Members of which bacterial phylum generated the first atmospheric oxygen on earth?

The correct answer is cyanobacteria (ancient cyanobacteria).

The huge concentration of oxygen in the atmosphere is believed to have been initially produced by the actions of ancient cyanobacteria. They are usually observed as symbionts with a number of another group of species, like coral, lichens (fungi), Azolla (pteridophytes), angiosperms, and others.

The atmosphere of oxygen was generated by various cyanobacteria at the time of Proterozoic and Archaean periods.

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.