Phosphorus Cycle

All living things require phosphorus. In the environment, phosphorus is often in the form of phosphate molecules, composed of one phosphorus atom and four oxygen atoms. One important function of phosphate groups of organic molecules within living organisms is energy storage. Adenosine triphosphate, or ATP, is an example. ATP, the "energy currency" of cells is used to transfer stored chemical energy from one molecule to another to perform work. The energy is stored in the phosphate portion of the molecule. The energy we derive from food, for example, is stored in the form of ATP. Phosphorus is also required for the formation of phospholipids of cells. Phospholipids are the major component of cell membranes. Also, phosphate groups activate and deactivate enzymes within cells that catalyze major chemical reactions. Phosphate is a mineral salt component of bones and teeth in vertebrate animals. In addition, phosphate is an important structural component of DNA itself. So, recycling of limited phosphorus is vital.

Unlike the carbon cycle, the phosphorus cycle does not include transition of phosphorus through the atmosphere as a gas. Phosphorus-containing gases are not common. Also, phosphate has a limited number of inorganic forms outside of living organisms, making its recycling scheme relatively simple. Weathering of rocks containing phosphate minerals is accomplished by rain. The erosion moves inorganic phosphate into soil where it is rapidly absorbed by plants and incorporated into organic molecules (DNA, ATP, phospholipids). Plants containing phosphorus die or are consumed by animals. When consumed by animals, the phosphorus is incorporated into animal mass. When phosphorus containing animals die, along with plants, their decomposition returns phosphorus from their tissues back into soil for new use by plants (or by fungi).

Not all of the phosphate eroded from rock is incorporated into plant and animal tissue directly. A portion of the run-off from phosphorus deposits in rock either enters streams and rivers that flow to the ocean, or leaches into the water table, gradually draining into the sea. Phosphates in the ocean very gradually build-up in sediments. Also, phosphorus in decaying aquatic organisms falls to the bottom to accompany the phosphorus built-up in inorganic sediment. Over extremely long periods of time, phosphorus-containing sediment is transformed into rock, buried deep in the ocean floor. Here, the phosphorus remains, not participating in the rest of the cycle. Most of the phosphorus on Earth is found here, at the bottom of the ocean as a part of the earth's crust. Periodically, violent geological shifts raise the once buried deposits. Now on land, exposed to wind and rain, the phosphorus minerals are free to participate in the rest of the cycle.