# Biosynthesis Progress of High-Energy-Density Liquid Fuels Derived from Terpenes
## 5 Conclusions and Outlook  
Fuel manufacturing is moving towards a greener direction, and the use of biomass as raw material for fuel synthesis has irreplaceable and significant advantages, leading to sustainable and environmentally friendly fuel production. Terpene-derived fuels, with complex and diverse structures, have great potential to replace traditional HED fuels, ensuring a sustainable and secure supply of HED fuels to a certain extent. While terpenes are widely present in nature, their natural content is insufficient to meet the demands for large-scale fuel production and application. However, with the advancement of synthetic biology and metabolic engineering, engineered microorganisms for fuel synthesis have rapidly progressed. The development of efficient microbial cell factories offers promising prospects for the supply of HED fuels derived from terpenes.  
Despite the considerable efforts made in research on monoterpene and sesquiterpene fuels, there are still some challenges that need to be addressed. Firstly, while there is a wide structural diversity of monoter and sesquiterpenes, there are not many reported terpene-derived fuel molecules that have undergone actual performance testing. The nature of fuel is closely related to its molecular structure, and the steric effects often result in significant differences between predicted performance and actual values, particularly in low-temperature performance, limiting their subsequent applications. Therefore, in the future, it is necessary to synthesize more types of terpene fuel molecules, determine their actual performance values and conduct in-depth research on the relationship between the molecular structure and properties of terpene-derived fuels to provide theoretical guidance for synthesizing high-performance fuels. Secondly, the current yields of terpenes in microorganisms, especially monoterrenes, are low, and the cost is still high, making it difficult to meet industrial production levels. Therefore, further research is needed to investigate the key biological elements in the pathway of terpene synthesis, design and reconstruct the metabolic pathways of chassis cells, optimize the synthetic pathway and metabolic network, and enhance fermentation processes and intelligent control strategies to increase the yield and productivity of terpene fuels. Thirdly, the comprehensive performance of some terpene-derived HED fuels is not as good as traditional petroleum-based HED fuels. However, due to the plasticity of terpene structures, their density and low-temperature performance can be improved in the future through condensation, cyclopropation, introducing side chains or controlling spatial configurations. Additionally, terpene-derived fuels can be used in combination with traditional fuels to enhance the overall performance of the fuel.  
J.L.: conceptualization, data curation, writing--original draft, writing--review and editing. M.L. and P.H.: formal analysis. G.Y. and H.J.: supervision, project administration. All authors have read and agreed to the published version of the manuscript.  
This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.  
There is no new data were created.  
The authors declare no conflicts of interest.