The Oxygen Cycle: Controlled by Plants

We explain the importance of oxygen, where it comes from, where it is found, and how (and in what form) it circulates through the different reservoirs on Earth. In the process, we explain both the fast and slow oxygen cycle and examine how they are affected by climate change.
Picture of Rainforest Jungle
Tropical rainforest, Nigeria. Trees & plants create oxygen from photosynthesis. Photo: © University of Southern Denmark

The oxygen cycle is one of the main biogeochemical cycles transporting vital chemicals through the environmental components of Earth. As usual, there is a fast oxygen cycle and a slow oxygen cycle.

The fast one circulates oxygen from the pedosphere (soil) and biosphere (living organisms), to the atmosphere and back. The slow one moves it from the atmosphere to the hydrosphere (oceans) and lithosphere (rocks of the Earth’s crust).

Other similar pathways circulating essential chemicals, include: the carbon cycle, the nitrogen cycle, the sulfur cycle, the phosphorus cycle and the water cycle.

What is Oxygen?

Oxygen is the third most abundant chemical element in the universe and readily reacts with other elements to form chemical compounds (e.g. oxides, dioxides), of which the most common is water (H2O). There are two types of oxygen: so-called “free oxygen” that isn’t combined with any other elements such as carbon or nitrogen, and oxygen that forms a compound with other chemicals, such as carbon dioxide (CO2) or nitrous oxide (N2O).

Oxygen is an essential chemical for all life on Earth. It makes up roughly 65 percent of the human body, mostly in the form of water (H2O), and accounts for roughly 21 percent of the atmosphere. Water (for example, in the oceans) contains 88.9 percent oxygen.

Where Does Oxygen Come From?

Almost all the oxygen on Planet Earth comes from photosynthesis, a metabolic process performed by plants and phytoplankton (marine plants), in which sunlight, water and CO2 are used to create food energy (sugars/carbs).

In the beginning (4.6 billion years BC), Planet Earth’s atmosphere contained little or free oxygen, even though tiny algae (cyanobacteria) were already generating the gas as a by-product of photosynthesis. 1 This appears to be because the oxygen was quickly absorbed by the weathering of ferrous minerals, such as olivine (magnesium iron silicate). 2

Then, cyanobacteria-produced oxygen in the atmosphere suddenly skyrocketed, increasing by about 10,000 times in less than 200 million years. According to a recent study, the increase may have been due to the disappearance of the mineral olivine. 3

The phenomenal surge in oxygen, known as the Great Oxidation Event, led to the formation of the protective ozone layer, and made possible the subsequent development of multicellular forms, including plants, animals and humans.

As plants and trees developed, photosynthesis became more common and the rate of oxygen production increased until about 580 million years BC, when it finally reached today’s levels.

Earth is unique among planets of the Solar System in having such high levels of oxygen gas in its atmosphere. Mars and Venus, for instance have much less. What’s more their oxygen is generated not by biological means, but solely by ultraviolet radiation breaking apart oxygen-containing molecules, such as carbon dioxide.

The Oxygen Cycle
The Oxygen Cycle. Image: © Cbusch01 (CC BY-SA 3.0)

Why is the Oxygen Cycle Important?

The oxygen cycle is important because it helps to maintain an adequate supply of the chemical in the atmosphere, for use by the terrestrial and marine biosphere. Here are a few of the valuable uses to which oxygen is put.

• BREATHING/RESPIRATION
Oxygen is critical to the survival of most life forms on Earth, including humans, because we need it to breathe. The scientific term for breathing is respiration. Respiration in plants, animals and humans, is the movement of oxygen from outside the body to the cells and tissues inside it, and the movement of carbon dioxide in the opposite direction. In a nutshell, living things breathe in O2 and breathe out CO2.

• DECOMPOSITION
When plants and animals die, their bodies are broken down into their basic chemical components by microbial decomposers, which allows important nutrients to be recycled back into the food web. Decomposition is an oxidation process: it uses up oxygen and releases carbon dioxide. This form of microbial oxidation leads to a significant uptake of O2 from the atmosphere.

• COMBUSTION
Oxygen is also necessary for combustion, such as the burning of fossil fuels, like coal, oil or natural gas. Combustion is a chain reaction which is chemically similar to photosynthesis, except in reverse.

To start a fire, you need: heat to start the burning process, plus fuel and oxygen to keep it going. The ignition and combustion of the fuel will continue as long as there is sufficient oxygen present. Like decomposition and rusting, combustion is an oxidation process. It uses up oxygen (oxidizes).

• ENERGY FOR FOOD WEB
Plants – Plants create the majority of the oxygen we breathe through a process called photosynthesis. In this process plants use carbon dioxide, sunlight, and water to create energy. In the process they also create oxygen which they release into the air.

• OZONE
Free oxygen is an essential ingredient in the production of the stratospheric ozone layer. Ozone absorbs harmful UV rays from the sun, thus making it possible for humans to survive on the surface of the planet. Ozone is formed by a 2-step process, as follows: first, solar ultraviolet radiation breaks apart one oxygen molecule (O2) into two oxygen atoms (2 x O); second, each of these oxygen atoms (O) combines with an oxygen molecule (O2) to produce an ozone molecule (O3).

• MOLECULAR BIOLOGY
Oxygen is also an essential element in proteins, and in nucleic acids such as RNA (ribonucleic acid) as well as DNA (deoxyribonucleic acid).

Where is Oxygen Found?

The four main reservoirs of the oxygen cycle are the atmosphere, biosphere, hydrosphere and lithosphere (crust and mantle).

The largest reservoir by far is the lithosphere, the rigid outer part of the Earth, consisting of the crust and mantle. This contains about 99.5 percent of Earth’s oxygen. The other environments share the remaining 0.5 percent. 4

Atmosphere
The atmosphere is 20.9 percent oxygen. This is mostly free O2 released from photosynthesis, but also exists as carbon dioxide (CO2), water vapor (H2O), ozone (O3), and sulfur or nitrogen oxides.

Biosphere
The biosphere is 22 percent oxygen, comprising mostly organic molecules and water.

Hydrosphere
The hydrosphere is 33 percent oxygen, mainly water molecules with dissolved free oxygen and carbonic acid.

Pedosphere
One of the main functions of the pedosphere is the cycling of elements that occur within soils, and the transfer of these chemicals to the atmosphere, lithosphere, biosphere, and hydrosphere. Soil acts as an interface between the atmosphere and lithosphere, between the biosphere and lithosphere, and between soil organisms and the atmosphere.

Lithosphere
In the lithosphere, oxygen occurs in a wide range of chemical compounds, such as oxides, silicates, carbonates, phosphates, sulfates, as well as other more complex compounds. The Earth’s crust is roughly 46.6 percent oxygen, mainly in the form of silicon dioxide and other oxides. The mantle is roughly 45 percent oxygen.

Earth’s outer core is 8–13 percent oxygen. 5 Earth’s inner core consists of a solid iron–nickel alloy.

How Does the Oxygen Cycle Work?

Fast Oxygen Cycle

The fast oxygen cycle operates between the atmosphere and biosphere. Its most important driver is photosynthesis, a metabolic process conducted exclusively by plants in the pedosphere, and marine phytoplankton in the hydrosphere.

Scientists estimate that at least 50 percent (perhaps as much as 80 percent) of the world’s oxygen is generated by oceanic plankton – drifting plants, algae, and photosynthetic bacteria. Most of the remainder (28 percent) is generated by the Amazon Rainforest and other tropical rainforests. One particular microscopic species of bacteria (Prochlorococcus) produces one fifth of the entire oxygen in our biosphere.

While photosynthesis emits oxygen into the atmosphere, respiration, decomposition, and combustion remove it from the atmosphere. Despite a reduction in the amount of natural vegetation on land and in coastal waters, due to deforestation and change of land use, the level of atmospheric oxygen is relatively stable due to the increase in plant productivity resulting from agricultural improvements around the globe.

But this equilibrium is likely to change as climate change takes hold. As ocean oxygen levels fall, due to warmer temperatures, photosynthetic productivity is projected to decrease, with uncertain consequences.

Slow Oxygen Cycle

In the slow cycle, oxygen is absorbed into the lithosphere mostly via the oceans. Marine organisms build shells out of calcium carbonate which is half oxygen. When the organism dies, the shell falls to the ocean floor where it forms sediment and eventually, over geological time, turns into limestone rock.

In addition, some oxygen is absorbed directly from the atmosphere via chemical weathering and other surface reactions, including the formation of iron oxides. Another small amount enters the lithosphere via the soil, as oxygen-rich waste from living organisms settles into the earth and eventually becomes lithified.

Map of Ocean Oxygen Levels
Map showing areas of ocean hypoxia (low oxygen levels) in the open ocean and coastal waters. Indicated by red dots are coastal sites where anthropogenic nutrients have exacerbated or caused oxygen declines to <2 mg/l (<63 μmol/l). Shown in blue, are ocean oxygen minimum zones, down to 300 meters. Image: © Breitburg et al. (2018) (CC BY-SA 4.0)

How Does Climate Change Affect the Oxygen Cycle?

Our climate crisis affects the oxygen cycle in several ways.

First, one of the most serious effects of deforestation is the reduction in net oxygen production. Once trees are cut down, they stop producing oxygen, and emit only CO2.

Second, the more coal is burned to produce electricity, the more greenhouse gas emissions are created and the more heat goes into the ocean. And the warmer the ocean, the less oxygen it absorbs.

Measurement of dissolved oxygen in coastal waters as well as the open ocean, over the past 50 years, shows a noticeable decline in oxygen levels. In fact, since 1950, over 500 coastal sites have seen oxygen concentrations drop below 2 mg per liter, which signifies hypoxic conditions. 6 This increase in the deoxygenation of the oceans has been quite rapid and affects all aerobic marine life.

Third, rising temperatures lead to ice melt and glacial runoff, which results in a surface layer that is less saline, thus less dense. This stratification limits the upwelling of nutrients by deep-water currents into the upper layer of the ocean, where most of the photosynthesis takes place, thus reducing phytoplankton productivity. 7

Warmer waters also raise the metabolism of marine organisms, leading to increased respiration. This is likely to result in less oxygen being transferred to the atmosphere.

Studies show that oceans have already lost 1-2 percent of their oxygen since the middle of the 20th century, and model simulations predict a decline of up to 7% in the global ocean O2 content over the next hundred years. 8

To understand more about the timeline of our planet and its oxygenation, please see: History of Earth in One Year (Cosmic Calendar).

References

  1. For a contrary view, see: “Deep-sea Rocks Point To Early Oxygen On Earth.” ScienceDaily. []
  2. “The oxygenation of the atmosphere and oceans”. Holland, H. D. (2006). Philosophical Transactions of the Royal Society B: Biological Sciences. 361 (1470): 903–915. []
  3. “Earth’s early O2 cycle suppressed by primitive continents.” Matthijs A. Smit & Klaus Mezger. Nature Geoscience, 2017. []
  4. Haynes, William M. (2016). “Abundance of Elements in the Earth’s Crust and in the Sea.” CRC Handbook of Chemistry and Physics (97th ed.). Taylor and Francis. ISBN 9781498754286. []
  5. “Temperature and composition of the Earth’s core”. Alfe, D.; Gillan, M. J.; Price, G. D. (2007). Contemporary Physics. 48 (2): 63–80. []
  6. “Declining oxygen in the global ocean and coastal waters”. Breitburg, D; et al. (2018). Science. 359 (6371). []
  7. “The role of nutricline depth in regulating the ocean carbon cycle.” Cermeno, Pedro, et al. Proceedings of the National Academy of Sciences 105.51 (2008): 20344-20349 []
  8. “Multiple stressors of ocean ecosystems in the 21st century: projections with CMIP5 models”. Bopp, L; Resplandy, L; Orr, JC; Doney, SC; Dunne, JP; Gehlen, M; Halloran, P; Heinze, C; Ilyina, T; Seferian, R; Tjiputra, J (2013). Biogeosciences. 10: pp.6625–6245. []
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