Prochlorococcus, a species of ocean-dwelling phytoplankton, only measures about 0.6 micrometers. It’s the world’s smallest organism capable of photosynthesis — so small that 20,000 or so can reside in a single water droplet. But its impacts are so huge that an estimated one out of every five breaths you take is thanks to this minuscule microbe. Prochlorococcus, along with many other types of plankton (organisms carried along by the tides and currents), create as much as 80% of the world’s oxygen. They also play a big role in sequestering carbon from the atmosphere, capturing about 40% of all the CO2 produced. That’s equivalent to the amount that would be captured by roughly four Amazon rainforests.
Phytoplankton such as Prochlorococcus produce oxygen through photosynthesis, the same way plants on land do, by soaking up the sun for energy and releasing oxygen into the ocean and atmosphere. Also like plants on land, phytoplankton are full of the compound chlorophyll, which gives some of the microbes their green color. The entire ocean ecosystem rests upon these vital, oxygen-burping organisms, which provide essential nutrients for beings from the smallest krill to the largest blue whale.