What is the Sulphur Cycle?
Sulphur is a yellow, brittle, non-metallic element found in rocks, minerals, and living organisms. It is an essential component of many proteins, making it vital for all forms of life. The sulphur cycle is the process through which sulphur moves between the environment (air, water, soil) and living organisms. Understanding this cycle helps us appreciate how sulphur is stored, transformed, and utilised by various life forms.
Importance of Sulphur
Protein Formation: Sulphur is present in amino acids such as methionine, cystine, and cysteine, which are building blocks of proteins.
Plant Growth: Plants absorb sulphates from soil and convert them into essential proteins and enzymes.
Environmental Impact: Sulphur compounds released from industrial activities and volcanic eruptions can affect air quality and contribute to acid rain.
Major Sources of Sulphur
Underground Rocks and Minerals: Sulphur is primarily stored in rocks, minerals, and salt domes. Weathering and geological activities release sulphur into the soil and water.
Volcanic Eruptions: Volcanoes release large amounts of sulphur dioxide (SO₂) into the atmosphere.
Fossil Fuels: Burning coal, oil, and natural gas emits sulphur oxides, which can influence the environment.
Biological Decomposition: When plants and animals die, their decomposition releases sulphur back into the soil or air.
Sulphur Cycle Steps
The sulphur cycle steps can be broadly divided into the following phases:
Weathering of Rocks
Sulphur is released into the soil and water when rocks containing sulphur undergo weathering.
This sulphur can then be converted into sulphates (SO₄²⁻) in the presence of air.
Uptake by Plants and Microbes
Plants absorb these sulphates directly from the soil through their roots.
Microorganisms also utilise sulphates to form organic sulphur compounds.
Movement through the Food Chain
Animals obtain sulphur by feeding on plants or other animals.
The sulphur present in these organisms is used to synthesise amino acids and proteins.
Decomposition
When organisms die, decomposers break down their bodies, releasing some sulphur back into the soil as sulphates.
Some sulphur also enters the tissues of microbes during decomposition.
Release into the Atmosphere
Activities like volcanic eruptions, burning of fossil fuels, and the breakdown of organic matter (e.g., in swamps) release sulphur-containing compounds, such as hydrogen sulphide (H₂S) and sulphur dioxide (SO₂), into the atmosphere.
These compounds can return to the Earth’s surface through precipitation (rainfall).
Detailed Biochemical Transformations
Decomposition of Organic Compounds
Proteins in dead organisms are broken down, releasing amino acids that contain sulphur.
Certain bacteria (e.g., Desulfotomaculum) reduce sulphates to hydrogen sulphide (H₂S) in the process.
Oxidation of Hydrogen Sulphide (H₂S) to Elemental Sulphur (S)
Hydrogen sulphide is oxidised to elemental sulphur by specific photosynthetic bacteria (e.g., from Chlorobiaceae and Chromatiaceae families).
Oxidation of Elemental Sulphur to Sulphates (SO₄²⁻)
Elemental sulphur in the soil is further converted into sulphates by chemolithotrophic bacteria.
Plants can then take up these sulphates.
Reduction of Sulphates to Hydrogen Sulphide
Bacteria such as Desulfovibrio desulfuricans reduce sulphates to hydrogen sulphide in two steps:
Sulphates are first converted to sulphites, using ATP.
Sulphites are then reduced to hydrogen sulphide.
Mnemonic to Remember the Main Stages
You can use a short mnemonic to recall the core sulphur cycle steps:
“We Often Observe Really Simple Transformations”
W – Weathering of rocks
O – Oxidation of hydrogen sulphide
O – Oxidation of elemental sulphur
R – Reduction of sulphates
S – Storage in rocks and minerals
T – Transfer through the food chain
This mnemonic combines both the major geological and biological processes in an easy-to-remember format.
Quick Quiz on the Sulphur Cycle
1. Which bacteria are involved in reducing sulphates to hydrogen sulphide?
Answer: Desulfovibrio desulfuricans.
2. Name one natural source and one human-made source of sulphur in the atmosphere.
Answer: Natural source: Volcanic eruptions. Human-made source: Burning of fossil fuels.
3. Which form of sulphur can plants directly absorb?
Answer: Plants can directly absorb sulphates (SO₄²⁻).
4. Which essential organic molecules contain sulphur?
Answer: Amino acids such as methionine, cystine, and cysteine.
5. What is the role of chemolithotrophic bacteria in the sulphur cycle?
Answer: They convert elemental sulphur to sulphates.
Related Topics
FAQs on Sulphur Cycle: Explanation for Students
1. How does sulphur affect the environment when released in large quantities?
Excessive sulphur release (especially in the form of sulphur dioxide) can contribute to air pollution and acid rain, which can harm plants, aquatic systems, and buildings.
2. Is sulphur always harmful to humans?
No, sulphur is essential for human health in the right amounts as it is part of certain amino acids. However, high levels of sulphur compounds like sulphur dioxide can irritate the respiratory system.
3. Can plants use elemental sulphur directly?
No, plants primarily use sulphates (SO₄²⁻). Elemental sulphur must be oxidised to sulphates before being absorbed by plant roots.
4. Which processes add sulphur to the soil?
Weathering of rocks, decomposition of dead organisms, and certain microbial activities add sulphur to the soil.
5. Why is sulphur important in amino acids?
Sulphur helps in maintaining the structure and function of proteins, influencing protein stability and enzyme activity.
