Most of the Earth's oxygen comes from the ocean, not the Amazon rainforest.

Marine organisms like phytoplankton, kelp, and algal plankton are the primary drivers of oxygen production on Earth. Specifically, a single species of cyanobacteria called Prochlorococcus is responsible for one out of every five breaths we take. This tiny organism is the most abundant photosynthetic inhabitant of the ocean and was discovered by researchers at MIT in 1986. While the Amazon rainforest is often called the 'lungs of the planet,' it is actually a closed system regarding oxygen. The vast amount of vegetation and wildlife within the rainforest consumes almost all the oxygen produced through photosynthesis during the process of cellular respiration. In contrast, the ocean's oxygen production enters the atmosphere because much of the organic matter created by phytoplankton sinks to the deep sea before it can be consumed. Data from NASA's MODIS satellite sensors help scientists track the 'greenness' of the ocean to estimate this global productivity. Estimates from the National Oceanic and Atmospheric Administration (NOAA) suggest that at least half of Earth's oxygen is produced by these marine plants. This process has been stable for millions of years, maintaining the atmospheric oxygen level at approximately 21%. Understanding this balance is crucial as rising ocean temperatures can affect the metabolic rates of these microscopic powerhouses.