What Is Plankton? The Drifting Life That Makes Half the World's Oxygen

The phytoplankton are the plants of the sea, though most are not plants: single-celled algae and photosynthetic bacteria that float in the sunlit surface layer and turn carbon dioxide and water into sugar and oxygen. They are outnumbered and outweighed by nothing. Diatoms, encased in glass shells of intricate design, dominate cold and nutrient-rich water and are the most productive; dinoflagellates, with two whipping tails, some of which glow at night and some of which produce the toxins of red tides, follow; coccolithophores wrap themselves in plates of chalk and, when they die, build the white cliffs of Dover; and cyanobacteria, the oldest photosynthesisers on Earth, fill the warm nutrient-poor oceans, where one species discovered only in 1988, Prochlorococcus, is probably the most abundant photosynthetic organism that exists, at around three billion billion billion cells. Together they carry out about half of all photosynthesis on the planet, from a total mass that is less than one percent of the land plants, because they live fast and are eaten within days.

The zooplankton are the animals that drift, and they include:

• •Copepods, shrimp-like crustaceans a millimetre or two long that are probably the most numerous animals on Earth and the main link between phytoplankton and fish
• •Krill, larger crustaceans that swarm in the Southern Ocean in shoals visible from space and feed whales, seals and penguins
• •The larvae of fish, crabs, lobsters, oysters, sea urchins and starfish, which drift as plankton before settling or growing into swimmers
• •Jellyfish, comb jellies and salps, gelatinous drifters that can outweigh everything else in a bloom
• •Foraminifera and radiolarians, single-celled animals with shells of chalk and glass whose fossils record the climate of the past in the seabed

Every night the largest migration on Earth takes place unseen. Zooplankton by the billion tonne rise from depths of several hundred metres to feed on the phytoplankton in the surface layer under cover of darkness, and sink again at dawn to hide from the fish that hunt by sight. The movement was discovered by wartime sonar operators, who found a false seabed that rose every evening, and it moves so much carbon downward, in bodies and droppings, that it is a measurable part of the planet's carbon budget. Whatever sinks past a kilometre is out of the atmosphere for centuries, and the marine snow of dead plankton drifting down through the dark is the food supply of the deep sea and, over millions of years, the origin of most petroleum.

The phytoplankton produce roughly half the oxygen in the atmosphere and take up a quarter of the carbon dioxide humans emit each year, and they also make the weather. Some species release a sulphur compound that escapes into the air and seeds cloud droplets, so that a bloom can brighten the sky above it; the smell of the sea is largely that compound. They respond to the ocean's changes within days. Warming water holds fewer nutrients and favours smaller species, satellite records since the 1990s show shifts in where and when blooms happen, and the acidification of the sea by absorbed carbon dioxide makes it harder for the chalk-shelled kinds to build their plates. Because everything in the ocean eats them or eats something that did, a change in the plankton is a change in the fisheries, the whales and the air, and it is not yet known which way the largest changes will go.

A litre of seawater holds up to a million phytoplankton cells and a few dozen animals, invisible until a drop is put under a microscope, where it becomes a menagerie of glass boxes, spinning cells and translucent larvae with eyes. Victorian naturalists drew them for months; a modern research ship tows nets of fine mesh and images every organism in the catch automatically, and the continuous plankton recorder, a torpedo towed behind merchant ships across the North Atlantic since 1931, holds the longest record of any marine life. Satellites see the whole: the green of chlorophyll in the water, mapped daily from orbit, is the plankton, and the swirls and blooms in those images are the ocean's food supply being made.

Plankton is everything that drifts in the sea rather than swimming against the current: the phytoplankton, single-celled algae and bacteria that carry out half the world's photosynthesis and produce half its oxygen, and the zooplankton, from copepods and krill to fish larvae and jellyfish, that eat them and feed everything larger. Their nightly migration and sinking remains carry carbon into the deep and made the oil under the ground, their sulphur seeds clouds, and their response to warming and acidification will reshape the oceans.