Phytoplankton, like plants, obtain energy through a process called Photosynthesis, and so must live in the well-lit surface layer (termed the euphotic zone) 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 (though see also Chemosynthesis). As a side note, one of the more remarkable food-chains in the ocean-remarkable because of the small number of links-is that of phytoplankton fed on by Krill (a type of shrimp) fed on by baleen whales.
In terms of numbers, the most important groups of phytoplankton include the diatoms, Cyanobacteria and dinoflagellates, although many other groups of algae are represented. One group, the coccolithophorids, is responsible (in part) for the release significant amounts of Dimethyl sulfide (DMS) into the atmosphere. DMS is converted to sulfate and these sulfate molecules act as cloud condensation nuclei, increasing general cloud cover.