Content Writer-SME
The biological phenomenon through which sulfur is produced through the weathering of rocks and is further passed into the atmosphere is called sulfur cycle. In other words, sulphur cycle is a type of biogeochemical cycle in which movement of sulphur takes place through all the spheres of earth (air, land, water, and living systems. Sulphur is directly released into the atmosphere through various natural processes such as volcanic eruptions, evaporation, etc. It is transformed into sulfates in the presence of air.
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Sulphur Cycle
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The movement of sulphur through the geosphere and biosphere is called the sulphur cycle. It is an important example of biogeochemical cycle in which transfer of sulphur through four reservoirs takes place. Sulphur is an inorganic substance that is mostly preserved under rocks, and minerals in the form of sulfates and sulfides. The extraction of sulphur is done through a process called weathering of rocks. Through this, sulphur is released into the soil.
Animals consume sulphur in the organic form and thus it becomes part of the food chain. Both atmospheric and terrestrial components are involved in the sulphur cycle. The two essential amino acids-methionine and cysteine, which are required for making proteins are derived from sulphur. Thus, the importance of sulphur cycle to the ecosystems are very diverse and valuable which includes:
- Improving water quality,
- Improving the productivity of ecosystems,
- Initiates other biogeochemical cycle.
Sulfur Cycle Diagram
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The entire sulphur cycle is completed in four important steps. The diagram of sulfur cycle is given below:
Steps of Sulphur Cycle
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Sulphur is first released through weathering of rocks and then enters the food chain and thus begins the process of sulpur cycle. The cycle is preceded by a series of events that are discussed below in detail:
Decomposition
The first step of the sulfur cycle is decomposition of organic matter through which sulfates are reduced to hydrogen sulfides by the action of bacteria.
- The amino acids, building blocks of protein which also contain sulfur, are released during this process.
- The bacteria involved in the degradation of organic matter is Desulfotomaculum bacteria.
Oxidation of Hydrogen Sulfide into Elemental Sulfur
This is a microbial process in which the hydrogen sulfide is oxidized into elemental sulfur by the action of photosynthetic bacteria. The family member of Chlorobiaceae and Chromatiaceae catalyzes the overall process of oxidation.
Hydrogen Sulfide + Oxygen → + Sulfur (S0) + Water
Oxidation of Elemental Sulfur
The elemental form of sulfur which is present in the soil can not be utilized by plants directly. Plants consume sulfur in the form of sulfates which are converted by the chemolithotrophic bacteria (Thiobacilus) present in the soil.
Sulphur (S0) + Oxygen → + Sulfate
Sulfate Reduction
Sulfate reduction is the last step of the sulfur cycle in which sulfates present in the soil are once again converted into hydrogen sulfide. This process is catalyzed by Desulfovibrio desulfuricans. Sulfate reduction is a two step process that involves:
- Reduction of sulfate into sulphite.
- Final reduction of sulphite into hydrogen sulfide.
Sulfate + Organic Matter → Hydrogen Sulfide + Carbon dioxide + Water
Importance of Sulfur Cycle
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Sulfur is an important component present in the atmosphere that plays an important role in various biological processes. The significance of sulfur cycle are as follows:
- Maintenance of sulfur is very important to keep the ecosystem healthy which can only be done through the sulfur cycle.
- Amino acids are building blocks of protein that play an important role in energy production.
- Methionine, and cysteine are the two examples of sulfur-containing amino acids that take part in various cellular processes.
- The maintenance of sulfur in the environment across the reservoirs of earth such as soil, water bodies, living organisms, etc. can only be done through the sulfur cycle.
- Sulfur enters the food chain of plants in the form of sulfate ions which are converted by the chemolithotrophic bacteria present in the soil.
- Animals consume plants to get enough sulfur for the normal functioning of their bodies.
Impact of Human Activities on Sulphur Cycle
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A naturally occurring biogeochemical cycle, the sulfur cycle involves the movement of sulfur across the hydrosphere, biosphere, lithosphere, and atmosphere. The sulfur cycle is greatly impacted by human activity, which also alters and disturbs the natural ecosystem. Here are some examples below of how idolatrous human activity affects the Sulphur cycle:
- Burning of Fossil Fuels: The remains of plants and animals that have been extinct for millions of years are the source of fossil fuels, also known as hydrocarbons. Carbon dioxide (CO₂) and methane, two main causes of global warming and climate change, are released when fossil fuels such as coal, oil and natural gas are burned.
- Industrial & Agricultural Activities: Sulphur added to soil by sulphate-based fertilizers (such as ammonium sulphate) can increase sulfur concentrations in runoff. Overuse may upset the microbial communities in the soil and alter the nutrient profile of plants.
- Improper Disposal of Organic Wastes: Improper disposal of organic waste causes anaerobic decomposition in landfills, which releases the poisonous and corrosive gas hydrogen sulfide (H₂S).
- Improper Disposal of Inorganic Wastes:Industrial waste that contains sulfur compounds such as sulphate or sulphide can leach into soil and water bodies if disposed off improperly.
- Mining Activities: The natural sulphur cycle can be seriously affected by mining activities, especially those that involve the extraction and processing of sulphur-containing minerals.
To maintain the balance of the sulfur cycle and preserve the ecosystem, improper waste disposal must be addressed.
Things to Remember
- Sulfur is an essential nutrient that plays a significant role in protein synthesis.
- Desulfotomaculum bacteria are involved in the process of degradation of organic matter.
- Sulfur is released into the atmosphere directly through natural processes like volcanic eruptions, breakdown of organic matter, etc.
- Elemental sulfur is not suitable for plant consumption, hence it is converted into sulfates by Thiobacillus bacteria.
Sample Questions
Ques. What are sulfur cycle reservoirs present in the environment? (2 marks)
Ans. The place where the movement and storage of sulfur takes place is called sulfur cycle reservoir. The important sulfur cycle reservoirs are as follows:
- Oceans
- Rock and minerals
- Soil
- Living organisms
- Volcanic deposits
- Fossil fuels
Ques. How have human activities affected the Sulphur cycle process? Explain. (3 marks)
Ans. Human activities have greatly impacted the sulfur cycle. Significant amounts of hydrogen sulfide gas are released into the atmosphere when large amounts of fossil fuels, particularly coal, are burnt.
The phenomenon known as acid rain occurs when rain falls across this gas. Acid rain disturbs the natural environment by decreasing the pH of lakes, which in turn harm humans and their surrounding environment.
Ques. What are the forms of Sulphur found in the Biosphere? (2 marks)
Ans. In the biosphere, sulfur may be found in three forms:
- Elemental Sulphur - This element is found in sulfur deposits and sulfide ores.
- Inorganic Sulfur – This is accessible as sulfate in aerobic soils and as sulfide in anaerobic soils.
- Organic Sulphur - Found in amino acids and plants/animals.
Ques. What is the importance of soil in the sulfur cycle? (1 mark)
Ans. Sulfur cycle is an example of a biogeochemical cycle in which soil is considered as one of the major sulfur cycle reservoirs. Organic as well as inorganic forms of sulfur are well stored in soil.
Ques. What is the importance of bacteria in the sulfur cycle? (1 mark)
Ans. Bacteria play an important role in the conversion of sulfur compounds into the more usable form of sulfur. Therefore, it is utilized in every step of the sulfur cycle.
Ques. What are the risk factors for excessive sulfur in the environment? (2 marks)
Ans. Excess of anything is dangerous and causes various health problems. Likewise, the presence of excess sulfur in the environment may cause respiratory problems such as bronchitis, asthma, nasal congestion, etc. They may also cause other severe problems like cardiovascular diseases and inflammation.
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