Tiny ocean organisms make a planetary difference
Much of the oxygen produced by photosynthesis comes from phytoplankton: microscopic, plant-like organisms drifting in sunlit water. The group includes tiny algae and photosynthetic bacteria. Individually they are nearly invisible, but they are spread across vast stretches of the ocean, where sunlight reaches the surface.
How photosynthesis releases oxygen
Phytoplankton use sunlight to turn carbon dioxide and water into energy-rich material for growth. In the process, they split water molecules and release oxygen. This is the same basic chemistry used by land plants, but it happens on an enormous scale across the ocean’s sunlit upper layer.
The organisms also form the base of many marine food webs. Some are eaten by small animals; others die and sink. Their growth depends on conditions such as light and nutrients, so oxygen production varies with place and season rather than coming from a steady, uniform ocean-wide output.
Production is not the same as accumulation
A crucial distinction is that oxygen made by photosynthesis does not all remain in the atmosphere. Phytoplankton, marine animals, and microbes use oxygen as they respire, and decomposers consume it while breaking down dead material. Over long periods, atmospheric oxygen builds up when some photosynthetically produced organic matter escapes that cycle—for example, by being buried in sediments before it can decay.
So the ocean’s microscopic producers are a major source of the oxygen continually made on Earth, while the amount that persists in the air depends on what happens to the carbon-rich material they leave behind.
