Atmospheric oxygen is a fundamental requirement for nearly all complex life forms, including humans. This gas is a component in respiration, the process that converts food into the energy needed for survival. Without a steady supply of oxygen, life as we know it would cease to exist.
The air we breathe is composed of approximately 20.95% oxygen by volume, with the majority being nitrogen at about 78%. Maintaining this high concentration requires continuous production. Oxygen is a highly reactive gas that naturally combines with other elements, necessitating constant replenishment by biological sources.
The Underlying Mechanism: Photosynthesis
Almost all free oxygen in the atmosphere is a byproduct of photosynthesis. This biological process is the foundational mechanism used by plants, algae, and certain bacteria to convert light energy into chemical energy. Photosynthesis reverses respiration by taking in carbon dioxide and water to create sugar molecules, which serve as food for the organism.
The transformation is powered by sunlight captured by the pigment chlorophyll, which gives plants their green color. Chlorophyll molecules absorb light energy and initiate reactions within specialized cell structures called chloroplasts. This energy is ultimately used to split water molecules, releasing oxygen as a waste product.
Oxygen production is inextricably linked to the fixation of carbon, where carbon dioxide is converted into organic compounds. The overall chemical equation shows that carbon dioxide plus water, in the presence of light energy, yields glucose and oxygen. This mechanism is the largest biological driver of atmospheric gas composition on Earth.
The Largest Global Producers: Marine Phytoplankton
The organisms that generate the greatest quantity of oxygen globally are microscopic, single-celled organisms floating in the ocean called phytoplankton. These organisms are responsible for producing between 50% and 80% of the world’s oxygen. Phytoplankton form the base of the marine food web and perform half of all global photosynthetic activity.
This massive output comes from two main groups: photosynthesizing bacteria, such as cyanobacteria, and various protists, including diatoms. Their immense contribution is due to their sheer numbers and rapid turnover rate across the vast surface area of the global ocean. Unlike trees, phytoplankton reproduce quickly, with life cycles often measured in days.
Phytoplankton live in the sunlit surface layer of the ocean, known as the euphotic zone, where light is available for photosynthesis. When they bloom, their numbers become so dense that they can be seen from space. The oxygen produced is first dissolved into the ocean water, but constant exchange ensures this gas is ultimately released to replenish the atmosphere.
The oceans cover nearly 71% of the planet’s surface, making them an effective oxygen factory. This enormous area, combined with the rapid life cycle of the microscopic producers, allows them to dominate global oxygen output. The open ocean is a significant part of this production, where the volume of these tiny organisms outweighs the output of all forests combined.
Terrestrial Oxygen Production and Net Contribution
Land-based plants, including large forests, are massive producers of oxygen through photosynthesis. However, their contribution to the net oxygen available in the atmosphere is much smaller than often assumed. While forests are sometimes called the “lungs of the Earth,” this title is misleading regarding their long-term impact on atmospheric oxygen levels.
Understanding the role of terrestrial ecosystems requires distinguishing between gross oxygen production and net oxygen contribution. Gross primary production is the total oxygen created during photosynthesis. Plants must also respire to live, consuming some oxygen to convert stored energy for growth and maintenance, a process called plant respiration.
The limiting factor for terrestrial ecosystems is decomposition and decay. When a plant dies, the organic material is broken down by decomposers, such as fungi and bacteria. These organisms consume oxygen in a process known as ecosystem respiration, which consumes the oxygen equivalent of the stored carbon.
Net primary production (NPP) is the gross production minus the oxygen lost through the plant’s own respiration. The net ecosystem exchange, which determines the long-term atmospheric contribution, is the gross production minus all respiration within the ecosystem, including that of the decomposers. Because terrestrial ecosystems accumulate biomass that eventually decays, their oxygen production is mostly balanced by consumption within the same system.
For a mature, stable forest, the amount of oxygen produced annually is nearly equal to the amount consumed by the respiration of the plants, animals, and decomposers. This means the net atmospheric contribution of an established forest is close to zero, acting more like an oxygen recycler than a net producer. Only during the rapid growth phase of a new forest does it become a true net oxygen contributor.