## 6 Conclusions and Future Prospects  
In recent years, a large number of studies have achieved the efficient conversion of renewable substrates into 3-HP using engineered microorganisms. As outlined in this paper, the design of microbial cell factories that can efficiently produce 3-HP is determined by selecting an appropriate biosynthetic pathway, production chassis, and high-quality carbon source. With continuing research on the microbial synthesis of 3-HP, the range of substrates that can be used by microbial cell factories has been gradually increased. Initially, most studies used glycerol and glucose as substrates, after which lignocellulosic, acetate, ethanol, fatty acids, and other alternative substrates were also used for 3-HP synthesis. Researchers are increasingly using renewable feedstocks that are cheaper, easier to obtain, and more environmentally friendly. Combining all reports on the microbial production of 3-HP, a conclusion was drawn that the fewer catalytic reactions required to move from substrate to product, the easier it seems to be to obtain high titers of 3-HP. For each 3-HP production pathway that has been developed, there should be one or more suitable carbon sources (inexpensive, converts to 3-HP by fewer reactions, and can be rapidly assimilated) corresponding to it. There is no doubt that glycerol is the most suitable carbon source for the glycerol pathway, and high 3-HP titers have been achieved in _K. _pneumoniae_ and _P. denitrificans_ that are able to naturally and rapidly metabolize glycerol. In particular, the 3-HP productivity of the _P. denitrifiers_ cell factory was excellent, reaching 2.5 g/(L-h) (which is the minimum requirement for the microbial 3-HP production to be economically competitive) [50]. Reports on the 3-HP production by recombinant _P. denitrifiers_ are scarce, and the production process needs to be further explored. For the malonyl-CoA pathway, it is more appropriate to use substrates that can be efficiently converted to acetyl-CoA, such as acetate and fatty acids. The synthesis of 3-HP from the above substrates is mainly performed in _E. coli_ due to its broad substrate spectrum and clear genetic background. The assimilation of acetate or fatty acids by _E. coli_ still needs to be further strengthened. The \(\beta\)-alanine pathway seems to be less competitive compared to the two pathways mentioned above because of the longer reaction route. A simplified pathway, the oxaloacetate pathway, was recently explored in _S. cerevisiae_, which showed excellent 3-HP production ability. It is worth testing the effects of the oxaloacetate pathway in other chassis. Finding an inexpensive carbon source that can be efficiently converted to oxaloacetate in a few steps is also essential in the future. In addition, engineered strains need to be matched with appropriate fermentation processes to avoid the accumulation of by-products and to balance production and growth. Dynamic regulation devices have also been developed to address these issues, and good 3-HP titers were achieved in each study [9; 28]. Designing or modifying dynamic conditioning devices is an effective strategy to improve the efficiency of engineered strains in the future. Third-generation microbial refineries that synthesize 3-HP directly from CO\({}_{2}\) have been successfully constructed, but the titers are currently at the mg/L level. Further modification of the third-generation microbial refineries for the synthesis of 3-HP to achieve high titers is necessary to unlock the future prospects of environmentally friendly and sustainable production. The efficiency of photosynthesis plays a decisive role in the productivity of cell factories based on cyanobacteria. In the future, it will become possible to optimize the components and pathways of the cyanobacterial photosynthetic system to enhance carbon uptake, sequester carbon, and reduce carbon loss [103]. Conceptualization, T.C. ; Writing--Original Draft Preparation, X.W. ; Writing--Review and Editing, T.C., Z.C., X.S. and Z.W. ; Supervision, T.C.