Most of Earth’s oxygen comes from the oceans
And no, it’s not the ‘O’ of H2O. About 70 percent of our planet’s entire supply of oxygen is a waste product created by marine-dwelling plankton. These minuscule creatures take in carbon dioxide and produce oxygen during photosynthesis, and this oxygen plays a vital role in the life of every oxygen-breathing creature on the planet. According to National Geographic, one type of plankton called Prochlorococcus produces so much oxygen that it’s most likely responsible for one of every five breaths we take.