Sulfur Cycle- Definition, Steps, Examples, Significance, Human Impacts

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Sulfur Cycle Definition

The sulfur cycle is a biogeochemical cycle consisting of various processes that together enable the movement of sulfur through different reservoirs like the atmosphere, biosphere, and lithosphere.

  • Sulfur is an important mineral in living organisms that is found in biomolecules like proteins.
  • Besides, sulfur is also an essential mineral found in rocks and soil as the lithosphere is the main reservoir of sulfur compounds.
  • The sulfur cycle is termed a biogeochemical cycle as the element in different oxidation states moves through various ecosystems on Earth, affecting both the biological and geological process.
  • As a result of the chemical stricture of sulfur, it remains stable in multiple oxidation states which enables the formation of a diverse group of organic and inorganic compounds.
  • Sulfur is the tenth most abundant element on the Earth’s surface, which is found in different forms in the rocks and soil.
  • The sulfur cycle consists of both the terrestrial and atmospheric processes; however, most of it remains in the lithosphere.
  • All of the reserves of sulfur are tied up in the lithosphere, from which they are slowly released by weathering processes.
  • Sulfur compounds formed during the cycle can act as oxidants or reductants depending on the oxidation state of the compounds.
  • Microorganisms play an important role in the sulfur cycle as these organisms have specialized enzyme systems and mechanisms to form different sulfur compounds.
  • Sulfur found in nature might even exist in a combined form with other elements like phosphorus and nitrogen.
  • The concentration of sulfur on Earth might be affected by human activities like the addition of sulfur-containing fertilizers to the soil.
  • Unlike inorganic phosphorus compounds, sulfur molecules are usually more soluble in water which increases their availability to plants and microorganisms found in soil.

Sulfur Cycle

Figure: Sulfur Cycle. Image Source: OpenStax.

Sulfur Cycle Steps

The sulfur cycle consists of various terrestrial and atmospheric processes consisting of different oxidation and reduction reactions. The following are the steps involved in the sulfur cycle:

1. In the atmosphere

  • Sulfur in the atmosphere is mostly present in the form of SO2. Most of the SO2 in the atmosphere arises from human activities like fossil fuel combustion.
  • Natural processes like volcanic eruptions also play an essential role in increasing the SO2 concentration in the atmosphere.
  • SO2 is followed by H2S, which represents another important gas that is present in the atmosphere.
  • The primary source of H2S in the atmosphere is the gas released by microbial action on dead decaying living organisms.
  • The microbes present both in the terrestrial and the aquatic habitats decompose organic and inorganic forms anaerobically.
  • The anaerobic decomposition results in the release of H2S, which then oxidizes in the air to form SO2.
  • The sulfate-reducing microorganisms involved in the decomposition process are usually anaerobic where they produce H2S from the oxidized form of sulfur.
  • Some of the sulfate-reducing bacteria include Desulfovibrio desulfuricans, Desulfovibrio vulgaris, Thermodesulfovibrio yellowstonii, Desulfotomaculum nigrificans, Desulfobacula toluolica, etc.

2. In the Biosphere

  • The sulfur in the biosphere enters the biosphere in one of two ways; from the atmosphere, rock weathering.
  • In either of the two ways, the sulfur eventually makes its way to the soil and then the ocean.
  • The sulfur in the atmosphere aids in the cloud formation by increasing the number of cloud droplets and the droplet size to decrease.
  • The sulfur particles, also called aerosols, fallout from the atmosphere to the biosphere.
  • The SO2 present in the atmosphere reaches the biosphere as the gas dissolves in the rainwater to form weak droplets of sulfuric acid.
  • Besides, chemical weathering in the pedogenesis process also results in the movement of sulfur from rocks to the soil and the water.
  • Weathering also causes the release of sulfur in the air as some of it is converted into sulfate.

3. Plant and Animal Uptake

  • Once the sulfur reaches the terrestrial and aquatic biosphere, it is taken up by plants and microorganisms.
  • A group of bacteria called the green sulfur bacteria, act as photoautotrophic bacteria, and utilize sulfur as a form of energy.
  • Other microorganisms in the soil also aid in making sulfur available to plants so that it can be taken up with water from the soil.
  • The sulfur taken up by living beings is then used in the formation of biomolecules like proteins and nucleotides.
  • In the ocean ecosystem, chemoautotrophic microorganisms utilize sulfur to produce organic compounds in the form of sulfates.

4. Lithification and Release

  • The sulfur in the biosphere then circulates through the food chain as the consumers feed on producers and then reach the microbial chains.
  • The sulfur that doesn’t circulate falls into the depths of terrestrial and marine habitats and remains there in the combined form (FeS) in rocks.
  • The sulfur in the food chain then undergoes the decomposition process, converting the sulfate into sulfides so that they can be returned back to the atmosphere.
  • Sulfur reducing bacteria act on the organic forms of sulfur to form inorganic forms like hydrogen sulfide (H2S), which is further reduced to S.
  • The sulfur in the lithosphere also releases back to the atmosphere as a result of volcanic activity.

Read Also: Phosphorus Cycle- Definition, Steps, Examples, Significance, Human Impacts

Sulfur Cycle Examples

The following are some of the examples of reservoirs of sulfur that are involved in the sulfur cycle;

1. Atmosphere

  • The atmosphere is one of the reservoirs of sulfur where sulfur is found in the form of sulfur dioxide and hydrogen sulfide.
  • The sulfur in the atmosphere is obtained either from human activities on Earth or natural processes like chemical weathering and microbial activities.
  • The sulfur in the atmosphere reaches the biosphere either via precipitation or as a fallout.
  • As the sulfur cycle continues, the sulfur circulates from the atmosphere to the biosphere and coming back to the atmosphere.

2. Biosphere

  • The biosphere is composed of living organisms that include plants, animals, and microorganisms. Out of these, microorganisms play an important role in the sulfur cycle.
  • The sulfur in the biosphere is present both in organic and inorganic forms, where the inorganic form remains in the soil while the organic form moves through the food chain.
  • The sulfur cycle comprises of cycling of sulfur from the atmosphere to the biosphere and back to the atmosphere.
  • Human activities, like the addition of sulfur-rich artificial fertilizers, also affect the sulfur cycle occurring in the biosphere.
  • However, the biosphere might also act as a sink for sulfur as many living organisms can take up sulfur as a mineral for their growth and development.

Sulfur Cycle Significance

  • Sulfur is an essential element that is an important component of biomolecules like proteins and nucleic acids. Thus, the understanding of the sulfur cycle enables the understanding of the biomolecules and their functions.
  • The sulfur cycle is essential as it balances the concentration of sulfur in different reservoirs so as to make the Earth a hospitable place for life.
  • Sulfur is found in nature in a combined state with other elements like nitrogen, iron, and phosphorus, so the sulfur cycle also affects the availability of other elements.
  • Sulfur bacteria are chemoautotrophic organisms that can convert the chemical energy into other forms through the food chain, which increases the biomass on the planet.
  • Mineralization of sulfur acts as a natural waste system where helps in the recycling of sulfur compounds.
  • Biological processes occurring in the terrestrial ecosystem in the sulfur cycle are responsible for increasing the availability of sulfur to plants and microorganisms.
  • Understanding the mechanism of the sulfur cycle helps to understand the physiology of different microorganisms involved in the process.
  • The sulfur metabolic pathways in various bacteria have important medical implications as some of the pathogenic microorganisms like Mycobacterium tuberculosis utilize sulfur as a source of energy.

Human impacts on the Sulfur Cycle

  • The burning of fossil fuels and other forms of combustion increases the concentration of SO2 in the atmosphere, which brings about some imbalance in the sulfur concentration on Earth.
  • The addition of artificial fertilizers to the soil can affect soil fertility, plant growth, and microbial activities in the soil.
  • Petroleum refining and industrial processing also cause the release of sulfuric acid and sulfur dioxide in the atmosphere.
  • Sulfur dioxide causes an increase in the Earth’s temperature as the gas droplets increase the particles in clouds. The increase of sulfur dioxide in clouds increases the extent of acid rain on Earth.
  • Industrial society, along with a rapidly rising world population and increasing mechanization of agriculture and forestry, modifies extensive areas of previously undisturbed landscape which then alter the terrestrial biosphere.

References and Sources

  • Kertesz, M. A., & Frossard, E. (2015). Biological Cycling of Inorganic Nutrients and Metals in Soils and Their Role in Soil Biogeochemistry. Soil Microbiology, Ecology and Biochemistry, 471–503.DOI:10.1016/b978-0-12-415955-6.00016-5 
  • Klotz MG, Bryant DA, Hanson TE. The microbial sulfur cycle. Front Microbiol. 2011;2:241. Published 2011 Dec 2. DOI:10.3389/fmicb.2011.00241
  • Kelly DP. The sulphur cycle: definitions, mechanisms and dynamics. Ciba Found Symp. 1979;(72):3-18. DOI: 10.1002/9780470720554.ch2. PMID: 398766.
  • Muyzer, G., Stams, A. The ecology and biotechnology of sulphate-reducing bacteria. Nat Rev Microbiol6, 441–454 (2008).
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About Author

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Anupama Sapkota

Anupama Sapkota has a bachelor’s degree (B.Sc.) in Microbiology from St. Xavier's College, Kathmandu, Nepal. She is particularly interested in studies regarding antibiotic resistance with a focus on drug discovery.

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