The air we breathe is a mixture of gases, with life-sustaining oxygen making up approximately 21% of our atmosphere. This oxygen is almost entirely generated through photosynthesis, a biological process carried out by living organisms. These organisms convert sunlight, water, and carbon dioxide into sugars for energy, releasing free oxygen as a byproduct. The sheer volume of this production cycles quadrillions of pounds of gas annually between the biosphere and the atmosphere. The question of which source contributes the most to this global supply has a surprising answer, shifting the focus from large, visible forests to the microscopic life that drifts unseen across the planet.
The Dominant Oxygen Producer
The vast majority of the oxygen added to Earth’s atmosphere comes from the ocean, specifically from microscopic organisms called phytoplankton. These single-celled photosynthesizers, which include algae, cyanobacteria, and diatoms, inhabit the sunlit upper layer of the world’s oceans. Scientists estimate that between 50% and 85% of the oxygen in our atmosphere is produced by these marine organisms.
One particular species, the cyanobacterium Prochlorococcus, is the smallest photosynthetic organism known, yet it is so numerous that it is responsible for generating up to 20% of the oxygen in the entire biosphere. The ocean’s immense surface area, covering over 70% of the planet, allows these fast-reproducing organisms to maintain a production rate far exceeding that of land plants.
Net Contribution and the Biological Pump
Phytoplankton have a rapid turnover rate, completing their life cycle in days rather than the decades or centuries required for trees. Furthermore, the ocean facilitates a mechanism known as the “biological pump,” which helps ensure a net oxygen contribution. When phytoplankton die, a portion of the organic carbon they fixed during photosynthesis sinks to the deep ocean before it can be consumed by respiration and decomposition. This sequestration leaves the corresponding oxygen free to enter the atmosphere.
Terrestrial Contribution and the Role of Forests
While land-based plants, including tropical rainforests, perform significant photosynthesis, their net contribution to atmospheric oxygen is often misunderstood. Forests are responsible for a large proportion of the gross oxygen production on land, generating oxygen during the day. This gross production is countered by an equally large consumption, making the crucial distinction the net contribution.
Plants consume oxygen through cellular respiration, especially at night when photosynthesis ceases. Microbes, fungi, and insects also consume oxygen as they break down dead organic matter, a process called heterotrophic respiration.
For a mature, stable forest ecosystem, the oxygen produced through growth is almost perfectly balanced by the oxygen consumed through respiration and decomposition. Consequently, the net addition of new oxygen to the global atmosphere from these ecosystems is near zero. The Amazon’s role is more accurately described as a recirculator of oxygen and a long-term carbon sink.
Oxygen Consumption and Global Balance
The current concentration of oxygen in the atmosphere is maintained by an equilibrium between global production and consumption processes. The primary mechanisms that consume oxygen are aerobic respiration by all living organisms, including animals, plants, and microbes, and the chemical process of oxidation.
A major source of current oxygen consumption is the burning of fossil fuels, which releases stored carbon and consumes oxygen in the process. Other oxidation events, such as wildfires and the slow chemical weathering of reduced elements in rocks, also continuously draw down the atmospheric oxygen supply.
Despite these massive fluxes of production and consumption, the atmospheric oxygen level has remained stable at about 20.9% for millions of years. The oxygen we breathe today is a vast reservoir that accumulated over geological time, primarily during the Great Oxidation Event billions of years ago. The stability of this reservoir is maintained by the burial of organic carbon in sediments, which removes carbon from the cycle before it can be oxidized. Protecting both the fast-cycling marine producers and the long-term carbon storage of terrestrial ecosystems is important for maintaining the overall stability of this global balance.