ATP Drives Efficient Terpene Biosynthesis in Marine Thraustochytrids

Genome-scale metabolic modeling of Thraustochytrium sp. RT2316-16: Effects of nutrients on metabolism

Marine thraustochytrids produce metabolically important lipids such as the long-chain omega-3 polyunsaturated fatty acids, carotenoids, and sterols. The growth and lipid production in thraustochytrids depends on the composition of the culture medium that often contains yeast extract as a source of amino acids. This work discusses the effects of individual amino acids provided in the culture medium as the only source of nitrogen, on the production of biomass and lipids by the thraustochytrid Thraustochytrium sp. RT2316-16. A reconstructed metabolic network based on the annotated genome of RT2316-16 in combination with flux balance analysis was used to explain the observed growth and consumption of the nutrients. The culture kinetic parameters estimated from the experimental data were used to constrain the flux via the nutrient consumption rates and the specific growth rate of the triacylglycerol-free biomass in the genome-scale metabolic model (GEM) to predict the specific rate of ATP production for cell maintenance. A relationship was identified between the specific rate of ATP production for maintenance and the specific rate of glucose consumption. The GEM and the derived relationship for the production of ATP for maintenance were used in linear optimization problems, to successfully predict the specific growth rate of RT2316-16 in different experimental conditions.

Enhancing the biomass and docosahexaenoic acid-rich lipid accumulation of Schizochytrium sp. in propionate wastewater

In order to find a more effective way to obtain docosahexaenoic acid (DHA) rich lipid from Schizochytrium sp., a widespread propionate wastewater (PW) is used. PW is a common industrial and domestic wastewater, and transforming it into valuable products is a potential treatment method. Schizochytrium sp. is a rapidly growing oleaginous organism, which has been used commercially for DHA production. Herein, PW is completely used for DHA production by Schizochytrium sp. by genetic engineering and fermentation optimization, which can alleviate the increasingly tense demand for water resources and environmental pollution caused by industrial wastewater. Firstly, the methylmalonyl-CoA mutase (MCM) was overexpressed in Schizochytrium sp. to enhance the metabolism of propionate, then the engineered strain of overexpressed MCM (OMCM) can effectively use propionate. Then, the effects of PW with different concentration of propionate were investigated, and results showed that OMCM can completely replace clean water with PW containing 5 g L-1 propionate. Furthermore, in the fed-batch fermentation, the OMCM obtained the highest biomass of 113.4 g L-1 and lipid yield of 64.4 g L-1 in PW condition, which is 26.8% and 51.7% higher than that of wild type (WT) in PW condition. Moreover, to verify why overexpression of MCM can promote DHA and lipid accumulation, the comparative metabolomics, ATP production level, the antioxidant system, and the transcription of key genes were investigated. Results showed that ATP induced by PW condition could drive the synthesis of DHA, and remarkably improve the antioxidant capacity of cells by enhancing the carotenoids production. Therefore, PW can be used as an effective and economical substrate and water source for Schizochytrium sp. to accumulate biomass and DHA.

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.

Metal cofactor regulation combined with rational genetic engineering of Schizochytrium sp. for high-yield production of squalene

The increasing market demand for squalene requires novel biotechnological production platforms. Schizochytrium sp. is an industrial oleaginous host with a high potential for squalene production due to its abundant native acetyl-CoA pool. We first found that iron starvation led to the accumulation of 1.5 g/L of squalene by Schizochytrium sp., which was 40-fold higher than in the control. Subsequent transcriptomic and lipidomic analyses showed that the high squalene titer is due to the diversion of precursors from lipid biosynthesis and increased triglycerides (TAG) content for squalene storage. Furthermore, we constructed the engineered acetyl-CoA C-acetyltransferase (ACAT)-overexpressing strain 18S::ACAT, which produced 2.79 g/L of squalene, representing an 86% increase over the original strain. Finally, a nitrogen-rich feeding strategy was developed to further increase the squalene titer of the engineered strain, which reached 10.78 g/L in fed-batch fermentation, a remarkable 161-fold increase over the control. To our best knowledge, this is the highest squalene yield in thraustochytrids reported to date.

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.

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.

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.

Efficient Terpene Production by Marine Thraustochytrids: Shedding Light on the Thermodynamic Driving Force

Terpenoids, such as squalene, are valuable compounds for cosmetic and drug industries, the supply of which is often limited by natural sources. Alternative production strategies have been investigated for decades but remain challenging due to low yields. In a recent study, Zhang and coworkers (A. Zhang, K. Mernitz, C. Wu, W. Xiong, et al., mBio 12:e0088121, 2021, https://doi.org/10.1128/mBio.00881-21) report the potential use of marine thraustochytrid metabolic thermodynamics in effective terpene engineering. Through comparative proteomics and metabolomics, as well as thermodynamic modeling, the authors demonstrated sodium-induced changes in thraustochytrid metabolism leading to a twofold increase in squalene accumulation. The differential abundances of the metabolic enzymes and metabolites, as well as higher respiration, indicated the metabolic shift from carbohydrate to lipid oxidation and increased ATP input to the mevalonate pathway and squalene synthesis. This breakthrough provides new important insights into microbial terpene metabolic engineering but above all displays thermodynamics as a valuable tool in metabolic engineering.