Answer:
They release oxygen into the atmosphere.
Step-by-step explanation:
Cyanobacteria are photosynthetic bacteria that harness the energy from the sun to make food. Using light energy that they trap with photosynthetic pigments, they convert carbon dioxide and water into sugar and oxygen. Therefore, they release oxygen into the atmosphere.
This is clear from the graph, as oxygen levels rise sharply shortly after the first oxygen producing cyanobacteria appear.