Although these creatures are small [some smaller than 0.0002 mm] they are the very foundation of marine life, they produce 50% of the earths oxygen. There are two main types of plankton, the zooplankton and the phytoplankton.
Phytoplankton are primary producers. This means they use energy from the sun, h2o and co2 to make sugars that form the tissues of their bodies and energy to grow. This process is called photosynthesis.
When they photosynthesise a byproduct is created - oxygen. Even though they are so small there are so many of them that they produce half of the worlds oxygen. Thats means every second breath we breath comes from plankton!