The oxygen in the upper atmosphere combine to form ozone layer (o3) that allows to colonize surface water.
Step-by-step explanation:
The fossil stomatolites are formed at about 3.0 ga laminated bacteria. they form the sediments to fall into a thin film of bacteria, these bacteria binds with the sediment and grows through it.
Cynobacteria uses water, CO2 and sunlight to create their own food and produce oxygen as a by product. The significance of stromatolites is that they are earliest fossil evidence life on earth.
They were first organism to photosynthesis and produce oxygen.