[Lewis Thomson (OP)]

Donald wants to find answers to this question.

"Earth's atmosphere changed during the "oxygen catastrophe" well before plants on land existed.  Trees and other land plants are not the significant source of oxygen, so what organisms are the major source of ongoing oxygen production? And what do they require to produce all this abundant gas for us oxygen dependent beasties. And why haven't the oxygen users multiplied sufficiently to counterbalance them to create a different equilibrium?  Billions of years, earth has had this imbalance. Sufficient time for organisms to evolve to exploit this resource."

Discuss in the comments below...

[evan_au]

Today's 20% oxygen content has not always been so.
- As mentioned, before the oxygenation event, the atmosphere was roughly 0% oxygen
- At some points in the past, atmospheric oxygen has reached as high as 33%
- During these periods, there have been some enormous insects; these would not be viable today, as insect oxygen intake depends to some extent on diffusion from the atmosphere.
See: https://en.wikipedia.org/wiki/Atmosphere_of_Earth#Third_atmosphere

It is thought that the source of oxygen for the Great Oxidation Event was cyanobacteria.
- Free-floating single-celled organisms don't tend to leave clear fossils, but accretions of single-celled organisms mixed with sediment can be identified in the geological record as something similar to modern stromatolites.
- Today, these stromatolites are only found in a few extreme locations where other organisms can't live. so they no longer make a significant contribution to oxygen production.
- Today, most oxygen is produced by plants, on land (trees, shrubs and grasses) or in the sea (seaweed, or single-celled algae).
See: https://en.wikipedia.org/wiki/Great_Oxidation_Event#Hypotheses