Phytoplankton refers to the autotrophic component of the plankton that drifts in the water column. Most phytoplankton are too small to be individually seen with the unaided eye. Phytoplankton, like plants, obtain energy through a process called photosynthesis, and so must live in the well-lit surface layer of an ocean, sea, or lake. Through photosynthesis, phytoplankton (and terrestrial plants) are responsible for much of the oxygen present in the Earth's atmosphere. Their cumulative energy fixation in carbon compounds (primary production) is the basis for the vast majority of oceanic and some freshwater food chains.