Caltech Question of the Month: Is it true that photosynthesis occurs in the ocean and supplies much of the oxygen we breathe on Earth?

Answered by Jenn Fletcher, research fellow in biology, Caltech.

Photosynthesis occurs in the ocean in tiny, single-celled plants called phytoplankton. Phytoplankton lie at the bottom of the oceanic food chain. They are eaten by tiny marine animals such as krill, a shrimplike organism, which are in turn eaten by other sea animals. There are two main types of phytoplankton, diatoms and dinoflagellates, which lend a green, yellow, or even reddish tint to the water. There are literally billions of phytoplankton living in the oceans—one milliliter of seawater contains hundreds of thousands or even millions of phytoplankton. A single phytoplankton is so small that one hundred of them could be lined up end to end across a single blade of grass.

Although the average depth of the ocean is over 200 meters, almost all phytoplankton live in the top 10 meters, floating in the sunlit waters near the surface like a thin skin spread over the seas. Fueled by the energy of sunlight, phytoplankton use their chlorophyll to convert carbon dioxide and water into a simple, sugary food on which they survive. Like other plants, phytoplankton give off oxygen as a byproduct of photosynthesis. It has been estimated that phytoplankton drifting in the ocean currents indeed account for much of the world's oxygen.