Nearly half the oxygen we breathe comes from photosynthetic plankton, not plants

https://earth.org/why-are-plankton-so-crucial-for-life-on-earth-and-how-is-climate-change-affecting-them/

Although the Amazon Rainforest is considered the “lungs of the Earth,” it only produces 20% of the planet’s oxygen, while phytoplankton, or microalgae, generate more than twice as much, despite making up less than 1% of the total mass of photosynthetic organisms on the planet. Through photosynthesis, microalgae also absorb up to 40% of global carbon emissions.