http://earthobservatory.nasa.gov/Newsroom/...p3?img_id=17297

Note especially:

Phytoplankton influence global climate by regulating gases in the atmosphere. Like all plants, phytoplankton absorb carbon dioxide and release oxygen as they grow. When the plants die, they sink to the ocean floor, carrying the absorbed carbon with them. Over the course of the Earth’s history, the oceans have become the primary sink for atmospheric carbon dioxide. Since carbon dioxide is a greenhouse gas (it traps heat at the Earth’s surface), the Earth would be a much warmer place without phytoplankton