Summary: Xanthophyllomyces dendrorhous is a promising source of natural astaxanthin due to its ability to accumulate high amounts of astaxanthin. This study showed that 6‐benzylaminopurine (6‐BAP) is an effective substrate that enhances cell biomass and astaxanthin accumulation in X. dendrorhous . In the current study, the biomass and astaxanthin content in X. dendrorhous were determined to be improved by 21.98% and 24.20%, respectively, induced by 6‐BAP treatments. To further understand the metabolic responses of X. dendrorhous to 6‐BAP, time‐course metabolomics and gene expression levels of X. dendrorhous cultures with and without 6‐BAP feeding were investigated. Metabolome analysis revealed that 6‐BAP facilitated glucose consumption, promoted the glycolysis, suppressed the TCA cycle, drove carbon flux of acetyl‐CoA into fatty acid and mevalonate biosynthesis, and finally facilitated the formation of astaxanthin. ROS analysis suggested that the antioxidant mechanism in X. dendrorhous can be induced by 6‐BAP. Additionally, the process of 6‐BAP significantly upregulated the expression of six key genes involved in pathways related to astaxanthin biosynthesis. This research demonstrates the metabolomic mechanism of phytohormone stimulation of astaxanthin production iNn X. dendrorhous and presents a new strategy to improve astaxanthin production to prevent the dilemma of choosing between accumulation of astaxanthin and cell biomass.

The polyketide synthase (PKS) cluster genes are supposed to synthesize polyunsaturated fatty acids (PUFAs) in S. limacinum. In this study, two enyolreductase (ER) genes located on PKS cluster were knocked out through homologous recombination to explore their functions. The knock-out of OrfB-ER (located on OrfB subunit) decreased lipid content and had obvious decrease on PUFAs content, indicating OrfB-ER domain played a vital role on PUFAs synthesis; the knock-out of OrfC-ER (located on OrfC subunit) decreased SFAs content and increased total lipid content, indicating OrfC-ER domain was likely to be related with SFAs synthesis, and lipid production could be improved by down-regulating OrfC-ER domain expression. Therefore, the addition of triclosan as a reported regulator of ER domain induced the increase of PUFAs production by 51.74% and lipids yield by 47.63%. Metabolic analysis indicated triclosan played its role through inhibiting the expression of OrfC-ER to reduce the feedback inhibition of SFAs and further to enhance NADPH synthesis for lipid production, and by weakening mevalonate pathway and tricarboxylic acid (TCA) cycle to shift precursors for lipid and PUFAs synthesis. This research illuminates functions of two ER domains in S. limacinum and provides a potential targets for improving lipid production.

Organic carbon sources have been reported to simultaneously increase the growth and lipid accumulation in microalgae. However, there have been no studies of the mixotrophic growth of Porphyridium purpureum in organic carbon media. In this study, three organic carbon sources, glucose, sodium acetate, and glycerol were used as substrates for the mixotrophic growth of P. purpureum. Moreover, a novel trait-based approach combined with Generalized Additive Modeling was conducted to determine the dosage of each organic carbon source that optimized the concentration of cell biomass or fatty acid. A 0.50% (w/v) dosage of glucose was optimum for the enhancement of the cell growth of P. purpureum, whereas sodium acetate performed well in enhancing cell growth, arachidonic acid (ARA) and eicosapentaenoic acid (EPA) content, and glycerol was characterized by its best performance in promoting both cell growth and ARA/EPA ratio. The optimum dosages of sodium acetate and glycerol for the ARA concentration were 0.25% (w/v) and 0.38% (v/v), respectively. An ARA concentration of 211.47 mg L⁻¹ was obtained at the optimum dosage of glycerol, which is the highest ever reported. The results suggested that a comprehensive consider of several traits offers an effective strategy to select an optimum dosage for economic and safe microalgae cultivation. This study represents the first attempt of mixotrophic growth of P. purpureum and proved that both biomass and ARA accumulation could be enhanced under supplements of organic carbon sources, which brightens the commercial cultivation of microalgae for ARA production.

Dietary consumption of omega-3 long-chain polyunsaturated fatty acids (PUFAs), particularly docosahexaenoic acid (DHA, 22:6n-3) and eicosapentaenoic acid (EPA, 20:5n-3), is associated with various health benefits. Numerous marine microorganisms, such as bacteria, yeast, microalgae, protists, fungi, and dinoflagellates, produce long-chain PUFAs. These PUFAs are biosynthesized via two pathways: the conventional aerobic fatty acid synthase (FAS) pathway in microalgae and the anaerobic PUFA synthase pathway, primarily in marine bacteria and eukaryotic microalgae. Understanding the mechanisms of long-chain PUFAs (LC-PUFAs) biosynthesis is crucial for developing effective strategies to engineer biosynthesis in both heterologous microbial species and native PUFAs producers. This review highlights recent findings on the biosynthetic mechanisms underlying long-chain PUFAs production, with an emphasis on marine bacteria and microalgae. Additionally, developments in metabolic engineering to enhance long-chain PUFAs production in marine microorganisms are discussed.