Here's why:

* Photosynthesis: Cyanobacteria are photosynthetic, meaning they produce energy from sunlight. This process requires oxygen as a byproduct.

* Oxygenic Photosynthesis: Cyanobacteria perform oxygenic photosynthesis, which specifically releases oxygen as a waste product.

* Oxygen Production: Cyanobacteria are responsible for the vast majority of the oxygen in Earth's atmosphere.

While some cyanobacteria can survive in anaerobic environments, they are fundamentally aerobic organisms and require oxygen for their metabolism.