# Biosynthesis Progress of High-Energy-Density Liquid Fuels Derived from Terpenes
## 3 Microbial Synthesis of Monoterpene and Sesquiterpene Fuels
### Microbial Synthesis of Sesquiterpenes  
The microbial synthesis research of sesquiterpenes is more extensive, and many sesquiterpene compounds have been synthesized (Table 2). The synthesis of \(\beta\)-farnesene has been achieved via commercial application by Amyris Inc., achieving a fermentation yield of 130 g/L in a 2000 L fermenter in _S. cerevisiae_[52]. They introduced four non-native metabolic reactions into _S. cerevisiae_ to rewrite the central carbon metabolism, enabling the biosynthesis of cytosolic acetyl-CoA with a reduced ATP requirement and loss of carbon to CO\({}_{2}\)-emitting reactions and improved the pathway redox balance [52]. These changes dramatically increased the productivity in industrial fermentations that are oxygen-constrained and provided a viable strategy for the large-scale production of other terpen in _S. cerevisiae_. In addition, other researchers have also synthesized \(\beta\)-farnesene using _Y. lipolytica_ as the host cell. [PERSON] et al. achieved a fermentation titer of 28.9 g/L \(\beta\)-farnesene in _Y. lipolytica_ by modulating the regeneration of NAD(P)H, the supply of precursors and ATP [53]. [PERSON] et al. achieved a high yield of \(\beta\)-farnesene (31.9 g/L) using waste edible oil as a carbon source by genetically engineering the lipid synthesis pathway and combining \(\beta\)-farnesene synthase engineering [54]. Apart from \(\beta\)-farnesene, the microbial synthesis of \(\alpha\)-farnesene has also shown good progress. The highest titer of \(\alpha\)-farnesene was achieved in _S. cerevisiae_, reaching 38.8 g/L [55]. Monocyclic sesquiterpenes such as bisabolene, germacrene and humulene have also been efficiently synthesized in microorganisms. The microbial synthesis of \(\alpha\)-bisabolene was achieved in _Y. lipolytica_ by positioning the \(\alpha\)-bisabolene synthetic pathway into peroxisomes and increasing the supply of ATP, achieving a yield of 15.5 g/L [55]. Recently, a new \(\gamma\)-bisabolene synthase, AcTPSS, was characterized from _Acremonium chrysogenum_, and the highest titer of \(\gamma\)-bisabolene, which reached 2.69 g/L, was achieved by expressing AcTPSS both in the cytosol and peroxisomes of _S. cerevisiae_[56]. The production of germacrene A reached 39 g/L by screened germacrene A synthase and increased the synthesis of acetylCoA by reconstructing the endogenous MVA pathway [57]. Our research group identified a new germacrene D synthase, AcTPS1, and by increasing the copy number of the limited enzyme of the MVA pathway and _Actps1_, together with inhibiting the expression of the competing pathway, the efficient synthesis of germacrene D was realized in _S. cerevisiae_, achieving a titer of 7.9 g/L [58]. [PERSON] et al. identified a bicyclogermacrene synthase in _Penicillium expansum_ and realized the synthesis of bicyclogermacrene in _E. coli_ with a titer of 188 mg/L [59]. In addition, [PERSON] et al. achieved the efficient synthesis of bicyclic sesquiterpenes including valencene and eremophilene in _S. cerevisiae_ through screening and engineering synthases and optimizing the synthetic pathway, achieving titers of 16.6 g/L and 34.6 g/L, respectively [60]. Recently, our research group synthesized 17 sesquiterpenes with high productions in _S. cerevisiae_, such as pentalenene, \(\alpha\)-santalene, presilphiperfol-1-ene and protoilludane, and all achieved titers exceeding 10 g/L [32]. With the discovery of more sesquiterpene synthases, more and more tricyclic sesquiterpenes, including oxygenated sesquiterpenes, have been successfully synthesized by microorganisms, including longifolene, zizaene, viridiflorol and patchoulol. [PERSON] et al. improved the translation efficiency of viridiflorol synthase and combined strain modification, achieving a titer of 25.7 g/L viridiflorol in _E. coli_[61]. The efficient synthesis of sesquiterpenes is crucial for their application in fuel development.