Phosphorus Cycle

Biogeochemical cycling of phosphorus is important for a number of reasons. All living cells require phosphorus for nucleic acids, and some lipids and polysaccharides. However, unlike carbon and nitrogen, phosphorus exists in a single valence state of 15 and has no gaseous component.

Indeed, all phosphorus is originally derived solely from the weathering of phosphate-containing rocks (figure). However, recent evidence suggests that phosphonates, which bear a C—P bond and are thus organic, may be a significant source of phosphorus for some marine microorganisms.

A Simplified Phosphorus Cycle
A Simplified Phosphorus Cycle

Phosphorus enters soil and water through the degradation of plants and animals, weathering of rocks, and fertilizer application. Phosphate in soil and waters is consumed by terrestrial and aquatic microbes and passed onto larger organisms. However, much of the soil phosphorus can leach great distances or, like phosphate in aquatic systems, complex with cations to form relatively insoluble compounds. Terrestrial chemolithotrophic microbes also take up soil phosphate but are not shown

Nonetheless, phosphorus is present in low concentrations and frequently limits growth. In soil, phosphorus exists in both inorganic and organic forms. Organic phosphorus includes not only that found in biomass but also that in materials such as humus and other organic compounds.

The phosphorus in such organic materials is readily recycled by microbial activity. Inorganic phosphorus is negatively charged, so it complexes with positively charged elements in the environment, such as iron, aluminum, and calcium.

These compounds are relatively insoluble, and their dissolution is pH dependent such that phosphate is most available to plants and microbes between pH 6 and 7. Microbes transform simple orthophosphate (PO4 3-) to more complex forms.

These include the polyphosphates present as inclusions as well as more familiar macromolecules such as nucleotides and phospholipids.