Choreography of multiple omics reveals the mechanism of lipid turnover in Schizochytrium sp. S31
Graphical abstract
Introduction
Schizochytrium, a genus of thraustochytrids, could produce more than 50% docosahexaenoic acid (DHA) of cellular lipids. DHA is a crucial n-3 polyunsaturated fatty acid and recognized as an essential nutrient by numerous clinical studies [1]. In recent years, various fermentation strategies to enhance DHA yield by using Schizochytrium have been reported, such as medium components, environment conditions, and genetic engineering [[2], [3], [4]]. In previous fed-batch cultivation of Schizochytrium sp. S31, three stages were identified, including cell growth, lipid accumulation, and lipid turnover [5]. The lipid turnover stage was also called a “DHA enrichment” stage due to the preferential degradation of saturated fatty acids and a reservation of polyunsaturated acids. Controlling the fermentation stage was therefore proposed to increase DHA concentration [6,7]. Recently, several studies have been conducted to facilitate the lipid migration in Schizochytrium. Ren et al. [8] firstly found that the transfer of neutral lipid into polar lipid, and the rise of unsaponifiable matters would happen during lipid turnover. After that, it was revealed that fatty acid oxidation process played a critical role in the stage of lipid turnover [9].
As we understand that the nutrition of DHA oil is not only determined by DHA concentration but also DHA distribution in glycerol backbones of triglycerides (TG) [10]. Lipid turnover facilities a preferential enrichment of DHA, and the distribution of DHA in TG structure may also be altered. Thus, uncovering the lipid turnover mechanism would contribute to the production of high-quality DHA oil in microalgae. To elucidate the phenomenon of lipid turnover, the involved lipid molecules should be identified in detail. Our previous studies firstly reported the profiles of TG and phospholipids (PL), which are the two major lipid classes produced by Schizochytrium [11,12]. We also optimized the cultivation conditions to investigate the influence on lipid turnover as well as the fermentation characteristics [[13], [14], [15]]. It was found that the lipid turnover phenomenon was more apparent when glycerol was used as the carbon source, and the carotenoid content was also enhanced. However, the explanation of the mystery is less well appreciated due to that lipid turnover is a complex process.
Recently, multiple omics were reported as a promising strategy in investigating the cellular metabolic mechanism [16]. The genomic analysis of several species of Schizochytrium was conducted [17,18]. But the genome of Schizochytrium sp. S31 remains uncovered. Though the pathways involved in the formation and degradation of lipids were demonstrated, existing literature was mainly limited to lipid synthesis process, fermentation optimization, or just a data report of the genome. Lipidomics is a powerful strategy and generally defined as the in-depth investigation of lipid networks in yeasts [19]. An extensive examination of lipid profile could provide insight into the linkage between lipase activities and gene expressions of Schizochytrium, thus contributing to the elucidation of lipid turnover mechanism. Transcriptomic is another strategy for understanding of gene expressions which can clarify the lipid metabolism during the turnover stage. Comparative transcriptomics in Schizochytrium have been undertaken, which have proved its wide-ranging adaptability in discovering molecular mechanisms [9,20,21]. In this study, multiple omics approaches, including lipidomics, transcriptomics, and genomics, were utilized to uncover the mechanism of lipid turnover in Schizochytrium sp. S31. Lipid profile changes and gene expression levels were illustrated at different cultivation stages. The genome of Schizochytrium sp. S31 was subsequently investigated to find the key lipases involved in this stage. We hope that the data generated from this study would contribute to a better understanding of lipid turnover mechanism and provide new insights for genetic engineering to improve the quality of algae oil produced by Schizochytrium sp. S31.
Section snippets
Microorganism and culture conditions
Schizochytrium sp. S31 (ATCC 20888) was supplied by the American Type Culture Collection. The seed culture medium (g/L): glucose, 30; yeast extract, 5; sodium glutamate, 5; NaCl, 0.3; vitamin B1, 0.004; vitamin B6, 0.003; vitamin B12, 0.005; Na2SO4, 15; K2SO4, 1; K2HPO4, 2; MgSO4·7H2O, 3; KH2PO4, 3; CaCl2, 0.02. The pH was adjusted to 6.5 by 10% NaOH before autoclaving at 121 °C for 20 min. The glucose was sterilized at 115 °C for 15 min separately. The vitamin B1, vitamin B6, and vitamin B12
Lipid turnover during fermentation
As shown in Fig. 1A, the biomass reached a maximum of 58.36 g/L at 72 h, and the total lipid content reached a peak of 51.69% at 96 h. Lipid turnover occurred at 120 h as indicated by glycerol depletion. During this stage, both the biomass and lipid content decreased drastically. Finally, the biomass decreased to 35.55 g/L, and the total lipid content decreased to 44.92% at 168 h. The fermentation process of Schizochytrium sp. S31 can be divided into three stages: cell growth, lipid
Conclusion
Uncover of lipid turnover mechanism and discovery of functional lipase are crucial for the subsequent design of high-quality algae oils. In this study, the special lipase STGL3 was found to be involved in the prior hydrolysis of FAS products in TG, leading to a reservation of PKS products in PL during the lipid turnover stage. The results shed new light on the lipid turnover mechanism and contribute to the customization of algae oil from Schizochytrium. However, further confirmations about the
CRediT authorship contribution statement
Ming Chang: Writing - original draft, Investigation, Funding acquisition.
Tao Zhang: Writing - original draft, Investigation, Data curation.
Leilei Li: Data curation.
Fei Lou: Data curation.
Meimei Ma: Data curation.
Ruijie Liu: Conceptualization, Funding acquisition.
Qingzhe Jin: Investigation, Conceptualization.
Xingguo Wang: Conceptualization, Writing – review & editing, Supervision, Project administration.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 31401619), the Natural Science Foundation of Jiangsu Province (Grant No. BK20140156).
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Authors contributed equally.