Excessive carbon dioxide (CO2) emissions have become a disaster, calling for an effective way to reduce atmospheric CO2 content. Cyanobacteria, also known as blue-green algae, are potent natural carbon sinks with efficient photosynthesis and produce valuable compounds. Herein, we demonstrated the excellent properties of Cyanobacterium aponinum PCC10605 (Capo) in maintaining growth and protein accumulation under salt stress and red light-emitting diodes (LEDs) illumination at 660 nm under direct air capture (DAC) condition. Multi-factor optimization from response surface methodology revealed the best condition of 25 °C, 70 μE light intensity, 17 g/L salt, 16 to 8 h for light to dark period, and 6 mM nitrogen to yield the highest combination of 1.49 g/L biomass and 19.1 % C-phycocyanin (C-PC) content. Capo also thrived in 50 % seawater from Penghu, Taiwan where its DAC capacity was confirmed using an elemental analyzer, showing that 48.2 % carbon content was equivalent to specific capture ratio of 1.75 g-CO2/g-DCW and volumetric efficiency of 2.62 g-CO2/L, respectively. Finally, scaling up cultivation in 2-liter photobioreactor was successfully carried out under red LED illumination. The potential of Capo in reducing atmospheric CO2 level represents an advancement in sustainable biotechnology for climate change mitigation.