Phytoplankton are microscopic plants that live in the ocean. They are the basis of oceanic biological productivity (the ocean's ability to support life) and they produce half of the world's oxygen through photosynthesis. (The other half is produced on land by trees and other plants.)

The Diatoms, a group of Phytoplankton, are responsible for 20% to 25% of all organic carbon fixation on the planet (turning carbon dioxide and water into sugars, using the sun's light energy). Because phytoplankton can double in population once a day, they respond very quickly to changes in their environment.

Despite their significant contribution to the planet's atmosphere and cycling, this group of organisms is not often studied or appreciated.

Sources:
http://news.nationalgeographic.com/news/2004/06/0607_040607_phytoplankton.html
http://earthobservatory.nasa.gov/Library/Phytoplankton/phytoplankton2.html