How do scientists estimate how much the plankton produce?

When we think of the plants that give us oxygen, we usually picture forests. Yet about half of the Earth's oxygen comes from the ocean.

Most of it is made by plankton: drifting plants, algae and some bacteria that, like plants on land, use sunlight to make food and give off oxygen as they do so. One of them, a bacterium called Prochlorococcus, is the smallest living thing on Earth that makes food from sunlight. Tiny as it is, it may produce up to 20 per cent of all the oxygen in the biosphere, more than all the world's tropical rainforests put together.

Scientists cannot count these organisms one by one, so they estimate how much the plankton are producing using images taken by satellites. The amount of plankton rises and falls with the seasons and changes with the nutrients in the water, its temperature and other conditions.

Not all the oxygen made in the ocean reaches the air. Roughly as much is used up by sea life, because fish and other sea animals breathe, just as land animals do, and oxygen is also used up when dead plants and animals decay. When a large bloom of algae dies, its decay can use up oxygen faster than it is replaced. The water then holds so little oxygen that most sea life cannot survive, and such places are called dead zones.

The oxygen we breathe today built up in the air over hundreds of millions of years.