Changes of lipid content and fatty acid composition of Schizochytrium limacinum in response to different temperatures and salinities

Harnessing oleaginous protist Schizochytrium for docosahexaenoic acid: Current technologies in sustainable production and food applications

Docosahexaenoic acid (DHA) exerts versatile roles in nutrition supplementation and numerous health disorders prevention. Global consumption demand for DHA has also been consistently increasing with enhanced health awareness. Oleaginous marine protist Schizochytrium is praised as a potential DHA source due to short growth cycle, convenient artificial culture, harmless to the human body, and easy manipulation of the DHA synthesis pathway. However, factors including strain performances, fermentation parameters, product harvest and extraction strategies, safety and stability maintenance, and also application limitations in health and functional properties affect the widespread adoption of Schizochytrium DHA products. This review provides a comprehensive summary of the current biotechnologies used for tackling factors affecting the Schizochytrium DHA production, with special focuses on Schizochytrium strain improvement technologies, fermentation optimization projects, DHA oil extraction strategies, safety evaluations and stability maintenance schemes, and DHA product application approaches in foods. Inspired by systematic literature investigations and recent advances, suggestive observations composed of improving strain with multiple breeding technologies, considering artificial intelligence and machine learning to optimize the fermentative process, introducing nanoparticles packing technology to improve oxidation stability of DHA products, covering up DHA odor defect with characteristic flavor foods, and employing synthetic biology to construct the structured lipids with DHA to exploit potential functions are formed. This review will give a guideline for exploring more Schizochytrium DHA and propelling the application development in food and health.

Microalgal metabolic engineering facilitates precision nutrition and dietary regulation

Microalgae have gained considerable attention as promising candidates for precision nutrition and dietary regulation due to their versatile metabolic capabilities. This review innovatively applies system metabolic engineering to utilize microalgae for precision nutrition and sustainable diets, encompassing the construction of microalgal cell factories, cell cultivation and practical application of microalgae. Manipulating the metabolic pathways and key metabolites of microalgae through multi-omics analysis and employing advanced metabolic engineering strategies, including ZFNs, TALENs, and the CRISPR/Cas system, enhances the production of valuable bioactive compounds, such as omega-3 fatty acids, antioxidants, and essential amino acids. This work begins by providing an overview of the metabolic diversity of microalgae and their ability to thrive in diverse environmental conditions. It then delves into the principles and strategies of metabolic engineering, emphasizing the genetic modifications employed to optimize microalgal strains for enhanced nutritional content. Enhancing PSY, BKT, and CHYB benefits carotenoid synthesis, whereas boosting ACCase, fatty acid desaturases, and elongases promotes polyunsaturated fatty acid production. Here, advancements in synthetic biology, evolutionary biology and machine learning are discussed, offering insights into the precision and efficiency of metabolic pathway manipulation. Also, this review highlights the potential impact of microalgal precision nutrition on human health and aquaculture. The optimized microalgal strains could serve as sustainable and cost-effective sources of nutrition for both human consumption and aquaculture feed, addressing the growing demand for functional foods and environmentally friendly feed alternatives. The tailored microalgal strains are anticipated to play a crucial role in meeting the nutritional needs of diverse populations and contributing to sustainable food production systems.

Marine microalgae Schizochytrium demonstrates strong production of essential fatty acids in various cultivation conditions, advancing dietary self-sufficiency

Introduction

Polyunsaturated fatty acids (PUFAs) are essential nutrients that humans obtain from their diet, primarily through fish oil consumption. However, fish oil production is no longer sustainable. An alternative approach is to produce PUFAs through marine microalgae. Despite the potential of algae strains to accumulate high concentrations of PUFAs, including essential fatty acids (EFAs), many aspects of PUFA production by microalgae remain unexplored and their current production outputs are frequently suboptimal.

Methods

In this study, we optimized biomass and selected ω-3 PUFAs production in two strains of algae, Schizochytrium marinum AN-4 and Schizochytrium limacinum CO3H. We examined a broad range of cultivation conditions, including pH, temperature, stirring intensity, nutrient concentrations, and their combinations.

Results

We found that both strains grew well at low pH levels (4.5), which could reduce bacterial contamination and facilitate the use of industrial waste products as substrate supplements. Intensive stirring was necessary for rapid biomass accumulation but caused cell disruption during lipid accumulation. Docosahexaenoic acid (DHA) yield was independent of cultivation temperature within a range of 28-34°C. We also achieved high cell densities (up to 9 g/L) and stable DHA production (average around 0.1 g/L/d) under diverse conditions and nutrient concentrations, with minimal nutrients required for stable production including standard sea salt, glucose or glycerol, and yeast extract.

Discussion

Our findings demonstrate the potential of Schizochytrium strains to boost industrial-scale PUFA production and make it more economically viable. Additionally, these results may pave the way for smaller-scale production of essential fatty acids in a domestic setting. The development of a new minimal culturing medium with reduced ionic strength and antibacterial pH could further enhance the feasibility of this approach.

Production of docosahexaenoic acid by a novel isolated Aurantiochytrium sp. 6-2 using fermented defatted soybean as a nitrogen source for sustainable fish feed development

We focused on the production of docosahexaenoic acid (DHA)-containing microbial lipids by Aurantiochytrium sp. using of defatted soybean (DS) as a nitrogen source. Defatted soybean is a plant biomass that could provide a sustainable supply at a low cost. Results showed that Aurantiochytrium sp. could not directly assimilate the DS as a nitrogen source but could grow well in a medium containing DS fermented with rice malt. When cultivated in a fermented DS (FDS) medium, Aurantiochytrium sp. showed vigorous growth with the addition of sufficient sulfate and chloride ions as inorganic nutrients without seawater salt. A novel isolated Aurantiochytrium sp. 6-2 showed 15.8 ± 3.4 g/L DHA productivity (in 54.8 ± 12.1 g/L total fatty acid production) in 1 L of the FDS medium. Therefore, DHA produced by Aurantiochytrium sp. using FDS enables a stable and sustainable DHA supply and could be an alternative source of natural DHA derived from fish oil.

In Vivo Absorption and Lymphatic Bioavailability of Docosahexaenoic Acid from Microalgal Oil According to Its Physical and Chemical Form of Vectorization

Docosahexaenoic acid (DHA) is an essential fatty acid (FA) with proven pro-health effects, but improving its bioavailability is becoming a public health issue. The bioavailability of DHA from microalgal (A) oil has been comprehensively assessed, particularly in terms of the molecular structuring capabilities offered by A-oil. Here, we explored the impact of five DHA-rich formulas differing in terms of (i) molecular structure, i.e., ethyl ester (EE), monoglyceride (MG), or triglyceride (TG), and (ii) supramolecular form, i.e., emulsified TG or TG + phospholipids (PL blend) on the lymphatic kinetics of DHA absorption and the lipid characteristics of the resulting lipoproteins. We demonstrated in rats that the conventional A-DHA TG structure afforded more effective DHA absorption than the EE structure (+23%). Furthermore, the A-DHA MG and A-DHA emulsions were the better DHA vectors (AUC: 89% and +42%, respectively) due to improved lipolysis. The A-DHA MG and A-DHA emulsion presented the richest DHA content in TG (+40%) and PL (+50%) of lymphatic chylomicrons, which could affect the metabolic fate of DHA. We concluded that structuring A-DHA in TG or EE form would better serve for tissue and hepatic metabolism whereas A-DHA in MG and emulsion form could better target nerve tissues.

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.

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.

Comparative transcriptomic and lipidomic analyses indicate that cold stress enhanced the production of the long C18–C22 polyunsaturated fatty acids in Aurantiochytrium sp

Aurantiochytrium sp. belonging to Thraustochytrids are known for their capacity to produce long-chain polyunsaturated fatty acids (PUFAs). However, effects of cold stress accompanied with staged-temperature control on the fatty acid metabolism in Aurantiochytrium sp. were rarely studied. In this study, cold stress (15°C, 5°C) was applied for Aurantiochytrium sp., with the physiological responses (morphology, growth, fatty acid profiling) and gene expression related FA synthesis, lipid metabolism, and regulatory processes was observed. Results showed that there is a significant change for the lipid types under 5°C (251 species) and 15°C (97 species) treatment. The 5°C treatment was benefit for the C18–C22 PUFAs with the yield of docosahexaenoic acid (DHA) increased to 1.25 times. After incubation at 15°C, the accumulation of eicosadienoic acid (EA) (20:2) was increased to 2.00-fold. Based on transcriptomic and qPCR analysis, an increase in genes involved in fatty acid synthase (FAS) and polyketide synthase (PKS) pathways was observed under low-temperature treatment. With upregulation of 3-ketoacyl-CoA synthase (2.44-fold), ketoreductase (2.50-fold), and dTDP-glucose 4,6-Dehydratase (rfbB) (2.31-fold) involved in PKS pathway, the accumulation of DHA was enhanced under 5°C. While, FAS and fatty elongase 3 (ELO) involved in the FAS pathway were upregulated (1.55-fold and 2.45-fold, respectively) to accumulate PUFAs at 15°C. Additionally, glycerol-3-phosphate acyltransferase (GPAT), lysophospholipid acyltransferase (LPAT), phosphatidic acid phosphatase (PAP), phosphatidylserine synthase (PSS), and phosphatidylserine decarboxylase (PSD) involved in glycerophospholipid biosynthesis were upregulated at 5°C increasing the accumulation of phosphatidic acid (PA), phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylglycerol (PG), and phosphatidylinositol (PI). However, glycolysis and the TCA cycle were inhibited under 5°C. This study provides a contribution to the application of two-staged temperature control in the Aurantiochytrium sp. fermentation for producing cold stress-enhancing PUFAs, in order to better understand the function of the key genes for future genetic engineering.

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.

Development of an alternative medium via completely replaces the medium components by mixed wastewater and crude glycerol for efficient production of docosahexaenoic acid by Schizochytrium sp

Exorbitant substrates for Schizochytrium culture result in the high cost of docosahexaenoic acid (DHA) production. In order to develop a feasible approach that is expected to reduce DHA production cost, Schizochytrium sp. S31 cultivation with a mixture of saline wastewater (SWW) and tofu whey wastewater (TWW) was investigated in this study. Using glucose as the carbon source, the maximum biomass and DHA yield in cultures using mixed wastewater containing 5% SWW reached 19.08 and 2.66 g/L, respectively, which were 2.29 and 2.66 times higher than those of cultures using control medium. Moreover, a good wastewater treatment performance was achieved as approximately 60% of the COD, TN, and TP were reduced in the cultures using mixed wastewater with a SWW ratio of 5%. The mixed wastewater presented better performance on DHA production than control medium using all tested carbon sources including glucose, fructose, and pure and crude glycerol. The components of control medium can be completely replaced by the mixed wastewater and crude glycerol. It is expected to effectively decrease the medium cost for DHA production and reduce the environmental risk of food processing wastewater.

Production of polyunsaturated fatty acids by Schizochytrium (Aurantiochytrium) spp

Diverse health benefits are associated with dietary consumption of omega-3 long-chain polyunsaturated fatty acids (ω-3 LC-PUFA), particularly docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). Traditionally, these fatty acids have been obtained from fish oil, but limited supply, variably quality, and an inability to sustainably increase production for a rapidly growing market, are driving the quest for alternative sources. DHA derived from certain marine protists (heterotrophic thraustochytrids) already has an established history of commercial production for high-value dietary use, but is too expensive for use in aquaculture feeds, a much larger potential market for ω-3 LC-PUFA. Sustainable expansion of aquaculture is prevented by its current dependence on wild-caught fish oil as the source of ω-3 LC-PUFA nutrients required in the diet of aquacultured animals. Although several thraustochytrids have been shown to produce DHA and EPA, there is a particular interest in Schizochytrium spp. (now Aurantiochytrium spp.), as some of the better producers. The need for larger scale production has resulted in development of many strategies for improving productivity and production economics of ω-3 PUFA in Schizochytrium spp. Developments in fermentation technology and metabolic engineering for enhancing LC-PUFA production in Schizochytrium spp. are reviewed.

Highly Valuable Polyunsaturated Fatty Acids from Microalgae: Strategies to Improve Their Yields and Their Potential Exploitation in Aquaculture

Microalgae have a great potential for the production of healthy food and feed supplements. Their ability to convert carbon into high-value compounds and to be cultured in large scale without interfering with crop cultivation makes these photosynthetic microorganisms promising for the sustainable production of lipids. In particular, microalgae represent an alternative source of polyunsaturated fatty acids (PUFAs), whose consumption is related to various health benefits for humans and animals. In recent years, several strategies to improve PUFAs' production in microalgae have been investigated. Such strategies include selecting the best performing species and strains and the optimization of culturing conditions, with special emphasis on the different cultivation systems and the effect of different abiotic factors on PUFAs' accumulation in microalgae. Moreover, developments and results obtained through the most modern genetic and metabolic engineering techniques are described, focusing on the strategies that lead to an increased lipid production or an altered PUFAs' profile. Additionally, we provide an overview of biotechnological applications of PUFAs derived from microalgae as safe and sustainable organisms, such as aquafeed and food ingredients, and of the main techniques (and their related issues) for PUFAs' extraction and purification from microalgal biomass.

Chemical and Physical Culture Conditions Significantly Influence the Cell Mass and Docosahexaenoic Acid Content of Aurantiochytrium limacinum Strain PKU#SW8

Thraustochytrids are well-known unicellular heterotrophic marine protists because of their promising ability to accumulate docosahexaenoic acid (DHA). However, the implications of their unique genomic and metabolic features on DHA production remain poorly understood. Here, the effects of chemical and physical culture conditions on the cell mass and DHA production were investigated for a unique thraustochytrid strain, PKU#SW8, isolated from the seawater of Pearl River Estuary. All the tested fermentation parameters showed a significant influence on the cell mass and concentration and yield of DHA. The addition of monosaccharides (fructose, mannose, glucose, or galactose) or glycerol to the culture medium yielded much higher cell mass and DHA concentrations than that of disaccharides and starch. Similarly, organic nitrogen sources (peptone, yeast extract, tryptone, and sodium glutamate) proved to be beneficial in achieving a higher cell mass and DHA concentration. PKU#SW8 was found to grow and accumulate a considerable amount of DHA over wide ranges of KH2PO4 (0.125-1.0 g/L), salinity (0-140% seawater), pH (3-9), temperature (16-36 °C), and agitation (140-230 rpm). With the optimal culture conditions (glycerol, 20 g/L; peptone, 2.5 g/L; 80% seawater; pH 4.0; 28 °C; and 200 rpm) determined based on the shake-flask experiments, the cell mass and concentration and yield of DHA were improved up to 7.5 ± 0.05 g/L, 2.14 ± 0.03 g/L, and 282.9 ± 3.0 mg/g, respectively, on a 5-L scale fermentation. This study provides valuable information about the fermentation conditions of the PKU#SW8 strain and its unique physiological features, which could be beneficial for strain development and large-scale DHA production.

Research advancement and commercialization of microalgae edible oil: a review

The global food crisis has led to a great deal of attention being given to microalgal oil as a sustainable natural food source. This article provides an overview of the progress and future directions in promoting the commercialization of microalgal edible oils, including microalgal triglyceride accumulation, suitable edible oil culture strategies for high nutritional value, metabolic engineering, production, and downstream technologies. The integration of the production process, biosafety, and the economic sustainability of microalgal oil production are analyzed for their critical roles in the commercialization of microalgal edible oil to provide a theoretical and scientific basis for the comprehensive development and utilization of microalgal edible oil. © 2021 Society of Chemical Industry.

Cultivation of microalgae on unhydrolysed waste molasses syrup using mass cultivation strategy for improved biodiesel

High cultivation cost and low lipid yield are framed as a major bottleneck for the production of microalgae biodiesel. Hence, we first and foremost highlight a trophic mode transition, coupled with a combinatorial effect of organic carbon, nitrogen and light (C/N/L) on an isolated microalga Chlorococcum sp. SVF in a one pot tri-phasic intermittent feeding system by developing a lab scale Raceway tank (40L). Hitherto, waste molasses syrup without hydrolysis is unexplored in algal bioenergy arena. The direct utilisation capability of sucrose, served by waste unhydrolysed molasses syrup (WUMS), effectively modulates the intrinsic biochemical and physiological characteristics towards microalgae biomass and lipid assimilation. Response surface methodology-central composite design (RSM-CCD) tool has been employed to observe the cumulative impact of light irradiation and nutrient sources (carbon and nitrogen) on cellular stoichiometric analysis. Experimental results exhibit a potentially achievable biomass (18.88 g L^-1) and lipid accumulation (80.34%) under the light intensity of 75.5 µmol m^-2 s^-1 with stepwise light attenuation strategy. Characterisation of fatty acid methyl esters (FAME) reveals the dominance of oleic acid (32.72%) and palmitic acid methyl esters (32.49%) in mixotrophic condition, which are considered as the upmost indicators of quality biodiesel. The biofuel properties were obtained in acquiescence with American and European standard. These findings are therefore a way forward towards the effective growth of Chlorococcum sp. SVF in sucrose rich inexpensive industrial waste stream that positively influences the lipid yield for large scale sustainable biodiesel production.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02823-7.

Sustainable production of food grade omega-3 oil using aquatic protists: Reliability and future horizons

Biotechnological production of omega-3 polyunsaturated fatty acids (PUFAs) has become a commercial alternative to fish oil in the past twenty years. Compared to PUFA production by fatty fishes, that from microorganisms has increased due to its promising sustainability and high product safety and to increasing awareness in the expanding vegan market. Although autotrophic production by microalgae seems to be more sustainable in the long term, to date most of the microbial production of omega-3 is carried out under heterotrophic conditions using conventional fermentation technologies. The present review critically analyzes the main reasons for this discrepancy and reports on the recent advances and the most promising approaches for its future development in the context of sustainability and circular economy.

Impact of low-intensity pulsed ultrasound on the growth of Schizochytrium sp. for omega-3 production

Schizochytrium sp. is a microalga that is known for its high content of oils or lipids. It has a high percentage of polyunsaturated fatty acids in the accumulated oil, especially docosahexaenoic acid (DHA). DHA is an important additive for the human diet. Large-scale production of Schizochytrium sp. can serve as an alternative source of DHA for humans as well as for fish feed, decreasing the burden on aqua systems. Therefore, research on improving the productivity of Schizochytrium attracts a lot of attention. We studied the potential of using low-intensity pulsed ultrasound (LIPUS) in the growth cycle of Schizochytrium sp. in shake flasks. Different intensities and treatment durations were tested. A positive effect of LIPUS on biomass accumulation was observed in the Schizochytrium sp. culture. Specifically, LIPUS stimulation at the ultrasound intensity of 400 mW/cm² with 20 min per treatment 10 times a day with equal intervals of 2.4 h between the treatments was found to enhance the growth of Schizochytrium biomass most effectively (by up to 20%). Due to the nature of cell division in Schizochytrium sp. which occurs via zoospore formation, LIPUS stimulation was inefficient if applied continuously during all 5 days of the growth cycle. Using microscopy, we studied the interval between zoospore formation in the culture and selected the optimal LIPUS application days (Days 0-1 and Days 4-5 of the 5-day growth cycle). Microscopic images have also shown that LIPUS stimulation enhances zoospore formation in Schizochytrium sp., leading to more active cell division in the culture. This study shows that LIPUS can serve as an additional tool for cost-efficiency improvement in the large-scale production of Schizochytrium as a sustainable and environmentally friendly source of omega-3 (DHA).

High-Density pH-Auxostat Fed-Batch Culture of Schizochytrium limacinum SR21 with Acetic Acid as a Carbon Source

Schizochytrium limacinum SR21 is an important strain for industrial production of docosahexaenoic acid (DHA), which is an important omega-3 fatty acid used in the nutraceutical and food industry. However, the high cost of carbon sources has limited its further application in the market with much larger volume, such as animal feed for aquaculture, poultry, and livestock. To seek low-cost carbon source, acetic acid is tested in the present study. The effect of different factors, including initial carbon source concentration, pH, aeration rate, and nitrogen sources, on biomass, lipid, and DHA production were tested. With optimized culture conditions, the biomass concentration of 146 g/L, total fatty acids (TFAs) of 82.3 g/L, and DHA content of 23.0 g/L were achieved with a pH-auxostat fed-batch cultivation. These results suggested that acetic acid is a promising feedstock for the low-cost production of DHA. Graphical Abstract.

The Application of Single-Cell Ingredients in Aquaculture Feeds—A Review

Single-cell ingredients (SCI) are a relatively broad class of materials that encompasses bacterial, fungal (yeast), microalgal-derived products or the combination of all three microbial groups into microbial bioflocs and aggregates. In this review we focus on those dried and processed single-cell organisms used as potential ingredients for aqua-feeds where the microorganisms are considered non-viable and are used primarily to provide protein, lipids or specific nutritional components. Among the SCI, there is a generalised dichotomy in terms of their use as either single-cell protein (SCP) resources or single-cell oil (SCO) resources, with SCO products being those oleaginous products containing 200 g/kg or more of lipids, whereas those products considered as SCP resources tend to contain more than 300 g/kg of protein (on a dry basis). Both SCP and SCO are now widely being used as protein/amino acid sources, omega-3 sources and sources of bioactive molecules in the diets of several species, with the current range of both these ingredient groups being considerable and growing. However, the different array of products becoming available in the market, how they are produced and processed has also resulted in different nutritional qualities in those products. In assessing this variation among the products and the application of the various types of SCI, we have taken the approach of evaluating their use against a set of standardised evaluation criteria based around key nutritional response parameters and how these criteria have been applied against salmonids, shrimp, tilapia and marine fish species.

Nutraceutical Status and Scientific Strategies for Enhancing Production of Omega-3 Fatty Acids from Microalgae and their Role in Healthcare

Adherence to Omega-3 fatty acids (O3FAs) as Nutraceuticals for medicinal applications provides health improvement. The prevention and treatment of diseases with O3FAs hold promise in clinical therapy and significantly reduces the risk of chronic disorders. Polyunsaturated fatty acids (PUFA) O3FAs have beneficial effects in the treatment of cardiovascular disorders, diabetic disease, foetal development, Alzheimer's disease, retinal problem, growth and brain development of infants and antitumor effects. Association to current analysis promotes the application of algal biomass for production of O3FAs, mode of action, fate, weight management, immune functions, pharmaceutical and therapeutic applications serving potent sources in healthcare management. A search of the literature was conducted in the databases of WHO website, Sci.org, PubMed, academics and Google. The authors performed search strategies and current scenario of O3FAs in health associated disorders. Promising outcomes and future strategies towards O3FAs may play a pivotal role in Nutraceutical industries in the cure of human health in the future.

High-added value products from microalgae and prospects of aquaculture wastewaters as microalgae growth media

Aquaculture plays an important role in human nutrition and economic development but is often expanded to the detriment of the natural environment. Several research projects, aimed at cultivating microalgae in aquaculture wastewaters (AWWs) to reduce organic loads and minerals, along with the production of microalgal cell mass and metabolic products, are underway. Microalgal cell mass is of high nutritional value and is regarded as a candidate to replace, partially at least, the fish meal in the fish feed. Also, microalgal cell mass is considered as a feedstock in the bio-fuel manufacture, as well as a source of high-added value metabolic products. The production of these valuable products can be combined with the reuse of AWWs in the light of environmental concerns related with the aquaculture sector. Many research papers published in the last decade demonstrate that plenty of microalgae species are able to efficiently grow in AWWs, mainly derived from fish and shrimp farms, and produce valuable metabolites reducing the AWW pollutant load. We conclude that bio-remediation of AWWs combining with the production of microalgae cell mass and specific metabolites is probably the most convenient and economical solution for AWWs management and can contribute to the sustainable growth of the aquaculture.

Nutrient recovery from tofu whey wastewater for the economical production of docosahexaenoic acid by Schizochytrium sp. S31

Docosahexaenoic acid plays a vital role in human health as it is essential for the proper function of the nervous system and for visual functions. To decrease the cost of docosahexaenoic acid production by Schizochytrium, the cost of the medium should be further decreased. In this study, the use of tofu whey wastewater to culture Schizochytrium sp. for docosahexaenoic acid production was tested, with the goal of reducing the medium cost. The results indicated that tofu whey wastewater presented a better culture performance with respect to biomass, lipid, and docosahexaenoic acid production compared with three traditional media. Through simple pH adjustment, the biomass and docosahexaenoic acid productivity reached 1.89 and 0.24 g/L/day, respectively, which were much higher than those obtained using traditional medium. The removal efficiency of chemical oxygen demand, total nitrogen, and total phosphorus reached 64.7, 66.0, and 59.3%, respectively. Due to the rich nutrients in tofu whey wastewater, the use of extra nitrogen source was avoided and the total medium cost for docosahexaenoic acid production in cultures using tofu whey wastewater was <1/3 of that of traditional media. This result indicated that tofu whey wastewater is an effective and economic basal medium for docosahexaenoic acid production by Schizochytrium sp.

Algal biorefinery models with self-sustainable closed loop approach: Trends and prospective for blue-bioeconomy

Microalgae due to its metabolic versatility have received a focal attention in the biorefinery and bioeconomy context. Microalgae products have broad and promising application potential in the domain of renewable fuels/energy, nutraceutical, pharmaceuticals and cosmetics. Biorefining of microalgal biomass in a circular loop with an aim to maximize resource recovery is being considered as one of the sustainable option that will have both economical and environmental viability. The expansive scope of microalgae cultivation with self-sustainability approach was discussed in this communication in the framework of blue-bioeconomy. Microalgae based primary products, cultivation strategies, valorization of microalgae biomass for secondary products and integrated biorefinery models for the production of multi-based products were discussed. The need and prospect of self-sustainable models in closed loop format was also elaborated.

The Dark Side of Microalgae Biotechnology: A Heterotrophic Biorefinery Platform Directed to ω-3 Rich Lipid Production

Microbial oils have been considered a renewable feedstock for bioenergy not competing with food crops for arable land, freshwater and biodiverse natural landscapes. Microalgal oils may also have other purposes (niche markets) besides biofuels production such as pharmaceutical, nutraceutical, cosmetic and food industries. The polyunsaturated fatty acids (PUFAs) obtained from oleaginous microalgae show benefits over other PUFAs sources such as fish oils, being odorless, and non-dependent on fish stocks. Heterotrophic microalgae can use low-cost substrates such as organic wastes/residues containing carbon, simultaneously producing PUFAs together with other lipids that can be further converted into bioenergy, for combined heat and power (CHP), or liquid biofuels, to be integrated in the transportation system. This review analyses the different strategies that have been recently used to cultivate and further process heterotrophic microalgae for lipids, with emphasis on omega-3 rich compounds. It also highlights the importance of studying an integrated process approach based on the use of low-cost substrates associated to the microalgal biomass biorefinery, identifying the best sustainability methodology to be applied to the whole integrated system.

Marine omega-3 (n-3) phospholipids: A comprehensive review of their properties, sources, bioavailability, and relation to brain health

For several decades, there has been considerable interest in marine-derived long chain n-3 fatty acids (n-3 LCPUFAs) due to their outstanding health benefits. n-3 LCPUFAs can be found in nature either in triglycerides (TAGs) or in phospholipid (PL) form. From brain health point of view, PL n-3 is more bioavailable and potent compared to n-3 in TAG form, as only PL n-3 is able to cross the blood-brain barrier and can be involved in brain biochemical reactions. However, PL n-3 has been ignored in the fish oil industry and frequently removed as an impurity during degumming processes. As a result, PL products derived from marine sources are very limited compared to TAG products. Commercially, PLs are being used in pharmaceutical industries as drug carriers, in food manufacturing as emulsifiers and in cosmetic industries as skin care agents, but most of the PLs used in these applications are produced from vegetable sources that contain less (without EPA, DPA, and DHA) or sometimes no n-3 LCPUFAs. This review provides a comprehensive account of the properties, structures, and major sources of marine PLs, and provides focussed discussion of their relationship to brain health. Epidemiological, laboratory, and clinical studies on n-3 LCPUFAs enriched PLs using different model systems in relation to brain and mental health that have been published over the past few years are discussed in detail.

Integration of lipidomic and transcriptomic profiles reveals novel genes and regulatory mechanisms of Schizochytrium sp. in response to salt stress

In this study, the effects of salt stress on the physiological, lipidomic and transcriptomic profiles of halophilic microalga Schizochytrium sp. were investigated. In general, Schizochytrium sp. could survive under high osmotic fermentation medium containing 30 g/L NaCl, and showed a significant increase in C14:0 percentage in total fatty acids. In lipidomic analysis, C14:0 was specifically enriched in phosphatidylcholine (PC), and membrane phospholipids participated in the salt stress response mostly. Specially, one novel signal lipid N-acylphosphatidylethanolamine (NAPE) (18:0/20:3/14:0) was upregulated significantly. Transcriptomic analysis revealed glycerol-3-phosphate acyltransferase (GPAT) and phospholipase ABHD3 (PLABDH3) were involved in C14:0 metabolism and NAPE biosynthesis. Signalling pathways they mediated were activated as evident by high expression level of Myristoyl-CoA: protein N-myristoyltransferase (NMT) and NAPE-hydrolyzing PLD (NAPE-PLD). This study gives us an insight in specific responses to salt stress in Schizochytrium sp. and provides a considerable proportion of novel genes that could commendably be used for engineering modification.

Effects of exogenous lipids and cold acclimation on lycopene production and fatty acid composition in Blakeslea trispora

Exogenous lipids serving as stimulators to improve lycopene production in Blakeslea trispora have been widely reported. However, the selection basis of exogenous lipids and their effects on intracellular lipids are not very clear. In this study, five plant oils with different fatty acid compositions were selected to investigate their effects on lycopene production, fatty acid composition and the desaturation degree of intracellular lipids. Among the oils, soybean oil, with a fatty acid composition similar to that of mycelium, exhibited the best stimulating effect on lycopene formation (improvement of 82.1%). The plant oils enhanced the total content of intracellular lipids and the desaturation degree of reserve lipids due to the alteration of fatty acid composition, especially in neutral lipids. Lycopene production was increased with the improved desaturation degree of intracellular lipids, which may be attributed to the enhancement of storage capacity for lycopene in storage lipid, thus reducing the feedback regulation of free lycopene. In addition, the increase of the desaturation degree of reserve lipids through temperature-changing fermentation also enhanced lycopene production. The present study could serve as a basis for a better understanding of the relationship between the fatty acid composition of reserve lipids and lycopene production.

The lipid metabolism in thraustochytrids

Thraustochytrids are unicellular heterotrophic marine protists of the Stramenopile group, often considered as non-photosynthetic microalgae. They have been isolated from a wide range of habitats including deep sea, but are mostly present in waters rich in sediments and organic materials. They are abundant in mangrove forests where they are major colonizers, feeding on decaying leaves and initiating the mangrove food web. Discovered 80 years ago, they have recently attracted considerable attention due to their biotechnological potential. This interest arises from their fast growth, their specific lipid metabolism and the improvement of the genetic tools and transformation techniques. These organisms are particularly rich in ω3-docosahexaenoic acid (DHA), an 'essential' fatty acid poorly encountered in land plants and animals but required for human health. To produce their DHA, thraustochytrids use a sophisticated system different from the classical fatty acid synthase system. They are also a potential source of squalene and carotenoids. Here we review our current knowledge about the life cycle, ecophysiology, and metabolism of these organisms, with a particular focus on lipid dynamics. We describe the different pathways involved in lipid and fatty acid syntheses, emphasizing their specificity, and we report on the recent efforts aimed to engineer their lipid metabolism.

Production of High-Value Polyunsaturated Fatty Acids Using Microbial Cultures

Microbes can produce not only commodity fatty acids, such as palmitic acid (16:0) and stearic acid (18:0), but also high-value fatty acids (essential fatty acids). Most high value fatty acids belong to long chain polyunsaturated fatty acids (PUFA), such as omega-3 fatty acids (e.g., eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)) and omega-6 fatty acids (e.g., arachidonic acid (ARA) and γ-linolenic acid (GLA)). EPA (20:5n-3) is a 20-carbon fatty acid with five double bonds, and the first double bond is in the n-3 position. DHA (22:6n-3) is a 22-carbon fatty acid with 6 double bonds and the first double bond is also in the n-3 position. Both EPA and DHA play an essential role in cardiovascular health including prevention of atherosclerotic disease development (Zehr and Walker, Prostaglandins Other Lipid Mediat 134:131-140, 2018). ARA (20:4n-6) is a 20-carbon fatty acid with four double bonds, and the first double bond is in the n-6 position. GLA (18:3n-6) is an 18-carbon fatty acid with three double bonds, and the first double bond is in the n-6 position. ARA and GLA have multiple biological effects, such as lowering blood cholesterol, and lowering cardiovascular mortality (Poli and Visioli, Eur J Lipid Sci Technol 117(11):1847-1852, 2015). This chapter provides details on microbial production of EAP, DHA, ARA, and GLA.

Delineating the molecular responses of a halotolerant microalga using integrated omics approach to identify genetic engineering targets for enhanced TAG production

BACKGROUND

Harnessing the halotolerant characteristics of microalgae provides a viable alternative for sustainable biomass and triacylglyceride (TAG) production. Scenedesmus sp. IITRIND2 is a fast growing fresh water microalga that has the capability to thrive in high saline environments. To understand the microalga's adaptability, we studied its physiological and metabolic flexibility by studying differential protein, metabolite and lipid expression profiles using metabolomics, proteomics, real-time polymerase chain reaction, and lipidomics under high salinity conditions.

RESULTS

On exposure to salinity, the microalga rewired its cellular reserves and ultrastructure, restricted the ions channels, and modulated its surface potential along with secretion of extrapolysaccharide to maintain homeostasis and resolve the cellular damage. The algal-omics studies suggested a well-organized salinity-driven metabolic adjustment by the microalga starting from increasing the negatively charged lipids, up regulation of proline and sugars accumulation, followed by direction of carbon and energy flux towards TAG synthesis. Furthermore, the omics studies indicated both de-novo and lipid cycling pathways at work for increasing the overall TAG accumulation inside the microalgal cells.

CONCLUSION

The salt response observed here is unique and is different from the well-known halotolerant microalga; Dunaliella salina, implying diversity in algal response with species. Based on the integrated algal-omics studies, four potential genetic targets belonging to two different metabolic pathways (salt tolerance and lipid production) were identified, which can be further tested in non-halotolerant algal strains.

Metabolic engineering to enhance biosynthesis of both docosahexaenoic acid and odd-chain fatty acids in Schizochytrium sp. S31

BACKGROUND

Docosahexaenoic acid (DHA, C22:6) and odd-chain fatty acids (OCFAs, C15:0 and C17:0) have attracted great interest, since they have been widely used in food and therapeutic industries, as well as chemical industry, such as biodiesel production and improvement. The oil-producing heterotrophic microalgae Schizochytrium sp. 31 is one of main DHA-producing strains. Recently, it was found that Schizochytrium can also synthesize OCFAs; however, contents and titers of DHA and OCFAs in Schizochytrium are still low, which limit its practical application.

RESULTS

In this study, we found that acetyl-CoA carboxylase suffered from a feedback inhibition by C16-CoA in Schizochytrium, and relief of the inhibition resulted in improved both lipid content and the ratio of OCFAs in total fatty acids. Based on this finding, a novel strategy for elevating both DHA and OCFAs contents was established. First, the total lipid accumulation was increased by overexpressing a malic enzyme from Crypthecodinium cohnii to elevate NADPH supply. Second, the inhibition effect on acetyl-CoA carboxylase was relieved by overexpressing a codon-optimized ELO3 gene from Mortierella alpina, which encodes an elongase enzyme responsible for converting C16 into C18 fatty acids. After the above two-step engineering, contents of DHA and OCFAs were increased by 1.39- and 3.30-fold, reaching a level of 26.70 and 25.08% of dry cell weight, respectively, which are the highest contents reported so far for Schizochytrium. The titers of DHA and OCFAs were elevated by 1.08- and 2.57-fold, reaching a level of 3.54 and 3.32 g/L, respectively. Notably, the OCFAs titer achieved was 2.66-fold higher than the highest reported in Escherichia coli (1.25 g/L), implying potential value for industry application. To reveal the potential metabolic mechanism for the enhanced biosynthesis of both DHA and OCFAs, LC-MS metabolomic analysis was employed and the results showed that the pentose phosphate pathway and the glycolysis pathway were strengthened and intracellular propionyl-CoA concentration were also significantly increased in the engineered Schizochytrium, suggesting an increased supply of NADPH, acetyl-CoA, and propionyl-CoA for DHA and OCFAs accumulation.

CONCLUSIONS

The discovery provides a new source of OCFAs production, and proposes a new strategy to improve contents and titers of both DHA and OCFAs in Schizochytrium. These will be valuable for improving commercial potential of Schizochytrium and guiding the engineering strategy in other fatty acids producing heterotrophic microalga.

Screening of chemical modulators for lipid accumulation in Schizochytrium sp. S31

Schizochytrium sp. is a promising candidate for docosahexaenoic acid (DHA) production due to its high content of lipid and DHA proportions. To further enhance the lipid accumulation, seven chemical modulators were screened to evaluate their roles on lipid accumulation. Notably, among the seven tested chemical modulators, the addition of naphthoxyacetic acid (BNOA) or jasmonic acid (JA) was able to increase the lipid accumulation of Schizochytrium sp. S31. In addition, the effects of BNOA and JA were demonstrated dose-dependent and time-dependent, achieving a highest increase in lipid content by 11.16% and 12.71% when 2.0 mg/L of BNOA or 20 mg/L of JA was added into culture at 48 h after inoculation, respectively. In addition, the combination of 2 mg/L BNOA and 20 mg/L JA further increased lipid accumulation up to 16.79%. These results provided valuable strategy on promoting the lipid accumulation and DHA production by chemical modulators in Schizochytrium sp. S31.

Influence of oxygen on the biosynthesis of polyunsaturated fatty acids in microalgae

As one of the most important environmental factors, oxygen is particularly important for synthesis of n-3 polyunsaturated fatty acids (n-3 PUFA) in microalgae. In general, a higher oxygen supply is beneficial for cell growth but obstructs PUFA synthesis. The generation of reactive oxygen species (ROS) under aerobic conditions, which leads to the peroxidation of lipids and especially PUFA, is an inevitable aspect of life, but is often ignored in fermentation processes. Irritability, microalgal cells are able to activate a number of anti-oxidative defenses, and the lipid profile of many species is reported to be altered under oxidative stress. In this review, the effects of oxygen on the PUFA synthesis, sources of oxidative damage, and anti-oxidative defense systems of microalgae were summarized and discussed. Moreover, this review summarizes the published reports on microalgal biotechnology involving direct/indirect oxygen regulation and new bioreactor designs that enable the improved production of PUFA.

Abiotic stresses as tools for metabolites in microalgae

Microalgae, due to various environmental stresses, constantly tune their cellular mechanisms to cope with them. The accumulation of the stress metabolites is closely related to the changes occurring in their metabolic pathways. The biosynthesis of metabolites can be triggered by a number of abiotic stresses like temperature, salinity, UV- radiation and nutrient deprivation. Although, microalgae have been considered as an alternative sustainable source for nutraceutical products like pigments and omega-3 polyunsaturated fatty acids (PUFAs) to cater the requirement of emerging human population but inadequate biomass generation makes the process economically impractical. The stress metabolism for carotenoid regulation in green algae is a 2-step metabolism. There are a few major stresses which can influence the formation of phycobiliprotein in cyanobacteria. This review would primarily focus on the cellular level changes under stress conditions and their corresponding effects on lipids (including omega-3 PUFAs), pigments and polymers.

Exploration of a mechanism for the production of highly unsaturated fatty acids in Scenedesmus sp. at low temperature grown on oil crop residue based medium

The ability of algae to produce lipids comprising of unsaturated fatty acids varies with strains and culture conditions. This study investigates the effect of temperature on the production of unsaturated fatty acids in Scenedesmus sp. grown on oil crop residue based medium. At low temperature (10°C), synthesis of lipids compromising of high contents of unsaturated fatty acids took place primarily in the early stage while protein accumulation mainly occurred in the late stage. This stepwise lipid-protein synthesis process was found to be associated with the contents of acetyl-CoA and α-KG in the algal cells. A mechanism was proposed and tested through simulation experiments which quantified the carbon flux allocation in algal cells at different cultivation stages. It is concluded that low culture temperature such as 10°C is suitable for the production of lipids comprising of unsaturated fatty acids.

Transcriptomic Analysis of the Regulation of Lipid Fraction Migration and Fatty Acid Biosynthesis in Schizochytrium sp

Schizochytrium sp. is the main source of docosahexaenoic acid-rich oil, which is widely used in food additive and pharmaceutical industry. In this study, using RNA-seq, comparative transcriptomic analyses were performed at four stages of DHA fermentation by Schizochytrium sp to get potential genes related to cell transition from cell growth to lipid accumulation and then to lipid turnover. 1406, 385, 1384 differently expressed genes were identified by comparisons in pairs of S2 vs S1, S3 vs S2 and S4 vs S3. Functional analysis revealed that binding and single-organism process might be involve in the cell transition from cell growth to lipid accumulation while oxidation-reduction process played an important role in the transition from lipid accumulation to lipid turnover. pfaC in the PKS pathway showed higher sensitivity to the environmental change, which might be the key regulator for enhancing PUFA biosynthesis in the future. Some other genes in signal transduction and cell transport were revealed to be related to lipid turnover, which would enrich the current knowledge regarding lipid metabolism and help to enhance the DHA production and enrich different lipid fractions by Schizochytrium in the future.