How do cyanobacteria affect the planet's atmosphere?
Those bacteria are part of the phytoplankton lives in salt and fresh water, which recovers more than 72% of the planet surface. There is so a lot of place for them to develop.
So the cyanobacteria dicrease the amount of CO2 in the athmosphere and expires O in it, it's because of them that the athmosphere is as it is today.
Have a good day :)