Squalene, as a natural triterpenoid with a range of biological activities (Dessì et al., 2002), was first discovered in the liver of deep-sea sharks in 1916 (Tsujimoto, 1916). Squalene is widely utilized in various fields, including the food (Sethi et al., 2016) and pharmaceutical sectors (Toelzer et al., 2020). Especially, squalene is used as the adjuvant in vaccines (Melody, 2020). By 2025, the market for squalene is expected to reach US $184 million (Markets and Markets, 2020). The traditional source of squalene from deep-sea shark liver oil is expensive, unsustainable, and detrimental to marine ecosystems (Pandarus et al., 2014). As an alternative, squalene can be extracted from plants such as olives and amaranth plants (Giacometti and Milin, 2007). However, due to the low productivity of squalene in plants and animals, researchers are now devoted to the development of microbial factories for squalene production. The evolution of squalene production methods is summarized and presented (Fig. 1). As shown in Table 1, the major companies engaged in the commercial production of squalene were summarized.

Many native microorganisms naturally produce squalene via two distinct squalene biosynthetic pathways, including the eukaryote mevalonate (MVA) pathway (Xia et al., 2022) and the prokaryote 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway (Kang et al., 2022). However, natural microorganisms, such as Saccharomyces cerevisiae, Torulaspora delbrueckii, and Methylomonas methanica, have a limited capacity to produce squalene at industrial-scale levels. For example, S. cerevisiae can produce 1.6 mg/g DCW squalene (Mantzouridou and Tsimidou, 2010), T. delbrueckii can produce 0.24 mg/g DCW squalene (Bhattacharjee et al., 2001), and M. methaica can produce 1.16 mg/g DCW squalene (Goldberg and Shechter, 1978). With advances in synthetic biology and metabolic engineering, various metabolic strategies have been used to engineered yeasts, especially using S. cerevisiae and Yarrowia lipolytica as cell factory for enhanced squalene production (Table 2, Table 3, Table 4, Table 5, Table 6). For example, the highest titer of squalene produced by engineered S. cerevisiae was 55.8 g/L (Tang et al., 2025). Similarly, the engineered Y. lipolytica was caple of producing squalene titer with 51.2 g/L (Ning et al., 2024).

Currently, researchers are focused on a series of metabolic engineering strategies aimed at boosting squalene production in yeasts. These strategies include: 1) upregulation of key genes in the pathway for squalene biosynthesis to enhance the flux towards squalene synthesis; 2) attenuation of squalene metabolism to minimize its degradation and maximize its accumulation; 3) compartmentalization engineering to provide sufficient intermediates and a suitable environment for catalytic reactions while bypassing competing metabolic pathways; 4) cofactor engineering to optimize the availability and recycling of essential cofactors required for squalene synthesis; 5) manipulation of squalene storage and efflux mechanisms to facilitate its accumulation and recovery from the yeasts cells; and 6) fermentation process engineering to optimize culture conditions and operational parameters for maximizing squalene titer. These strategies are illustrated in Fig. 2. Over the past five years, several reviews have systematically summarized advances in microbial squalene production (Elsharawy and Refat, 2024; Gohil et al., 2019; Mendes et al., 2022; Paramasivan and Mutturi, 2022; Patel et al., 2022; Wang et al., 2023; Chai et al., 2024). However, notable limitations in the current research remain unaddressed. For example, although Wang et al. provided a broad perspective on enhancing squalene synthesis efficiency through metabolic regulation and process optimization, this review encompassed diverse microbial systems without focusing specifically on yeast as chassis (Wang et al., 2024). Similarly, Elsharawy and Refat highlighted the potential of Y.lipolytica as a robust platform for squalene synthesis, nevertheless, this review did not systematically compare the applied engineering strategies among different yeast hosts (Elsharawy and Refat, 2024). The yeasts like S. cerevisiae and Y. lipolytica are recognized as promising chassis for squalene production due to the native MVA pathways, redox capacity, and substrate flexibility (Moser and Pichler, 2019; Elsharawy and Refat, 2024; Liu et al., 2015). Although the dedicated and systematic review that integrates recent advances, compares various engineering approaches across various yeast species, and provides a modular metabolic engineering framework for yeast derived-squalene production is still lacking.

This review aims to address this critical limitation by offering the focused overviews of engineering strategies in yeasts, thereby establishing a foundational resource for future research and industrial applications in yeast-based squalene production. Firstly, the characteristics of squalene biosynthesis pathways within yeast cells were introduced. Secondly, metabolic engineering strategies for promoting squalene production were summarized. Thirdly, advanced genetic engineering tools for boosting squalene and other terpenoids production are investigated. Fourthly, the potential of emerging other yeasts for squalene synthesis is explored. Finally, the potential approaches for improving squalene yield are discussed. This review will offer guidance for the sustainable production of squalene via yeasts fermentation.