of the fatty acids of the microbial lipid were unsaturated and corresponded mainly to oleic, palmitic and linoleic acids. Linolenic acid (C18:3) was produced in significant quantities (between 10% and 20%, wt/wt of dry cell weight (DCW)) by strains H917 and Po1dL.

Ethanol and H2O2 stresses enhance lipid production in an oleaginous Rhodotorula toruloides thermotolerant mutant L1-1

Stress tolerance is a desired characteristic of yeast strains for industrial applications. Stress tolerance has been well described in Saccharomyces yeasts but has not yet been characterized in oleaginous Rhodotorula yeasts even though they are considered promising platforms for lipid production owing to their outstanding lipogenicity. In a previous study, the thermotolerant strain L1–1 was isolated from R. toruloides DMKU3-TK16 (formerly Rhodosporidium toruloides). In this study, we aimed to further examine the ability of this strain to tolerate other stresses and its lipid productivity under various stress conditions. We found that the L1–1 strain could tolerate not only thermal stress but also oxidative stress (ethanol and H2O2), osmotic stress (glucose) and a cell membrane disturbing reagent (DMSO). Our results also showed that the L1–1 strain exhibited enhanced ability to maintain ROS homeostasis, stronger cell wall strength and increased levels of unsaturated membrane lipids under various stresses. Moreover, we also demonstrated that ethanol-induced stress significantly increased the lipid productivity of the thermotolerant L1–1. The thermotolerant L1–1 was also found to produce a higher lipid titer under the dual ethanol-H2O2 stress than under non-stress conditions. This is the first report to indicate that ethanol stress can induce lipid production in an R. toruloides thermotolerant strain.

Coproducts of algae and yeast-derived single cell oils: A critical review of their role in improving biorefinery sustainability

Oleaginous microalgae and yeast are of increasing interest as a renewable resource for single cell oils (SCOs). These have applications in fuels, feed and food products. In order to become cost competitive with existing terrestrial oils, a biorefinery approach is often taken where several product streams are valorised alongside the SCO. Whilst many life cycle assessment (LCA) and Techno-economic (TEA) studies have employed this biorefinery approach to SCO production, a systematic analysis of their implications is missing. This review evaluates the economic and environmental impacts associated with the use of coproducts. Overall, protein production plays the greatest role in determining viability, with coproduct strategy crucial to considering in the early stages of research and development.

Production of added-value microbial metabolites during growth of yeast strains on media composed of biodiesel-derived crude glycerol and glycerol/xylose blends

A total of 11 yeast strains of Yarrowia lipolytica, Metschnikowia sp., Rhodotorula sp. and Rhodosporidium toruloides were grown under nitrogen-limited conditions with crude glycerol employed as substrate in shake flasks, presenting interesting dry cell weight (DCW) production. Three of these strains belonging to Metschnikowia sp. accumulated significant quantities of endopolysaccharides (i.e. the strain V.V.-D4 produced 11.0 g/L of endopolysaccharides, with polysaccharides in DCW ≈ 63% w/w). A total of six Y. lipolytica strains produced either citric acid or mannitol. Most of the screened yeasts presented somehow elevated lipid and polysaccharides in DCW values at the early steps of growth despite nitrogen appearance in the fermentation medium. Lipid in DCW values decreased as growth proceeded. R. toruloides DSM 4444 cultivated on media presenting higher glycerol concentrations presented interesting lipid-accumulating capacities (maximum lipid = 12.5 g/L, maximum lipid in DCW = 43.0–46.0% w/w, conversion yield on glycerol = 0.16 g/g). Replacement of crude glycerol by xylose resulted in somehow decreased lipid accumulation. In xylose/glycerol mixtures, xylose was more rapidly assimilated from glycerol. R. toruloides total lipids were mainly composed of triacylglycerols. Total cellular fatty acid composition on xylose presented some differences compared with that on glycerol. Cellular lipids contained mainly oleic and palmitic acid.

Orange peel waste-based liquid medium for biodiesel production by oleaginous yeasts

Orange peel waste (OPW), the primary byproduct of the juice extraction process, is annually generated in massive amounts (21 Mton), and its aqueous extraction in biorefining operations yields a liquid fraction, referred to as orange peel extract (OPE). Although OPE contains significant amounts of easily assimilable carbohydrates, such as fructose, glucose, and sucrose, no investigations have been conducted yet to assess its possible use in biodiesel production by oleaginous yeasts. Consequently, the objective of the present study was to assess whether OPE might act as the basis of a liquid medium for microbial lipid production. A screening conducted with 18 strains of oleaginous yeasts in shaken flask on the OPE-based medium showed that Rhodosporidium toruloides NRRL 1091 and Cryptococcus laurentii UCD 68-201 gave the best results in terms of lipid production (5.8 and 4.5 g L^-1, respectively) and accumulation (77 and 47% on a dry matter basis, respectively). The subsequent scale transfer of the process to a 3-L STR operated in batch mode halved the time required to reach the lipid peak with the ensuing increase in volumetric productivities in R. toruloides NRRL 1091 (3646 mg L^-1 day^-1) and C. laurentii UCD 68-201 (2970.7 mg L^-1 day^-1). The biodiesel yields from the lipids of the former and the latter strain were 36.9 and 31.9%, respectively. Based on multivariate analysis of fatty acid methyl ester compositions, the lipids from the former and the latter strain were highly resembling those of Jatropha and palm oils, two commonly used feedstocks for biodiesel manufacturing.

Valorization of Crude Glycerol, Residue Deriving from Biodiesel- Production Process, with the Use of Wild-type New Isolated Yarrowia lipolytica Strains: Production of Metabolites with Pharmaceutical and Biotechnological Interest

BACKGROUND & OBJECTIVE

Crude glycerol (Glol), used as substrate for screening eleven natural Yarrowia lipolytica strains in shake-flask experiments. Aim of this study was to assess the ability of the screened strains to produce biomass (dry cell weight; X), lipid (L), citric acid (Cit), mannitol (Man), arabitol (Ara) and erythritol (Ery), compounds presenting pharmaceutical and biotechnological interest, in glycerol-based nitrogen-limited media, in which initial glycerol concentration had been adjusted to 40 g/L.

METHODS

Citric acid may find use in biomedical engineering (i.e. drug delivery, tissue engineering, bioimaging, orthopedics, medical device coating, wound dressings). Polyols are considered as compounds with non-cariogenic and less calorigenic properties as also with low insulin-mediated response. Microbial lipids containing polyunsaturated fatty acids (PUFA) are medically and dietetically important (selective pharmaceutical and anticancer properties, aid fetal brain development, the sight function of the eye, hormonal balance and the cardio-vascular system, prevent reasons leading to type-2 diabetes, present healing and anti-inflammatory effects).

RESULTS

All strains presented satisfactory microbial growth (Xmax=5.34-6.26 g/L) and almost complete substrate uptake. The principal metabolic product was citric acid (Citmax=8.5-31.7 g/L). Production of cellular lipid reached the values of 0.33-0.84 g/L. Polyols were also synthesized as strain dependent compounds (Manmax=2.8-6.1 g/L, Aramax ~2.0 g/L, Erymax= 0.5-3.8 g/L). The selected Y. lipolytica strain ACA-DC 5029 presented satisfactory growth along with synthesis of citric acid and polyols, thus, was further grown on media presenting an increased concentration of Glol~75 g/L. Biomass, lipid and citric acid production presented significant enhancement (Xmax=11.80 g/L, Lmax=1.26 g/L, Citmax=30.8 g/L), but conversion yield of citric acid produced per glycerol consumed was decreased compared to screening trials. Erythritol secretion (Erymax=15.6 g/L) was highly favored, suggesting a shift of yeast metabolism from citric acid accumulation towards erythritol production. Maximum endopolysaccharides (IPS) concentration was 4.04 g/L with yield in dry weight 34.2 % w/w.

CONCLUSION

Y. lipolytica strain ACA-YC 5029 can be considered as a satisfactory candidate grown in high concentrations of crude glycerol to produce added-value compounds that interest pharmaceutical and biotechnology industries.

Redox governed electro-fermentation improves lipid production by the oleaginous yeast Rhodosporidium toruloides

Electro-fermentation (EF) is a promising technique to increase the performance of bioprocesses. Here, the effect of EF on the lipid production by the yeast Rhodosporidium toruloides is studied. First, an in silico analysis was performed to unveil possible lipid yield increase and metabolic shifts by EF. Subsequently, cathodic EF (CEF) and anodic EF (AEF) were experimentally tested at different pO₂ levels. CEF enabled artificial lowering of the extracellular redox potential to less than -200 mV even under strictly aerobic conditions. CEF and AEF both positively affected lipid yield and productivity. Additional CEF cultivations with the redox mediator Neutral Red yielded an immense increase in the ratio of saturated fatty acids (from 37% to 50%). Overall, this work demonstrates that EF offers broad potential to improve microbial lipid production. In this context, the use of redox mediators might be of special future interest for the production of cocoa-butter equivalents.

Effect of Salt Addition upon the Production of Metabolic Compounds by Yarrowia lipolytica Cultivated on Biodiesel-Derived Glycerol Diluted with Olive-Mill Wastewaters

One of the major environmental problems is the highly toxic agro-industrial waste called olive mill wastewater (OMW), deriving from olive oil production. On the other hand, the continuous development of the biological liquid fuel industry (biodiesel and bioethanol) makes it mandatory the process and exploitation of their main by-products, crude glycerol. This study dealt with the biotechnological conversions of biodiesel-derived crude glycerol with the use of the non-conventional yeast Yarrowia lipolytica in media that had been diluted with OMWs. OMWs, employed as simultaneous liquid medium and substrate, is a new trend recently appearing in Industrial Biotechnology, where value-added metabolites could be produced with simultaneous partial detoxification (i.e. decolorization and phenol removal) of the used residue. In the present study, diluted OMWs (containing 2.0 g/L of total phenolic compounds) blended with 70.0 g/L crude glycerol were employed as substrates. Production of value-added compounds by Y. lipolytica strain ACA-YC 5031 was studied in nitrogen-limited media favoring the production of secondary metabolites (i.e. citric acid, polyols, microbial lipids, polysaccharides). Batch-flask cultures were carried out and the impact of the addition of different NaCl concentrations (1.0%, 3.0%, 5.0% w/w) added upon the biochemical behavior of the strain was studied. Remarkable biomass production was observed in all trials, while in the “blank” experiment (no OMWs and no salt added), the metabolism was shifted toward the synthesis of polyols (Σpolyols = mannitol + arabitol + erythritol > 20 g/L and maximum total citric acid-Cit (sum of citric and isocitric acid) = 10.5 g/L). Addition of OMWs resulted in Citmax = 32.7 g/L, while Σpolyols concentration dropped to <15 g/L. Addition of salt in the OMW-based media slightly reduced the produced biomass, while Cit production drastically increased, reaching a final value of 54.0 g/L (conversion yield of Cit produced per unit of glycerol consumed = 0.82 g/g) in the trial with addition of 5.0% NaCl. Finally, significant color and phenols removal were observed, evaluating the yeast as a decontamination medium for the OMW and a great candidate for the production of value-added compounds.

Improvement of lipid production in oleaginous yeast Rhodosporidium toruloides by ultraviolet mutagenesis

The oleaginous yeast Rhodosporidium toruloides AS 2.1389 is viewed as desirable industrial microorganisms that can accumulate a high content of lipids for biodiesel production. In this study, we attempted to improve lipid accumulation in the yeast Rhodosporidium toruloides by UV irradiation mutagenesis and selection based on lithium chloride tolerance or ethanol-H₂O₂ tolerance. The biomass concentration, lipid yield and glucose consumption of mutant R. toruloides were determined. The transcription levels of lipid accumulation-related genes in the wild-type and mutant strains were also determined. The lithium chloride-tolerant strain R-ZL2 and the ethanol-H₂O₂-resistant strain R-ZY13 were generated by UV mutagenesis. The two mutant strains showed greater lipid productivity and lipid yield compared to the wild type. Transcriptional analysis revealed that IDP1, GPD1 and GND were expressed at significantly higher levels in the two high-lipid-producing mutants. In conclusion, lipid productivity and lipid yield in R. toruloides were successfully improved via UV mutagenesis and selection. We also identified some lipid accumulation-related genes for improving lipid productivity through genetic engineering.

Production of Added-Value Chemical Compounds through Bioconversions of Olive-Mill Wastewaters Blended with Crude Glycerol by a Yarrowia lipolytica Strain

Olive mill wastewaters (OMW) are the major effluent deriving from olive oil production and are considered as one of the most challenging agro-industrial wastes to treat. Crude glycerol is the main by-product of alcoholic beverage and oleochemical production activities including biodiesel production. The tremendous quantities of glycerol produced worldwide represent a serious environmental challenge. The aim of this study was to assess the ability of Yarrowia lipolytica strain ACA-DC 5029 to grow on nitrogen-limited submerged shake-flask cultures, in crude glycerol and OMW blends as well as in media with high initial glycerol concentration and produce biomass, cellular lipids, citric acid and polyols. The rationale of using such blends was the dilution of concentrated glycerol by OMW to (partially or fully) replace process tap water with a wastewater stream. The strain presented satisfactory growth in blends; citric acid production was not affected by OMW addition (Citmax~37.0 g/L, YCit/Glol~0.55 g/g) and microbial oil accumulation raised proportionally to OMW addition (Lmax~2.0 g/L, YL/X~20% w/w). Partial removal of color (~30%) and phenolic compounds (~10% w/w) of the blended media occurred. In media with high glycerol concentration, a shift towards erythritol production was noted (Erymax~66.0 g/L, YEry/Glol~0.39 g/g) simultaneously with high amounts of produced citric acid (Citmax~79.0 g/L, YCit/Glol~0.46 g/g). Fatty acid analysis of microbial lipids demonstrated that OMW addition in blended media and in excess carbon media with high glycerol concentration favored oleic acid production.

Aseptic production of citric and isocitric acid from crude glycerol by genetically modified Yarrowia lipolytica

The unconventional yeast Yarrowia lipolytica is known for its capacity to produce citric or isocitric acid from glycerol. In this study a reduction of production cost was achieved by using cheap crude glycerol and conducting the production at pH 3 to prevent bacterial contamination. In this study a Y. lipolytica strain overexpressing Gut1 and Gut2 was used. For the modified strain, crude glycerol proved to be an excellent substrate for production of citric/isocitric acids in aseptic conditions, as the final concentration of these compounds reached 75.9 ± 1.8 g L^-1 after 7 days of batch production. Interestingly, the concentration of isocitric acid was 42.5 ± 2.4 g L^-1, which is one of the highest concentrations of isocitric acid obtained from a waste substrate. In summary, these data show that organic acids can be efficiently produced by the yeast Y. lipolytica from crude glycerol without any prior purification in aseptic conditions.

2,4-dichlorophenoxyacetic acid-induced oxidative stress: Metabolome and membrane modifications in Umbelopsis isabellina, a herbicide degrader

The study reports the response to herbicide of the 2,4-dichlorophenoxyacetic acid (2,4-D)-degrading fungal strain Umbelopsis isabellina. A comparative analysis covered 41 free amino acids as well as 140 lipid species of fatty acids, phospholipids, acylglycerols, sphingolipids, and sterols. 2,4-D presence led to a decrease in fungal catalase activity, associated with a higher amount of thiobarbituric acid-reactive substances (TBARS). Damage to cells treated with the herbicide resulted in increased membrane permeability and decreased membrane fluidity. Detailed lipidomic profiling showed changes in the fatty acids composition such as an increase in the level of linoleic acid (C18:2). Moreover, an increase in the phosphatidylethanolamine/phosphatidylcholine ratio was observed. Analysis of fungal lipid profiles revealed that the presence of 2,4-D was accompanied by the accumulation of triacylglycerols, a decrease in ergosterol content, and a considerable rise in the level of sphingolipid ceramides. In the exponential phase of growth, increased levels of leucine, glycine, serine, asparagine, and hydroxyproline were found. The results obtained in our study confirmed that in the cultures of U. isabellina oxidative stress was caused by 2,4-D. The herbicide itself forced changes not only to membrane lipids but also to neutral lipids and amino acids, as the difference of tested compounds profiles between 2,4-D-containing and control samples was consequently lower as the pesticide degradation progressed. The presented findings may have a significant impact on the basic understanding of 2,4-D biodegradation and may be applied for process optimization on metabolomic and lipidomic levels.

A Role of a Newly Identified Isomerase From Yarrowia lipolytica in Erythritol Catabolism

Erythritol is a natural sweetener produced by microorganisms as an osmoprotectant. It belongs to the group of polyols and it can be utilized by the oleaginous yeast Yarrowia lipolytica. Despite the recent identification of the transcription factor of erythritol utilization (EUF1), the metabolic pathway of erythritol catabolism remains unknown. In this study we identified a new gene, YALI0F01628g, involved in erythritol assimilation. In silico analysis showed that YALI0F01628g is a putative isomerase and it is localized in the same region as EUF1. qRT-PCR analysis of Y. lipolytica showed a significant increase in YALI0F01628g expression during growth on erythritol and after overexpression of EUF1. Moreover, the deletion strain ΔF01628 showed significantly impaired erythritol assimilation, whereas synthesis of erythritol remained unchanged. The results showed that YALI0F1628g is involved in erythritol assimilation; thus we named the gene EYI1. Moreover, we suggest the metabolic pathway of erythritol assimilation in yeast Y. lipolytica.

Fermentation Conditions and Media Optimization for Isocitric Acid Production from Ethanol by Yarrowia lipolytica

Isocitric acid exists in the form of four stereoisomers, of which only the threo-Ds-form (ICA) is a natural active compound, an intermediate of Krebs cycle, and suitable for nutritional and pharmaceutical use. In this paper, we propose a method for ICA production from ethanol by yeast Yarrowia lipolytica. The effects of temperature, pH of the medium, and aeration on the growth of the producer Y. lipolytica VKM Y-2373 and synthesis of ICA were studied. An optimal fermentation regime, which ensures a good growth of the producer and directed synthesis of the target product, was determined. The producer is advised to carry out cultivation at 29°C and various pH of the medium and the oxygen concentration (pH 5 and pO₂ 20–25% (of saturation) during the growth period and pH 6 and pO₂ 50–55% (of saturation) during the acid formation) on a nutrient medium containing an increased content of zinc (0.6 mg/L), iron (1.2 mg/L), and 30 mM itaconic acid (inhibitor of isocitrate lyase—the key enzyme of ICA metabolism) should also be introduced into the nutrition medium. Such fermentation production mode provides 90.5 g/L ICA with process selectivity of 80%, mass yield (Y_ICA) of 0.77 g/g, and energy yield (η_ICA) of 0.278 g/g.