Sulfur cycle

Cycle definition

Most sulfur exists as rocks. As these rocks are weathered over time, they release sulfate ions (SO4^2-) that producers can take up and assimilate. This assimilated sulfur then passes through the food web. Volcanoes, the burning of fossil fuels, and the mining of copper put sulfur dioxide (SO2) into the atmosphere. In the atmosphere, sulfur dioxide combines with water to form sulfuric acid (H2SO4) This sulfuric acid is carried back to Earth when it rains or snows.

Sulfur supports the growth and development of…

living organisms, as it forms the structure of amino acids, proteins, and vitamins

steps in order

1. Decomposition of Organic Compounds

2. Oxidation of Hydrogen Sulfide to Elemental Sulfur

3. Oxidation of Elemental Sulfur to Sulfates

4. Assimilation in organic compounds 

1. Decomposition of Organic Compounds

• Dead plants and animals contain sulfur in the form of organic compounds and decomposer microorganisms break down these organic compounds during the process of decomposition. This breakdown releases sulfur-containing compounds, including amino acids from proteins. The sulfates gets reduced to hydrogen sulfide by the action of Desulfotomaculum bacteria. 

2. Oxidation of Hydrogen Sulfide to Elemental Sulfur

• Through microbial and chemical processes Hydrogen sulfide (H₂S) is oxidized to elemental sulfur (S⁰). Photosynthetic bacteria, such as those from the families Chlorobiaceae and Chromatiaceae, are involved in this oxidation.

3. Oxidation of Elemental Sulfur to Sulfates

• In the soil the sulphur is present in the elemental form that is not directly usable by the plants. Chemolithotrophic bacteria, including species like Thiobacillus, oxidize elemental sulfur to sulfates (SO₄²⁻). This conversion makes sulfur available in a form that plants can assimilate. 

4. Assimilation in organic compounds 

• By the cation of certain bacteria, such as Desulfovibrio desulfuricans, the sulfates in the soil are reduced back to hydrogen sulfide. The reduction occurs in two steps: firstly, sulfates are converted to sulfites (SO⁻), and secondly, sulfites are further reduced to hydrogen sulfide. During sulfate reduction the hydrogen sulfide (H2S) that is produced is released into the atmosphere, completing the cycle.

Human impact on the cycle:

Impact on the atmosphere:

Impact on the terrestrial ecosystems:

Impact on aquatic ecosystems: