The Sulfur Cycle

As a gaseous molecule, sulfur cycles between the atmosphere, terrestrial, and aquatic ecosystems.

One form, sulfur dioxide, or SO2, enters the terrestrial system from the atmosphere, either as weak sulfuric acid dissolved in precipitation or directly deposited in a process called fallout.

The weathering of sulfur containing rocks made by geological uplift of ocean sediments also contributes sulfur into the terrestrial ecosystem.

From the soil, sulfur then enter the food chain through the roots of plants as sulfates which are consumed by heterotrophs and released into the atmosphere again as hydrogen sulfide by decomposers after the organisms die.

Excess sulfur in the soil and from underground geothermal vents enters the ocean as runoff where it precipitates into sediments on the ocean floor or is used as biological energy source by marine chemoautotrophs.

Volcanic activity and geothermal vents also release sulfur into the atmosphere in the form of hydrogen sulfide gas as a natural process. However, the burning of fossil fuels contributes an unnaturally large amount into the atmosphere which returns to the ground as acid rain, damaging ecosystems.