The sulfur cycle describes the continuous movement of sulfur through Earth’s diverse systems, including the atmosphere, oceans, land, and living organisms. Sulfur is a fundamental nutrient for all life forms, playing a significant role in the structure of amino acids like cysteine and methionine, and various vitamins. This constant circulation ensures sulfur remains available for biological processes.
Mineralization from Organic Matter
The recycling journey of sulfur begins with its release from organic materials through mineralization. When plants and animals die, the sulfur within their complex organic molecules becomes locked away. Decomposers, mainly bacteria and fungi, then break down this dead organic matter.
These microorganisms secrete enzymes that dismantle the larger organic compounds. This decomposition converts organic sulfur back into inorganic forms, notably hydrogen sulfide (H₂S) and other sulfide compounds. This liberates sulfur from its organic bonds, making it accessible for further transformations within the environment.
Transformation in Soil and Water
Following mineralization, the inorganic sulfur compounds undergo further transformations within the soil and aquatic environments. Chemosynthetic bacteria are active in these conversions. These microbes derive energy by oxidizing various sulfur compounds.
In an oxygen-rich environment, these bacteria convert sulfides, such as hydrogen sulfide (H₂S), into elemental sulfur (S) and then oxidize it into sulfate (SO₄²⁻). Sulfate is the primary form of sulfur that most plants can readily absorb through their roots. This chemical change from a reduced form like hydrogen sulfide to a more oxidized, soluble sulfate ion makes sulfur available for biological uptake.
Atmospheric Deposition and Return
A parallel pathway for sulfur recycling occurs through the atmosphere, involving natural sources and human activities. Natural sources contribute sulfur compounds to the air, including sulfur dioxide (SO₂) and hydrogen sulfide from volcanic eruptions. Marine phytoplankton also release dimethyl sulfide (DMS), which oxidizes in the atmosphere to form other sulfur gases.
Human activities increase atmospheric sulfur levels, primarily through the combustion of fossil fuels like coal and oil. This burning releases large amounts of sulfur dioxide (SO₂) into the atmosphere. Once airborne, sulfur dioxide reacts with water vapor and oxygen, forming sulfuric acid (H₂SO₄).
This sulfuric acid then returns to the Earth’s surface through various deposition processes. Wet deposition occurs when sulfuric acid dissolves in rainwater, snow, or fog, falling as precipitation often called acid rain. Dry deposition involves the direct settling of sulfur-containing particles and gases onto surfaces. These atmospheric processes redistribute sulfur widely, returning it to land and aquatic ecosystems, where it can re-enter the terrestrial and aquatic cycles.
Assimilation into the Food Web
The final stage in the sulfur cycle involves sulfur’s re-entry into living systems. Plants absorb water-soluble sulfate (SO₄²⁻) ions from the soil through their root systems. This uptake ensures the bioavailable form of sulfur is acquired.
Once inside the plant, absorbed sulfate is transformed and incorporated into complex organic molecules. This includes the synthesis of sulfur-containing amino acids, such as cysteine and methionine, which are building blocks of proteins. These amino acids are utilized by the plant for growth and metabolic functions.
Sulfur then moves through the food web as organisms consume one another. Herbivores obtain sulfur by feeding on plants. Carnivores acquire sulfur by preying on herbivores or other carnivores. This dietary transfer ensures that sulfur, initially assimilated by plants, cycles through various trophic levels, supporting the structure and function of all living organisms.