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When organisms higher on the food chain -- like human beings -- eat plants and other organisms lower on the food chain, they absorb and utilize the nitrogen that was originally fixed by these bacteria (Harrison 2003). In this way, the nitrogen from the atmosphere is able to make it through the entire biosphere. The cycle is not complete, however, until the nitrogen is returned back to the atmosphere. This occurs when higher organisms die, and decay (as well as metabolic process of other bacteria and fungi) converts the fixed nitrogen back to ammonia, or even further into nitrates which will eventually break down further and become nitrogen gas again, returning to the atmosphere (Kimball 2008). Sometimes the nitrogen products of decay return to the soil or directly to plants, and other byproducts can actually build up to toxic levels in soil and in the world's oceans (Harrison 2003). In this latter regard, much if the blame lies with the artificial manipulation of the nitrogen cycle through human industrial activities and fertilization. It is an ingredient in every single amino acid, and thus is one of the most basic ingredients in proteins -- the most fundamental organic compound (UC 2003). As an ingredient in nucleic acids such as DNA and RNA, it is also necessary for proper cell division and the continuation of genetic material. It is for these reasons that nitrogen is so essential to life; human beings' alteration of the nitrogen cycle is leading to toxic forms of the element, but a healthy amount is an absolute necessity.
References
Harrison, J. (2003). "The nitrogen cycle." Accessed 4 January 2010. http://www.visionlearning.com/library/module_viewer.php?mid=98
Kimball, J. (2008). "The nitrogen cycle." Accessed 4 January 2010. http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/N/NitrogenCycle.html
UC. (2003). "Nitrogen." Accessed 4 January 2010. http://periodic.lanl.gov/elements/7.html
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