More than half of the oxygen you breathe comes from plants and algae in the ocean, like phytoplankton and seaweed, called marine photosynthesizers. Both use carbon dioxide, water and energy from the sun to make food for themselves, releasing oxygen in the process. One of the most impressive photosynthesizers is a cyanobacterium called Prochlorococcus. It is estimated to be more abundant than any other photosynthesizer on the planet, and to be responsible for producing 20 percent of the oxygen in the atmosphere. One in every five breaths you take, you owe to Prochlorococcus.