Molecular oxygen (\(O_2\)), which makes up about 21% of the atmosphere, is fundamental to complex life on Earth. Its constant presence supports aerobic respiration, the metabolic process organisms use to efficiently convert food into energy. This continuous supply of breathable air is a biological output, maintained by a vast network of living organisms. The source of this gas is photosynthesis, a process linking light energy, water, and simple compounds.
The Fundamental Process of Photosynthesis
The mechanism responsible for nearly all free oxygen on Earth is photosynthesis, a complex chemical reaction powered by sunlight. This process uses light energy, carbon dioxide, and water to create energy-rich sugar molecules, releasing oxygen as a gaseous byproduct. Chlorophyll, a green pigment within photosynthetic organisms, captures the necessary light energy. This captured energy initiates the light-dependent reactions of photosynthesis.
During this initial phase, water molecules (\(H_2O\)) are split apart, a process known as photolysis. The oxygen atoms from the water molecules are released as diatomic oxygen (\(O_2\)). The remaining hydrogen atoms are used later in the process to form the sugar molecule. This oxygen byproduct, which is waste for the organism, sustains life across the planet.
The Major Sources: Ocean Versus Land
While terrestrial forests are often called the “lungs of the planet,” the ocean is the source of the majority of global oxygen production. Estimates suggest that marine organisms contribute between 50% and 80% of the Earth’s oxygen supply.
The primary oxygen producers in the ocean are microscopic organisms called phytoplankton, which include tiny, drifting plants, algae, and bacteria. These single-celled organisms live in the sunlit upper layer of the ocean, performing photosynthesis year-round. An example is the bacterium Prochlorococcus, one of the most abundant photosynthetic organisms on Earth. This single species is estimated to produce up to 20% of the oxygen in the biosphere.
Land-based organisms like trees and grasses produce the remaining fraction of oxygen. Although terrestrial plants are larger, the sheer volume and widespread distribution of phytoplankton result in the ocean having the greater overall output.
Maintaining the Earth’s Oxygen Balance
The supply of atmospheric oxygen is maintained through a complex, dynamic equilibrium known as the oxygen cycle. This cycle describes the movement of oxygen through the atmosphere, biosphere, and lithosphere. The constant production of oxygen through photosynthesis is balanced by its consumption through a few primary mechanisms.
The largest consumer of oxygen is aerobic respiration, the process used by animals, plants, and many microorganisms to convert food into usable energy. Respiration is chemically the inverse of photosynthesis, consuming oxygen and releasing carbon dioxide (\(CO_2\)). Plants themselves consume some of the oxygen they produce when they respire to power their own cellular functions.
Oxygen is also consumed during decomposition, where fungi and bacteria break down dead organic matter. This process uses oxygen to oxidize carbon compounds, releasing carbon dioxide back into the environment. Over geological timescales, some oxygen is removed from the atmosphere when it reacts with minerals exposed during the weathering of rocks. The balance between this continuous production and consumption ensures that the atmospheric concentration of oxygen remains relatively stable, supporting life across the globe.