The air we breathe is composed of approximately 21% molecular oxygen ($O_2$), a gas fundamental for supporting life. This gas powers the cellular respiration of nearly all complex organisms, enabling the high energy demands necessary for multicellularity and movement. Its presence in Earth’s atmosphere is not a permanent geological feature but rather a transient biological phenomenon sustained by continuous production. Understanding the source of this atmospheric supply requires tracing a journey that spans billions of years, driven by microscopic organisms and vast global cycles. The story of oxygen is intrinsically linked to the history of life itself, from the first microbes that evolved to weaponize sunlight to the global balance that exists today.
Photosynthesis: The Primary Production Mechanism
Photosynthesis is the singular process responsible for generating virtually all the atmospheric oxygen on Earth, converting light energy, water, and carbon dioxide into chemical energy in the form of sugars. The creation of oxygen is not derived from carbon dioxide, but rather a direct result of the light-driven splitting of water molecules, a reaction known as photolysis. This process is orchestrated by a massive protein complex called Photosystem II (PSII), which is housed within the thylakoid membranes of chloroplasts in plants and algae, or within the cell membranes of cyanobacteria.
The initial capture of solar energy is performed by pigments like chlorophyll, which funnels the light energy to PSII to begin the chemical cascade. PSII uses this captured energy to strip electrons from water, initiating the electron transport chain that generates the organism’s chemical fuel. The specific site of the water-splitting reaction is a specialized catalytic cluster that binds two molecules of water. This cluster sequentially removes four electrons and four protons using the energy from four absorbed photons.
Once the electrons are removed, the oxygen atoms from the two water molecules are combined to form one molecule of diatomic $O_2$. This molecular oxygen is a waste product of the water-splitting reaction, which is subsequently released into the surrounding environment. The ability of PSII to use readily available water as an electron source, instead of scarcer compounds like hydrogen sulfide, made this type of oxygenic photosynthesis transformative for the planet.
The Great Oxygenation Event: How Earth Breathed
For nearly two billion years of Earth’s history, the atmosphere contained virtually no free molecular oxygen. The monumental shift known as the Great Oxygenation Event (GOE) began approximately 2.4 to 2.1 billion years ago, fundamentally changing the planet’s chemistry. This geological transformation was driven entirely by the evolution of oxygenic photosynthesis in tiny aquatic microbes called cyanobacteria, which were the first organisms to use water as an electron donor.
These single-celled organisms began producing $O_2$ as a waste product in the ancient oceans, where they often formed layered, mound-shaped fossils known as stromatolites. Initially, the oxygen did not accumulate in the air but was chemically scavenged by highly reactive elements dissolved in the seawater, most notably ferrous iron. This reaction produced massive amounts of oxidized iron minerals that settled on the ocean floor, eventually forming the distinctive geological layers known as banded iron formations.
Once the oceanic iron sink was saturated, the free oxygen began escaping into the atmosphere, leading to a rapid environmental crisis. For the strictly anaerobic life forms that dominated the planet at the time, $O_2$ was a potent poison, causing the world’s first mass extinction event. The rise of oxygen also reacted with atmospheric methane, a powerful greenhouse gas, which significantly reduced the atmosphere’s heat-trapping capacity and led to a cooling event known as the Huronian glaciation.
Land vs. Sea: Where Most Oxygen is Created
Despite the common perception of tropical rainforests as the “lungs of the Earth,” the majority of the planet’s oxygen is generated in its oceans. Current estimates suggest that marine photosynthetic organisms produce between 50% and 80% of the molecular oxygen found in the atmosphere. The primary producers responsible for this immense output are phytoplankton, a diverse collection of microscopic, single-celled organisms that drift in the upper, sunlit layers of the water column.
This vast biomass includes organisms like diatoms, coccolithophores, and cyanobacteria, all capable of performing oxygenic photosynthesis. The ocean dominates global production due to its enormous surface area, the rapid reproductive rate of the microalgae, and the concept of net oxygen production. While land plants produce large amounts of $O_2$, nearly all of that gas is quickly consumed by the plants themselves or by bacteria and fungi that decompose the dead organic matter through aerobic respiration.
In contrast, a significant fraction of the organic matter produced by phytoplankton sinks out of the surface waters and is buried in deep-sea sediments. This process effectively removes its carbon and associated oxygen consumption from the surface cycle. This creates a net surplus of oxygen that is sustained over geological time scales, ensuring the oceans contribute the larger share of the steady atmospheric supply.
The Atmospheric Oxygen Cycle
While photosynthesis continually replenishes atmospheric oxygen, the gas is simultaneously consumed through several natural processes, maintaining a stable concentration of about 21%. The most significant biological sink is aerobic respiration, where nearly all complex life forms use $O_2$ to metabolize sugars for energy, releasing carbon dioxide as a byproduct. Oxygen is also consumed during decomposition, as bacteria and fungi break down organic matter in a process similar to respiration.
Non-biological processes also act as chemical sinks that continuously remove oxygen from the atmosphere over long periods. These processes include combustion, such as natural wildfires, and the chemical weathering of rocks, which can oxidize minerals like iron sulfides. The long-term stability of atmospheric oxygen levels results from a near-perfect global balance between biological production and consumption by respiration and various geological sinks.