27.11:
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.
27.11:
The Sulfur Cycle
Sulfur, an important element in the chemical makeup of proteins, is recycled through the atmosphere and aquatic and terrestrial environments. Found in the atmosphere as sulfur dioxide (SO2), sulfur is released by decaying organisms, weathered rocks, geothermal vents, volcanos, and burning fossil fuels. It is deposited into the ecosystem, cycled through the biotic community, and either released back into the atmosphere as gas or deposited in marine sediment for long-term storage and eventual release back into the soil and atmosphere.
The Biogeochemical Sulfur Cycle
Sulfur is essential to biological systems and is a component of certain amino acids, such as cysteine, which plays an important role in the structure of proteins. Sulfur is distributed to terrestrial (i.e., land) ecosystems by the precipitation of weak sulfuric acid, direct fallout from the atmosphere, weathering of sulfur-containing rocks, and geothermal vents.
From the soil, it is taken up by microorganisms and plants and converted into organic forms that can be used by consumers in the ecosystem. When organisms die, decomposers break the organic sulfur compounds down into gases, such as hydrogen sulfide, which is oxidized in the atmosphere to form sulfur dioxide. Additionally, some chemoautotrophs use sulfur as a biological energy source and recycle sulfur molecules directly through ecosystems.
Sulfur is introduced into marine ecosystems through runoff from land, direct fallout from the atmosphere, and underwater geothermal vents. Some of this sulfur cycles through the food chain and is released into the atmosphere as sea spray. The remainder is deposited as sediment on the ocean floor, where it is stored for extended time periods. Over geological time, uplift can transfer the sediment back to land, where sulfur is released by erosion.
Human Impacts on the Sulfur Cycle
The burning of fossil fuels, particularly coal, adds an unnatural quantity of hydrogen sulfide gas into the atmosphere, resulting in a higher concentration of sulfur dioxide that manifests as acid rain. Acid rain damages the environment by lowering the pH of lakes and rivers, harming both aquatic and terrestrial fauna.
Suggested Reading
Wasmund, Kenneth, Marc Mußmann, and Alexander Loy. “The Life Sulfuric: Microbial Ecology of Sulfur Cycling in Marine Sediments.” Environmental Microbiology Reports 9, no. 4 (August 2017): 323–44. [Source]
Durham, Bryndan P., Shalabh Sharma, Haiwei Luo, Christa B. Smith, Shady A. Amin, Sara J. Bender, Stephen P. Dearth, et al. “Cryptic Carbon and Sulfur Cycling between Surface Ocean Plankton.” Proceedings of the National Academy of Sciences 112, no. 2 (January 13, 2015): 453–57. [Source]