关键词: 蓝细菌, 聚球藻, 合成生物学, 底盘细胞, 碳中和, 光驱动细胞工厂

Abstract:

CO₂ is both the primary greenhouse gas and an abundant carbon resource. Highly efficient CO₂ utilization technologies, which can alleviate the urgent pressure of energy and environment, are considered as the crucial reliances for getting the goal of “carbon peak and neutrality”. Photoautotrophic cyanobacteria can directly convert CO₂ into organic compounds only using solar energy. It is the main microbial chassis for developing light-driven cell factories that can produce useful compounds by capturing CO₂. As a typical representative of cyanobacteria, Synechococcus possesses many advantages: fast growth rate, clear genetic background, and low nutritional requirements. It is currently a hotspot of cyanobacterial synthetic biology. In the context of “carbon peak and neutrality,” research of Synechococcus chassis is ushering unprecedented opportunities. This review discusses the rationality and opportunities in developing cyanobacterial chassis from the perspectives of natural evolution, historical geologic limitations, climate dependence, and energy conversion efficiency. The application potentials in energy production, chemical manufacturing, and carbon sequestration are proposed and discussed. The metabolic potential of cyanobacteria is also discussed for their carbon fixation, light utilization, and biodiversity. Then, we systematically review the significant research advances in cyanobacterial chassis development and application. First, we describe the recently developed gene-editing methods of cyanobacteria, which are very important for constructing and remodeling cyanobacteria chassis. The feasibility of developing base editing technology that can facilitate multiplex editing in cyanobacteria is discussed. The CRISPRi technology for Synechococcus is also summarized. Second, we review the adaptive evolution in cyanobacteria. Researches on direct chassis evolution based on continuous cultivation and genetic element evolution based on phage and error-prone PCR are summarized. We also discuss the potential of adaptive evolution in cyanobacteria. Third, we review the stress tolerance of Synechococcus, especially the resistance to multiple stresses. The reported genetic elements responsible for stress factors, such as intense light, alkali, low pH, high temperature, and high salinity, are described. Some newly identified chassis are discussed on their unique characteristics. We propose some strategies for the directed engineering, which are practible for enhancing the stress tolerance of Synechococcus. Fourth, we review the progress of cyanobacterial cell factories and describe the recent production of various compounds by cyanobacteria, including bulk chemicals and fine chemicals. Moreover, we review new methods for developing cyanobacterial cell factories. The coculture method is discussed on its advantages and applications. The nanoparticle-mediated NADP regeneration is also reviewed for its application in enhancing the efficiency of the cyanobacterial cell factories. The existing problems and challenges are also listed with corresponding proposed solutions and coping strategies. We believe that the in-depth exploration of these problems and challenges will promote the advancement of cyanobacterial synthetic biology. It is expected that breakthroughs will soon be made in light energy capture, carbon fixation, stress resistance, and metabolic reprogramming. It is also expected that we can eventually design and build an efficient photosynthetic chassis surpassing natural evolution, based on which next-generation light-driven microbial factories can be constructed. This will significantly propel the realization of “carbon neutrality”.

Key words: Cyanobacteria, Synechococcus, synthetic biology, chassis cell, carbon neutrality, light-driven cell factory