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Sulphur Cycle Definition

A biogeochemical cycle, the sulphur cycle, identifies the various steps and processes that facilitate the movement of sulphur across the various reservoirs. These reservoirs include the lithosphere, biosphere, and atmosphere. 

Sulphur is an important macromolecule involved in a number of biological processes. It is a crucial component found in vitamins and proteins. The amino acids that constitute proteins consist of sulphur atoms. For the enzymes and proteins found in plants, sulphur is an essential component contributing to their functioning. Animas also indirectly absorb sulphur upon consumption of these plants and are able to maintain and regulate their bodily processes. 

On this page, we will discuss the sulphur cycle in detail with the aid of the sulphur cycle diagram explaining the process of the sulphur cycle. 


Sulphur Cycle Diagram

To provide you with a clear insight into the sulphur cycle steps, we have provided a detailed sulphur cycle diagram below. The sulphur cycle diagram will help you to visualize the sulphur cycle steps and build your understanding of the same.


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Sulphur Cycle Process

The sulfur cycle consists of various terrestrial and atmospheric processes along with oxidation and reduction reactions. The sulphur cycle steps majorly involve the atmosphere, the biosphere, uptake by living organisms, and release. The sulphur cycle explanation has been provided below for each of the steps involved:

Atmosphere: Sulphur dioxide is the primary source of sulphur in the atmosphere and is mostly found due to human activities such as the combustion of fossil fuels. In addition to anthropogenic causes, volcanic activity is also responsible for increasing the concentration of sulphur dioxide in the atmosphere. Another source of sulphur in the atmosphere is the hydrogen sulphide gas mostly released by the action of microorganisms on dead and decaying organic matter. In terrestrial as well as aquatic habitats, carry out the process of decomposition inorganically which results in hydrogen sulphide formation. This hydrogen sulphide later oxides to form sulphur dioxide in the air. 

Biosphere: The sulphur then enters the biosphere either through the weathering of rocks or through the atmosphere. Sulphur present in the atmosphere aids in the formation of clouds by increasing the count of cloud droplets as well as increasing the size of the droplet. Sulphur thus dissolves in the rainwater forming weak sulphuric acid droplets and reaches the biosphere. Aerosols or sulphur particles also fall out into the biosphere from the atmosphere. Additionally, in the process of pedogenesis, chemical weathering causes sulphur to move to soil and water from the rocks. Some sulphur gets converted into sulphate and is released into the air in the process of weathering as well. So, we can see that sulphur in either of the ways involved is eventually transported to soil and then water bodies. 

Uptake by Living Organisms: As the sulphur gets transported to the soil and the oceans, it becomes available for consumption by microorganisms, animals, and plants. Sulphur is taken up by the green sulfur bacteria that utilize it as a form of energy.  Plants absorb sulphur from the soil by means of their roots. Several microorganisms present in the soil also contribute to making sulphur for absorption by plants. Animals including human beings end up consuming sulphur indirectly as they consume plants. Sulphur consumed is utilized for the formation of biomolecules such as nucleotides and proteins in living organisms. In the terrestrial biosphere, sulphur gets utilized by chemoautotrophic organisms for the production of organic compounds in sulphate form. 

Release: This is the final step in the sulphur cycle process. Sulphur which is present in the biosphere travels through the food chain by means of consumers feeding on producers and then ultimately reaching the decomposers. This sulphur through the process of decomposition gets converted into sulphides from the sulphate form and is released back into the atmosphere. The organic forms of sulphur are converted into inorganic forms such as H2S and further into elemental sulphur by the action of the sulphate reducing bacteria. The sulphur that does not become a part of the food chain layers on in the rocks found deep in the marine and the terrestrial habitats. Such sulphur is usually found in the combined FeS form. The lithospheric sulphur is also returned to the atmosphere by means of natural volcanic activity. 


Importance of Sulfur Cycle

The sulphur cycle is of paramount importance as it provides an insight into the understanding of how the different biomolecules functions. The sulphur cycle provides a balance in the sulphur concentration among the various reservoirs as which is necessary for the ecological balance on earth and its continuity as a hospitable place. The availability of several other elements is also affected by the sulphur cycle as sulphur is often found in a combined state with other elements such as iron, phosphorus, nitrogen, etc., in nature. 

The terrestrial part of the sulphur cycle comprises a series of biological processes that play a significant role in increasing the sulphur available for microbial life and plants. The chemoautotrophic sulphur bacteria in the food chain convert the chemical energy into several other forms which lead to a biomass increase on the planet. 

The physiology of different microorganisms involved in the process of sulphur conversion can be understood through the sulphur cycle. Sulphur is a major source of energy in certain pathogenic bacteria such as Mycobacterium tuberculosis, and studying the metabolic pathways of sulphur in such bacteria has various crucial medical implications. The sulphur mineralization process of the sulphur cycle is a natural system of waste disposal as the sulphur compounds are regenerated in the same. 

FAQs (Frequently Asked Questions)

1. What are Some of the Anthropogenic Impacts On the Sulphur Cycle?

Ans. Some of the impacts caused by human activities on the sulphur cycle include: 

  1. The concentration of sulphur is made disproportionate owing to human activities such as excessive burning of fossil fuels as it leads sulphur dioxide concentrations to rise in the atmosphere. 

  2. The industrial process and refining of petroleum products also cause the release of sulphuric acid to the atmosphere to increase.

  3. An increase in sulphur dioxide concentrations in the clouds leads to acid rain on Earth, which gives rise to a series of other impacts including global warming as the sulphur cycle gets adversely affected.  

2. What are the Main Reservoirs Involved in the Sulfur Cycle?

Ans. The major reservoirs involved in the sulphur cycle are the biosphere and the atmosphere.

In the atmosphere, sulphur is mainly found in the form of sulphur dioxide and hydrogen sulphide. Natural processes as chemical weathering, human activities, and microbial activity are the major sources of sulphur found in the atmosphere. Sulphur in the atmosphere is transported to the biosphere either as fallout or precipitation. At the end of the sulphur cycle, sulphur from the biosphere is transported back into the atmosphere. 


The biosphere comprises living organisms, of which microorganisms play a very important role in the sulphur cycle. In the biosphere, sulphur is available in organic as well as inorganic forms. The organic form of sulphur travels through the food chain whereas the inorganic form is present in the soil. The biosphere, at times, can also act as a sulphur sink as any living organisms take up sulphur for their growth process and development. 

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