Atmospheric oxygen

The amount of oxygen in Earth’s atmosphere is linked to the biological cycling of carbon via photosynthesis and respiration, as well as to the geological cycling of carbon through the planet’s crust, oceans, and atmosphere. As a result, periods of high atmospheric oxygen concentration tend to be periods of low concentration for CO2, an important GHG.

Background

Oxygen exists in Earth’s atmosphere mainly as dioxygen gas (O₂). It is a by-product of photosynthesis and accumulated in the primordial atmosphere primarily as a result of the photosynthetic activities of cyanobacteria in the oceans. and plants are also important contributors of atmospheric oxygen.

In photosynthesis, organisms take up from the air and water and minerals from the soil, and, using the energy of sunlight, they convert these substances into chemical compounds from which they build their cellular constituents. The oxygen released in the process is used in respiration by animals, including humans, as well as by plants and many other organisms, to retrieve the energy stored in chemical bonds. CO₂ is given off as a respiratory by-product. In cyanobacteria, algae, and plants, photosynthesis outpaces respiration, yielding a net gain of oxygen by the atmosphere.

The Early Atmosphere and Photosynthesis

When the Earth formed, some 4.5 billion years ago, the atmosphere contained almost no free oxygen. This early atmosphere probably consisted of CO₂, water vapor, nitrogen, hydrogen, and trace gases. Earth’s first living things, bacteria, appeared around 3.8 billion years ago, in the oceans. These early prokaryotes were unable to photosynthesize. They probably derived their nutrients from chemical compounds in seawater, and their energy through fermentation. Some prokaryotes developed the ability to photosynthesize, and, about 2.8 billion years ago, one group of these, the cyanobacteria, began to produce oxygen as a by-product. These microbes still exist today.

Effects of Atmospheric Oxygen on Life

Oxygen reached appreciable levels in the atmosphere by about 2.3 billion years ago. It changed the course of evolution. Because it is a highly reactive gas, oxygen was a deadly poison to many of the earliest bacteria, but some bacteria harnessed it to break the chemical bonds in their food, yielding energy. Thus, they developed respiration, an aerobic pathway for energy production. Respiration is more efficient than fermentation. It became life’s predominant energy-producing pathway.

The buildup of oxygen in the atmosphere aided life in another way: It led to the formation of the planet’s ozone layer. Ozone (O₃) is produced by the interaction of the Sun’s ultraviolet radiation with O₂ gas. The shields terrestrial life from ultraviolet radiation.

About 1.5 billion years ago, the accumulation of abundant free oxygen was accompanied by the evolution of a new, more complex cell type, the eukaryote. Those eukaryotes that acquired photosynthetic endosymbionts became algae. By 600 million years ago, multicellular eukaryotic forms had arisen, and the organismal groups that dominate the globe today—animals, plants, and fungi—became established.

Atmospheric Oxygen and the Geological Carbon Cycle

The countervailing processes of photosynthesis and respiration help stabilize the atmospheric concentrations of oxygen and CO₂ on timescales of single years to tens of thousands of years. Over longer timescales—millions of years—recycling of the Earth’s crust regulates carbon exchange between rocks, oceans, and the atmosphere and affects the balance of atmospheric gases. As plant matter and sulfur in rocks and sediments are alternately buried and oxidized, oxygen is added to the atmosphere or removed from it. CO₂’s abundance is generally inversely related to oxygen’s abundance in these processes.

The concentration of oxygen in Earth’s atmosphere is about 20.95 percent. The remainder of the atmosphere consists mainly of the relatively unreactive gas nitrogen, at 78.09 percent. The other atmospheric components are trace gases, which include CO₂, at about 0.035 percent, and water vapor, which fluctuates in concentration. Although a slight decrease in atmospheric oxygen has been recorded during recent decades, attributable to the burning of fossil fuels, the reservoir of atmospheric oxygen is so large that significant change cannot readily occur. Conversely, atmospheric CO₂ is present in such small concentrations that minor changes make a large proportional difference.

There is evidence that, in the distant past, the atmospheric oxygen concentration was not completely stable. About 540 million years ago, oxygen composed about 15 percent of the atmosphere. Then, around 300 million years ago, it reached about 35 percent, as a result of the conquest of the land by plants and the attendant increase in global photosynthesis. Oxygen concentration also increased by the burial of forests in coal swamps, which sequestered vast amounts of organic carbon from the atmosphere. Under normal circumstances, the trees would decay, and their carbon would be oxidized to CO₂ and water, but the swamps prevented this from happening. By around 200 million years ago, the atmospheric oxygen concentration had plummeted back to about 15 percent, accompanied by a major at the Permian-Triassic boundary.

Context

Oxygen’s existence has been known only since 1774, when Englishman Joseph Priestley isolated the gas and noted its special properties, including its ability to make a flame burn particularly brightly. In Priestley’s day, the atmosphere was just beginning to be understood as a collection of individual gases, rather than the single, uniform “air” it had been supposed to be. Priestley was also the first to note that plants and animals exist in a reciprocal relationship, mediated by the gases of photosynthesis and respiration.

It took nearly two centuries following Priestley’s discovery of oxygen for the origin of this gas to be understood: Only in the 1960s did the idea that photosynthesis was responsible for the buildup of atmospheric oxygen over the eons become widely accepted by scientists.

Key Concepts

• aerobic: in the presence of oxygen
• anaerobic: in the absence of oxygen
• endosymbiont: an organism living inside a cell or body of another organism
• eukaryote: an advanced cell, containing a nucleus and other membrane-bound organelles
• fermentation: the biochemical generation of energy from organic compounds in the absence of oxygen
• organic matter: carbon-containing compounds produced by life processes
• photosynthesis: the production of food from light energy, carbon dioxide, and water
• prokaryote: a primitive cell (bacterium), lacking a nucleus and other membrane-bound organelles
• respiration: the biological generation of energy, using oxygen

Bibliography

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Berkner, Lloyd V., and Lauriston C. Marshall. “On the Origin and Rise of Oxygen Concentration in the Earth’s Atmosphere.” Journal of the Atmospheric Sciences 22, no. 3 (1965): 225-261.

Brasted, Robert C. "Oxygen." Britannica, 25 Nov. 2024, www.britannica.com/science/oxygen. Accessed 20 Dec. 2024.

Johnson, Steven. The Invention of Air: A Story of Science, Faith, Revolution, and the Birth of America. New York: Riverhead Books, 2008.

Lane, Nick. Oxygen: The Molecule That Made the World. New York: Oxford University Press, 2002.

Morton, Oliver. Eating the Sun: How Plants Power the Planet. New York: HarperCollins, 2008.

Walker, Gabrielle. An Ocean of Air: Why the Wind Blows, and Other Mysteries of the Atmosphere. Orlando, Fla.: Harcourt, 2007.

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