Cyanobacteria increased the levels of carbon dioxide in the atmosphere. True or False?

False!

Cyanobacteria (blue-green algae) increased the levels of oxygen. It is believed they were the first organisms able to produce oxygen in the process of photosynthesis. They use carbon dioxide for photosynthesis and release oxygen. Cyanobacteria live in the seawater, so oxygen produced in photosynthesis first enters to water, and then from water escapes to the atmosphere.