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. (continuation of overlong headline) The invisible forest Phytoplankton are tiny, individually invisible, and almost unimaginably numerous. A single teaspoon of seawater can hold more than a million microscopic organisms, of which phytoplankton are a large part, although the count rises and falls enormously with season, sunlight and the nutrients in the water. One group stands out. Prochlorococcus, the smallest known photosynthetic organism, is so abundant that, by NOAA’s account, it alone produces up to a fifth...