Where does the oxygen in the atmosphere come from?

The Earth’s atmosphere is approximately 78% nitrogen and 21% oxygen. The remaining 1% is occupied by other gases, mainly carbon dioxide and argon. Oxygen in the atmosphere is consumed by all animals and many other organisms that use it in their metabolism.

In addition to being used by aerobic life forms, oxygen also reacts with Earth’s minerals and elements. Through these processes, oxygen is removed from the atmosphere and replaced by several ways, mainly the photosynthesis of plants, algae and some bacteria.

Photosynthesis, the main source of oxygen

It is estimated that approximately 98% of the oxygen in the atmosphere comes from photosynthesis, specifically oxygenated photosynthesis, a process carried out by so-called oxygenated photoautotrophic organisms (to differentiate them from other organisms that carry out anoxygenic photosynthesis and from other autotrophs that carry out chemosynthesis).

Currently, the known oxygenated photoautotrophic agencies are the plants, algae (also called lower plants) and cyanobacteria (such as spirulina). These organisms absorb water (HdoisO) and carbon dioxide (CO2) from the medium and with these molecules form organic compounds. The energy needed for biosynthesis reactions is obtained from solar radiation.

In photosynthesis, in addition to organic molecules, molecular oxygen (Odois) is released into the medium (air and water). This process has been happening on planet Earth for thousands of years, oxygenating the atmosphere and allowing the life of other living beings that need oxygen, including human beings and all animals on the planet.

In 2009, American scientists published in the journal Geoscience of Nature the discovery of samples of an iron mineral (hematite) in the Pilbara Craton (Northwestern Australia) with an approximate age of 3,460 million years. According to these researchers, the presence of this mineral evidences the possible existence since then of oxygenic photosynthetic organisms, as they need an aqueous medium rich in oxygen to form.

Of all photosynthetic organisms, the greatest contribution to atmospheric oxygen is cyanobacteria and algae from oceanic phytoplankton and land plants. The proportion of atmospheric oxygen that comes from the ocean and land environment is up for debate. Some scientists believe that the contribution of each medium is approximately 50%, while others point out that the ocean contributes 1/3 and land plants 2/3. What is clear is that these numbers can vary from one area of ​​the planet to another depending on the balance between the different forms of life that exist.

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Increased oxygen levels

Cyanobacteria are believed to be the first organisms to initiate oxygenated photosynthesis. In the beginning, when the population of these organisms was still small, the oxygen produced would be quickly consumed in oxidative reactions with substances in the environment, mainly iron from the soil, rocks and oceans, forming oxidized compounds of iron and other minerals.

Geologists can estimate the amount of oxygen present in the atmosphere since ancient times by studying the types of iron compounds present in rocks. In the absence of oxygen, iron tends to combine with sulfur to form sulfides such as pyrite. When oxygen is present, these compounds degrade and the iron combines with oxygen to form oxides. Therefore, pyrites from ancient rocks indicate low levels of oxygen while the presence of iron oxides indicates the presence of significant amounts of oxygen.

As the amounts of iron available to combine with oxygen were depleted, gaseous oxygen built up in the atmosphere. It is thought that 2300-25 billion years ago, oxygen made up only 1% of the air. The accumulation of oxygen in the atmosphere would continue for millions of years until a certain balance between formation and consumption was achieved.

With the increase in atmospheric oxygen, new forms of life could evolve, heterotrophic living beings, such as humans, which consume organic matter and oxidize it to obtain energy through the process known as aerobic respiration. In this process, organic carbon, which ultimately comes from that formed in photosynthesis, is oxidized and carbon dioxide (CO) is released.two), an inorganic form of carbon that will be used again by photosynthetic organisms.

It seems that, due to this consumption of oxygen by other forms of life, photosynthesis alone could not have been sufficient for the initial increase in oxygen. One possible explanation is that large amounts of organic carbon remained buried and unavailable to aerobic organisms, which could tip the balance in favor of oxygen production over oxygen consumption.

At some later point in planet Earth’s history, oxygen levels appear to have increased rapidly to current concentrations. Some scientists believe this may have been around 600 million years ago, when complex multicellular living things that required high levels of oxygen began to appear. However, it is not known very well what caused this change.

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One theory relates this rapid rise to the coming out of an ice age, a phase in which glaciers, in their advance and retreat, would break up rocks rich in minerals with phosphorus that would end up in the oceans. Phosphorus is one of the main nutrients of phytoplankton and it is believed that it could have caused a rapid increase in the population of photosynthetic organisms in the oceans while on the Earth’s surface there would be relatively little oxygen-consuming life, as it was covered in ice. This theory is still being discussed.

Other more recent studies point to the accumulation of millions of tons of oxygen in iron dioxide crystals deep within the Earth, between the core and the mantle. According to this research, the subduction of the earth’s crust drags hydrated minerals with it; When water molecules encounter iron in the core at high pressure and temperature, iron dioxide crystals can form, which accumulate year after year until they reach sizes comparable to continents.

Certain geological phenomena can cause these reserves of oxygen to be expelled into the atmosphere, as could have happened 25 billion years ago in what is known as the Great Oxygenation.

Decreased atmospheric oxygen

There are several studies that show a continuous drop in ocean oxygen levels, and atmospheric oxygen measurements show a global decrease of 0.0317% between 1990 and 2008. This drop is mainly attributed to the intense combustion of hydrocarbons and fossil fuels such as oil and coal. This combustion uses oxygen and emits COdoiso which promotes a decrease in the concentration of Odois in the atmosphere.

The measured decrease, however, appears to be less than might be expected from the large amount of these fuels consumed during these years. One possibility is that the increase in carbon dioxide, possibly combined with the use of agricultural fertilizers, may have contributed to the faster growth of some plants whose photosynthesis partially compensated for oxygen consumption. It is believed that even if all of the planet’s fossil fuel reserves were burned, there would be no major impact on the concentration of oxygen in the atmosphere.

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Deforestation is another major concern. Although the destruction of large areas of rainforest has very harmful effects on the environment, the same cannot be said for the effect on the concentration of oxygen in the atmosphere. Some of the explanations for this low impact could be that forests, in addition to trees and photosynthetic plants, also harbor a large amount of oxygen-consuming aerobic life, so their overall contribution to atmospheric oxygen would be close to neutral. But let it be clear that deforestation is a serious environmental problem, although not because of oxygen levels, but for many other reasons, such as desertification and loss of biodiversity.

A problem of much greater magnitude, with regard to oxygen, seems to be the impact of human activity on phytoplankton, which, according to some sources such as the one cited above, would currently be the main global producer of atmospheric oxygen. The increase in CO2 in the atmosphere would not only contribute to an increase in the average temperature of the oceans, but also to its acidification, which would negatively affect the development of phytoplankton. However, this theory is unclear why some phytoplanktonic organisms could decrease in numbers while others would be positively affected.


Global distribution of photosynthesis in aquatic and terrestrial environments Evolution of oxygen in the atmosphere in the history of planet Earth Diagram of the gaseous composition of the atmosphere

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