OXYGEN CYCLE: CHARACTERISTICS, RESERVOIRS AND STAGES - ENVIRONMENT - 2025

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 molecular oxygen of two atoms (O ₂). 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, from Wikimedia Commons

O ₂ is used by living organisms in cellular respiration, generating the production of carbon dioxide (CO ₂), 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 ₂ molecule interacts with an oxygen atom, ozone (O ₃) is produced. Ozone makes up the so-called ozone layer.

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 O ₂.

O ₂ includes three stable isotopes: ¹⁶ O, ¹⁷ O and ¹⁸ O. The predominant form in the universe is ¹⁶ O. On Earth it represents 99.76% of total oxygen. The ¹⁸ O represents 0.2%. The ¹⁷ O form is very rare (~ 0.04%).

Origin

Oxygen is the third most abundant element in the universe. Production of the ¹⁶ O isotope began in the first generation of solar helium burning 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 (H ₂ O) 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 the 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. The 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 the planetary differential heating and causes the marine and atmospheric currents.

Relationship with other biogeochemical cycles

Due to its abundance and high reactivity, the oxygen cycle is connected with other cycles such as CO ₂, nitrogen (N ₂) and the water cycle (H ₂ O). This gives it a multicyclic character.

The O ₂ and CO ₂ reservoirs 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 the concentration of O ₂ 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 a significant fraction of the Earth's mantle and crust.

• Earth's core: in the liquid outer mantle of the Earth's core there are, in addition to iron, other elements, including oxygen.
• The 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 made up of oxygen in the form of dioxygen (O ₂). The other forms of atmospheric presence of oxygen are water vapor (H ₂ O), carbon dioxide (CO ₂) and ozone (O ₃).

• Water vapor: the concentration of water vapor is variable, depending on temperature, atmospheric pressure and atmospheric circulation currents (water cycle).
• Carbon dioxide: CO ₂ represents approximately 0.03% of the volume of air. Since the beginning of the Industrial Revolution, the concentration of CO ₂ in the atmosphere has increased by 145%.
• Ozone: it is a molecule that is present in the stratosphere in a low quantity (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. CO _{2 is} also dissolved in water and is subject to 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.

OR

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, 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 as 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 CO ₂. Combustion processes, especially industrial processes, also consume molecular oxygen and contribute CO ₂ to the atmosphere.

In the exchange between the atmosphere and the hydrosphere, the dissolved oxygen in water masses passes into the atmosphere. For its part, atmospheric CO ₂ 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 ₂ molecules. These are split into free oxygen atoms (O).

These free O atoms react with O ₂ molecules and produce ozone (O ₃). This reaction is reversible. Due to the effect of ultraviolet radiation, O ₃ decomposes into free oxygen atoms again.

Oxygen as a component of atmospheric air is part of various oxidation reactions, integrating various terrestrial compounds. A major 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 ocean 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 fuel.

The oxygen that is part of the water molecule (H ₂ O) follows the water cycle in the processes of evaporation-transpiration and condensation-precipitation.

- 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 (H ₂ O).

By taking hydrogen (H) from water, oxygen (O ₂) is released 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 (O ₂) produced during photosynthesis comes from the water used in the process. In photosynthesis, CO ₂, solar energy and water (H ₂ O) are consumed, and oxygen (O ₂) is released.

-Atmospheric return stage

The O ₂ 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 (CO ₂).

-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 molecules of carbohydrates (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. In respiration, oxygen (O ₂) is consumed and CO ₂ and water (H ₂ O) are released, producing useful energy.

CO ₂ and water (water vapor) are released through stomata in plants. In animals, CO ₂ is released through the nostrils and / or the mouth, and water through perspiration. In algae and bacteria, CO ₂ 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 the concentration of CO ₂ with respect to the concentration of O ₂.

Photorespiration establishes a negative energy balance for the plant. It consumes O ₂ and chemical energy (produced by photosynthesis) and releases CO ₂. 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 O ₂ 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 metal structures.

Oxygen is an element with a high oxidative capacity. Although oxygen molecules are very stable due to the fact that they form a double bond, since oxygen has a high electronegativity (the ability to attract electrons), it has a high reactive capacity. Due to this high electronegativity, oxygen takes part 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 humans. 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 (O ₃) in the stratosphere is the protective shield of the atmosphere against the entry of excess ultraviolet radiation. This highly energetic radiation increases the warming of the Earth.

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

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