The lipid metabolism in thraustochytrids

Mushroom oils: A review of their production, composition, and potential applications

This review delves into the world of mushroom oils, highlighting their production, composition, and versatile applications. Despite mushrooms' overall low lipid content, their fatty acid composition, rich in essential fatty acids like linoleic acid and oleic acid, proves nutritionally significant. Variations in fatty acid profiles across mushroom species and the prevalence of unsaturated fats contribute to their cardiovascular health benefits. The exploration extends to mushroom essential oils, revealing diverse volatile compounds through extraction methods like hydrodistillation and solvent-assisted flavor evaporation (SAFE). The identification of 1-octen-3-ol as a key contributor to the distinct "mushroom flavor" adds a nuanced perspective. The focus broadens to applications, encompassing culinary and industrial uses with techniques like cold pressing and supercritical fluid extraction (SFE). Mushroom oils, with their unique nutritional and flavor profiles, enhance gastronomic experiences. Non-food applications in cosmetics and biofuels underscore the oils' versatility. The nutritional composition, enriched with essential fatty acids, bioactive compositions, and trace elements, is explored for potential health benefits. Bioactive compounds such as phenolic compounds and terpenes contribute to antioxidant and anti-inflammatory properties, positioning mushroom oils as nutritional powerhouses. In short, this concise review synthesizes the intricate world of mushroom oils, emphasizing their nutritional significance, extraction methodologies, and potential health benefits. The comprehensive overview underscores mushroom oils as a promising area for further exploration and utilization. The characteristics of mushroom biomass oil for the use in various industries are influenced by the mushroom species, chemical composition, biochemical synthesis of mushroom, and downstream processes including extraction, purification and characterization. Therefore, further research and exploration need to be done to achieve a circular bioeconomy with the integration of SDGs, waste reduction, and economic stimulation, to fully utilize the benefits of mushroom, a valuable gift of nature.

Transcriptional, post-transcriptional, and post-translational regulation of polyunsaturated fatty acid synthase genes in Aurantiochytrium limacinum strain BL10: Responses to nitrogen starvation

Under nitrogen deficient conditions, the Aurantiochytrium limacinum strain BL10 greatly increases the production of docosahexaenoic acid (DHA) and n-6 docosapentaenoic acid. Researchers have yet to elucidate the mechanism by which BL10 promotes the activity of polyunsaturated fatty acid synthase (Pfa), which plays a key role in the synthesis of polyunsaturated fatty acid (PUFA). Analysis in the current study revealed that in nitrogen-depleted environments, BL10 boosts the transcription and synthesis of proteins by facilitating the expression of pfa genes via transcriptional regulation. It was also determined that BL10 adjusts the lengths of the 5'- and 3'-untranslated regions (suggesting post-transcriptional regulation) and modifies the ratio of two Pfa1 isoforms to favor PUFA production via post-translational regulation (ubiquitination). These findings clarify the exceptional DHA production of BL10 and provide additional insights into the regulatory mechanisms of PUFA biosynthesis in Aurantiochytrium.

Selectively superior production of docosahexaenoic acid in Schizochytrium sp. through engineering the fatty acid biosynthetic pathways

BACKGROUND

Schizochytrium sp. is commercially used for production of docosahexaenoic acid (DHA). Schizochytrium sp. utilizes the polyketide synthase complex (PKS) and a single type I fatty acid synthase (FAS) to synthesize polyunsaturated fatty acids and saturated fatty acids, respectively. The acyl carrier protein (ACP) domains of FAS or PKS are used to load acyl groups during fatty acids biosynthesis. Phosphopantetheinyl transferase (PPTase) transfers the pantetheine moiety from Coenzyme A to the conserved serine residue of an inactive ACP domain to produce its active form.

RESULTS

In this study, in order to improve production and content of DHA, we decreased the expression of fas, strengthened the expression of the PKS pathway, and enhanced the supply of active ACP in Schizochytrium sp. ATCC20888. Weakening the expression of fas or disruption of orfA both led to growth defect and reduction of lipid yields in the resulting strains WFAS and DPKSA, indicating that both FAS and PKS were indispensable for growth and lipid accumulation. Although WFAS had a higher DHA content in total fatty acids than the wild-type strain (WT), its growth defect and low DHA yield hinders its use for DHA production. Overexpression of the orfAB, orfC, orfC-DH (truncated orfC), or ppt promoted DHA and lipid production, respectively. The yields and contents of DHA were further increased by combined overexpression of these genes. Highest values of DHA yield (7.2 g/L) and DHA content (40.6%) were achieved in a recombinant OPKSABC-PPT, ⁓56.5% and 15.3% higher than the WT values, respectively.

CONCLUSIONS

This study demonstrates that genetic engineering of the fatty acid biosynthetic pathways provides a new strategy to enhance DHA production in Schizochytrium.

Thraustochytrids as a promising source of fatty acids, carotenoids, and sterols: bioactive compound biosynthesis, and modern biotechnology

Thraustochytrids are eukaryotes and obligate marine protists. They are increasingly considered to be a promising feed additive because of their superior and sustainable application in the production of health-benefiting bioactive compounds, such as fatty acids, carotenoids, and sterols. Moreover, the increasing demand makes it critical to rationally design the targeted products by engineering industrial strains. In this review, bioactive compounds accumulated in thraustochytrids were comprehensively evaluated according to their chemical structure, properties, and physiological function. Metabolic networks and biosynthetic pathways of fatty acids, carotenoids, and sterols were methodically summarized. Further, stress-based strategies used in thraustochytrids were reviewed to explore the potential methodologies for enhancing specific product yields. There are internal relationships between the biosynthesis of fatty acids, carotenoids, and sterols in thraustochytrids since they share some branches of the synthetic routes with some intermediate substrates in common. Although there are classic synthesis pathways presented in the previous research, the metabolic flow of how these compounds are being synthesized in thraustochytrids still remains uncovered. Further, combined with omics technologies to deeply understand the mechanism and effects of different stresses is necessary, which could provide guidance for genetic engineering. While gene-editing technology has allowed targeted gene knock-in and knock-outs in thraustochytrids, efficient gene editing is still required. This critical review will provide comprehensive information to benefit boosting the commercial productivity of specific bioactive substances by thraustochytrids.

Enhanced fatty acid storage combined with the multi-factor optimization of fermentation for high-level production of docosahexaenoic acid in Schizochytrium sp

Schizochytrium sp. hasreceived much attention for itsability to synthesize and accumulate high-level docosahexaenoic acid (DHA), which can reach nearly 40 % of total fatty acids. In this study, the titer of DHA in Schizochytrium sp. was successfully improved by enhancing DHA storage through overexpressing the diacylglycerol acyltransferase (ScDGAT2C) gene, as well as optimizing the supply of precursors and cofactors required for DHA synthesis by response surface methodology. Notably, malic acid, citric acid, and biotin showed synergistic and time-dependent effects on DHA accumulation. The maximum lipid and DHA titers of the engineered Schizochytrium sp. strain reached 84.28 ± 1.02 g/L and 42.23 ± 0.69 g/L, respectively, with the optimal concentration combination (1.62 g/L malic acid + 0.37 g/L citric acid + 8.28 mg/L biotin) were added 48 h after inoculation. This study provides an effective strategy for improving lipid and DHA production in Schizochytrium sp.

The Nitrogen Content in the Fruiting Body and Mycelium of Pleurotus Ostreatus and Its Utilization as a Medium Component in Thraustochytrid Fermentation

Following the idea of a circular bioeconomy, the use of side streams as substitutes for cultivation media (components) in bioprocesses would mean an enormous economic and ecological advantage. Costly compounds in conventional media for the production of the triterpene squalene in thraustochytrids are the main carbon source and complex nitrogen sources. Among other side streams examined, extracts from the spent mycelium of the basidiomycete Pleurotus ostreatus were best-suited to acting as alternative nitrogen sources in cultivation media for thraustochytrids. The total nitrogen (3.76 ± 0.01 and 4.24 ± 0.04%, respectively) and protein (16.47 ± 0.06 and 18.57 ± 0.18%, respectively) contents of the fruiting body and mycelium were determined. The fungal cells were hydrolyzed and extracted to generate accessible nitrogen sources. Under preferred conditions, the extracts from the fruiting body and mycelium contained 73.63 ± 1.19 and 89.93 ± 7.54 mM of free amino groups, respectively. Cultivations of Schizochytrium sp. S31 on a medium using a mycelium extract as a complex nitrogen source showed decelerated growth but a similar squalene yield (123.79 ± 14.11 mg/L after 216 h) compared to a conventional medium (111.29 ± 19.96 mg/L, although improvable by additional complex nitrogen source).

The Biosynthesis of Astaxanthin Esters in Schizochytrium sp. is Mediated by a Bifunctional Diacylglycerol Acyltransferase

Astaxanthin esters are a major form of astaxanthin found in nature. However, the exact mechanisms of the biosynthesis and storage of astaxanthin esters were previously unknown. We found that Schizochytrium sp. synthesized both astaxanthin and docosahexaenoic acid (DHA)-enriched lipids. The major type of astaxanthin produced was free astaxanthin along with astaxanthin-DHA monoester and other esterified forms. DHA accounted for 41.0% of the total fatty acids from astaxanthin monoesters. These compounds were deposited mainly in lipid droplets. The biosynthesis of the astaxanthin esters was mainly carried out by a novel diacylglycerol acyltransferase ScDGAT2-1, while ScDGAT2-2 was involved only in the production of triacylglycerol. We also identified astaxanthin ester synthases from the astaxanthin-producing algae Haematococcus pluvialis and Chromochloris zofingiensis, as well as a thraustochytrid Hondaea fermentalgiana with an unknown carotenoid profile. This investigation enlightens the application of thraustochytrids for the production of both DHA and astaxanthin and provides enzyme resources for the biosynthesis of astaxanthin esters in the engineered microbes.

Enhancing docosahexaenoic acid production in Aurantiochytrium species using atmospheric and room temperature plasma mutagenesis and comprehensive multi-omics analysis

Aurantiochytrium sp. belongs to marine heterotrophic single-cell protist, which is an important decomposer in marine ecosystem. Aurantiochytrium sp. has gained notoriety because of its ability to accumulate high-value docosahexaenoic acid (DHA), but the key factors of DHA synthesis were unclear at present. In this study, Atmospheric and Room Temperature Plasma technology was applied to the mutagenic breeding of Aurantiochytrium sp., and transcriptomics and proteomics were adopted to analyze the DHA-biosynthesis mechanism. According to the growth and DHA accumulation profiles, the mutant strain Aurantiochytrium sp. R2A35 was selected. The DHA content in total lipids was greatly improved from 49.39 % of the wild strain R2 to 63.69 % of the mutant strain. Moreover, the DHA content in the biomass of Aurantiochytrium sp. R2A35 as 39.72 % was the highest DHA productivity reported so far. The differentially expressed genes distinguished from transcriptome and the TMT-identified differential proteins distinguished from proteome confirmed that the expression of acetyl-CoA carboxylase and ketoacyl reductase was up-regulated by 4.78-fold and 6.95-fold, respectively and the fatty acid synthase was concurrently down-regulated by 2.79-fold, so that more precursor was transported to the polyketide synthase pathway, thereby increasing the DHA yield in Aurantiochytrium sp. R2A35. This research would provide reference for the DHA metabolism process and contribute to the understanding of the decomposer - Aurantiochytrium sp. in marine ecosystems.

Squalene production under oxygen limitation by Schizochytrium sp. S31 in different cultivation systems

The triterpene squalene is widely used in the food, cosmetics and pharmaceutical industries due to its antioxidant, antistatic and anti-carcinogenic properties. It is usually obtained from the liver of deep sea sharks, which are facing extinction. Alternative production organisms are marine protists from the family Thraustochytriaceae, which produce and store large quantities of various lipids. Squalene accumulation in thraustochytrids is complex, as it is an intermediate in sterol biosynthesis. Its conversion to squalene 2,3-epoxide is the first step in sterol synthesis and is heavily oxygen dependent. Hence, the oxygen supply during cultivation was investigated in our study. In shake flask cultivations, a reduced oxygen supply led to increased squalene and decreased sterol contents and yields. Oxygen-limited conditions were applied to bioreactor scale, where squalene accumulation and growth of Schizochytrium sp. S31 was determined in batch, fed-batch and continuous cultivation. The highest dry matter (32.03 g/L) was obtained during fed-batch cultivation, whereas batch cultivation yielded the highest biomass productivity (0.2 g/L*h-1). Squalene accumulation benefited from keeping the microorganisms in the growth phase. Therefore, the highest squalene content of 39.67 ± 1.34 mg/g was achieved by continuous cultivation (D = 0.025 h-1) and the highest squalene yield of 1131 mg/L during fed-batch cultivation. Volumetric and specific squalene productivity both reached maxima in the continuous cultivation at D = 0.025 h-1 (6.94 ± 0.27 mg/L*h-1 and 1.00 ± 0.03 mg/g*h-1, respectively). Thus, the choice of a suitable cultivation method under oxygen-limiting conditions depends heavily on the process requirements. KEY POINTS: • Measurements of respiratory activity and backscatter light of thraustochytrids • Oxygen limitation increased squalene accumulation in Schizochytrium sp. S31 • Comparison of different cultivation methods under oxygen-limiting conditions.

Integration analysis of ATAC-seq and RNA-seq provides insight into fatty acid biosynthesis in Schizochytrium limacinum under nitrogen limitation stress

BACKGROUND

Schizochytrium limacinum holds significant value utilized in the industrial-scale synthesis of natural DHA. Nitrogen-limited treatment can effectively increase the content of fatty acids and DHA, but there is currently no research on chromatin accessibility during the process of transcript regulation. The objective of this research was to delve into the workings of fatty acid production in S. limacinum by examining the accessibility of promoters and profiling gene expressions.

RESULTS

Results showed that differentially accessible chromatin regions (DARs)-associated genes were enriched in fatty acid metabolism, signal transduction mechanisms, and energy production. By identifying and annotating DARs-associated motifs, the study obtained 54 target transcription factor classes, including BPC, RAMOSA1, SPI1, MYC, and MYB families. Transcriptomics results revealed that several differentially expressed genes (DEGs), including SlFAD2, SlALDH, SlCAS1, SlNSDHL, and SlDGKI, are directly related to the biosynthesis of fatty acids, meanwhile, SlRPS6KA, SlCAMK1, SlMYB3R1, and SlMYB3R5 serve as transcription factors that could potentially influence the regulation of fatty acid production. In the integration analysis of DARs and ATAC-seq, 13 genes were identified, which were shared by both DEGs and DARs-associated genes, including SlCAKM, SlRP2, SlSHOC2, SlTN, SlSGK2, SlHMP, SlOGT, SlclpB, and SlDNAAF3.

CONCLUSIONS

SlCAKM may act as a negative regulator of fatty acid and DHA synthesis, while SlSGK2 may act as a positive regulator, which requires further study in the future. These insights enhance our comprehension of the processes underlying fatty acid and DHA production in S. limacinum. They also supply a foundational theoretical framework and practical assistance for the development of strains rich in fatty acids and DHA.

Biochemical characterization of lipid metabolic genes of Aurantiochytrium limacinum

Docosahexaenoic acid (DHA) is known to have numerous health benefits and immense dietary value. There is a pressing need to have a deeper understanding of DHA metabolism. Acyl CoA: Diacylglycerol Acyltransferase (DGAT) is an important enzyme of lipid anabolism and an essential piece of the puzzle. Aurantiochytrium limacinum, a primary producer of DHA, is a good model for studying DHA metabolism. Thus, we aimed to investigate important lipid metabolic genes from A. limacinum. We cloned four putative DGATs (DGAT2a, DGAT2b, DGAT2c, and DGAT2d) from A. limacinum and performed detailed in vivo and in vitro characterization. Functional characterization showed that not all the studied genes exhibited DGAT activity. DGAT2a and DGAT2d conferred DGAT activity whereas DGAT2b showed wax synthase (WS) activity and DGAT2c showed dual function of both WS and DGAT. Based on their identified function, DGAT2b and DGAT2c were renamed as AlWS and AlWS/DGAT respectively. DGAT2a was found to exhibit a preference for DHA as a substrate. DGAT2d was found to have robust activity and emerged as a promising candidate for genetic engineering aimed at increasing oil yield. The study enriches our knowledge of lipid biosynthetic enzymes in A. limacinum, which can be utilized to design suitable application strategies.

Nutritional enrichment of fruit peel wastes using lipid accumulating Aurantiochytrium strain as feed for aquaculture in the North-East Region of India

Utilization of fruit peel wastes to grow thraustochytrids for nutritional enrichment of wastes will lower environmental and economic costs associated with feedstock specific for aquaculture industries. In this study, high-carbohydrate content agricultural wastes, such as orange, pineapple, banana, and mausambi fruit peels were enriched with essential fatty acids producing thraustochytrids Aurantiochytrium sp. ATCC276. Characterizations of fruit peels revealed the presence of high carbohydrate content (9-16%) and reducing sugars essential for the growth of thraustochytrids. Optimization for lipid production of Aurantiochytrium sp. ATCC276 was carried out using response surface methodology (RSM) in combination with different concentrations of fruit peels in solid-state fermentation (SSF) conditions. Fruit peels composed of SSF experiments were designed using a central composite design. Aurantiochytrium sp. ATCC276 cells efficiently utilized the sugar components of fruit peels for their growth and lipid accumulation. Different SSF composites made of fruit peels were significantly enriched with fatty acids of Aurantiochytrium sp. ATCC276 cells. Culturing Aurantiochytrium sp. ATCC276 cells with these waste materials demonstrated distinct responses towards lipid accumulation at different compositions. The optimized SSF composite consists of 9.91 g 100 mL-1 orange, 5 g 100 mL-1 mausambi, 4.12 g 100 mL-1 pineapple, and 8.01 g 100 mL-1 banana peels and was enriched with 8.37% of Aurantiochytrium sp. ATCC276-derived lipids. This study expands the benefits and bioprocessing potential of essential fatty acids producing Aurantiochytrium sp. ATCC276 along with fruit peel wastes which a frontier in circular bioeconomy and valorizing waste for usage.

Legal Aspects of Microalgae in the European Food Sector

The interest in microalgae as food in Europe is growing due to its remarkable features that can foster a sustainable economy. The lack of tradition on their use among Europeans is changing and a demand for more sustainable products is increasing. The legal framework from the microalgae stakeholders' point of view has been consistently identified as a bottleneck, regardless of its nutritional value and potential to provide added-value metabolites. Microalgae-based products have been mostly consumed as food supplements, which are characterized by some general uncertainty with regards to food security of products sourced from non-European countries. The novel foods regulation is a landmark in Europe's food law defining the conditions in which a new type of food can be commercialized. Currently, a more simplified and centralized version is in place, and around eleven microalgae-based products are on the market; however, more than half are represented by Schizochytrium sp. derived products (DHA-rich oil). Microalgae have immense potential as a sustainable food source; nonetheless, there is limited experience in assessing the safety of these microorganisms, considering the uncertainty around undesirable substances present in the way they are produced and their diverse metabolites. Here, we overview the regulatory use of microalgae as food in Europe with a focus on market introduction, highlighting the administrative procedures and scientific requirements to assess food safety. We also discuss the implications of the Transparency regulation related to microalgae as novel foods and provide considerations for a more solid interaction between academia and industry.

The Clinical Promise of Microalgae in Rheumatoid Arthritis: From Natural Compounds to Recombinant Therapeutics

Rheumatoid arthritis (RA) is an invalidating chronic autoimmune disorder characterized by joint inflammation and progressive bone damage. Dietary intervention is an important component in the treatment of RA to mitigate oxidative stress, a major pathogenic driver of the disease. Alongside traditional sources of antioxidants, microalgae-a diverse group of photosynthetic prokaryotes and eukaryotes-are emerging as anti-inflammatory and immunomodulatory food supplements. Several species accumulate therapeutic metabolites-mainly lipids and pigments-which interfere in the pro-inflammatory pathways involved in RA and other chronic inflammatory conditions. The advancement of the clinical uses of microalgae requires the continuous exploration of phytoplankton biodiversity and chemodiversity, followed by the domestication of wild strains into reliable producers of said metabolites. In addition, the tractability of microalgal genomes offers unprecedented possibilities to establish photosynthetic microbes as light-driven biofactories of heterologous immunotherapeutics. Here, we review the evidence-based anti-inflammatory mechanisms of microalgal metabolites and provide a detailed coverage of the genetic engineering strategies to enhance the yields of endogenous compounds and to develop innovative bioproducts.

Enhancing the accumulation of lipid and docosahexaenoic acid in Schizochytrium sp. by co-overexpression of phosphopantetheinyl transferase and ω-3 fatty acid desaturase

Docosahexaenoic acid (DHA) as one of ω-3 polyunsaturated fatty acids (PUFAs), plays a key role in brain development, and is widely used in food additives and the pharmaceutical industry. Schizochytrium sp. is often considered as a satisfactory strain for DHA industrialization. The aim of this study was to assess the feasibility of phosphopantetheinyl transferase (PPTase) and ω-3 fatty acid desaturase (FAD) for regulating DHA content in Schizochytrium sp. PPTase is essential to activate the polyketide-like synthase (PKS) pathway, which can transfer apo-acyl-carrier protein (apo-ACP) into holo-ACP, and plays a key role in DHA synthesis. Moreover, DHA and docosapentaenoic acid (DPA) are synthesized by the PKS pathway simultaneously, so high DPA synthesis limits the increase of DHA content. In addition, the detailed mechanisms of PKS pathway have not been fully elucidated, so it is difficult to improve DHA content by modifying PKS. However, ω-3 FAD can convert DPA into DHA, and it is the most direct and effective way to increase DHA content and reduce DPA content. Based on this, PPTase was overexpressed to enhance the synthesis of DHA by the PKS pathway, overexpressed ω-3 FAD to convert the co-product of the PKS pathway into DHA, and co-overexpressed PPTase and ω-3 FAD. With these strategies, compared with wild type, the final lipid, and DHA titer were 92.5 and 51.5 g L-1 , which increased by 46.4% and 78.1%, respectively. This study established an efficient DHA production strain, and provided some feasible strategies for industrial DHA production in Schizochytrium sp.

Characterization of thraustochytrid-specific sterol O-acyltransferase: modification of DGAT2-like enzyme to increase the sterol production in Aurantiochytrium limacinum mh0186

IMPORTANCE

Since the global market for sterols and vitamin D are grown with a high compound annual growth rate, a sustainable source of these compounds is required to keep up with the increasing demand. Thraustochytrid is a marine oleaginous microorganism that can synthesize several sterols, which are stored as SE in lipid droplets. DGAT2C is an unconventional SE synthase specific to thraustochytrids. Although the primary structure of DGAT2C shows high similarities with that of DGAT, DGAT2C utilizes sterol as an acceptor substrate instead of diacylglycerol. In this study, we examined more detailed enzymatic properties, intracellular localization, and structure-activity relationship of DGAT2C. Furthermore, we successfully developed a method to increase sterol and provitamin D3 productivity of thraustochytrid by more than threefold in the process of elucidating the function of the DGAT2C-specific N-terminal region. Our findings could lead to sustainable sterol and vitamin D production using thraustochytrid.

Development of sustainable downstream processing for nutritional oil production

Nutritional oils (mainly omega-3 fatty acids) are receiving increased attention as critical supplementary compounds for the improvement and maintenance of human health and wellbeing. However, the predominant sources of these oils have historically shown numerous limitations relating to desirability and sustainability; hence the crucial focus is now on developing smarter, greener, and more environmentally favourable alternatives. This study was undertaken to consider and assess the numerous prevailing and emerging techniques implicated across the stages of fatty acid downstream processing. A structured and critical comparison of the major classes of disruption methodology (physical, chemical, thermal, and biological) is presented, with discussion and consideration of the viability of new extraction techniques. Owing to a greater desire for sustainable industrial practices, and a desperate need to make nutritional oils more available; great emphasis has been placed on the discovery and adoption of highly sought-after 'green' alternatives, which demonstrate improved efficiency and reduced toxicity compared to conventional practices. Based on these findings, this review also advocates new forays into application of novel nanomaterials in fatty acid separation to improve the sustainability of nutritional oil downstream processing. In summary, this review provides a detailed overview of the current and developing landscape of nutritional oil; and concludes that adoption and refinement of these sustainable alternatives could promptly allow for development of a more complete 'green' process for nutritional oil extraction; allowing us to better meet worldwide needs without costing the environment.

Bioprocess conditions and regulation factors to optimize squalene production in thraustochytrids

Squalene is a widely distributed natural triterpene, as it is a key precursor in the biosynthesis of all sterols. It is a compound of high commercial value worldwide because it has nutritional, medicinal, pharmaceutical, and cosmetic applications, due to its different biological properties. The main source of extraction has been shark liver oil, which is currently unviable on a larger scale due to the impacts of overexploitation. Secondary sources are mainly vegetable oils, although a limited one, as they allow low productive yields. Due to the diversity of applications that squalene presents and its growing demand, there is an increasing interest in identifying sustainable sources of extraction. Wild species of thraustochytrids, which are heterotrophic protists, have been identified to have the highest squalene content compared to bacteria, yeasts, microalgae, and vegetable sources. Several studies have been carried out to identify the bioprocess conditions and regulation factors, such as the use of eustressors that promote an increase in the production of this triterpene; however, studies focused on optimizing their productive yields are still in its infancy. This review includes the current trends that also comprises the advances in genetic regulations in these microorganisms, with a view to identify the culture conditions that have been favorable in increasing the production of squalene, and the influences that both bioprocess conditions and applied regulation factors partake at a metabolic level.

Transcriptome Analysis Reveals an Eicosapentaenoic Acid Accumulation Mechanism in a Schizochytrium sp. Mutant

Eicosapentaenoic acid (EPA) is an omega-3 long-chain polyunsaturated fatty acid (PUFA) essential for human health. Schizochytrium is a marine eukaryote that has been widely utilized for the synthesis of PUFAs. The current low potency and performance of EPA production by fermentation of Schizochytrium spp. limits its prospect in commercial production of EPA. Since the synthesis pathway of EPA in Schizochytrium spp. is still unclear, mutagenesis combined with efficient screening methods are still desirable. In this study, a novel screening strategy was developed based on a two-step progressive mutagenesis method based on atmospheric and room temperature plasma (ARTP) and diethyl sulfate (DES) after multiple stresses (sethoxydim, triclosan and 2,2'-bipyridine) compound screening. Finally, the mutant strain DBT-64 with increased lipid (1.57-fold, 31.71 g/L) and EPA (5.64-fold, 1.86 g/L) production was screened from wild-type (W) strains; the docosahexaenoic acid (DHA) content of mutant DBT-64 (M) was 11.41% lower than that of wild-type strains. Comparative transcriptomic analysis showed that the expression of genes related to the polyketide synthase, fatty acid prolongation, and triglyceride synthesis pathways was significantly upregulated in the mutant strain, while the expression of genes involved in the β-oxidation pathway and fatty acid degradation pathway was downregulated in favor of EPA biosynthesis in Schizochytrium. This study provides an effective strain improvement method to enhance EPA accumulation in Schizochytrium spp. IMPORTANCE Schizochytrium, a marine eukaryotic microorganism, has emerged as a candidate for the commercial production of PUFAs. EPA is an omega-3 PUFA with preventive and therapeutic effects against cardiovascular diseases, schizophrenia, and other disorders. Currently, the low potency and performance of EPA production by Schizochytrium spp. limits its commercialization. In this study, we performed two-step progressive mutagenesis based on ARTP and DES and screened multiple stresses (sethoxydim, triclosan, and 2,2'-bipyridine) to obtain the EPA-high-yielding Schizochytrium mutant. In addition, high expression of the polyketide synthase pathway, fatty acid elongation pathway, and triglyceride synthesis pathway in the mutants was confirmed by transcriptomic analysis. Therefore, the multistress screening platform established in this study is important for breeding EPA-producing Schizochytrium spp. and provides valuable information for regulating the proportion of EPA in microalgal lipids by means of genetic engineering.

Bioprocess engineering to produce essential polyunsaturated fatty acids from Thraustochytrium sp

In recent studies, thraustochytridhas emerged as a sustainable substitute to fish oil or polyunsaturated fatty acid(PUFA) sources: docosapentaenoic acid (DPA) eicosapentaenoic acid(EPA), anddocosahexaenoic acid(DHA). Due to growing health concerns, there is increasing demand for commercial application of PUFA to several diseases, aquaculture feeds, and dietary products. Thraustochytrium sp. found a sustainable source for considerable PUFA and SFA production and is expected to meet omega PUFA demand globally. This study aims to increase PUFA yield by glucose carbon with an appropriate nitrogen ratio (10:1). The maximum biomass and lipid obtained from 40 g/L glucose, with 7.47±0.3 g/L and 4.63 g/L (60.84±1.4%) yields, respectively. However, maximum relative lipid, DHA and DPA yields were from 30 g/L glucose i.e, 67.6±1.9 % and 963.58±24 and 693.10±24 mg/L respectively with complete glucose assimilation. Thus, this could be a potential source of commercial DPA and DHA producers under the biorefinery scheme.

Enhanced docosahexaenoic acid production from cane molasses by engineered and adaptively evolved Schizochytrium sp

Cane molasses (CM) is a sugar-rich agro-industrial byproduct. The purpose of this study is to synthesize docosahexaenoic acid (DHA) in Schizochytrium sp. by using CM. The single factor analysis showed that sucrose utilization was the main factor limiting the utilization of CM. Therefore, the endogenous sucrose hydrolase (SH) was overexpressed in Schizochytrium sp., which enhanced the sucrose utilization rate 2.57-fold compared to the wild type. Furthermore, adaptive laboratory evolution was used to further improve sucrose utilization from CM. Comparative proteomics and RT-qPCR were used out to analyze the metabolic differences of evolved strain grown on CM and glucose, respectively. Finally, a constant flow rate CM feeding strategy was implemented, whereby the DHA titer and lipid yield of the final strain OSH-end reached 25.26 g/L and 0.229 g/g sugar, respectively. This study demonstrated the CM is a cost-effective carbon source for industrial DHA fermentation.

Revealing holistic metabolic responses associated with lipid and docosahexaenoic acid (DHA) production in Aurantiochytrium sp. SW1

Aurantiochytrium sp. SW1, a marine thraustochytrid, has been regarded as a potential candidate as a docosahexaenoic acid (DHA) producer. Even though the genomics of Aurantiochytrium sp. are available, the metabolic responses at a systems level are largely unknown. Therefore, this study aimed to investigate the global metabolic responses to DHA production in Aurantiochytrium sp. through transcriptome and genome-scale network-driven analysis. Of a total of 13,505 genes, 2527 differentially expressed genes (DEGs) were identified in Aurantiochytrium sp., unravelling the transcriptional regulations behinds lipid and DHA accumulation. The highest number of DEG were found for pairwise comparison between growth phase and lipid accumulating phase where a total of 1435 genes were down-regulated with 869 genes being up-regulated. These uncovered several metabolic pathways that contributing in DHA and lipid accumulation including amino acid and acetate metabolism which involve in the generation of crucial precursors. Upon applying network-driven analysis, hydrogen sulphide was found as potential reporter metabolite that could be associated with the genes related to acetyl-CoA synthesis for DHA production. Our findings suggest that the transcriptional regulation of these pathways is a ubiquitous feature in response to specific cultivation phases during DHA overproduction in Aurantiochytrium sp. SW1.

Saturated and Polyunsaturated Fatty Acids Production by Aurantiochytrium limacinum PKU#Mn4 on Enteromorpha Hydrolysate

Thraustochytrids are unicellular marine heterotrophic protists, which have recently shown a promising ability to produce omega-3 fatty acids from lignocellulosic hydrolysates and wastewaters. Here we studied the biorefinery potential of the dilute acid-pretreated marine macroalgae (Enteromorpha) in comparison with glucose via fermentation using a previously isolated thraustochytrid strain (Aurantiochytrium limacinum PKU#Mn4). The total reducing sugars in the Enteromorpha hydrolysate accounted for 43.93% of the dry cell weight (DCW). The strain was capable of producing the highest DCW (4.32 ± 0.09 g/L) and total fatty acids (TFA) content (0.65 ± 0.03 g/L) in the medium containing 100 g/L of hydrolysate. The maximum TFA yields of 0.164 ± 0.160 g/g DCW and 0.196 ± 0.010 g/g DCW were achieved at 80 g/L of hydrolysate and 40 g/L of glucose in the fermentation medium, respectively. Compositional analysis of TFA revealed the production of equivalent fractions (% TFA) of saturated and polyunsaturated fatty acids in hydrolysate or glucose medium. Furthermore, the strain yielded a much higher fraction (2.61-3.22%) of eicosapentaenoic acid (C20:5n-3) in the hydrolysate medium than that (0.25-0.49%) in the glucose medium. Overall, our findings suggest that Enteromorpha hydrolysate can be a potential natural substrate in the fermentative production of high-value fatty acids by thraustochytrids.

Low dissolved oxygen supply functions as a global regulator of the growth and metabolism of Aurantiochytrium sp. PKU#Mn16 in the early stages of docosahexaenoic acid fermentation

BACKGROUND

Thraustochytrids accumulate lipids with a high content of docosahexaenoic acid (DHA). Although their growth and DHA content are significantly affected by the dissolved oxygen (DO) supply, the role of DO on the transcriptional regulation of metabolism and accumulation of intracellular metabolites remains poorly understood. Here we investigate the effects of three different DO supply conditions (10%, 30%, and 50%) on the fed-batch culture of the Aurantiochytrium PKU#Mn16 strain to mainly reveal the differential gene expressions and metabolite profiles.

RESULTS

While the supply of 10% DO significantly reduced the rates of biomass and DHA production in the early stages of fermentation, it achieved the highest amounts of biomass (56.7 g/L) and DHA (6.0 g/L) on prolonged fermentation. The transcriptome analyses of the early stage (24 h) of fermentation revealed several genes involved in the central carbon, amino acid, and fatty acid metabolism, which were significantly downregulated at a 10% DO level. The comparative metabolomics results revealed the accumulation of several long-chain fatty acids, amino acids, and other metabolites, supporting the transcriptional regulation under the influence of a low oxygen supply condition. In addition, certain genes involved in antioxidative systems were downregulated under 10% DO level, suggesting a lesser generation of reactive oxygen species that lead to oxidative damage and fatty acid oxidation.

CONCLUSIONS

The findings of this study suggest that despite the slow growth and metabolism in the early stage of fermentation of Aurantiochytrium sp. PKU#Mn16, a constant supply of low dissolved oxygen can yield biomass and DHA content better than that with high oxygen supply conditions. The critical information gained in this study will help to further improve DHA production through bioprocess engineering strategies.

Screening and Identification of Coastal Chilean Thraustochytrids for Arachidonic Acid Production: Biotechnological Potential of Ulkenia visurgensis Lng2-Strain

Thraustochytrids are unicellular heterotrophic marine protists that have been described as producing a high content of polyunsaturated fatty acids (PUFAs). Among them, arachidonic acid (ARA) stands out as a precursor of several mediators of pivotal importance for the immune system. However, the biotechnological potential of thraustochytrids for ARA production has not been developed. The objective of this study is to isolate and identify native strains from different Chilean coastal environments and evaluate in vitro the effect of culture parameters such as C/N ratio (19 and 33) and temperature (15 °C and 23 °C) on biomass production and arachidonic acid content. A total of nine strains were identified and classified into four genera of the Thraustochitridae family. The Lng2 strain with 99% identity belongs to the species Ulkenia visurgenis and was the most prominent one for ARA production. Temperature had an effect on the PUFA profile but not on the ARA content nor on the biomass yield. Additionally, the C/N ratio has been identified as a key parameter. The ARA productivity increased by 92% (from 0.6 to 8.3 ARA mg/g-DW) and its total biomass by 62.7% (from 1.9 to 5.1 g/L) at a high C/N ratio (33) as compared to the control.

Function of the Polyketide Synthase Domains of Schizochytrium sp. on Fatty Acid Synthesis in Yarrowia lipolytica

It is well known that polyunsaturated fatty acids (PUFAs) in Schizochytrium sp. are mainly synthesized via the polyketide synthase (PKS) pathway. However, the specific mechanism of PKS in fatty acid synthesis is still unclear. In this work, the functions of ORFA, ORFB, ORFC, and their individual functional domain genes on fatty acid synthesis were investigated through heterologous expression in Yarrowia lipolytica. The results showed that the expression of ORFA, ORFB, ORFC, and their individual functional domains all led to the increase of the very long-chain PUFA content (mainly eicosapentaenoic acid). Furthermore, the transcriptomic analysis showed that except for the 3-ketoacyl-ACP synthase (KS) domain of ORFB, the expression of an individual functional domain, including malonyl-CoA: ACP acyltransferase, 3-hydroxyacyl-ACP dehydratase (DH), 3-ketoacyl-ACP reductase, and KS domains of ORFA, acyltransferase domains of ORFB, and two DH domains of ORFC resulted in upregulation of the tricarboxylic acid cycle and pentose phosphate pathway, downregulation of the triacylglycerol biosynthesis, fatty acid synthesis pathway, and β-oxidation in Yarrowia lipolytica. These results provide a theoretical basis for revealing the function of PKS in fatty acid synthesis in Y. lipolytica and elucidate the possible mechanism for PUFA biosynthesis.

Enhanced production of high-value polyunsaturated fatty acids (PUFAs) from potential thraustochytrid Aurantiochytrium sp

Due to growing health concerns, the urban population is utterly inclined towards a healthy lifestyle and incorporated nutritional food supplements to lower common health risks. The ω-3 and ω-6 PUFAs consumption is increasing, hence alternative commercial production is essentially developed. The microbial source is an emerging platform to overcome the global demand for omega PUFAs. Marine oleaginous protist Aurantiochytrium sp. found a potential source to produce substantial DHA and SFA. The objective of the present research was to enhance the PUFA yield by optimizing maximum tolerable glucose concentration with a suitable nitrogen ratio (10:1). The maximum lipid and DHA yield and content were determined 4.30, 1.34 g/L, and 62.4, 33.49 % of total biomass and lipid at 30 g/L glucose respectively, which is one of among highest reported, however relative PUFA was maximum 46.97 % (DHA) in total lipid at 10 g/L glucose. Remaining 42-53.6 % SFA could be used for biodiesel.

Nitrogen Sources Affect the Long-Chain Polyunsaturated Fatty Acids Content in Thraustochytrium sp. RT2316-16

The psychrophilic marine microorganism Thraustochytrium sp. RT2316-16 can produce carotenoids as well as lipids containing the omega-3 polyunsaturated fatty acids (PUFA) eicosapentaenoic acid and docosahexaenoic acid. This work reports on the effects of the composition of the culture medium, including certain amino acids, on growth and lipid synthesis by RT2316-16. Compared with the culture on glutamate, the use of lysine, alanine, or serine, increased the content of the omega-3 PUFA in total lipids. In the media that contained yeast extract, glutamate, and glucose, lipid accumulation occurred when organic ammonium was exhausted earlier than glucose. In contrast, lipid mobilization was promoted if glucose was exhausted while organic ammonium (supplied by yeast extract and glutamate) remained in the medium. The total content of carotenoids in the lipid-free biomass decreased during the first 12 to 24 h of culture, simultaneously with a decrease in the total lipid content of the biomass. The experimental data suggested a possible interrelationship between the metabolism of carotenoids and lipids. A high content of omega-3 PUFA in the total lipids could be obtained by growing the thraustochytrid in a medium with a low glucose concentration (6 g L^-1) and a high concentration of organic nitrogen (yeast extract 12 g L^-1; glutamate 1.06 g L^-1), after glucose was exhausted. These observations may guide the development of a strategy to enhance omega-3 PUFA in the biomass.

Efficient co-production of EPA and DHA by Schizochytrium sp. via regulation of the polyketide synthase pathway

Presently, the supply of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) traditionally produced by marine fisheries will be insufficient to meet their market demand in food industry. Thus a sustainable alternative source is urgently required. Schizochytrium sp. is an ideal producer of DHA; however, its ability to co-produce DHA and EPA has not yet been proved. Herein, we first described a cobalamin-independent methionine synthase-like (MetE-like) complex, which contains independent acyltransferase and 3-ketoacyl synthase domains, independent of the traditional polyketide synthase (PKS) system. When the MetE-like complex was activated, the EPA content was increased from 1.26% to 7.63%, which is 6.06-folds higher than that in the inactivated condition. Through lipidomics, we find that EPA is more inclined to be stored as triglyceride. Finally, the EPA production was enhanced from 4.19 to 29.83 (mg/g cell dry weight) using mixed carbon sources, and the final yield reached 2.25 g/L EPA and 9.59 g/L DHA, which means that Schizochytrium sp. has great market potential for co-production of EPA and DHA.

Deciphering and engineering the polyunsaturated fatty acid synthase pathway from eukaryotic microorganisms

Polyunsaturated fatty acids (PUFAs) are important nutrients that play important roles in human health. In eukaryotes, PUFAs can be de novo synthesized through two independent biosynthetic pathways: the desaturase/elongase pathway and the PUFA synthase pathway. Among them, PUFAs synthesized through the PUFA synthase pathway typically have few byproducts and require fewer reduction equivalents. In the past 2 decades, numerous studies have been carried out to identify, analyze and engineer PUFA synthases from eukaryotes. These studies showed both similarities and differences between the eukaryotic PUFA synthase pathways and those well studied in prokaryotes. For example, eukaryotic PUFA synthases contain the same domain types as those in prokaryotic PUFA synthases, but the number and arrangement of several domains are different; the basic functions of same-type domains are similar, but the properties and catalytic activities of these domains are somewhat different. To further utilize the PUFA synthase pathway in microbial cell factories and improve the productivity of PUFAs, many challenges still need to be addressed, such as incompletely elucidated PUFA synthesis mechanisms and the difficult genetic manipulation of eukaryotic hosts. In this review, we provide an updated introduction to the eukaryotic PUFA synthase pathway, summarize the functions of domains and propose the possible mechanisms of the PUFA synthesis process, and then provide future research directions to further elucidate and engineer the eukaryotic PUFA synthase pathway for the maximal benefits of humans.

Computationally Guided Enzymatic Studies on Schizochytrium-Sourced Malonyl-CoA:ACP Transacylase

The malonyl-CoA:ACP transacylase (MAT) domain is responsible for the selection and incorporation of malonyl building blocks in the biosynthesis of polyunsaturated fatty acids (PUFAs) in eukaryotic microalgae (Schizochytrium) and marine bacteria (Moritella marina, Photobacterium profundum, and Shewanella). Elucidation of the structural basis underlying the substrate specificity and catalytic mechanism of the MAT will help to improve the yield and quality of PUFAs. Here, a methodology guided by molecular dynamics simulations was carried out to identify and mutate specificity-conferring residues within the MAT domain of Schizochytrium. Combining mutagenesis, cell-free protein synthesis, and in vitro biochemical assay, we dissected nearby interactions and molecular mechanisms relevant for binding and catalysis and found that the reorientation of the Ser154 C^β-O^γ bond establishes distinctive proton-transfer chains (His153-Ser154 and Asn235-His153-Ser154) for catalysis. Gln66 can be replaced by tyrosine to shorten the distance between His153 (N^ε2) and Ser154 (O^γ), which facilitates a faster proton-transfer rate, allowing better use of acyl substrates than the wild type. Furthermore, we screened a mutant that displayed an 18.4% increase in PUFA accumulation. These findings provide important insights into the study of MAT through protein engineering and will benefit dissecting the molecular mechanisms of other PUFA-related catalytic domains.

Nitrogen Starvation Enhances the Production of Saturated and Unsaturated Fatty Acids in Aurantiochytrium sp. PKU#SW8 by Regulating Key Biosynthetic Genes

Nitrogen deprivation is known to improve lipid accumulation in microalgae and thraustochytrids. However, the patterns of fatty acid production and the molecular mechanisms underlying the accumulation of unsaturated and saturated fatty acids (SFAs) under nitrogen starvation remain largely unknown for thraustochytrids. In this study, batch culture experiments under nitrogen replete and nitrogen starvation conditions were performed, and the changes in the transcriptome of Aurantiochytrium sp. PKU#SW8 strain between these conditions were investigated. Our results showed improved yields of total fatty acids (TFAs), total unsaturated fatty acids, and total SFAs under nitrogen starvation, which suggested that nitrogen starvation favors the accumulation of both unsaturated and saturated fatty acids in PKU#SW8. However, nitrogen starvation resulted in a more than 2.36-fold increase of SFAs whereas a 1.7-fold increase of unsaturated fatty acids was observed, indicating a disproportionate increase in these groups of fatty acids. The fabD and enoyl-CoA hydratase genes were significantly upregulated under nitrogen starvation, supporting the observed increase in the yield of TFAs from 2.63 ± 0.22 g/L to 3.64 ± 0.16 g/L. Furthermore, the pfaB gene involved in the polyketide synthase (PKS) pathway was significantly upregulated under nitrogen starvation. This suggested that the increased expression of the pfaB gene under nitrogen starvation may be one of the explanations for the increased yield of docosahexaenoic acid by 1.58-fold. Overall, our study advances the current understanding of the molecular mechanisms that underlie the response of thraustochytrids to nitrogen deprivation and their fatty acid biosynthesis.

The Porifera microeukaryome: Addressing the neglected associations between sponges and protists

While bacterial and archaeal communities of sponges are intensively studied, given their importance to the animal's physiology as well as sources of several new bioactive molecules, the potential and roles of associated protists remain poorly known. Historically, culture-dependent approaches dominated the investigations of sponge-protist interactions. With the advances in omics techniques, these associations could be visualized at other equally important scales. Of the few existing studies, there is a strong tendency to focus on interactions with photosynthesizing taxa such as dinoflagellates and diatoms, with fewer works dissecting the interactions with other less common groups. In addition, there are bottlenecks and inherent biases in using primer pairs and bioinformatics approaches in the most commonly used metabarcoding studies. Thus, this review addresses the issues underlying this association, using the term "microeukaryome" to refer exclusively to protists associated with an animal host. We aim to highlight the diversity and community composition of protists associated with sponges and place them on the same level as other microorganisms already well studied in this context. Among other shortcomings, it could be observed that the biotechnological potential of the microeukaryome is still largely unexplored, possibly being a valuable source of new pharmacological compounds, enzymes and metabolic processes.

Emerging prospects of microbial production of omega fatty acids: Recent updates

Healthy foods containing omega-3/omega-6 polyunsaturated fatty acids (PUFAs) have been in great demand because of their unique dietary and health properties. Reduction in chronic inflammatory and autoimmune diseases has shown their therapeutic and health-promoting effects when consumed under recommended ratio 1:1-1:4, however imbalanced ratios (>1:4, high omega-6) enhance these risks. The importance of omega-6 is apparent however microbial production favors larger production of omega-3. Current research focus is prerequisite to designing omega-6 production strategies for better application prospects, for which thraustochytrids could be promising due to higher lipid productivity. This review provides recent updates on essential fatty acids production from potential microbes and their application, especially major insights on omega research, also discussed the novel possible strategies to promote omega-3 and omega-6 accumulation via engineering and omics approaches. It covers strategies to block the conversion of omega-6 into omega-3 by enzyme inhibition, nanoparticle-mediated regulation and/or metabolic flux regulation, etc.

Acyl-CoA:diacylglycerol acyltransferase: Properties, physiological roles, metabolic engineering and intentional control

Acyl-CoA:diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) catalyzes the last reaction in the acyl-CoA-dependent biosynthesis of triacylglycerol (TAG). DGAT activity resides mainly in membrane-bound DGAT1 and DGAT2 in eukaryotes and bifunctional wax ester synthase-diacylglycerol acyltransferase (WSD) in bacteria, which are all membrane-bound proteins but exhibit no sequence homology to each other. Recent studies also identified other DGAT enzymes such as the soluble DGAT3 and diacylglycerol acetyltransferase (EaDAcT), as well as enzymes with DGAT activities including defective in cuticular ridges (DCR) and steryl and phytyl ester synthases (PESs). This review comprehensively discusses research advances on DGATs in prokaryotes and eukaryotes with a focus on their biochemical properties, physiological roles, and biotechnological and therapeutic applications. The review begins with a discussion of DGAT assay methods, followed by a systematic discussion of TAG biosynthesis and the properties and physiological role of DGATs. Thereafter, the review discusses the three-dimensional structure and insights into mechanism of action of human DGAT1, and the modeled DGAT1 from Brassica napus. The review then examines metabolic engineering strategies involving manipulation of DGAT, followed by a discussion of its therapeutic applications. DGAT in relation to improvement of livestock traits is also discussed along with DGATs in various other eukaryotic organisms.

Recent Progress in Microalgal Squalene Production and Its Cosmetic Application

Squalene, [oxidized form squalane] is a terpenoid with biological activity that produced by animals and plants. In the human body, a significant excretion named as sebum includes squalene in 12 percent. This bioactive compound shows anti-inflammatory, detoxifying, moisturizing and antioxidant effects on the human body. In addition to having these properties, it is known that squalene production decreases as less sebum is produced with age. Because of that, the need for supplementation of squalene through products has arisen. As a result, squalene production has been drawn attention due to its many application possibilities by cosmetic, cosmeceutical and pharmaceutical fields. At this point, approximately 3,000 of sharks, the major and the most popular source of squalene must be killed to obtain 1 ton of squalene. These animals are on the verge of extinction. This situation has caused to focus on finding microalgae strains, which are sustainable producers of squalene as alternative to sharks. This review paper summarizes the recent progresses in the topic of squalene. For this purpose, it contains information on squalene producers, microalgal squalene production and cosmetic evaluation of squalene.

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.

Optimal NaCl Medium Enhances Squalene Accumulation in Thraustochytrium sp. ATCC 26185 and Influences the Expression Levels of Key Metabolic Genes

Squalene, a natural lipid of the terpenoid family, is well-recognized for its roles in regulating cholesterol metabolism, preventing tumor development, and improving immunity. For large-scale squalene production, the unicellular marine protists—thraustochytrids—have shown great potential. However, the growth of thraustochytrids is known to be affected by salt stress, which can eventually influence the squalene content. Here, we study the effects of an optimal concentration of NaCl on the squalene content and transcriptome of Thraustochytrium sp. ATCC 26185. Under the optimal culture conditions (glucose, 30 g/L; yeast extract, 2.5 g/L; and NaCl, 5 g/L; 28°C), the strain yielded 67.7 mg squalene/g cell dry weight, which was significantly greater than that (5.37 mg/g) under the unoptimized conditions. NaCl was determined as the most significant (R = 135.24) factor for squalene production among glucose, yeast extract, and NaCl. Further comparative transcriptomics between the ATCC 26185 culture with and without NaCl addition revealed that NaCl (5 g/L) influences the expression of certain key metabolic genes, namely, IDI, FAS-a, FAS-b, ALDH3, GS, and NDUFS4. The differential expression of these genes possibly influenced the acetyl-CoA and glutamate metabolism and resulted in an increased squalene production. Through the integration of bioprocess technology and transcriptomics, this report provides the first evidence of the possible mechanisms underscoring increased squalene production by NaCl.

Patchy Blooms and Multifarious Ecotypes of Labyrinthulomycetes Protists and Their Implication in Vertical Carbon Export in the Pelagic Eastern Indian Ocean

Labyrinthulomycetes protists are an important heterotrophic component of microeukaryotes in the world’s oceans, but their distribution patterns and ecological roles are poorly understood in pelagic waters. This study employed flow cytometry and high-throughput sequencing to characterize the abundance, diversity, and community structure of Labyrinthulomycetes in the pelagic Eastern Indian Ocean. The total Labyrinthulomycetes abundance varied much more among stations than did the abundance of prokaryotic plankton, reaching over 1,000 cells mL⁻¹ at a few “bloom” stations. The total Labyrinthulomycetes abundance did not decline with depth throughout the whole water column (5 to 2,000 m) like the abundance of prokaryotic plankton did, and the Labyrinthulomycetes average projected biomass over all samples was higher than that of the prokaryotic plankton. However, Labyrinthulomycetes diversity showed obvious vertical variations, with richness, Shannon diversity, and evenness greatest in the upper epipelagic, lower epipelagic, and deep waters, respectively. Many abundant phylotypes were detected across multiple water layers, which aligned with the constant vertical Labyrinthulomycetes biomass, suggesting potential sinking and contribution to the biological pump. Hierarchical clustering revealed distinct ecotypes partitioning by vertical distribution patterns, suggesting their differential roles in the carbon cycle and storage processes. Particularly, most phylotypes showed patchy distributions (occurring in only few samples) as previously found in the coastal waters, but they were less associated with the Labyrinthulomycetes blooms than the prevalent phylotypes. Overall, this study revealed distinct patterns of Labyrinthulomycetes ecotypes and shed light on their importance in the pelagic ocean carbon cycling and sequestration relative to that of the prokaryotic plankton.

IMPORTANCE While prokaryotic heterotrophic plankton are well accepted as major players in oceanic carbon cycling, the ecological distributions and functions of their microeukaryotic counterparts in the pelagic ocean remain largely unknown. This study focused on an important group of heterotrophic (mainly osmotrophic) protistan microbes, the Labyrinthulomycetes, whose biomass can surpass that of the prokaryotic plankton in many marine ecosystems, including the bathypelagic ocean. We found patchy horizontal but persistent vertical abundance profiles of the Labyrinthulomycetes protists in the pelagic waters of the Eastern Indian Ocean, which were distinct from the spatial patterns of the prokaryotic plankton. Moreover, multiple Labyrinthulomycetes ecotypes with distinct vertical patterns were detected and, based on the physiologic, metabolic, and genomic understanding of their cultivated relatives, were inferred to play multifaceted key roles in the carbon cycle and sequestration, particularly as contributors to the vertical carbon export from the surface to the dark ocean, i.e., the biological pump.

Microbial genetic engineering approach to replace shark livering for squalene

Squalene is generally sourced from the liver oil of deep sea sharks (Squalus spp.), in which it accounts for 40-70% of liver mass. To meet the growing demand for squalene because of its beneficial effects for human health, three to six million deep sea sharks are slaughtered each year, profoundly endangering marine ecosystems. To overcome this unsustainable practice, microbial sources of squalene might offer a viable alternative to plant- or animal-based squalene, although only a few microorganisms have been found that are capable of synthesizing up to 30% squalene of dry biomass by native biosynthetic pathways. These squalene biosynthetic pathways, on the other hand, can be genetically manipulated to transform microorganisms into 'cellular factories' for squalene overproduction.

Genetic regulation and fermentation strategy for squalene production in Schizochytrium sp

Squalene, as an important terpenoid, is extensively used in the medicine and health care fields owing to its functions of anti-oxidation, blood lipid regulation and cancer prevention. The marine microalgae, Schizochytrium sp., which acts as an excellent strain with potential of high squalene production was selected as the starting strain. The overexpressed strain with sqs gene got the reduced biomass and lipid, while the squalene titer was increased by 79.6% ± 4.7% to 12.8 ± 0.2 mg/L. In order to further increase squalene production, the recombinant strain (HS strain) with sqs and hmgr gene co-overexpression was further constructed. The biomass and squalene titer of the HS strain were increased by 13.6% ± 1.2% and 88.8% ± 5.3%, respectively, which indicated the carbon flux of the mevalonate pathway was enhanced for squalene accumulation. Regarding the squalene synthesis is completely coupled with cell growth, fermentation strategy to prolong the logarithmic growth phase was conducive to improve squalene production. Under the condition of optimal composition and concentrated medium, the squalene titer of HS strain was 27.0 ± 1.3 mg/L, which was 2.0 times that of the basal medium condition (13.5 ± 0.4 mg/L). This study which combined the metabolic engineering and fermentation strategy provides a new strategy for squalene production in Schizochytrium sp. KEY POINTS: •The overexpression of sqs and hmgr genes promoted carbon metabolism for squalene. •The optimal and concentrated media can increase squalene yield.

Structural and Molecular Characterization of Squalene Synthase Belonging to the Marine Thraustochytrid Species Aurantiochytrium limacinum Using Bioinformatics Approach

The marine microorganisms thraustochytrids have been explored for their potential in the production of various bioactive compounds, such as DHA, carotenoids, and squalene. Squalene is a secondary metabolite of the triterpenoid class and is known for its importance in various industrial applications. The bioinformatic analysis for squalene synthase (SQS) gene (the first key enzyme in the tri-terpenoid synthesis pathway), that is prevailing among thraustochytrids, is poorly investigated. In-silico studies combining sequence alignments and bioinformatic tools helped in the preliminary characterization of squalene synthases found in Aurantiochytrium limacinum. The sequence contained highly conserved regions for SQS found among different species indicated the enzyme had all the regions for its functionality. The signal peptide sequence and transmembrane regions were absent, indicating an important aspect of the subcellular localization. Secondary and 3-D models generated using appropriate templates demonstrated the similarities with SQS of the other species. The 3-D model also provided important insights into possible active, binding, phosphorylation, and glycosylation sites.

Co-cultivation of Phaeodactylum tricornutum and Aurantiochytrium limacinum for polyunsaturated omega-3 fatty acids production

Marine protist Aurantiochytrium limacinum produces docosahexaenoic acid (DHA) as main polyunsaturated fatty acid (PUFA) and lacks any monounsaturated fatty acids (MUFA), while eicosapentaenoic acid (EPA) and MUFA's are produced by Phaeodactylum tricornutum. The marine diatom P. tricornutum was co-cultured with A.limacinum to match the EPA:DHA ratio of fish oil. Modulation in initial cell density ratio overcame the dominance of A.limacinum during co-cultivation and led to regulated proliferation of both species. Media engineering with nitrate and glycerol concentration yielded 2:1 (56.44: 30.11) mg g^-1 and 1:1 (47.43: 49.61) mg g^-1 EPA: DHA ratio. The oil and biomass obtained from co-cultivation comprised of MUFA's such as palmitoleic acid (2.65 mg g^-1) and oleic acid (1.25 mg g^-1) along with pigments like fucoxanthin (367.18 µg g^-1), β-carotene (8.98 µg g^-1) and astaxanthin (0.77 µg g^-1). Thus, co-cultivation of P. tricornutum with A. limacinum represented a unique strategy towards achieving desired fatty acid composition.

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

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

An emerging simple and effective approach to increase the productivity of thraustochytrids microbial lipids by regulating glycolysis process and triacylglycerols' decomposition

BACKGROUND

The oleaginous microorganism Schizochytrium sp. is widely used in scientific research and commercial lipid production processes. However, low glucose-to-lipid conversion rate (GLCR) and low lipid productivity of Schizochytrium sp. restrict the feasibility of its use.

RESULTS

Orlistat is a lipase inhibitor, which avoids triacylglycerols (TAGs) from hydrolysis by lipase. TAGs are the main storage forms of fatty acids in Schizochytrium sp. In this study, the usage of orlistat increased the GLCR by 21.88% in the middle stage of fermentation. Whereas the productivity of lipid increased 1.34 times reaching 0.73 g/L/h, the saturated fatty acid and polyunsaturated fatty acid yield increased from 21.2 and 39.1 to 34.9 and 48.5 g/L, respectively, indicating the advantages of using a lipase inhibitor in microbial lipids fermentation. Similarly, the system was also successful in Thraustochytrid Aurantiochytrium. The metabolic regulatory mechanisms stimulated by orlistat in Schizochytrium sp. were further investigated using transcriptomics and metabolomics. The results showed that orlistat redistributed carbon allocation and enhanced the energy supply when inhibiting the TAGs' degradation pathway. Therefore, lipase in Schizochytrium sp. prefers to hydrolyze saturated fatty acid TAGs into the β-oxidation pathway.

CONCLUSIONS

This study provides a simple and effective approach to improve lipid production, and makes us understand the mechanism of lipid accumulation and decomposition in Schizochytrium sp., offering new guidance for the exploitation of oleaginous microorganisms.