Sulfur cycle: Definition and description

Sulfur cycle


Plants, animals, and various types of microorganisms are made up of different types of organic matter. These organic matters are necessary for survival the most important of these are hydrogen, oxygen, and carbon. Other essential elements include nitrogen, phosphorus, sulfur, calcium, magnesium, iron, potassium, etc. There are also various salts and basic elements that play a vital role in the growth of the organism. The topic of the discussion below is the sulfur cycle.

Of all these elements, 6 elements- carbon, hydrogen, nitrogen, sulfur, phosphorus, and oxygen are required in large quantities and these 6 elements make up about 95% of the biomass of plants, animals, and microorganisms. These elements are circulating in the organism and its environment as a result, a cycle is created known as the biogeochemical cycle.

There are two types of biogeochemical cycles observed in nature such as the gaseous cycle and the sedimentary cycle. Carbon, oxygen, and nitrogen cycles are gaseous cycles, whereas sulfur and phosphorus cycles are sedimentary cycles (1) & (3).

What is Sulfur Cycle? 

Sulfur plays an important role in the ecosystem and in the animal and plant kingdom. It is the third most element in the human body as all the proteins and vitamins are formed by this sulfur. Sulfur flows in cycles from the organic environment to the physical environment and again from the physical environment to the organic environment to form the sulfur cycle. There are four steps involved in the process of the sulfur cycle these are as follows

  1. Transformation of sulfur from organic state to inorganic state.
  2. Oxidation of sulfide compounds and sulfur elements.
  3. Reduction sulfate to sulfide.
  4. Accumulation of sulfide in organic form (2) & (4).

Definition of the sulfur cycle

The controlled cyclic process by which natural sulfur circulates from the environment to the organism and from the organism to the environment and maintains the balance of Sulfur in the environment is called the Sulfur cycle (5).

Sources of Sulfur

Sulfur is an essential element. The sea is the main source of Sulfur in this one of the sedimentary cycles. It is present in the form of different compounds in physical and biological environments. There are different sources of sulfur that are

  • It is found in the atmosphere in the form of gaseous hydrogen sulfide (H₂S) and sulfur dioxide (SO₂). Volcanic eruptions, decomposition of organic matter, combustion of fossil fuels, etc. are sources of sulfur in the atmosphere.
  • Sulfur is found in the form of sulfate in the lithosphere and hydrosphere. Acid rain, direct fall from the atmosphere, rock weathering, and geothermal vents are the four sources of sulfur in the lithosphere and hydrosphere.
  • And it is also present in organic molecules to form proteins in the living environment. Animal and plant-based proteins are the sources of sulfur in the human body (7) & (8).

Sulfur cycle

The Sulphur cycle is an important biogeochemical cycle. This cycle is completed by the inflow of sulfur from the physical environment to the living environment and the outflow of sulfur from the living environment to the physical environment. There are two phases in the sulfur cycle

Phase 1: Sulfur enters the organism from the physical environment or atmosphere

These phases are divided into two parts.

1. Atmosphere phase

  • Atmospheric sulfur is in the form of gaseous hydrogen sulfide (H₂S) and sulfur dioxide (SO₂).
  • More toxic sulfur compounds like sulfurous acid, sulfuric acid (H₂SO₄), and sulfate molecules are formed in further reactions.
  • In addition to sulfur dioxide (SO₂), the atmosphere also contains a small amount of sulfur trioxide (SO₃).
  • The main sources of sulfur in the atmosphere are the combustion of fossil fuels, volcanic eruptions, and the decomposition of organic matter.
  • Combustion of excess fossil fuels such as wood, coal, etc. produces large amounts of sulfur dioxide (SO₂) and hydrogen sulfide (H₂S), which is released into the atmosphere by the air (6).

This sulfur present in the atmosphere is mixed in various ways on the surface.

  • Sulfur dioxide (SO₂) present in the atmosphere reacts with the water present in the atmosphere to produce sulfuric acid (H₂SO₄). This sulfuric acid (H₂SO₄) falls as acid rain on the surface or biosphere.
  • Besides acid rains, sulfur is often mixed with the earth’s surface or biosphere by rock-weathering (3) & (9).

2. Biosphere phase

  • Green plants absorb the sulfur that dissolved in water and soil biosphere.
  • This sulfur is used to make protein molecules which is an important element in the body of all plants and animals.
  • It is needed for the formation of root nodules of leguminous plants.
  • Sulfur is also used in plant nitrogen assimilation. The sulfur is oxidized to sulfate by the chemoautotrophic bacteria.
  • This sulfate is absorbed by plants from water and soil. During the life cycle, the food chain in the environment helps the sulfur stored in the plants to spread to different animals according to the food. Thus sulfur enters the organism from the physical environment or atmosphere (7).

Phase 2: Addition of sulfur to the environment or atmosphere

  • When plants and animals are degraded, the sulfur in the organism is converted to H₂S and released.
  • This decomposition process is done by bacteria. Aspergillus, Neurospora, etc. unicellular fungi in the aerobic environment, and Escherichia and Proteus bacteria in the anaerobic environment participate in the decomposition.
  • Sulfate is decomposed into sulfur or sulfide in an anaerobic environment. In this case, bacteria such as Desulfovibrio, Escherichia, Azotobacter, and bacteria are active. Hydrogen sulfide (H₂S) is oxidized by sulfur-oxidizing bacteria and becomes sulfur.
  • The Beggiatoa bacteria also oxidize H₂S and convert it into sulfur.
  • Thiobacillus bacteria present in the soil oxidizes sulfide to sulfur and even in the combustion of fossil fuels very little sulfur is released into the environment.
  • Sulfur is deposited in the sedimentary rock in the form of iron sulfide (FeS). This deposited sulfur is broken down by some microorganisms and added to the environment.
  • In this way, sulfur is mixed with the physical environment or atmosphere from the biosphere or living environment.
  • Thus, sulfur is continuously circulating from the atmosphere to the biosphere and again from the biosphere to the atmosphere in a cyclic way and forms the sulfur cycle. In this way, the supply of sulfur elements to the environment remains intact (3) & (6).

Impact of Sulfur cycle

Sulfur is a very significant element for the environment and organisms. The sulfur cycle is a moving natural process. The importance of the sulfur cycle is immense.

The positive impact of the sulfur cycle

  • Sulfur is a necessary element of all organisms. It is a special component of many amino acids in the body of plants and animals.
  • It is a key component of various enzymes and vitamin B-complex that provide nutrients to the body.
  • This element is a major component of skeletal minerals and body fluids.
  • Sulfur is needed for the formation of root nodules of leguminous plants (5) & (7).

So above all this reason sulfur cycle is very important for all the organisms and the environment. This cycle maintains the balance of sulfur between the organism and the environment.

The negative impact of the sulfur cycle

Along with the positive impact of the sulfur cycle on the environment. Some of the negative impacts of this cycle are also present.

  • One of the negative sides of this cycle is acid rain. Atmospheric sulfur is mixed due to volcanic eruptions, combustion of fossil fuels at factories etc. This sulfur reacts with the water in the atmosphere to form sulfuric acid. This sulfuric acid falls to the surface in the form of acid rain with precipitation. Acid rain damages forests decrease the pH of soils, kills insects and aquatic life, as well human health.
  • Sulfur dioxide (SO₂) affects the ozone layer. Air pollution is caused by sulfur dioxide (SO₂). Air pollution affects all the environments and organisms.
  • The sulfur cycle has a significant impact on climate change. The sulfur dioxide (SO₂) generated by volcanic eruptions increases the oxidizing capacity of the atmosphere. As a result, global warming occurs.
  • There is a negative impact of hydrogen sulfide (H₂S) on plants. Hydrogen sulfide (H₂S) levels found in pollution zones contribute significantly to plant growth. It disrupts plant growth and has a negative effect on survival.
  • In some ecosystems, large amounts of H₂S are produced through raid protein analysis. For example, in the Black sea, at a depth of about 200 m, the thickness of H₂S and H₂SO₄ is very high. And no other organism can survive there except sulfur bacteria. As a result, the environment gradually becomes polluted (3).

The sulfur cycle is carried out by the sulfur element in various forms from the atmosphere to the surface soil and water, from soil to plants, from plants to animals, and from the soil, water, plants, and animals to the atmosphere again. This natural cycle of sulfur never stops, it continues and maintains the balance of the environment (3) & (9).

Written by: Manisha Bharati

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