SULFUR CYCLE: STAGES AND IMPORTANCE - BIOLOGY - 2025

The sulfur cycle is the set of processes by which sulfur is transported through nature in various molecules. Sulfur travels through the air, soil, water, and living things. This biogeochemical cycle includes the mineralization of organic sulfur to sulfur, its oxidation to sulfate and its reduction to sulfur.

Sulfur is taken up by microbes and forms different organic compounds. Sulfur is a very abundant element in the Universe; It is considered a non-metal, its color is yellow and it has no odor. Sulfur is released into the atmosphere by burning fossil fuels, such as coal.

In the atmosphere, sulfur is in the form of sulfur dioxide (SO2) and can enter it in three ways: from the decomposition of organic molecules, from volcanic activity and geothermal vents, and from the burning of fossil fuels. By humans.

Sulfur atoms are an important part of the structure of proteins. Sulfur is found in the amino acid cysteine ​​and is involved in the formation of a type of bond called a disulfide bridge. These bonds are essential in determining the three-dimensional structure of proteins.

Stages

The sulfur cycle involves the movement of this element in many directions through the atmosphere, the hydrosphere, the lithosphere and the biosphere. In the lithosphere, the erosion processes of the rocks occur that release the stored sulfur.

Sulfur undergoes a series of chemical transformations as it is transported through different media. Throughout its journey, sulfur goes through four fundamental chemical stages:

- Mineralization of organic sulfur to inorganic form, such as hydrogen sulfide, elemental sulfur and other sulfur-based minerals.

- Oxidation of hydrogen sulfide, elemental sulfur and sulfate-related minerals.

- Reduction of sulfate to sulfur.

- Microbial immobilization of sulfur compounds and subsequent incorporation into the organic form of sulfur.

Sulfur flow

Despite its complexity, the flow of sulfur can be summarized in three large groups:

The sulfur that is found forming compounds

In this group are atmospheric sulfur, organic sulfur, inorganic sulfur (minerals), reduced sulfur and sulfur that forms sulfates.

Sulfate is absorbed by plants and microorganisms, which incorporate them into their organic molecules. Animals then consume these organic forms through the food they eat, moving the sulfur along the food chain.

Sulfur entering the soil

Sulfur is incorporated into the soil in different ways; for example, by atmospheric deposition, by using fertilizers of animal origin, by plant residues, by the use of mineral fertilizers and by the erosion of rocks.

The sulfur that is coming out of the ground

Sulfur is removed from the soil in various ways. For example, when plants absorb sulfates through their roots, when crops are harvested, and when some reduced compounds are volatilized.

Another part of the sulfur in the soil is lost through seepage, runoff and erosion. Volcanoes and some gases produced by organic decomposition are another source of sulfur that is transferred directly to the atmosphere.

However, most of the sulfur on Earth is stored in rocks, minerals, and sulfate salts buried deep in ocean sediments.

Importance

Main component in chemical compounds

Sulfur is an important nutrient for organisms because it is a fundamental component of the amino acids cysteine ​​and methionine, as well as other biochemical compounds.

Plants meet their nutritional needs for sulfur by assimilating mineral compounds from the environment.

Associated with the productivity of plants

In certain situations, particularly in intensive agriculture, the availability of biologically useful forms of sulfur can be a limiting factor for plant productivity; consequently, the application of sulfate-based fertilizers is necessary.

Recognition of the importance of sulfate for plant growth and vigor, as well as the nutritional importance of sulfur for human and animal diets, has led to a greater emphasis on research on sulfate absorption, transport, and assimilation processes..

Necessary to build proteins

After entering the plant, sulfate is the main form of sulfur transported and stored. Sulfur is necessary for the construction of proteins, enzymes and vitamins, it is also a key ingredient in the formation of chlorophyll.

Crops that are deficient in sulfur typically show growth limitations. Thus, plants with a lack of sulfur appear thinner and smaller, their younger leaves turn yellow and the number of seeds is reduced.

Commercial uses

Apart from the production of fertilizers, sulfur has other commercial uses, for example: in gunpowder, matches, insecticides and fungicides.

Additionally, sulfur is involved in the production of fossil fuels due to its ability to act as an oxidizing or reducing agent.

Associated with environmental damage

Sulfur compounds can also be associated with significant environmental damage, such as sulfur dioxide that damages vegetation, or acid drains associated with sulfides that degrade ecosystems.

Human impact on the sulfur cycle

Human activities have played an important role in altering the balance of the global sulfur cycle. Burning large amounts of fossil fuels, especially coal, releases large amounts of hydrogen sulfide gases into the atmosphere.

When this gas is traversed by rain, acid rain is produced, which is a corrosive precipitation caused by rainwater that falls to the ground through sulfur dioxide, turning it into weak sulfuric acid that causes damage to aquatic ecosystems.

Acid rain damages the environment by reducing the pH of lakes, which kills much of the fauna that lives there. It also affects unnatural man-made structures, such as the chemical degradation of buildings and statues.

Many marble monuments, such as the Lincoln Memorial in Washington, DC, have suffered significant damage from acid rain over the years.

These examples show the far-reaching effects of human activities on our environment and the challenges that remain for our future.

References

1. Butcher, S., Charlson, R., Orians, G. & Wolfe, G. (1992). Global biogeochemical cycles. Academic Press.
2. Cunningham, W. & Cunningham, M. (2009). Environmental Science: A Global Concern (11th ed.). McGraw-Hill.
3. Jackson, A. & Jackson, J. (1996). Environmental Science: The Natural Environment and Human Impact.
4. Loka Bharathi, PA (1987). Sulfur Cycle. Global Ecology, (1899), 3424–3431.
5. Meyer, B. (2013). Sulfur, Energy and Environment.
6. O'Neill, P. (1998). Environmental Chamistry (3rd ed.). CRC Press.