Phytoplankton

Phytoplankton are the plant-like part of the plankton: tiny organisms, mostly single cells, that float in the sunlit upper layer of seas, lakes and rivers and make their own food by photosynthesis. Like land plants they contain chlorophyll, absorb carbon dioxide and give off oxygen. They are not one related group. They include cyanobacteria, which are bacteria, and many kinds of algae and other protists, such as diatoms with glassy silica shells, dinoflagellates that swim with whip-like flagella, and coccolithophores covered in chalky plates.

Phytoplankton are the main primary producers of the oceans and carry out roughly half of all the photosynthesis on Earth. Zooplankton, krill, shellfish and small fish graze on them, so they ultimately feed almost everything else in the sea, up to whales. They also move huge amounts of carbon: some of the carbon they fix sinks into the deep ocean when they die or are eaten, a process called the biological pump that helps regulate climate. Their growth is limited by light and by nutrients such as nitrate, phosphate and iron.

When nutrients are plentiful, phytoplankton can multiply into blooms visible from satellites. Some blooms release toxins, as in red tides, and the decay of large blooms can strip water of oxygen, part of the process called eutrophication.