Phytoplankton
The rain forests are pretty and important and all, but lets give some credit to these tiny creatures as well!
It is actually estimated that about 70% of the oxygen in the atmosphere is produced in the oceans, thanks to the photosynthetic activity of phytoplankton: Read more!
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Sekerci and Petrovskii (2015) “Mathematical Modelling of Plankton–Oxygen Dynamics Under the Climate Change” https://doi.org/10.1007/s11538-015-0126-0