A magnified view of phytoplankton
Almost 70 percent of the oxygen we breathe comes from microscopic plant-like organisms called phytoplankton (or micro algae) that live on the surface of oceans and lakes. One type of phytoplankton, Prochlorococcus, produces up to 20 percent of the oxygen found in our entire biosphere. It is so small that millions can fit in a drop of water. These drifting marine organisms photosynthesise, using carbon dioxide, water and energy from the sun, releasing tonnes of oxygen, a process they’ve been at since billions of years.