CARBON CYCLE: CHARACTERISTICS, STAGES, IMPORTANCE - ENVIRONMENT - 2025

The carbon cycle is the process of circulation of this chemical element in air, water, soil and living things. It is a gas-type biogeochemical cycle and the most abundant form of carbon in the atmosphere is carbon dioxide (CO2).

The largest stores of carbon are in the oceans, fossil fuels, organic matter, and sedimentary rocks. Likewise, it is essential in the body structure of living organisms and enters the trophic chains as CO2 through photosynthesis.

Photosynthesizers (plants, phytoplankton, and cyanobacteria) absorb carbon from atmospheric CO2, and then herbivores take it from these organisms. These are consumed by carnivores and finally all dead organisms are processed by decomposers.

In addition to the atmosphere and living beings, carbon is found in the soil (edaphosphere) and in water (hydrosphere). In the oceans, phytoplankton, macroalgae and aquatic angiosperms take the CO2 dissolved in the water to carry out photosynthesis.

Carbon cycle illustration

CO2 is reintegrated into the atmosphere or water through the respiration of terrestrial and aquatic living beings respectively. Once living beings are dead, carbon is reintegrated into the physical environment as CO2 or as part of sedimentary rocks, coal or oil.

The carbon cycle is very important because it performs different functions such as being part of living beings, helping to regulate the planetary temperature and the acidity of the water. Likewise, it contributes to the erosive processes of sedimentary rocks and serves as a source of energy for the human being.

characteristics

Carbon

This element ranks sixth in abundance in the Universe and its structure allows it to form bonds with other elements such as oxygen and hydrogen. It is formed by four electrons (tetravalent) that form covalent chemical bonds capable of constituting polymers with complex structural forms.

The atmosphere

Carbon is found in the atmosphere mainly as carbon dioxide (CO2) in a proportion of 0.04% of the composition of air. Although the concentration of atmospheric carbon has changed substantially in the last 170 years due to human industrial development.

Before the industrial period, the concentration ranged from 180 to 280 ppm (parts per million) and today it exceeds 400 ppm. In addition, there is methane (CH4) in a much smaller proportion and carbon monoxide (CO) in small traces.

CO2 and methane (CH4)

These carbon-based gases have the property of absorbing and radiating long-wave energy (heat). For this reason, its presence in the atmosphere regulates the planetary temperature, by preventing the escape into space of the heat radiated by the Earth.

Of these two gases, methane captures more heat, but CO2 plays the most determining role due to its relative abundance.

The biological world

Most of the structure of living organisms is made up of carbon, essential in the formation of proteins, carbohydrates, fats and vitamins.

The lithosphere

Carbon is part of the organic matter and air in the soil, it is also found in elemental form such as carbon, graphite and diamond. In the same way, it is a fundamental part of the hydrocarbons (oil, bitumens) found in deep deposits.

Carbon formation

As vegetation dies in lake basins, swamps or shallow seas, plant debris accumulates in layers covered by water. A slow anaerobic decomposition process caused by bacteria is then generated.

The sediments cover the layers of decomposing organic material that undergoes a progressive process of carbon enrichment over millions of years. This passes through a stage of peat (50% carbon), lignite (55-75%), coal (75-90%) and finally anthracite (90% or more).

Oil formation

It begins with a slow aerobic decomposition, then there is an anaerobic phase, consisting of remains of plankton, animals and marine or lake plants. This organic matter was buried by sedimentary layers and subjected to high temperatures and pressures inside the Earth.

However, given its lower density, oil rises through the pores of sedimentary rocks. Eventually, it either becomes trapped in impermeable areas or forms shallow bituminous outcrops.

The hydrosphere

The hydrosphere maintains a gaseous exchange with the atmosphere, especially oxygen and carbon in the form of CO2 (soluble in water). Carbon is found in water, especially in the oceans, mainly in the form of bicarbonate ions.

Bicarbonate ions play an important role in regulating the pH of the marine environment. On the other hand, on the seabed there are large amounts of methane trapped as methane hydrates.

Acid rain

Carbon also penetrates between the gaseous medium and the liquid, when CO2 reacts with atmospheric water vapor and forms H2CO3. This acid precipitates with rainwater and acidifies soils and waters.

Stages of the carbon cycle

Carbon capture and storage. Source: Carbon_sequestration-2009-10-07.svg: * LeJean Hardin and Jamie Paynederivative work: Jarl Arntzen (talk) derivative work: Ortisa / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Like any biogeochemical cycle, the carbon cycle is a complex process made up of a network of relationships. Their separation into defined stages is only a means for their analysis and understanding.

- Geological stage

Tickets

Carbon inputs to this stage come to a lesser extent from the atmosphere, by acid rain and air filtered to the ground. However, the main input is the contributions of living organisms, both by their excrement and by their bodies when they die.

Storage and circulation

In this stage, carbon is stored and moves in deep layers of the lithosphere such as coal, oil, gas, graphite and diamonds. It is also part of carbonate rocks, trapped in permafrost (frozen ground layer in polar latitudes) and is dissolved in the water and air of the soil pores.

In the dynamics of plate tectonics, carbon also reaches the deeper layers of the mantle and is part of the magma.

Departures

The action of rain on calcareous rocks erodes them and calcium is released along with other elements. The calcium from the erosion of these carbonate rocks is washed into the rivers and from there to the oceans.

Similarly, CO ₂ is released when the permafrost thaws or by over-plowing the soil. However, the main output is driven by man by extracting coal, oil and gas from the lithosphere, to burn them as fuel.

Human activity, based on the consumption of hydrocarbons, releases carbon into the atmosphere

- Hydrological stage

Tickets

When atmospheric CO ₂ comes into contact with the water surface, it dissolves forming carbonic acid and methane from the seabed enters the lithosphere, as has been detected in the Arctic. In addition, HCO ₃ ions enter rivers and oceans due to the erosion of carbonate rocks in the lithosphere and soil washing.

When it rains, the water carries carbon in the form of carbon dioxide from the atmosphere and from the rocks. Upon reaching the ocean, corals, plankton and other aquatic animals use it to grow. These living things - corals, plankton and aquatic animals - die and enter the carbon into the soil

Storage and circulation

The CO2 dissolves in water forming carbonic acid (H2CO3), dissolving the calcium carbonate of the shells, forming calcium acid carbonate (Ca (HCO3) 2). Therefore, carbon is found and circulates in water mainly as CO2, H2CO3 and Ca (HCO3) 2.

On the other hand, marine organisms maintain a constant exchange of carbon with their aquatic environment via photosynthesis and respiration. Also, large reserves of carbon are in the form of methane hydrates on the seabed, frozen by low temperatures and high pressures.

Departures

The ocean exchanges gases with the atmosphere, including CO2 and methane, and part of the latter is released into the atmosphere. Recently, an increase in oceanic methane leakage has been detected at depths of less than 400 m, such as off the coast of Norway.

The increase in global temperature is heating the water in depths no greater than 400 m and releasing these methane hydrates. A similar process occurred in the Pleistocene, releasing large amounts of methane, warming the Earth more and causing the end of the Ice Age.

- Atmospheric stage

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Carbon enters the atmosphere from the respiration of living beings and from bacterial methanogenic activity. Similarly, due to vegetation fires (biosphere), exchange with the hydrosphere, burning of fossil fuels, volcanic activity and release from the ground (geological).

Release of geological carbon into the atmosphere by an erupting volcano. Author: Ciencia1.com

Storage and circulation

In the atmosphere, carbon is mainly in the gaseous form such as CO2, methane (CH4) and carbon monoxide (CO). Similarly, you can find carbon particles suspended in the air.

Departures

The main carbon outputs from the atmospheric stage are the CO2 that dissolves in ocean water and that used in photosynthesis.

- Biological stage

Tickets

Carbon enters the biological stage as CO2 through the photosynthesis process carried out by plants and photosynthetic bacteria. Likewise, the Ca2 + and HCO3- ions that reach the sea by erosion and are used by various organisms in the manufacture of shells.

Plants and microorganisms absorb carbon dioxide from the atmosphere and convert it into oxygen and energy through photosynthesis

Storage and circulation

Each cell and therefore the bodies of living beings are made up of a high proportion of carbon, constituting proteins, carbohydrates and fats. This organic carbon circulates through the biosphere through trophic webs from primary producers.

Angiosperms, ferns, liverworts, mosses, algae, and cyanobacteria incorporate it by photosynthesis. Then these organisms are consumed by herbivores, which will be food for carnivores.

Herbivorous animals consume plants and release carbon dioxide into the atmosphere. When these animals die they reintegrate carbon into the soil. The same thing happens with coral and plankton on the ocean floor

Departures

The main carbon leakage from this stage to others in the carbon cycle is the death of living beings that reintegrates it into the soil, water and the atmosphere. A massive and drastic form of carbon death and release is forest fires that produce large amounts of CO2.

On the other hand, the most important source of methane into the atmosphere is the gases expelled by livestock in their digestive processes. Likewise, the activity of methanogenic anaerobic bacteria decomposing organic matter in swamps and rice crops is a source of methane.

Importance

The carbon cycle is important due to the relevant functions that this element fulfills on planet Earth. Its balanced circulation allows to regulate all these relevant functions for the maintenance of planetary conditions as a function of life.

In living beings

Carbon is the main element in the structure of cells as it is part of carbohydrates, proteins and fats. This element is the basis of all the chemistry of life, from DNA to cell membranes and organelles, tissues and organs.

The regulation of the earth's temperature

CO2 is the main greenhouse gas, which makes it possible to maintain a suitable temperature for life on Earth. Without atmospheric gases such as CO2, water vapor and others, the heat emitted by the Earth would completely escape into space and the planet would be a frozen mass.

Global warming

On the other hand, an excess of CO2 emitted into the atmosphere, such as that currently caused by humans, breaks the natural balance. This causes the planet to overheat, which alters the global climate and negatively affects biodiversity.

Regulation of oceanic pH

CO2 and methane dissolved in water are part of the complex mechanism for regulating the pH of water in the oceans. The higher the content of these gases in the water, the pH becomes more acidic, which is negative for aquatic life.

Power source

Coal is an essential part of fossil fuels, both mineral coal, oil and natural gas. Although its use is questioned due to the negative environmental effects it produces, such as global overheating and the release of heavy metals.

Economic value

Coal is a mineral that generates sources of work and economic profits for its use as fuel and the economic development of Humanity is based on the use of this raw material. On the other hand, in its crystallized form of diamond, much rarer, it is of great economic value for its use as a precious stone.

References

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4. Margalef, R. (1974). Ecology. Omega editions.
5. Miller, G. and TYLER, JR (1992). Ecology and Environment. Grupo Editorial Iberoamérica SA de CV
6. Odum, EP and Warrett, GW (2006). Fundamentals of ecology. Fifth edition. Thomson.