By plants and planktonMost of the Earth's oxygen is produced by plants, both on surface and under water. Algae. read more
Oxygen Supply. Phytoplankton need two things for photosynthesis and thus their survival: energy from the sun and nutrients from the water. Phytoplankton absorb both across their cell walls. In the process of photosynthesis, phytoplankton release oxygen into the water. Half of the world's oxygen is produced via phytoplankton photosynthesis. read more