If someone were to ask, “where does Earth's oxygen come from?” your thoughts may jump to the plants, the trees, the rainforest. Vegetation does take in carbon dioxide and release oxygen, after all. It's what we were all taught in school—and it's undoubtedly true—but this is only half the story.
To address this gap, our attention must shift from the land to the oceans. This is the place where tiny single-celled organisms called plankton live.
There are two types of plankton: phytoplankton and zooplankton. Phytoplankton are classified as plants. Zooplankton, on the other hand, are classified as animals and feed on phytoplankton.
Because phytoplankton are plants, they need light to survive and, therefore, live close to the surface of the ocean where the sunlight is plentiful. Here they are able to take in carbon dioxide for energy and release oxygen into the atmosphere.
What's incredible about this is that the phytoplankton are so massive in number that the oxygen they release actually accounts for about 50% of the oxygen on our planet; they make 50% of the oxygen we breathe.
To get an idea of how prevalent phytoplankton are, take a look at the image below. Scientists take advantage of the fact that phytoplankton contain chlorophyll. Chlorophyll absorbs light differently than surrounding water, so scientists use NASA's Aqua satellite (specifically the MODIS instrument) to scan the oceans for these differences in absorption, allowing them to detect areas where phytoplankton thrive.
Blue areas correspond to little/no phytoplankton while yellows correspond to very high concentrations.
The MODIS instrument has been taking measurements since 2002. This website includes a great animation of phytoplankton concentration data from 2002 thru 2015. As it plays through the seasons, you can see the whole system shift between the northern and southern hemispheres, corresponding with the planet's angle toward the sun during different times of the year.