What was first responsible for changing the formerly poisonous atmosphere to an oxygen-rich atmosphere, as we have today? meteor impacts volcanic gases earthquakes photosynthetic bacteria (cyanobacteria)

The correct answer is photosynthetic bacteria (cyanobacteria).

Explanation:

Blue-green algae or cyanobacteria are said to become the first reason to produce oxygen by photosynthesis, about 27 million years ago.  

Cyanobacteria are found mainly in the seawater. It is said that they have been the very first species on Earth to facilitate oxygenic photosynthesis. In this process, they generate organic molecules and release oxygen in the process. The oxygen moves to seawater to the atmosphere.

Thus, the correct answer is photosynthetic bacteria (cyanobacteria).