Scientists estimate that 50-80% of the oxygen manufacturing on Earth comes from the ocean. The vast majority of this manufacturing is from oceanic plankton — drifting crops, algae, and a few micro organism that may photosynthesize. One specific species, Prochlorococcus, is the smallest photosynthetic organism on Earth. However this little micro organism produces as much as 20% of the oxygen in our whole biosphere. That’s a better proportion than all the tropical rainforests on land mixed.