If you were asked where the majority of oxygen in the Earth’s atmosphere comes from what would you say?
The rain forest?
Scientists estimate that between 50-80% of the oxygen on Earth is produced by the ocean. More specifically, it is produced by plankton and other tiny micro-plants that drift with the oceans currents and float close to the surface.
The reason being that these plants, similar to others on land, grow and survive through photosynthesis (from carbon dioxide and sunlight) and produce oxygen as a by-product.
Given the vast expanses of the ocean and the manner in which such plants can travel on currents that spread them wide across the planet’s surface they are major producers of oxygen.
That said, marine life are major consumers of oxygen (they also need it to breathe) while many also feast on plankton.