Oxygen cycle characteristics, reservoirs and stages

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Sherman Hoover
Oxygen cycle characteristics, reservoirs and stages

The oxygen cycle refers to the circulatory movement of oxygen on Earth. It is a gaseous biogeochemical cycle. Oxygen is the second most abundant element in the atmosphere after nitrogen, and the second most abundant in the hydrosphere after hydrogen. In this sense, the oxygen cycle is connected to the water cycle..

The circulatory movement of oxygen includes the production of dioxygen or two-atom molecular oxygen (Otwo). This occurs due to hydrolysis during photosynthesis carried out by the different photosynthetic organisms..

Oxygen reservoir: Cloud forest, Waraira Repano National Park, Venezuela. Arnaldo Noguera Sifontes [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons

The Otwo It is used by living organisms in cellular respiration, generating the production of carbon dioxide (COtwo), the latter being one of the raw materials for the photosynthesis process.

On the other hand, in the upper atmosphere, photolysis (hydrolysis activated by solar energy) of water vapor caused by ultraviolet radiation from the sun occurs. Water decomposes releasing hydrogen that is lost in the stratosphere and oxygen is integrated into the atmosphere.

When an O molecule interactstwo With an oxygen atom, ozone (O3). Ozone makes up the so-called ozone layer.

Article index

  • 1 Features
    • 1.1 Origin
    • 1.2 Primitive atmosphere
    • 1.3 Energies that drive the cycle
    • 1.4 Relationship with other biogeochemical cycles
  • 2 Reservoirs
    • 2.1 Geosphere
    • 2.2 Atmosphere
    • 2.3 Hydrosphere
    • 2.4 Cryosphere
    • 2.5 Living organisms
  • 3 Stages
    • 3.1 -Environmental stage of reservoir and source: atmosphere-hydrosphere-cryosphere-geosphere
    • 3.2-Photosynthetic stage
    • 3.3 -Atmospheric return stage
    • 3.4 -Respiratory stage
  • 4 Importance
  • 5 Alterations
    • 5.1 Greenhouse effect
  • 6 References

Characteristics

Oxygen is a non-metallic chemical element. Its atomic number is 8, that is, it has 8 protons and 8 electrons in its natural state. Under normal conditions of temperature and pressure it is present in the form of dioxygen gas, colorless and odorless. Its molecular formula is Otwo.

The Otwo includes three stable isotopes: 16OR, 17Or and 18O. The predominant form in the universe is the 16O. On Earth it represents 99.76% of the total oxygen. The 18Or represents 0.2%. The shape 17Or is it very rare (~ 0.04%).

Source

Oxygen is the third most abundant element in the universe. Isotope production 16Or it started in the first generation of solar helium burn that occurred after the Big Bang.

The establishment of the carbon-nitrogen-oxygen nucleosynthesis cycle in later generations of stars has provided the predominant source of oxygen on the planets..

High temperatures and pressures produce water (HtwoO) in the Universe by generating the reaction of hydrogen with oxygen. Water is part of the makeup of the Earth's core.

Magma outcrops give off water in the form of steam and this enters the water cycle. Water is decomposed by photolysis into oxygen and hydrogen through photosynthesis, and by ultraviolet radiation in the upper levels of the atmosphere..

Primitive atmosphere

The primitive atmosphere before the evolution of photosynthesis by cyanobacteria was anaerobic. For living organisms adapted to that atmosphere, oxygen was a toxic gas. Even today an atmosphere of pure oxygen causes irreparable damage to cells.

Photosynthesis originated in the evolutionary lineage of today's cyanobacteria. This began to change the composition of Earth's atmosphere approximately 2.3-2.7 billion years ago..

The proliferation of photosynthesizing organisms changed the composition of the atmosphere. Life evolved towards adaptation to an aerobic atmosphere.

Energies that drive the cycle

The forces and energies that act to drive the oxygen cycle can be geothermal, when magma expels water vapor, or it can come from solar energy.

The latter provides the fundamental energy for the photosynthesis process. Chemical energy in the form of carbohydrates resulting from photosynthesis, in turn drives all living processes through the food chain. In the same way, the Sun produces differential planetary heating and causes marine and atmospheric currents..

Relationship with other biogeochemical cycles

Due to its abundance and high reactivity, the oxygen cycle connects with other cycles such as COtwo, nitrogen (Ntwo) and the water cycle (HtwoOR). This gives it a multicyclic character.

The reservoirs of Otwo and COtwo  they are linked by processes that involve the creation (photosynthesis) and destruction (respiration and combustion) of organic matter. In the short term, these oxidation-reduction reactions are the major source of variability in O concentration.two in the atmosphere.

Denitrifying bacteria obtain oxygen for their respiration from nitrates in the soil, releasing nitrogen.

Reservoirs

Geosphere

Oxygen is one of the main components of silicates. Therefore, it constitutes an important fraction of the Earth's mantle and crust..

  • Earth core: in the liquid outer mantle of the Earth's core there are, in addition to iron, other elements, including oxygen.
  • Soil: in the spaces between particles or pores of the soil the air is diffused. This oxygen is used by the soil microbiota.

Atmosphere

21% of the atmosphere is composed of oxygen in the form of dioxygen (Otwo). The other forms of atmospheric oxygen presence are water vapor (HtwoO), carbon dioxide (COtwo) and ozone (O3).

  • Water steam: the concentration of water vapor is variable, depending on the temperature, atmospheric pressure and atmospheric circulation currents (water cycle).
  • Carbon dioxide: the COtwo It represents approximately 0.03% of the volume of air. Since the beginning of the Industrial Revolution the concentration of CO has increasedtwo in the atmosphere by 145%.
  • Ozone: is a molecule that is present in the stratosphere in a low amount (0.03 - 0.02 parts per million by volume).

Hydrosphere

71% of the earth's surface is covered by water. More than 96% of the water present on the earth's surface is concentrated in the oceans. 89% of the mass of the oceans is made up of oxygen. The COtwo It is also dissolved in water and undergoes an exchange process with the atmosphere..

Cryosphere

The cryosphere refers to the mass of frozen water that covers certain areas of the Earth. These ice masses contain approximately 1.74% of the water in the earth's crust. On the other hand, ice contains varying amounts of trapped molecular oxygen.

ORliving organisms

Most of the molecules that make up the structure of living things contain oxygen. On the other hand, a high proportion of living things is water. Therefore, the terrestrial biomass is also an oxygen reserve.

Stages

In general terms, the cycle that oxygen follows as a chemical agent comprises two large areas that make up its character as a biogeochemical cycle. These areas are represented in four stages.

The geoenvironmental area encompasses the displacements and containment in the atmosphere, hydrosphere, cryosphere and geosphere of oxygen. This includes the environmental stage of reservoir and source, and the stage of return to the environment..

Oxygen cycle. Eme Chicano [CC0], from Wikimedia Commons

Two stages are also included in the biological area. They are associated with photosynthesis and respiration.

-Environmental stage of reservoir and source: atmosphere-hydrosphere-cryosphere-geosphere

Atmosphere

The main source of atmospheric oxygen is photosynthesis. But there are other sources from which oxygen can enter the atmosphere..

One of these is the liquid outer mantle of the Earth's core. Oxygen reaches the atmosphere in the form of water vapor through volcanic eruptions. Water vapor rises to the stratosphere where it undergoes photolysis as a result of high-energy radiation from the sun and free oxygen is produced..

On the other hand, respiration emits oxygen in the form of COtwo.  Combustion processes, especially industrial processes, also consume molecular oxygen and contribute COtwo to the atmosphere.

In the exchange between the atmosphere and the hydrosphere, dissolved oxygen in water masses passes into the atmosphere. For its part, the COtwo Atmospheric is dissolved in water as carbonic acid. Dissolved oxygen in water comes mainly from the photosynthesis of algae and cyanobacteria.

Stratosphere

In the upper levels of the atmosphere, high-energy radiation hydrolyzes water vapor. Short-wave radiation activates O moleculestwo. These are split into free oxygen atoms (O).

These free O atoms react with O moleculestwo and produce ozone (O3). This reaction is reversible. Due to the effect of ultraviolet radiation, O3 breaks down into free oxygen atoms again.

Oxygen as a component of atmospheric air is part of various oxidation reactions, becoming part of various terrestrial compounds. An important sink for oxygen is the oxidation of gases from volcanic eruptions..

Hydrosphere

The largest concentration of water on Earth is the oceans, where there is a uniform concentration of oxygen isotopes. This is due to the constant exchange of this element with the earth's crust through hydrothermal circulation processes..

At the limits of the tectonic plates and the oceanic ridges, a constant process of gas exchange is generated.

Cryosphere

Land ice masses, including polar ice masses, glaciers, and permafrost, constitute a major sink for oxygen in the form of solid-state water..

Geosphere

Likewise, oxygen participates in the gas exchange with the soil. There it constitutes the vital element for the respiratory processes of soil microorganisms..

An important sink in the soil is the processes of mineral oxidation and the burning of fossil fuels..

The oxygen that is part of the water molecule (HtwoO) follows the water cycle in the evaporation-transpiration and condensation-precipitation processes.

-Photosynthetic stage

Photosynthesis takes place in chloroplasts. During the light phase of photosynthesis, a reducing agent is required, that is, a source of electrons. Said agent in this case is water (HtwoOR).

By taking hydrogen (H) from water, oxygen (Otwo) as a waste product. Water enters the plant from the soil through the roots. In the case of algae and cyanobacteria, it comes from the aquatic environment.

All molecular oxygen (Otwo) produced during photosynthesis comes from the water used in the process. CO is consumed in photosynthesistwo, solar energy and water (HtwoO), and oxygen is released (Otwo).

-Atmospheric return stage

The Otwo generated in photosynthesis is expelled into the atmosphere through the stomata in the case of plants. Algae and cyanobacteria return it to the environment by membrane diffusion. Similarly, respiratory processes return oxygen to the environment in the form of carbon dioxide (COtwo).

-Respiratory stage

To perform their vital functions, living organisms need to make effective the chemical energy generated by photosynthesis. This energy is stored in the form of complex carbohydrate molecules (sugars) in the case of plants. The rest of the organisms obtain it from the diet

The process by which living beings unfold chemical compounds to release the required energy is called respiration. This process takes place in cells and has two phases; one aerobic and one anaerobic.

Aerobic respiration takes place in the mitochondria in plants and animals. In bacteria it is carried out in the cytoplasm, since they lack mitochondria.

The fundamental element for respiration is oxygen as an oxidizing agent. Respiration consumes oxygen (Otwo) and CO is releasedtwo and water (HtwoO), producing useful energy.

The COtwo and water (water vapor) are released through stomata in plants. In animals COtwo it is released by the nostrils and / or the mouth, and the water by perspiration. In algae and bacteria COtwo is released by membrane diffusion.

Photorespiration

In plants, in the presence of light, a process that consumes oxygen and energy called photorespiration develops. Photorespiration increases with increasing temperature, due to the increase in CO concentration.two with respect to the concentration of Otwo.

Photorespiration establishes a negative energy balance for the plant. Consume Otwo and chemical energy (produced by photosynthesis) and releases COtwo. For this reason, they have developed evolutionary mechanisms to counteract it (C4 and CAN metabolisms)..

Importance

Today the vast majority of life is aerobic. Without the circulation of Otwo in the planetary system, life as we know it today would be impossible.

In addition, oxygen constitutes a significant proportion of the earth's air masses. Therefore, it contributes to the atmospheric phenomena linked to it and its consequences: erosive effects, climate regulation, among others..

Directly, it generates oxidation processes in the soil, of volcanic gases and on artificial metallic structures..

Oxygen is an element with a high oxidative capacity. Although oxygen molecules are very stable because they form double bonds, since oxygen has a high electronegativity (ability to attract electrons), it has a high reactive capacity. Due to this high electronegativity, oxygen intervenes in many oxidation reactions.

Alterations

The vast majority of combustion processes that occur in nature require the participation of oxygen. Likewise in those generated by the human being. These processes fulfill both positive and negative functions in anthropic terms.

The combustion of fossil fuels (coal, oil, gas) contributes to economic development, but at the same time represents a serious problem due to its contribution to global warming.

Large forest fires affect biodiversity, although in some cases they are part of natural processes in certain ecosystems.

Greenhouse effect

The ozone layer (O3) in the stratosphere, is the protective shield of the atmosphere against the entry of excess ultraviolet radiation. This highly energetic radiation increases global warming.

On the other hand, it is highly mutagenic and harmful to living tissues. In humans and other animals it is carcinogenic.

The emission of various gases causes the destruction of the ozone layer and therefore facilitate the entry of ultraviolet radiation. Some of these gases are chlorofluorocarbons, hydrochlorofluorocarbons, ethyl bromide, nitrogen oxides from fertilizers, and halons..

References

  1. Anbar AD, Y Duan, TW Lyons, GL Arnold, B Kendall, RA Creaser, AJ Kaufman, WG Gordon, S Clinton, J Garvin and R Buick (2007) A Whiff of Oxygen Before the Great Oxidation Event? Science 317: 1903-1906.
  2. Bekker A, HD Holland, PL Wang, D Rumble, HJ Stein, JL Hannah, LL Coetzee, and NJ Beukes. (2004) Dating the rise of atmospheric oxygen. Nature 427: 117-120.
  3. Farquhar J and DT Johnston. (2008) The Oxygen Cycle of the Terrestrial Planets: Insights into the Processing and History of Oxygen in Surface Environments. Reviews in Mineralogy and Geochemistry 68: 463-492.
  4. Keeling RF (1995) The atmospheric oxygen cycle: The oxygen isotopes of atmospheric COtwo and Otwo and the Otwo/ Ntwo Reviws of Geophysics, supplement. U.S: National Report to International Union of Geodesy and Geophysics 1991-1994. pp. 1253-1262.
  5. Purves WK, D Sadava, GH Orians and HC Heller (2003) Life. The Science of Biology. 6th Edt. Sinauer Associates, Inc. and WH Freeman and Company. 1044 p.

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