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Commercial application of cyanobacterial polyhydroxybutyrate (PHB) production directly from CO2 is hampered by slow growth rate and low photoautotrophic PHB productivity of natural cyanobacteria species. Herein, a novel PHB-producing cyanobacterial strain was constructed by harnessing a fast-growing cyanobacterium Synechococcus elongatus UTEX 2973 with introduction of heterologous phaCAB genes. Under photoautotrophic condition, the recombinant cyanobacteria produced 420 mg L-1 (16.7% of dry cell weight) with the highest productivity of 75.2 mg L-1 d-1. When compared with a representative natural producer, Synechocystis sp. PCC 6803, the engineered strain exhibited 2.4-fold higher in PHB productivity. The scalability of the engineered strain was investigated by indoor large-scale cultivation in a photobioreactor, resulting in 278 mg L-1 of PHB (21.1% of dry cell weight). The demonstration of outdoor cultivation directly utilizing industrial flue gas as the sole carbon source further highlighted the potential of the constructed strain as a Carbon Capture and Utilization (CCU) platform. This study can provide a promising solution that addresses both petroleum-based plastic waste and CO2 pollution, the two of the most confound environmental issues today. 

1. Introduction	1
2. Research Background	7
 2.1. Polyhydroxybutyrate	6
 2.2. Conventional production of PHB	11
 2.3. Cyanobacterial production of PHB	13
 2.4. Challenges in cyanobacterial PHB production	16
3. Materials and Methods	18
 3.1. Chemicals and reagents	18
 3.2. Strain construction	19
  3.2.1 Plasmid and strain construction	19
  3.2.2 Conjugation of Synechococcus 2973	21
 3.3. Lab-scale cultivation	23
 3.4. Culture conditions for the large-scale experiments	24
  3.4.1. Indoor cultivation in a scalable photobioreactor	24
  3.4.2. Outdoor cultivation using industrial flue gas	25
 3.5. Quantification of dry weight	26
 3.6. Quantification of PHB	27
 3.7. Quantification of glycogen	28
 3.8. Visualization of PHB granules using Transmission Electron Microscope (TEM)	29
 3.9. RNA extraction and real-time qPCR	30
4. Results and Discussion	32
 4.1. Development of PHB-producing Synechococcus 2973 strain	32
 4.2. Verification of PHB synthesis in the engineered Synechococcus 2973	37
 4.3. Characterization of the engineered Synechococcus 2973	42
 4.4 Photoautotrophic production of PHB in the engineered Synechococcus 2973	47
 4.5. Relative expression levels of PHB genes in the engineered Synechococcus 2973 and Synechocystis 6803 strains	52
 4.6. Large-scale indoor PHB production	55
 4.7. Large-scale outdoor PHB production using industrial 
flue gas	59

5. Conclusion	64

References	65
Acknowledgments	74