Step 1 : Introduction to the question "What organism contributes 70 to 80 percent of Earth's oxygen?"
Tiny microscopic plants are at the base of the world's marine food chain. These phytoplankton are a form of algae and they produce between 70 and 80 percent of the oxygen in our atmosphere. Good ol' pond scum, the green slick riding the waves, and even some marine seaweed beds are colonies of algae. Trees and other green plants supply oxygen and are important to Earth's ecosystem, but without marine algae, oxygen-dependent life would not survive on planet Earth. Source: Ecology.com