The ocean produces roughly half of Earth’s oxygen — not the rainforest, which uses up most of what it makes — through photosynthesis by phytoplankton, microscopic marine organisms so abundant that a single teaspoon of seawater can contain as many as a million of them.

The ocean produces roughly half of Earth’s oxygen — not the rainforest, which uses up most of what it makes — through photosynthesis by phytoplankton, microscopic marine organisms so abundant that a single teaspoon of seawater can contain as many as a million of them.

About half of the oxygen on Earth comes from the ocean. Not from the rainforests, which consume close to as much oxygen as they make, but from phytoplankton, the microscopic algae and bacteria that drift through the sunlit surface of the sea and photosynthesise on a scale that is hard to picture. The figure is real, and widely cited by agencies including the United States National Oceanic and Atmospheric Administration. It also comes with a foot…