Carbon Cycle: Definition, Steps, and Importance

Carbon is one of the essential components that is found in every life on Earth. Scientists believe that the vast majority, approximately 99.9%, of all life forms on Earth are comprised of carbon. The carbon content in a planet is limited (i.e., 0.03%). Therefore, to fulfill the carbon requirement on the Earth, it is continuously used and recycled, termed the carbon cycle by Joseph Priestley and Antoine Lavoisier.

Therefore, the carbon cycle is a bio-geo-chemical cycle in which carbon travels (in the form of CO2) from the atmosphere into organisms and, after their death, back to the atmosphere. This cycle is essential for maintaining the Earth’s ecosystem by sustaining plants and animals. It occurs through various processes such as; oceanic gas exchange, photosynthesis, respiration, decomposition, combustion, rock weathering, and volcanism.

Steps of the Carbon Cycle

The carbon cycle only completes in a single step. To compete for a single carbon cycle, many steps take place that is as follows;

General steps of carbon cycle
Carbon cycle
  1. Carbon enters the atmosphere as CO2: Carbon enters the atmosphere through respiration by living organisms (exhales CO2) via industries that release CO2.
  2. Carbon present in the atmosphere is absorbed by autotrophs (which use CO2 to make their food): Autotrophs utilize the absorbed CO2 via the process known as photosynthesis. Photosynthesis converts atmospheric CO2 into glucose molecules. These glucose molecules are either converted into other substances or used to provide energy to produce other biologically important molecules. CO2 + H2O + energy → (CH2O)n +O2
  1. Animals consume autotrophs: Carbon present in plants moves through the food chain. The animal consumes plants as a carbon source. Animals that eat other animals also get the carbon from their food. Furthermore, most of the carbon from animals is released into the atmosphere through respiration in the form of CO2.
  2. Carbon released to the atmosphere: When these plants and animals die, and upon decomposition by decomposers, carbon is released back into the atmosphere.
  3. Some carbon becomes fossil fuel: In some cases, dead plants and animals remain on the Earth as fossil fuel used for future combustion.

Role of Microorganisms in the Carbon Cycle

Microorganisms play an essential role in the carbon cycle. These microorganisms are involved in various steps of the carbon cycle that include;

  1. Carbon fixation: Some microorganisms, such as cyanobacteria and algae, can fix carbon through photosynthesis. Such organisms capture carbon dioxide from the atmosphere and convert it into organic carbon compounds.
  2. Respiration: Microorganisms release carbon dioxide as a byproduct during the metabolic process in order to obtain energy. Therefore, respiration helps to release carbon dioxide in the atmosphere.
  3. Decomposition: Microorganisms play a vital role in decay. They help to decay dead plants and animals into simpler forms, such as amino acids and sugars, by secreting enzymes.

Methane production: Some microorganisms, such as methanogenic archaea and methanogens under anaerobic conditions, decompose organic matter into methane. Thus, released methane contributes to the carbon cycle by remaining in the soil or being released back into the atmosphere.

Some examples of microorganisms that are involved in the carbon cycle are 

  1. Algae: Cyanobacteria, Diatoms, etc.
  2. Bacteria: Bacillus spp, Pseudomonas spp, etc. 
  3. Fungi: Aspergillus spp, Trichoderma, etc.

Carbon Cycle in Water

The carbon cycle is essential not only for terrestrial life it equally plays a significant role in aquatic life. The carbon cycle occurs in different ways in marine life (i.e., in low-depth rivers and the deepest oceans). In normal aquatic life, the carbon cycle initiates when aquatic plants use carbon dioxide to make food. Animals then eat these plants. When the animal dies, their body decomposes, and carbon is finally returned to the atmosphere.

Whereas, at the deepest level of the ocean, the oceanic carbon cycle occurs differently; carbon intake is more than releasing it to the atmosphere, which is known as a carbon sink. Marine animal utilizes carbon to calcium carbonate to build hard shells. When an organism with a hard shell dies, its body decomposes, but the hard shell accumulates in sea ground, turning into limestone under high pressure. Thus formed limestones, when exposed to the atmosphere, get weathered. As a result, carbon is released back into the atmosphere as carbon dioxide.

Role of Microorganisms

In the oceanic carbon cycle, microorganisms transfer and exchange carbon between the atmosphere, ocean, and marine organisms. The primary role of microorganisms in the oceanic carbon cycle are as follows;

  1. Carbon fixation: Some of the phytoplanktons are microscopic that help to convert atmospheric carbon dioxide to organic compounds through photosynthesis.
  2. Respiration: Ocean microorganisms contribute to the carbon cycle by releasing carbon dioxide into the atmosphere as a byproduct during the metabolic process.
  3. Export of carbon: Microorganisms are also involved in carbon exportation from the surface ocean to the deep sea. 
  4. Decomposition: Majorly bacteria decompose organic matter through an enzymatic process in the ocean. The decay helps release carbon dioxide, nutrients, and other substances that dissolve into the water.

Some examples of microorganisms involved in the oceanic carbon cycle are;

  • Algae: Diatoms, Green algae (e.g., Chlamydomonas and Spirogyra), Coccolithophores, Red Algae (e.g., Corallina and Porphyra)
  • Bacteria: SAR11 Bacteria, Prochlorococcus, Rosebacter, Thaumarchea, Sulphate reducing bacteria, etc.
  • Fungi: Aspergillus spp.


The carbon cycle plays a crucial role in the Earth’s ecosystem that is as follows;

  1. It helps to regulate the Earth’s temperature: The carbon cycle is one of the important ways of maintaining the concentration of carbon dioxide in the atmosphere. Therefore, it significantly impacts reducing global warming and the greenhouse effect on the Earth.
  2. It helps to make food to sustain us: Almost all foods (i.e., plants or animals) are the carbon source.
  3. It helps to provide energy: Carbon is also the fuel source. Therefore, it also plays a significant role in sustaining the global economy.
  4. It facilitates the existence of all life on Earth.


  1. What is the carbon cycle? National Ocean Service. Retrieved on 5th June. Retrieved from:
  2. Archer, D. (2008, May). Carbon cycle: Checking the thermostat. Nature Geoscience, 1, 289-290.

Samikshya Acharya

Hello, I am Samikshya Sharma. I have completed my post-graduate study in medical microbiology at the central department of microbiology, TU, Nepal. I hope my articles are helpful to you. Thank you!!

We love to get your feedback. Share your queries or comments

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Recent Posts