Which microscopic ocean organism produces much of the oxygen in Earth’s atmosphere?

Phytoplankton are microscopic photosynthetic organisms that produce roughly half of Earth’s oxygen through photosynthesis.

They include marine algae and cyanobacteria that use sunlight, carbon dioxide and water to make organic matter. Their activity releases oxygen and forms the foundation of most open-ocean food webs. Tiny animals called zooplankton graze on them, and larger animals ultimately depend on this energy.

Important phytoplankton groups include diatoms, dinoflagellates and cyanobacteria such as Prochlorococcus. They vary greatly in size and shape, and their abundance changes with sunlight, nutrients, temperature and ocean mixing.

Phytoplankton do not produce all of Earth’s oxygen in the ocean alone; land plants and freshwater organisms contribute too. Also, much of the oxygen they make is consumed by respiration and decomposition. Their global role is nevertheless central to both climate and marine life.