Oxygen is primarily produced through photosynthesis by organisms both in the ocean and on land. Roughly half of Earth's oxygen comes from oceanic phytoplankton—tiny marine plants and bacteria—and the other half comes from land plants, including forests. These photosynthetic organisms capture sunlight to convert carbon dioxide and water into oxygen and organic matter. In addition to photosynthesis, recent discoveries show that oxygen is also produced on the deep ocean floor by certain metallic minerals, even in the absence of light, supporting aerobic life in the deep sea. Thus, oxygen production happens mainly in:
- The ocean surface by phytoplankton (about 50%)
- Terrestrial plants and forests (about 50%)
- Deep ocean floor minerals (to a lesser extent)
These processes have collectively maintained the oxygen levels in Earth's atmosphere over millions of years.