The ozone layer protects ocean plankton, which generate more than half of the world's oxygen.

Phytoplankton, specifically groups like diatoms and cyanobacteria such as Prochlorococcus, are responsible for an estimated 50% to 80% of the oxygen in Earth's atmosphere. Prochlorococcus is the smallest known photosynthetic organism, yet it produces the oxygen for one out of every five breaths we take. These organisms live in the euphotic zone of the ocean, which is the upper layer where sunlight can penetrate.The stratospheric ozone layer, located approximately 15 to 35 kilometers above Earth, absorbs about 97% to 99% of the sun's medium-frequency ultraviolet light. Without this protection, increased UV-B radiation would penetrate the water column and damage the DNA and reproductive cycles of phytoplankton. Research by the National Science Foundation has shown that even a small reduction in ozone can lead to a significant decrease in primary productivity in the Southern Ocean.A study published in the journal Nature highlighted that UV radiation inhibits photosynthesis by damaging the proteins involved in the process. This is critical because phytoplankton form the base of the marine food web, supporting everything from krill to blue whales. By maintaining the ozone layer through international agreements like the 1987 Montreal Protocol, humanity protects the biological pump that regulates global carbon dioxide and oxygen levels.