Biogeochemical cycle

From New World Encyclopedia


It would be good to do a brief summary of each of the biogeochemical cycles, just stating the overall basics of the cycle. The more detailed summary will be done under each cycle, but there should be some overview here of the basic cycles: oxygen, carbon, nitrogen, etc.

In ecology, a biogeochemical cycle is a circuit or pathway by which a chemical element or molecule moves through both biotic ("bio-") and abiotic ("geo-") compartments of an ecosystem. In effect, the element is recycled, although in some such cycles there may be places (called "sinks") where the element is accumulated or held for a long period of time.

All chemical elements occurring in organisms are part of biogeochemical cycles. In addition to being a part of living organisms, these chemical elements also cycle through abiotic factors of ecosystems such as water (hydrosphere), land (lithosphere), and the air (atmosphere); the living factors of the planet can be referred to collectively as the biosphere. All the chemicals, nutrients, or elements—such as carbon, nitrogen, oxygen, phosphorus—used in ecosystems by living organisms operate on a closed system, which means that these chemicals are recycled instead of lost such as in an open system. The energy of an ecosystem occurs on an open system; the sun constantly gives the planet energy in the form of light while it is eventually used and lost in the form of heat throughout the trophic levels of a food web.

Although components of the biogeochemical cycle cannot be lost as in the energy system, they can be held for long periods of time in one place. This place is called a reservoir, which, for example, includes such things as coal deposits that are storing carbon for a long period of time. When chemicals are held for only short periods of time, they are being held in exchange pools. Generally, reservoirs are abiotic factors while exchange pools are biotic factors. Examples of exchange pools include plants and animals, which temporarily use carbon in their systems and release it back into a particular reservoir. Carbon is held for a relatively short time in plants and animals when compared to coal deposits. The amount of time that a chemical is held in one place is called its residence time.

The most well-known and important biogeochemical cycles, for example, include the carbon cycle, the nitrogen cycle, the oxygen cycle, the phosphorus cycle, and the water cycle.

Biogeochemical cycles always involve equilibrium states: a balance in the cycling of the element between compartments. However, overall balance may involve compartments distributed on a global scale.

Biogeochemical cycles of particular interest in ecology are:

Nitrogen Cycle: A quick Summary

See main article at Nitrogen cycle

This is just a summation of the complicated nitrogen cycle. For more complete information, visit the nitrogen cycle page. The nitrogen cycle is a much more complicated biogeochemical cycle but also cycles through living parts and nonliving parts including the water, land, and air. Nitrogen is a very important element in that it is part of both proteins, present in the composition of the amino acids that make up proteins, as well as nucleic acids such as DNA and RNA, present in nitrogenous bases. The largest reservoir of nitrogen is the atmosphere, in which about 78% of nitrogen is contained as nitrogen gas (N2). Nitrogen gas is “fixed,” in a process called nitrogen fixation. Nitrogen fixation combines nitrogen with oxygen to create nitrates (NO3).

Nitrates can then be used by plants or animals (which eat plants or eat animals that have eaten plants). Nitrogen can be fixed either by lightning, industrial methods (such as for fertilizer), in free nitrogen-fixing bacteria in the soil, as well as in nitrogen-fixing bacteria present in roots of legumes (such as rhizobium). Nitrogen-fixing bacteria use certain enzymes that are capable of fixing nitrogen gas into nitrates and include free bacteria in soil, symbiotic bacteria in legumes, and also cyanobacteria, or blue-green algae, in water.

After being used by plants and animals, nitrogen is then disposed of in decay and wastes. Detritivores and decomposers decompose the detritius from plants and animals, nitrogen is changed into ammonia, or nitrogen with 3 hydrogen atoms (NH3). Ammonia is toxic and cannot be used by plants or animals, but nitrite bacteria present in the soil can take ammonia and turn it into nitrite, nitrogen with two oxygen atoms (NO2). Although nitrite is also unusable by most plants and animals, nitrate bacteria changes nitrites back into nitrates, usable by plants and animals. Some nitrates are also converted back into nitrogen gas through the process of denitrification, which is the opposite of nitrogen-fixing, also called nitrification. Certain denitrifying bacteria are responsible for this.

Sulfur Cycle: A brief Summary

Sulfur is one of the constituents of many proteins and vitamins and hormones. It recycles like other biogeochemical cycles.

The essential steps of the sulfur cycle are:

  • Mineralization of organic sulfur to the inorganic form, hydrogen sulfide: (H2S).
  • Oxidation of sulfide and elemental sulfur (S) and related compounds to sulfate (SO42-).
  • Reduction of sulfate to sulfide.
  • Microbial immobilization of the sulfur compounds and subsequent incorporation into the organic form of sulfur.

These are often termed as follows:

Assimilative sulfate reduction in which sulfate (SO42-) is reduced to organic sulfhydryl groups (R-SH) by plants, fungi and various prokaryotes. The oxidation states of sulfur are +6 in sulfate and -2 in R-SH.
Desulfuration in which organic molecules containing sulfur can be desulfurated, producing hydrogen sulfide gas (H2S), oxidation state = -2. Note the similarity to deamination.
Oxidation of hydrogen sulfide produces elemental sulfur (So), oxidation state = 0. This reaction is done by the photosynthetic green and purple sulfur bacteria and some chemolithotrophs (organisms using inorganic compounds for ATP production).
Further oxidation of elemental sulfur by sulfur oxidizers produces sulfate.
Dissimilative sulfur reduction in which elemental sulfur can be reduced to hydrogen sulfide.
Dissimilative sulfate reduction in which sulfate reducers generate hydrogen sulfide from sulfate.

Human impact on the sulfur cycle is primarily in the production of sulfur dioxide (SO2) from industry (e.g. burning coal) and the internal combustion engine. Sulfur dioxide can precipitate onto surfaces where it can be oxidized to sulfate in the soil (it is also toxic to some plants), reduced to sulfide in the atmosphere, or oxidized to sulfate in the atmosphere as sulfuric acid, a principal component of acid rain.

References
ISBN links support NWE through referral fees

External Links

Sulfur Cycle

Biogeochemical cycles
Carbon cycle - Hydrogen cycle - Nitrogen cycle
Oxygen cycle - Phosphorus cycle - Sulfur cycle - Water cycle


Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

The history of this article since it was imported to New World Encyclopedia:

Note: Some restrictions may apply to use of individual images which are separately licensed.