16.4F: The Sulfur Cycle
- Describe the sulfur cycle
The Sulfur Cycle
The sulfur cycle describes the movement of sulfur through the atmosphere, mineral forms, and through living things. Although sulfur is primarily found in sedimentary rocks or sea water, it is particularly important to living things because it is a component of many proteins.
Sulfur is released from geologic sources through the weathering of rocks. Once sulfur is exposed to the air, it combines with oxygen, and becomes sulfate SO 4. Plants and microbes assimilate sulfate and convert it into organic forms. As animals consume plants, the sulfur is moved through the food chain and released when organisms die and decompose.
Some bacteria – for example Proteus, Campylobacter, Pseudomonas and Salmonella – have the ability to reduce sulfur, but can also use oxygen and other terminal electron acceptors. Others, such as Desulfuromonas, use only sulfur. These bacteria get their energy by reducing elemental sulfur to hydrogen sulfide. They may combine this reaction with the oxidation of acetate, succinate, or other organic compounds.
The most well known sulfur reducing bacteria are those in the domain Archea, which are some of the oldest forms of life on Earth. They are often extremophiles, living in hot springs and thermal vents where other organisms cannot live. Lots of bacteria reduce small amounts of sulfates to synthesize sulfur-containing cell components; this is known as assimilatory sulfate reduction. By contrast, the sulfate-reducing bacteria considered here reduce sulfate in large amounts to obtain energy and expel the resulting sulfide as waste. This process is known as dissimilatory sulfate reduction. In a sense, they breathe sulfate.
Sulfur metabolic pathways for bacteria have important medical implications. For example, Mycobacterium tuberculosis (the bacteria causing tuberculosis) and Mycobacterium leprae (which causes leoprosy) both utilize sulfur, so the sulfur pathway is a target of drug development to control these bacteria.
Key Points
- The sulfur cycle describes the movement of sulfur through the geosphere and biosphere. Sulfur is released from rocks through weathering, and then assimilated by microbes and plants. It is then passed up the food chain and assimilated by plants and animals, and released when they decompose.
- Many bacteria can reduce sulfur in small amounts, but some specialized bacteria can perform respiration entirely using sulfur. They use sulfur or sulfate as an electron receptor in their respiration, and release sulfide as waste. This is a common form of anaerobic respiration in microbes.
- Sulfur reducing pathways are found in many pathogenic bacteria species. Tuberculosis and leprosy are both caused by bacterial species that reduce sulfur, so the sulfur reduction pathway is an important target of drug development.
Key Terms
- extremophile : An organism that lives under extreme conditions of temperature, salinity, and so on. They are commercially important as a source of enzymes that operate under similar conditions.
- assimilatory sulfate reduction : The reduction of 3′-Phosphoadenosine-5′-phosphosulfate, a more elaborated sulfateester, leads also to hydrogen sulfide, the product used in biosynthesis (e.g., for the production of cysteine because the sulfate sulfur is assimilated).