Sulfur cycle
The sulfur cycle is a natural process involving the transformation and movement of sulfur through various forms and locations in the environment, including the Earth's crust and atmosphere. It begins with the release of hydrogen sulfide (H₂S) into the atmosphere, originating from sources like volcanic eruptions, the decomposition of organic materials, and fossil fuel combustion. Once in the atmosphere, H₂S is oxidized to sulfur dioxide (SO₂), which then reacts with water vapor to form sulfuric acid, returning sulfur to the Earth through precipitation. This sulfur is taken up by plants and incorporated into proteins, forming part of the food chain. When plants die or are consumed, sulfur returns to its original forms through decomposition, completing the cycle.
The sulfur cycle has significant implications for climate change, as SO₂ contributes to cloud formation and can influence global temperatures. Additionally, the production of dimethyl sulfide (DMS) by marine phytoplankton can affect rainfall patterns by forming sulfate particles in the atmosphere. However, human activities, especially in industrial regions, have led to increased sulfur emissions, resulting in environmental issues like acid rain that harm ecosystems. Notably, volcanic eruptions can also cause dramatic climate effects, exemplified by the Mount Tambora eruption in 1816, which resulted in widespread food shortages due to global cooling. Understanding the sulfur cycle is crucial for addressing its environmental impacts and the broader implications for climate dynamics.
Subject Terms
Sulfur cycle
Definition
The ancients referred to hell as a place of fire and burning brimstone. Brimstone was their term for the rotten-egg-smelling gas of sulfur dioxide (SO2). Sulfur is also present in other, less odious forms owing to a series of transforming oxidation-reduction reactions, and in one form or another it is in constant between Earth and sky and back again to Earth. This is the sulfur cycle. Technically, the sulfur cycle describes the mineralization of organic sulfur to sulfide, its oxidation to sulfate, and the reduction of sulfate to sulfide, followed by microbial conversion into organic compounds.
Oxidation is most simply defined as the addition of oxygen, while reduction is the removal of oxygen from a compound. However, chemists more narrowly define oxidation-reduction reactions as the transfer of electrons between compounds. The term organic refers to chemical compounds having a carbon basis, which implies having come from living organisms. During its cycle, electrons recombine to move sulfur in and out of organic and inorganic compounds.
From the Earth’s crust, sulfur finds its way into the atmosphere in the form of hydrogen sulfide (H2S). It does this in a variety of ways: It comes from volcanic eruptions, decomposition of organic material, fossil fuel combustion, and gas exchange from the ocean. Once it is in the atmosphere, H2S is oxidized to SO2. SO2 then combines with water vapor to become sulfuric acid, allowing sulfur to return to Earth in drops of rain. Because living things require low levels of sulfur, plants then take up the sulfur and incorporate it into protein, and it becomes part of the food chain. These organic sulfur compounds are returned to the land or water after the plants die or are consumed by animals. Ultimately, decomposition returns sulfur to its H2S form, and the cycle begins again. The source of sulfur is the Earth’s crust. Rocks and salts contain most of the planet’s sulfur, but it is also found in ocean sediments and in an organic form in protein.
Significance for Climate Change
SO2 when dissolved in the moisture of clouds transforms into weak sulfuric acid. These acid-laden clouds reflect more of the Sun’s radiant energy than do uncontaminated clouds in a process called cloud brightening. The result is climate cooling.
Another form of sulfur of environmental concern is dimethyl sulfide (DMS). Marine life in the form of phytoplankton produces DMS in great quantities. When DMS escapes into the troposphere, it oxidizes into sulfate particles. Scientists have found that these particles play a role in forming condensation nuclei, which promote cloud growth and increased rainfall. This suggests that elevated ultraviolet radiation from large-scale climate change might damage phytoplankton to the extent that DMS levels are reduced, thereby lowering rainfall amounts.
Highly populated and industrial areas of the United States, Eastern Europe, and—increasingly—Asia have the highest levels of sulfur emissions. The associated problems are likely to increase as areas such as industrialized China continue to grow. One specific problem is that of acid rain. These rain droplets containing sulfuric acid have caused great damage to freshwater ecosystems.
The issue of climate impact from sulfur is not in doubt, but authorities debate whether discharged from burning raises climate temperatures beyond the cooling effects of the sulfur dioxide that is also discharged. Millions of metric tons of SO2 gas can reach the from a major volcanic eruption. The year 1816 witnessed the great Mount Tambora eruption. Because of the lingering ash cloud that had spewed into the atmosphere, thereby lowering temperatures around the world, the following year became known as the year without a summer. Crops failed in the United States, and food shortages became commonplace around the globe. More recently, environmentalists think it likely that greenhouse warming was delayed for a few years by the eruption of Mount Pinatubo in 1991. Research reveals that volcanic ash clouds together with sulfur dioxide emissions can force dramatic temperature changes.
A certain type of bacteria known as Thiobacillus ferroxidans is capable of harvesting energy for its growth and reproduction through the oxidation of sulfides into sulfates, leaving large amounts of caustic acid in its wake. In heavy metal or coal mining, large amounts of acid are produced in this way that are often referred to as acid mine drainage. Many treatment systems are being developed to treat coal-mine drainage in order to raise the of contaminated water and control the precipitation of dissolved metals.
Bibliography
Bianchi, Thomas S. Biogeochemistry of Estuaries. New York: Oxford University Press, 2007.
Cunningham, William, and Barbara Woodworth Saigo. Environmental Science: A Global Concern. 6th ed. New York: McGraw-Hill, 2001.
Mateos, Katherine, et al. "The Evolution and Spread of Sulfur Cycling Enzymes Reflect the Redox State of the Early Earth." Science Advances, vol. 9, no. 27, 7 July 2023, doi: 10.1126/sciadv.ade4847. Accessed 11 Dec. 2024.
Mitchell, Stephen C. Biological Interactions of Sulfur Compounds. New York: Routledge, 2006.