The Phosphorus Cycle

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Global Phosphorus pools

Most Phosphorus is stored in the crust as the mineral Apatite (Calcium Phosphate -- Ca5(PO4)3), which is common in granites, but also occurs in metamorphic and sedimentary rocks.  Apatite is found in teeth and bones, and sedimentary rocks containing these can also be a good source of Phosphate.

Some Phosphorus is also dissolved in fresh and ocean water, or is stored in soil or in organic matter.

Biological uses of Phosphorus

Nucleic Acids, which are made of nitrogen containing compounds combined with a phosphate group and a sugar (N compound-CH8O2-PO4). DNA looks like a twisted ladder. The rungs of the ladder are nucleic acids and the side rails of the ladder are phosphate groups that hold the rungs apart.

NASA created a storm in 2011 when researchers announced the discovery of microorganisms that use arsenic in place of phosphorus. Although the microorganisms are extremely arsenic-tolerant, it has not yet been shown that they use arsenic the same way other organisms use phosphorus.

Adenosene di-Phosphate (ADP) and Adenosene tri-Phosphate (ATP), which is made of Adenine, a ribose sugar, and 2-3 Phosphate groups.  These compounds are essential to life as they carry energy from mitochondria (who liberate energy in food) to the places in the cell where the energy is used (muscles, metabolic processes, cell growth, etc.).  Without these compounds, no life on Earth could live.

Redfield Ratios also consider the optimum ratio of Carbon to Phosphorus needed by life.  Because the energy demands of land and water life is the same, the optimum ratio of C:P is 106 C:1 P for both.

Thus, the full Redfield Ratio (optimum ratio of C to N to P) for land and water life is:

Land:  106 C: 16 N: 1 P
Water:  106 C: 13 N: 1 P

We have already seen that N demands are greater in land life, as they need more proteins to make solid body structure.  The other side of this is that as N - demands are less in aquatic systems, the relative demand for P is higher, as P is equally required by both land ans water life.  Thus, P usually limits growth of aquatic systems.  This is why there is so much concern about limiting use of Phosphate detergents, as they will fertilize aquatic ecosystems, and allow some algae to take over.

The Phosphorus Cycle

Chemically, this is a much easier nutrient to deal with than N, as it only exists in one chemical state,   Phosphate (PO4-3). This is the only major essential element without an atmospheric component.

The only complex feature is that not all Phosphate molecues can readily dissolve.  If the phosphate molecule does not dissolve, the phosphate con not be absorbed by living things.

Based on this, Phosphorus resources can be divided into 3 categories:

Factors which effect P levels in soil and water:


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Created 2 September 2011, Last Update 02 September 2011

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