Storage lipid and polysaccharide metabolism in Yarrowia lipolytica and Umbelopsis isabellina

Single-Cell Protein and Ethanol Production of a Newly Isolated Kluyveromyces marxianus Strain through Cheese Whey Valorization

The present work examined the production of single-cell protein (SCP) by a newly isolated strain of Kluyveromyces marxianus EXF-5288 under increased lactose concentration of deproteinized cheese whey (DCW) and different temperatures (in °C: 20.0, 25.0, 30.0 and 35.0). To the best of the authors' knowledge, this is the first report examining the ability of Kluyveromyces marxianus species to produce SCP at T = 20.0 °C. Different culture temperatures led to significant differences in the strain's growth, while maximum biomass and SCP production (14.24 ± 0.70 and 6.14 ± 0.66 g/L, respectively) were observed in the cultivation of K. marxianus strain EXF-5288 in shake-flask cultures at T = 20.0 °C. Increased DCW lactose concentrations (35.0-100.0 g/L) led to increased ethanol production (Ethmax = 35.5 ± 0.2 g/L), suggesting that K. marxianus strain EXF-5288 is "Crabtree-positive". Batch-bioreactor trials shifted the strain's metabolism to alcoholic fermentation, favoring ethanol production. Surprisingly, K. marxianus strain EXF-5288 was able to catabolize the produced ethanol under limited carbon presence in the medium. The dominant amino acids in SCP were glutamate (15.5 mg/g), aspartic acid (12.0 mg/g) and valine (9.5 mg/g), representing a balanced nutritional profile.

Molecular phylogenetics of the Umbelopsis genus-identification of new species and evaluation of their oil application value

AIMS

The aim of this study was to reconstruct the evolutionary framework of the genus Umbelopsis by using modern taxonomic strategies and evaluating the quality of oil and prospective uses of three distinct species.

METHODS AND RESULTS

Three species of Umbelopsis were identified based on morphological characteristics and phylogenetic evidence obtained from three genes (ITS, LSU, and ACT). A new species of Umbelopsis was described and illustrated, and subsequently named U. ophiocordycipiticola. The characteristics of U. ophiocordycipiticola exhibited sporangia with a diameter ranging from 8 to 17 µm. and sporangiospores that were oval to ellipsoidal in shape, irregularly angular, with dimensions of ∼1.9-2.9 × 1.7-3.0 µm. Gas chromatography and mass spectrometry (GC-MS) were used to examine the composition of fatty acids. Notably, U. ophiocordycipiticola showed a significantly higher oil content of 50.89% in dry cell weight (DCW) compared to U. vinacea and U. ramanniana. The mean proportion of polyunsaturated fatty acids (PUFAs) in U. ophiocordycipiticola was 32.38%, and the maximum levels of γ-linolenic acid (GLA), arachidonic acid (ARA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) in U. ophiocordycipiticola were found to be 14.51, 0.24, 0.54, and 0.53%, respectively. The biodiesel quality from all three species complied with applicable standards set by the American Association for Testing and Materials (ASTM 6751) and the Brazilian National Petroleum Agency (ANP 255).

CONCLUSIONS

The establishment of a novel species, U. ophiocordycipiticola, was strongly supported by morphological and molecular evidence. Umbelopsis ophiocordycipiticola exhibited a high-value PUFA content. Additionally, three Umbelopsis species demonstrated good quality for biodiesel production.

Altering environmental conditions induce shifts in simulated deep terrestrial subsurface bacterial communities-Secretion of primary and secondary metabolites

The deep terrestrial subsurface (DTS) harbours a striking diversity of microorganisms. However, systematic research on microbial metabolism, and how varying groundwater composition affects the bacterial communities and metabolites in these environments is lacking. In this study, DTS groundwater bacterial consortia from two Fennoscandian Shield sites were enriched and studied. We found that the enriched communities from the two sites consisted of distinct bacterial taxa, and alterations in the growth medium composition induced changes in cell counts. The lack of an exogenous organic carbon source (ECS) caused a notable increase in lipid metabolism in one community, while in the other, carbon starvation resulted in low overall metabolism, suggesting a dormant state. ECS supplementation increased CO2 production and SO4 2- utilisation, suggesting activation of a dissimilatory sulphate reduction pathway and sulphate-reducer-dominated total metabolism. However, both communities shared common universal metabolic features, most probably involving pathways needed for the maintenance of cell homeostasis (e.g., mevalonic acid pathway). Collectively, our findings indicate that the most important metabolites related to microbial reactions under varying growth conditions in enriched DTS communities include, but are not limited to, those linked to cell homeostasis, osmoregulation, lipid biosynthesis and degradation, dissimilatory sulphate reduction and isoprenoid production.

Challenges and prospects of yeast-based microbial oil production within a biorefinery concept

Biodiesel, unlike to its fossil-based homologue (diesel), is renewable. Its use contributes to greater sustainability in the energy sector, mainly by reducing greenhouse gas emissions. Current biodiesel production relies on plant- and animal-related feedstocks, resulting in high final costs to the prices of those raw materials. In addition, the production of those materials competes for arable land and has provoked a heated debate involving their use food vs. fuel. As an alternative, single-cell oils (SCOs) obtained from oleaginous microorganisms are attractive sources as a biofuel precursor due to their high lipid content, and composition similar to vegetable oils and animal fats. To make SCOs competitive from an economic point of view, the use of readily available low-cost substrates becomes essential. This work reviews the most recent advances in microbial oil production from non-synthetic sugar-rich media, particularly sugars from lignocellulosic wastes, highlighting the main challenges and prospects for deploying this technology fully in the framework of a Biorefinery concept.

What can be lost? Genomic perspective on the lipid metabolism of Mucoromycota

Mucoromycota is a phylum of early diverging fungal (EDF) lineages, of mostly plant-associated terrestrial fungi. Some strains have been selected as promising biotechnological organisms due to their ability to produce polyunsaturated fatty acids and efficient conversion of nutrients into lipids. Others get their lipids from the host plant and are unable to produce even the essential ones on their own. Following the advancement in EDF genome sequencing, we carried out a systematic survey of lipid metabolism protein families across different EDF lineages. This enabled us to explore the genomic basis of the previously documented ability to produce several types of lipids within the fungal tree of life. The core lipid metabolism genes showed no significant diversity in distribution, however specialized lipid metabolic pathways differed in this regard among different fungal lineages. In total 165 out of 202 genes involved in lipid metabolism were present in all tested fungal lineages, while remaining 37 genes were found to be absent in some of fungal lineages. Duplications were observed for 69 genes. For the first time we demonstrate that ergosterol is not being produced by several independent groups of plant-associated fungi due to the losses of different ERG genes. Instead, they possess an ancestral pathway leading to the synthesis of cholesterol, which is absent in other fungal lineages. The lack of diacylglycerol kinase in both Mortierellomycotina and Blastocladiomycota opens the question on sterol equilibrium regulation in these organisms. Early diverging fungi retained most of beta oxidation components common with animals including Nudt7, Nudt12 and Nudt19 pointing at peroxisome divergence in Dikarya. Finally, Glomeromycotina and Mortierellomycotina representatives have a similar set of desaturases and elongases related to the synthesis of complex, polyunsaturated fatty acids pointing at an ancient expansion of fatty acid metabolism currently being explored by biotechnological studies.

Solid-state fermentation with Zygomycetes fungi as a tool for biofortification of apple pomace with γ-linolenic acid, carotenoid pigments and phenolic antioxidants

In the last few years, there has been a growing interest in the more efficient utilization of agricultural and food by-products. Apples are among the most processed fruits in the world that generate huge quantities of processing waste biomasses. Therefore, the objective of this study was to improve the nutritional value of apple pomaces with γ-linolenic acid (GLA) and carotenoid pigments by solid-state fermentation (SSF) using two Zygomycetes fungi (Actinomucor elegans and Umbelopsis isabellina). The impact of fermentation periods on the polyphenol content and antioxidant capacity of the bioprocessed apple pomace was also investigated. The accumulated lipids were composed primarily of neutral fractions (mostly triacylglycerols). SSF with U. isabellina yielded a 12.72% higher GLA content than with A. elegans (3.85 g GLA/kg DW of pomace). Contrary to the lipogenic capacity, A. elegans showed higher carotenoids and phenolic antioxidants productivity than U. isabellina. The maximum concentrations for β-carotene (433.11 μg/g DW of pomace-SSF with A. elegans and 237.68 μg/g DW of pomace-SSF with U. isabellina), lutein (374.48 μg/g DW- A. elegans and 179.04 μg/g DW- U. isabellina) and zeaxanthin (247.35 μg/g DW- A. elegans and 120.41 μg/g DW- U. isabellina) were registered on the 12th day of SSFs. In the case of SSF with A. elegans, the amount of total phenolics increased significantly (27%) by day 4 from the initial value (2670.38 μg of gallic acid equivalents/g DW) before slowly decreasing for the remaining period of the fungal growth. The experimental findings showed that a prolonged fermentation (between 8 and 12 days) should be applied to obtain value-added apple pomaces (rich in GLA and carotenoids) with potential pharmaceutical and functional food applications. Moreover, the SSF processes of simultaneous bioaccumulation of valuable fatty acids, carotenoids and phenolic antioxidants proposed in the present study may open up new challenges for biotechnological production of industrially important biomolecules using abundant and unexploited apple pomaces.

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.

Redistributing Carbon Flux by Impairing Saccharide Synthesis to Enhance Lipid Yield in Oleaginous Fungus Mortierella alpina

Increasing carbon flux toward target metabolites is important in improving microbial productivity and economic value. To improve the efficiency of lipid production in Mortierella alpina, we knocked down genes for trehalose-6-phosphate synthetase (Matps) and phosphoenolpyruvate carboxykinase (Mapepck) in the major pathways for saccharide synthesis. The knockdown of Matps reduced trehalose content by an average of 31.87%, while the knockdown of Mapepck reduced the total saccharide content by 28.6%, and both recombinant strains showed more than 20% increased lipid yield. Trehalose plays a vital role in stress resistance, but a higher polyunsaturated fatty acid-rich lipid content was found to partly compensate for the loss of trehalose after Matps knockdown. As compared with Matps knockdown, the knockdown of Mapepck gave better lipid production by bringing forward the time to maximum lipid yield by three days in a scale-up test. The arachidonic acid yield after the Mapepck knockdown reached 1.23 g/L, which was 39.9% higher than that of the original strain. The present research provided an efficient strategy for redistributing carbon flux among different metabolites and therefore promoted microbial lipid yield in a shorter fermentation period.

Application of Adaptive Laboratory Evolution in Lipid and Terpenoid Production in Yeast and Microalgae

Due to the complexity of metabolic and regulatory networks in microorganisms, it is difficult to obtain robust phenotypes through artificial rational design and genetic perturbation. Adaptive laboratory evolution (ALE) engineering plays an important role in the construction of stable microbial cell factories by simulating the natural evolution process and rapidly obtaining strains with stable traits through screening. This review summarizes the application of ALE technology in microbial breeding, describes the commonly used methods for ALE, and highlights the important applications of ALE technology in the production of lipids and terpenoids in yeast and microalgae. Overall, ALE technology provides a powerful tool for the construction of microbial cell factories, and it has been widely used in improving the level of target product synthesis, expanding the range of substrate utilization, and enhancing the tolerance of chassis cells. In addition, in order to improve the production of target compounds, ALE also employs environmental or nutritional stress strategies corresponding to the characteristics of different terpenoids, lipids, and strains.

The monitoring of oil production process by deep learning based on morphology in oleaginous yeasts

BACKGROUND

Monitoring jar fermenter-cultured microorganisms in real time is important for controlling productivity of bioproducts in large-scale cultivation settings. Morphological data is used to understand the growth and fermentation states of these microorganisms during monitoring. Oleaginous yeasts are used for their high productivity of single-cell oils but the relationship between lipid productivity and morphology has not been elucidated in these organisms.

RESULTS

In this study, we investigated the relationship between the morphology of oleaginous yeasts (Lipomyces starkeyi and Rhodosporidium toruloides were used) and their cultivation state in a large-scale cultivation setting using a real-time monitoring system. We combined this with deep learning by feeding a large amount of high-definition cell images obtained from the monitoring system to a deep learning algorithm. Our results showed that the cell images could be grouped into 7 distinct groups and that a strong correlation existed between each group and its biochemical activity (growth and oil-productivity).

CONCLUSIONS

This is the first report describing the morphological variations of oleaginous yeasts in a large-scale cultivation, and describes a promising new avenue for improving productivity of microorganisms in large-scale cultivation through the use of a real-time monitoring system combined with deep learning.

KEY POINTS

• A real-time monitoring system followed the morphological change of oleaginous yeasts. • Deep learning grouped them into 7 distinct groups based on their morphology. • A correlation between the cultivation state and the shape of the yeast was observed.

Phosphate limitation as crucial factor to enhance yeast lipid production from short-chain fatty acids

Microbial lipids for chemical synthesis are commonly obtained from sugar-based substrates which in most cases is not economically viable. As a low-cost carbon source, short-chain fatty acids (SCFAs) that can be obtained from food wastes offer an interesting alternative for achieving an affordable lipid production process. In this study, SCFAs were employed to accumulate lipids using Yarrowia lipolytica ACA DC 50109. For this purpose, different amounts of SCFAs, sulfate, phosphate and carbon: phosphate ratios were used in both synthetic and real SCFAs-rich media. Although sulfate limitation did not increase lipid accumulation, phosphate limitation was proved to be an optimal strategy for increasing lipid content and lipid yields in both synthetic and real media, reaching a lipid productivity up to 8.95 g/L h. Remarkably, the highest lipid yield (0.30 g/g) was achieved under phosphate absence condition (0 g/L). This fact demonstrated the suitability of using low-phosphate concentrations to boost lipid production from SCFAs.

Key role of fluorescence quantum yield in Nile Red staining method for determining intracellular lipids in yeast strains

Background

Microbial lipids are found to be an interesting green alternative to expand available oil sources for the chemical industry. Yeasts are considered a promising platform for sustainable lipid production. Remarkably, some oleaginous yeasts have even shown the ability to grow and accumulate lipids using unusual carbon sources derived from organic wastes, such as volatile fatty acids. Recent research efforts have been focused on developing rapid and accurate fluorometric methods for the quantification of intracellular yeast lipids. Nevertheless, the current methods are often tedious and/or exhibit low reproducibility.

Results

This work evaluated the reliability of different fluorescence measurements (fluorescence intensity, total area and fluorescence quantum yield) using Nile Red as lipid dye in two yeast strains (Yarrowia lipolytica ACA-DC 50109 and Cutaneotrichosporon curvatum NRRL-Y-1511). Different standard curves were obtained for each yeast specie. Fermentation tests were carried with 6-month difference to evaluate the effect of the fluorometer lamp lifetime on lipid quantification.

Conclusions

Fluorescence quantum yield presented the most consistent measurements along time and the closer estimations when compared with lipids obtained by conventional methods (extraction and gravimetrical determination). The need of using fluorescence quantum yield to estimate intracellular lipids, which is not the common trend in studies focused on microbial lipid production, was stressed. The information here provided will surely enable more accurate results comparison.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13068-022-02135-9.

Different factors drive the assembly of pine and Panax notoginseng-associated microbiomes in Panax notoginseng-pine agroforestry systems

Land-use conversion affects the composition and assembly of plant-associated microbiomes, which in turn affects plant growth, development, and ecosystem functioning. However, agroforestry systems, as sustainable land types, have received little attention regarding the dynamics of different plant-associated microbes. In this study, we used high-throughput sequencing technology to analyze the assembly mechanisms and the driving factors of pine- and Panax notoginseng (P.n.)-associated microbiomes during the conversion of different pine forests (Pinus kesiya var. langbianensis and Pinus armandii) into P.n.-pine agroforestry systems. The results showed that the conversion of pure pine forest into P.n.-pine agroforestry systems significantly altered the diversity of pine-associated fungi rather than the community structure, and the community structure of P.n.-associated fungi rather than the diversity. Additionally, plant-associated fungi were more responsive to land-use change than bacteria. Main effect analysis revealed that compartment rather than genotype was the driving factor of pine- and P.n.-associated microbiomes, but P.n. cultivation also significantly affected the assembly of pine-associated microbiomes. In addition, there was a transfer of P.n. endophytes to pine trees in agroforestry systems and the beneficial microbiomes (Massilia, Marmoricola, Herbaspirillum, etc.) were enlarged in pine roots. Therefore, the diversity of the assembly mechanisms of P.n.- and pine-associated microbiomes played an important role in the P.n.--pine agroforestry systems and were the basis for the sustainable development of the P.n.--pine agroforestry systems.

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.

In-situ transesterification of single-cell oil for biodiesel production: a review

In recent years, biodiesel synthesis and production demands have increased because of its high degradability, cleaner emissions, non-toxicity, and an alternative to petroleum diesel. In this context, Single Cell Oil (SCO) has been identified as an alternative feedstock, having the advantage of accumulating high intracellular lipid. SCO/microbial lipids are potential alternatives for sustainable biodiesel production. The traditional technique for biodiesel production from the oils obtained from microbes generally requires two steps: lipid extraction and transesterification. In-situ transesterification is an innovative and renewable process for biodiesel production. It rules out the need to isolate and refine the feedstock lipid, as it directly uses biomass in a single step, i.e., the pretreated biomass will be subjected to in-situ transesterification in the presence of catalysts. Hence, the production cost can be reduced by eliminating the lipid extraction procedure. The current review focuses on the basic features and advantages of in-situ transesterification of SCO for biodiesel production with the aid of short-chain alcohols along with different acid, base, and enzyme catalysts. In addition, a comparative study was carried out to highlight the merits of in-situ transesterification over conventional transesterification.

Supercritical fluid extraction (SCFE) as green extraction technology for high-value metabolites of algae, its potential trends in food and human health

Application of high-value algal metabolites (HVAMs) in cosmetics, additives, pigments, foods and medicines are very important. These HVAMs can be obtained from the cultivation of micro- and macro-algae. These metabolites can benefit human and animal health in a physiological and nutritional manner. However, because of conventional extraction methods and their energy and the use of pollutant solvents, the availability of HVAMs from algae remains insufficient. Receiving their sustainability and environmental benefits have recently made green extraction technologies for HVAM extractions more desirable. But very little information is available about the technology of green extraction of algae from these HVAM. This review, therefore, highlights the supercritical fluid extraction (SCFE) as principal green extraction technologyand theirideal parameters for extracting HVAMs. In first, general information is provided concerning the HVAMs and their components of macro and micro origin. The review also includes a description of SCFE technology's properties, instrumentation operation, solvents used, and the merits and demerits. Moreover, there are several HVAMs associated with their numerous high-level biological activities which include high-level antioxidant, anti-inflammatory, anticancer and antimicrobial activity and have potential health-beneficial effects in humans since they are all HVAMs, such as foods and nutraceuticals. Finally, it provides future insights, obstacles, and suggestions for selecting the right technologies for extraction.

SNF1β-Modulated Glucose Uptake and the Balance between Polyunsaturated Fatty Acids and Carbohydrates in Mortierella alpina

A sucrose nonfermenting protein kinase 1 (SNF1) complex is an important metabolic regulator in fungi that is critical to cell metabolism and stress response. In this study, the role of an SNF1 β-subunit in the oleaginous fungus Mortierella alpina (MaSip2) was investigated. The MaSip2 contained a glycogen-binding domain and a conserved SNF1-complex interaction region; its transcriptional level during lipogenesis shared high consistency with a previously reported SNF1 γ-subunit (MaSnf4). Overexpression of MaSip2 in M. alpina significantly promoted glucose uptake and resulted in 34.1% increased total biomass, leading to 44.8% increased arachidonic acid yield after 7 day fermentation. MaSip2 also regulated the balance between polyunsaturated fatty acids and carbohydrates in M. alpina. Intracellular metabolite analysis revealed increased carbohydrate-related metabolite accumulation in MaSip2 overexpression strains. On the contrary, knockdown of MaSip2 increased the total fatty acid unsaturation degree, especially under low-temperature conditions. This research improved our knowledge of SNF1 complex in M. alpina and provided a target gene for enhancing glucose utilization and modulating fatty acid composition for better application of oleaginous fungi.

Adaptive laboratory evolution principles and applications in industrial biotechnology

Adaptive laboratory evolution (ALE) is an innovative approach for the generation of evolved microbial strains with desired characteristics, by implementing the rules of natural selection as presented in the Darwinian Theory, on the laboratory bench. New as it might be, it has already been used by several researchers for the amelioration of a variety of characteristics of widely used microorganisms in biotechnology. ALE is used as a tool for the deeper understanding of the genetic and/or metabolic pathways of evolution. Another important field targeted by ALE is the manufacturing of products of (high) added value, such as ethanol, butanol and lipids. In the current review, we discuss the basic principles and techniques of ALE, and then we focus on studies where it has been applied to bacteria, fungi and microalgae, aiming to improve their performance to biotechnological procedures and/or inspect the genetic background of evolution. We conclude that ALE is a promising and efficacious method that has already led to the acquisition of useful new microbiological strains in biotechnology and could possibly offer even more interesting results in the future.

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.

Recent advances in biotechnological production of polyunsaturated fatty acids by Yarrowia lipolytica

Owing to the important physiological functions, polyunsaturated fatty acids (PUFAs) play a vital role in protecting human health, such as preventing cancer, cardiovascular disease, and diabetes. Specifically, Yarrowia lipolytica has been identified as the most popular non-conventional oleaginous yeast, which can accumulate the abundant intracellular lipids, indicating that has great potential as an industrial host for production of PUFAs. Notably, some novel engineering strategies have been applied to endow and improve the abilities of Y. lipolytica to synthesize PUFAs, including construction and optimization of PUFAs biosynthetic pathways, improvement of preucrsors acetyl-coA and NADPH supply, inhibition of competing pathways, knockout of β-oxidation pathways, regulation of oxidative stress defense pathways, and regulation of genes involved in upstream lipid metabolism. Besides, some bypass approaches, such as strain mating, evolutionary engineering, and computational model based on omics, also have been proposed to improve the performance of engineering strains. Generally, in this review, we summarized the recent advances in engineering strategies and bypass approaches for improving PUFAs production by Y. lipolytica. In addition, we further summarized the latest efforts of CRISPR/Cas genome editing technology in Y. lipolytica, which is aimed to provide its potential applications in PUFAs production.

Carbohydrate analysis of Mortierella alpina by colorimetry and HPLC-ELSD to reveal accumulation differences of sugar and lipid

OBJECTIVES

To establish reliable methods for the extraction and quantification of the total carbohydrate and intracellular saccharides from Mortierella alpina and study the changes between carbohydrate and lipid in fermentation process.

RESULTS

The extraction of mycelia with HCl following a photometric phenol-sulphuric acid reaction was identified as an optimal method for total carbohydrate analysis in Mortierella alpina, which the extraction efficiency performed 1.1-3.6 fold than other five methods. The total carbohydrate content increased from initial 19.26 to 25.86% during early fermentation process and declined gradually thereafter, while the fatty acid was increasing from 8.47 to 31.03%. For separation and qualitative estimation of intracellular saccharides, the acetonitrile/water freeze-thaw method for extraction and Sugar-Pak I column for separation proved to be possible. With the glucose rapidly decreasing at the beginning of growth, the trehalose accumulated rapidly from 1.63 to 5.04% and then decreased slightly but maintain above 4% of dry biomass.

CONCLUSIONS

This work established comprehensive carbohydrate extraction and analysis methods of Mortierella alpina and identified the main saccharide in fermentation process which indicated that the accumulation of fatty acids was related to the change of intracellular carbohydrate content.

A fattening factor to quantify the accumulation ability of microorganisms under N-starvation

Many microorganisms can accumulate biomass in the form of lipids and polysaccharides, which can be used for biofuels, bioplastics, food and feed. Some innovative bioprocesses exploit the competitive advantage provided by such accumulation ability, mainly under N-starvation, to select high-accumulating strains against biological contaminants, by using uncoupled nutrient feeding. However, there is no general and easily comparable parameter available to compare biomass accumulation ability among different microbial strains, which could measure the competitive advantage. Here, a parameter termed "fattening factor" (η_x) is described to quantify such strain-specific biomass accumulation ability in bacteria, yeasts and microalgae. This parameter measures how many fold a microbial population can increase its biomass just as the result of accumulation. It is derived from considerations about the main metabolic aspects of cells' response to N-starvation, which induces variations in cell cycle, biomass production and biochemical composition. The fattening factor described here should be easily estimatable in N-starvation for every culturable microbial strain, by measuring the amount of accumulated biomass.

Cultivation processes to select microorganisms with high accumulation ability

The microbial ability to accumulate biomolecules is fundamental for different biotechnological applications aiming at the production of biofuels, food and bioplastics. However, high accumulation is a selective advantage only under certain stressful conditions, such as nutrient depletion, characterized by lower growth rate. Conventional bioprocesses maintain an optimal and stable environment for large part of the cultivation, that doesn't reward cells for their accumulation ability, raising the risk of selection of contaminant strains with higher growth rate, but lower accumulation of products. Here in this work the physiological responses of different microorganisms (microalgae, bacteria, yeasts) under N-starvation and energy starvation are reviewed, with the aim to furnish relevant insights exploitable to develop tailored bioprocesses to select specific strains for their higher accumulation ability. Microorganism responses to starvation are reviewed focusing on cell cycle, biomass production and variations in biochemical composition. Then, the work describes different innovative bioprocess configurations exploiting uncoupled nutrient feeding strategies (feast-famine), tailored to maintain a selective pressure to reward the strains with higher accumulation ability in mixed microbial populations. Finally, the main models developed in recent studies to describe and predict microbial growth and intracellular accumulation upon N-starvation and feast-famine conditions have been reviewed.

Effect of Selenium on the Growth and Lipid Accumulation of Yarrowia lipolytica Yeast

Nowadays, there is an increase attention on the effect of selenium (Se) on metabolic processes of microorganisms. Strains belonging to the genus of Yarrowia are of great biotechnological interest for various industries. In this study, we evaluated the effect of 10 mg/L of Se on the growth and lipid production of two Yarrowia lipolytica strains: the ACA DC 50109 and one more with increased oleagenicity, derived after ALE methodology (referred here as Y. lipolytica ALE_70). The presence of Se in the growth medium negatively affected both cell mass production and total lipid accumulation, for both Y. lipolytica strains. Fractionation of total lipids showed an inhibition on neutral lipid (NL) synthesis and consequently, an increase of polar lipids (glycolipids plus sphingolipids, and phospholipids) on the lipids of the Se-enriched ACA DC 50109 strain; however, the NL/polar ratio of the Se-enriched ALE_70 indicated that Se, apart from the inhibition of NL synthesis, provoked also the accumulation of polar lipids in this strain. In addition, the fatty acid (FA) composition was differently affected by Se. Se-enriched total lipids of the ALE_70 strain were enriched in linoleic acid (C18:2 n-6), which resulted in increase of the unsaturated index. On the other hand, Se-enriched lipids of the ACA DC 50109 strain were more saturated, as the percentage of palmitic (C16:0) and stearic (C18:0) acids increased in the total FAs. Moreover, it seems that Se influenced the activity or the expression of desaturases and elongase in both strains. Finally, the supplementation of growth medium with Se affected cell morphology, as well as the size and distribution of lipid droplets inside the yeast cells. According to our opinion, Se caused stress conditions and the consequence of that was the occurrence of metabolic disorders that affected cell mass, lipid content, and/or morphological structures. The results of the present study suggest that further research should be carried out to understand the background of the lipogenesis process in yeast cells cultured under stress conditions.

Microbial lipids from organic wastes: Outlook and challenges

Microbial lipids have recently drawn a lot of attention as renewable sources for biochemicals production. Strong research efforts have been addressed to efficiently use organic wastes as carbon source for microbial lipids, which would definitively increase the profitability of the production process and boost a bio-based economy. This review compiles interesting traits of oleaginous microorganisms and highlights current trends on microbial- and process-oriented approaches to maximize microbial oil production from inexpensive substrates like lignocellulosic sugars, volatile fatty acids and glycerol. Furthermore, downstream processes such as cell harvesting or lipid extraction, that are decisive for the cost-effectiveness of the process, are discussed. To underpin microbial oils within the so demanded circular economy, associated challenges, recent advances and possible industrial applications that are also identified in this review.

Valorization of volatile fatty acids derived from low-cost organic waste for lipogenesis in oleaginous microorganisms-A review

To meet environmental sustainability goals, microbial oils have been suggested as an alternative to petroleum-based products. At present, microbial fermentation for oil production relies on pure sugar-based feedstocks. However, these feedstocks are expensive and are in limited supply. Volatile fatty acids, which are generated as intermediates during anaerobic digestion of organic waste have emerged as a renewable feedstock that has the potential to replace conventional sugar sources for microbial oil production. They comprise short-chain (C2 to C6) organic acids and are employed as building blocks in the chemical industry. The present review discusses the use of oleaginous microorganisms for the production of biofuels and added-value products starting from volatile fatty acids as feedstocks. The review describes the metabolic pathways enabling lipogenesis from volatile fatty acids, and focuses on strategies to enhance lipid accumulation in oleaginous microorganisms by tuning the ratios of volatile fatty acids generated via anaerobic fermentation.

Single Cell Oil Production by Oleaginous Yeasts Grown in Synthetic and Waste-Derived Volatile Fatty Acids

Four yeast isolates from the species—Apiotrichum brassicae, Candida tropicalis, Metschnikowia pulcherrima, and Pichia kudriavzevii—previously selected by their oleaginous character and growth flexibility in different carbon sources, were tested for their capacity to convert volatile fatty acids into lipids, in the form of single cell oils. Growth, lipid yields, volatile fatty acids consumption, and long-chain fatty acid profiles were evaluated in media supplemented with seven different volatile fatty acids (acetic, butyric, propionic, isobutyric, valeric, isovaleric, and caproic), and also in a dark fermentation effluent filtrate. Yeasts A. brassicae and P. kudriavzevii attained lipid productivities of more than 40% (w/w), mainly composed of oleic (>40%), palmitic (20%), and stearic (20%) acids, both in synthetic media and in the waste-derived effluent filtrate. These isolates may be potential candidates for single cell oil production in larger scale applications by using alternative carbon sources, combining economic and environmental benefits.

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.

Highly Effective, Regiospecific Hydrogenation of Methoxychalcone by Yarrowia lipolytica Enables Production of Food Sweeteners

We describe the impact of the number and location of methoxy groups in the structure of chalcones on the speed and efficiency of their transformation by unconventional yeast strains. The effect of substrate concentration on the conversion efficiency in the culture of the Yarrowia lipolytica KCh 71 strain was tested. In the culture of this strain, monomethoxychalcones (2′-hydroxy-2″-, 3″- and 4″-methoxychalcone) were effectively hydrogenated at over 40% to the specific dihydrochalcones at a concentration of 0.5 g/L of medium after just 1 h of incubation. A conversion rate of over 40% was also observed for concentrations of these compounds of 1 g/L of medium after three hours of transformation. As the number of methoxy substituents increases in the chalcone substrate, the rate and efficiency of transformation to dihydrochalcones decreased. The only exception was 2′-hydroxy-2″,5″-dimethoxychalcone, which was transformed into dihydrochalcone by strain KCh71 with a yield comparable to that of chalcone containing a single methoxy group.

Production of tailor-made fatty acids from crude glycerol at low pH by Yarrowia lipolytica

Nowadays, single cell oil (SCO) can play two distinct roles, first as a supplier of functional oils, and second as a feedstock for the biodiesel industry. These two distinct functions require a different fatty acids (FA) profile in the lipid pool. Moreover, to exploit their potential for industrialization, it is necessary to employ a low-cost substrate. Crude glycerol is the main side-product of biodiesel production. This renewable feedstock is one of Yarrowia lipolytica favorable substrates. In this study we improved polyunsaturated fatty acids (PUFA) synthesis by overexpression of the glycerol phosphate acyltransferase gene (SCT1). Here, we established a method to alter the quantity and FA composition of SCO. The engineered strain showed a 10-fold improvement (>20%) in linoleic acid synthesis (C18:2) in a shake-flask experiment. In a fermenter study co-overexpression of glycerol kinase (GUT1) and SCT1 allowed for 3-fold improvement in C18:2 synthesis from crude glycerol and at low pH.

Microbial sources of polyunsaturated fatty acids (PUFAs) and the prospect of organic residues and wastes as growth media for PUFA-producing microorganisms

The discovery of non-fish sources of polyunsaturated fatty acids (PUFAs) is of great biotechnological importance. Although various oleaginous microalgae and fungi are able of accumulating storage lipids (single cell oils - SCOs) containing PUFAs, the industrial applications utilizing these organisms are rather limited due to the high-fermentation cost. However, combining SCO production with other biotechnological applications, including waste and by-product valorization, can overcome this difficulty. In the current review, we present the major sources of fungi (i.e. members of Mucoromycota, fungoid-like Thraustochytrids and genetically modified strains of Yarrowia lipolytica) and microalgae (e.g. Isochrysis, NannochloropsisandTetraselmis) that have come recently to the forefront due to their ability to produce PUFAs. Approaches adopted in order to increase PUFA productivity and the potential of using various residues, such as agro-industrial, food and aquaculture wastes as fermentation substrates for SCO production have been considered and discussed. We concluded that several organic residues can be utilized as feedstock in the SCO production increasing the competitiveness of oleaginous organisms against conventional PUFA producers.

High Efficacy of Ozonated Oils on the Removal of Biofilms Produced by Methicillin-Resistant Staphylococcus aureus (MRSA) from Infected Diabetic Foot Ulcers

Ozone has a high wound healing capacity and antibacterial properties and can be used as a complementary treatment in infections. Methicillin-resistant S. aureus (MRSA) is the most common pathogen found in infected diabetic foot ulcers. Most of MRSA are resistant to several classes of antibiotics and, therefore, there is a need for new, effective, and well-tolerated agents. Thus, we aimed evaluate the antimicrobial and antibiofilm potentials of ozonated vegetable oils against MRSA strains isolated from diabetic foot ulcers. Six ozonated oils were produced with concentrations of ozone ranging from 0.53 to 17 mg of ozone/g of oil. The peroxide values were determined for each oil. Ozonated oils content on fatty acid was determined by gas chromatography equipped with a flame ionization detector. The antimicrobial susceptibility testing was performed by the Kirby–Bauer disk diffusion method and the effect of ozonated oils on biofilm formation ability and on established biofilms was investigated. In general, the content in identified unsaturated fatty acid in oils decreased with the increase of ozonation time and, consequently, the peroxide value increased. Most bacterial strains were inhibited by ozonated oil at a concentration of 4.24 mg/g. Ozonated oils had moderate to high ability to remove adhered cells and showed a high capacity to eradicate 24 h old biofilms. Our results show promising use of ozonated oils on the treatment of infections, in particular those caused by multidrug-resistant MRSA strains.

Patterns of Lignocellulosic Sugar Assimilation and Lipid Production by Newly Isolated Yeast Strains From Chilean Valdivian Forest

Three yeast strains were isolated from decaying wood of Chilean Valdivian forest and identified as Meyerozyma guilliermondii, Scheffersomyces coipomensis, and Sugiyamaella paludigena. These strains were able to efficiently grow on the major monomers contained in Pinus spp. and Eucalyptus spp. wood that includes glucose (Glc), xylose (Xyl), and mannose (Man), showing at 28 °C higher uptake rates for Man, and in some cases for Glc, than for Xyl, used as single carbon sources. Nevertheless, in cultures performed on sugar mixtures, the strains displayed a notable preference for Glc. Additionally, in sugar mixtures, the absence of regulatory mechanisms in sugar assimilation (e.g., catabolic repression) was observed and documented when the activities of several enzymes involved in sugar assimilation (i.e., phosphoglucose isomerase, phosphomannose isomerase, and xylulokinase) were determined. The activity of the key enzymes involved in the onset of lipid accumulation (i.e., NAD⁺-ICDH) and in fatty acid (FA) biosynthesis (i.e., ATP:CL) indicated a significant accumulation of storage lipids (i.e., up to 24%, w/w) containing oleic and palmitic acids as the major components. The present paper is the first report on the potential of M. guilliermondii, S. coipomensis, and S. paludigena as oleaginous yeasts. We conclude that the new isolates, being able to simultaneously assimilate the major lignocellulosic sugars and efficiently convert them into oily biomass, present a biotechnological potential which deserve further investigation.

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.

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.

Heterologous overexpression of bacterial hemoglobin VHb improves erythritol biosynthesis by yeast Yarrowia lipolytica

Background

Yarrowia lipolytica is an unconventional yeast with a huge industrial potential. Despite many advantages for biotechnological applications, it possesses enormous demand for oxygen, which is a bottleneck in large scale production. In this study a codon optimized bacterial hemoglobin from Vitreoscilla stercoraria (VHb) was overexpressed in Y. lipolytica for efficient growth and erythritol synthesis from glycerol in low-oxygen conditions. Erythritol is a natural sweetener produced by Y. lipolytica under high osmotic pressure and at low pH, and this process requires high oxygen demand.

Results

Under these conditions the VHb overexpressing strain showed mostly yeast-type cells resulting in 83% higher erythritol titer in shake-flask experiments. During a bioreactor study the engineered strain showed higher erythritol productivity (Q_ERY = 0.38 g/l h) and yield (Y_ERY = 0.37 g/g) in comparison to the control strain (Q_ERY = 0.30 g/l h, Y_ERY = 0.29 g/g). Moreover, low stirring during the fermentation process resulted in modest foam formation.

Conclusions

This study showed that overexpression of VHb in Y. lipolytica allows for dynamic growth and efficient production of a value-added product from a low-value substrate.

Sources of microbial oils with emphasis to Mortierella (Umbelopsis) isabellina fungus

The last years a constantly rising number of publications have appeared in the literature in relation to the production of oils and fats deriving from microbial sources (the "single cell oils"-SCOs). SCOs can be used as precursors for the synthesis of lipid-based biofuels or employed as substitutes of expensive oils rarely found in the plant or animal kingdom. In the present review-article, aspects concerning SCOs (economics, biochemistry, substrates, technology, scale-up), with emphasis on the potential of Mortierella isabellina were presented. Fats and hydrophilic substrates have been used as carbon sources for cultivating Zygomycetes. Among them, wild-type M. isabellina strains have been reported as excellent SCO-producers, with conversion yields on sugar consumed and lipid in DCW values reported comparable to the maximum ones achieved for genetically engineered SCO-producing strains. Lipids produced on glucose contain γ-linolenic acid (GLA), a polyunsaturated fatty acid (PUFA) of high dietary and pharmaceutical importance, though in low concentrations. Nevertheless, due to their abundance in oleic acid, these lipids are perfect precursors for the synthesis of 2nd generation biodiesel, while GLA can be recovered and directed to other usages. Genetic engineering focusing on over-expression of Δ6 and Δ12 desaturases and of C16 elongase may improve the fatty acid composition (viz. increasing the concentration of GLA or other nutritionally important PUFAs) of these lipids.

Understanding lipogenesis by dynamically profiling transcriptional activity of lipogenic promoters in Yarrowia lipolytica

Lipogenesis is a complicated process involving global transcriptional reprogramming of lipogenic pathways. It is commonly believed that nitrogen starvation triggers a metabolic shift that reroutes carbon flux from Krebs cycles to lipogenesis. In this study, we systematically surveyed and dynamically profiled the transcriptional activity of 22 lipogenic promoters aiming to delineate a picture how nitrogen starvation regulates lipogenesis in Y. lipolytica. These lipogenic promoters drive the expression of critical pathways that are responsible for the generation of reducing equivalents (NADPH), carbon backbones (acetyl-CoA, malonyl-CoA, DHAP, etc.), synthesis and degradation of fatty acids. Specifically, our investigated promoters span across an array of metabolic pathways, including glycolysis, Krebs cycle, pentose phosphate pathway, mannitol cycle, glutamine-GABA cycle, fatty acid and lipid synthesis, glyoxylate, β-oxidation, and POM (pyruvate-oxaloacetate-malate) cycle. Our work provides evidences that mannitol cycle, glutamine-GABA cycle and amino acid degradation, pyruvate oxidation, and acetate assimilation pathways are lipogenesis-related steps involved in generating cytosolic NADPH and acetyl-CoA precursors. This systematic investigation and dynamic profiling of lipogenic promoters may help us better understand lipogenesis, facilitate the formulation of structure-based kinetic models, as well as develop efficient cell factories for fuels and chemical production in oleaginous species.

Microbial oil production from acidified glycerol pretreated sugarcane bagasse by Mortierella isabellina

An integrated microbial oil production process consisting of acidified glycerol pretreatment of sugarcane bagasse, enzymatic hydrolysis, microbial oil production by Mortierella isabellina NRRL 1757 and oil recovery by hydrothermal liquefaction (HTL) of fungal biomass in fermentation broth was assessed in this study. Following pretreatment, the effect of residual pretreatment hydrolysate (containing glycerol) on enzymatic hydrolysis was firstly studied. The residual pretreatment hydrolysate (corresponding to 2.0–7.5% glycerol) improved glucan enzymatic digestibilities by 10–11% compared to the enzymatic hydrolysis in water (no buffer). Although residual pretreatment hydrolysate at 2.0–5.0% glycerol slightly inhibited the consumption of glucose in enzymatic hydrolysate by M. isabellina NRRL 1757, it did not affect microbial oil production due to the consumption of similar amounts of total carbon sources including glycerol. When the cultivation was scaled-up to a 1 L bioreactor, glucose was consumed more rapidly but glycerol assimilation was inhibited. Finally, HTL of fungal biomass in fermentation broth without any catalyst at 340 °C for 60 min efficiently recovered microbial oils from fungal biomass and achieved a bio-oil yield of 78.7% with fatty acids being the dominant oil components (∼89%). HTL also led to the hydrogenation of less saturated fatty acids (C18:2 and C18:3) to more saturated forms (C18:0 and C18:1).

A microbial oil production process consisting of acidified glycerol pretreatment of sugarcane bagasse, enzymatic hydrolysis, microbial oil production by M. isabellina NRRL 1757 and oil recovery by hydrothermal liquefaction of fungal biomass in fermentation broth was assessed.

Fish farm effluents are suitable growth media for Nannochloropsis gaditana, a polyunsaturated fatty acid producing microalga

Fish farm effluents may be used as culture media for marine microalgae, the cell mass of which constitute an excellent fish feed rich in bioactive compounds. In the current investigation different fish farm effluents were tested as culture media for Nannochloropsis strains. Among them, Nannochloropsis gaditana grew well on the effluent released from the sedimentation tank (EST), which is the final step of the wastewater treatment. Mono-algal but non-aseptic cultures were conducted in two types of photo-bioreactors, namely stirred tank reactor (STR) and open pond simulating reactor (OPSR) working under various photoperiods. N. gaditana grew well under full illumination mode on phosphate rich EST in the STR, producing 847.0 mg/L of dry cell mass containing 7.8%, w/w lipids, while when cultivated on phosphate limited EST, cell mass production was slightly lower but lipid biosynthesis was favored, with the lipid content reaching 24.7%, w/w in dry cell mass. In all trials, Nannochloropsis cell mass contained significant quantities of proteins and polysaccharides. Neutral lipids were predominant over polar lipids. Both glycolipid and phospholipid fractions were rich in polyunsaturated fatty acids, especially in eicosapentaenoic acid. We conclude that fish farm wastewaters can be re-used as microalgae growth media, which is of financial and environmental importance.