The two correct statements that describe cyanobacteria are:
- Cyanobacteria were the first photosynthesizing organisms.
- Cyanobacteria lived in the ocean.
Cyanobacteria are known to be among the first organisms to perform photosynthesis, which contributed to the oxygenation of the Earth's atmosphere. They are also found in marine environments, as well as fresh water. The other two statements are incorrect; cyanobacteria increased oxygen levels in the atmosphere and do not have complex cells with a nucleus, as they are prokaryotic organisms.