This bacterium is too small to see, yet it makes 1 in 5 breaths of oxygen
Prochlorococcus is barely half a micron across, smaller than most bacteria, yet it's the most abundant photosynthetic organism on the planet. Trillions of these cells drift through sunlit ocean water worldwide, and together they generate a substantial share of the oxygen entering Earth's atmosphere. It wasn't identified by science until the 1980s, missed for decades because it was too small for standard lab filters.