Sulphur Cycle

One of the most prevalent elements on the planet is sulphur. It’s a non-metal that’s yellow, brittle, tasteless, and odourless. Sulphur can be found in a variety of proteins. Sulphur-containing amino acids like methionine, cystine, and cysteine are directly absorbed by plants. 

Occurrence of sulphur:

The burning of fossil fuels, volcanic activity, and the decomposition of organic molecules all release sulphur into the atmosphere. Sulphur is trapped underground in rocks and minerals on land. Precipitation, rock weathering, and geothermal vents all release it. 

Sulphur cycle:

Sulphur flows between rocks, waterways, and living systems in the sulphur cycle, which is a biogeochemical cycle. The sulphur cycle is a sedimentary cycle, but the gaseous components are hydrogen sulphide (H2S) and sulphur dioxide (SO2). Sulphur is trapped in organic (coal, oil, peat) and inorganic (water) forms in the soil and sediments (pyrite, sulphur rock). Sulphates, sulphides, and organic sulphur are the forms in which they are stored. 

Process of sulphur cycle:

The sulphur cycle is explained as follows: 

1. The weathering of rocks releases the sulphur.

2. When sulphur comes into contact with air, it forms sulphates.

3. Plants and microorganisms absorb sulphates and convert them to organic forms.

4. Sulphur goes up the food chain as the organic form of sulphur is absorbed by animals through their food.

5. Some of the sulphur is released by decomposition when the animals die, while others enter the tissues of microorganisms.

6. Sulphur is released directly into the atmosphere through a variety of natural sources, including volcanic eruptions, water evaporation, and the breakdown of organic waste in swamps. Rainfall brings this sulphur to the surface of the soil. 

Steps of sulphur cycle:

The major steps in the sulphur cycle are as follows: 

Decomposition of organic compounds:

Sulphur-containing amino acids are released during protein breakdown. Desulfotomaculum bacteria convert sulphates to hydrogen sulphide (H2S). 

Oxidation of hydrogen sulphide to element sulphur:

Elemental sulphur is formed when hydrogen sulphide is oxidised. The oxidation process is started by photosynthetic bacteria from the Chlorobiaceae and Chromatiaceae families. 

Oxidation of elemental sulphur:

The plants are unable to use the element sulphur that is present in the soil. Chemolithotrophic bacteria convert it to sulphates as a result. 

Reduction of sulphates:

Desulfovibrio desulfuricans converts sulphates to hydrogen sulphide. This is accomplished in two steps: 

1. To begin, ATP is used to convert sulphates to sulphites.

2. Second, sulphite is reduced to hydrogen sulphide. 

Economic importance of sulphur:

1. Each year, millions of tonnes of sulphur are manufactured, largely for the production of sulfuric acid, a widely used industrial chemical.

2. Because of its potential to operate as an oxidising or reducing agent, sulphur is directly involved in the formation of fossil fuels and the majority of metal deposits.

3. One of the limiting criteria for the concentration of precious metals in the source is the presence or absence of sulphur. 

Impact of human activities on sulphur cycle:

1. The global sulphur cycle is being influenced significantly by human activities. The amount of sulphur in the atmosphere and ocean has increased dramatically as a result of the burning of coal, natural gas, and fossil fuels. The sedimentary rock sink has also been depleted.

2. Without the human factor, sulphur would be trapped in rocks for millions of years until it was raised by tectonic activities, after which it would be liberated through erosion and weathering. However, it is now being dug out and burned at an excessively high rate. 

3. There has been a 30-fold rise in sulphate deposition in the world’s worst polluted locations. Mining coal and extracting petroleum at such a high rate has resulted in a 100-fold increase in global sulphur flux.

Conclusion:

Sulphur is increasingly employed as a component of fertilisers since it is a critical nutrient for plants. Sulphur deficiency has recently become common in many European countries. As a result of actions taken to reduce acid rain, atmospheric sulphur inputs continue to decline. As a result, unless sulphur fertilisers are used, the sulphur input deficit is likely to worsen.