Question

I need help.... Cyanobacteria formed colonies called stromatolites. These used to be plentiful. What role did they contribute to the atmosphere?

Answer 1

Answer - Cyanobacteria aka Algae that contributed in creating O2 for the atmosphere as a byproduct thru Photosynthetic.

Reason - Of course Algae make up mainly the whole world in waters and they do photosynthesis to give off oxygen in the atmosphere.