* Cyanobacteria (blue-green algae): These single-celled organisms are responsible for producing the majority of the oxygen in Earth's early atmosphere. They still play a significant role in oxygen production today.
* Some algae: Like cyanobacteria, certain types of algae, such as phytoplankton, are also capable of photosynthesis and produce oxygen as a byproduct.
* Certain bacteria: Some bacteria, particularly those involved in the process of photosynthesis, can also produce oxygen. This is less significant than the contribution of plants and algae.
It's important to note that while these other sources exist, plants are by far the largest contributors to the oxygen in our atmosphere. They are estimated to produce about 99% of the oxygen we breathe.
