The Phosphorus Cycle

Phosphorus is needed by all organisms to make ATP, nucleic acids (DNA and RNA) and phospholipids in cell membranes. Unlike carbon and nitrogen, phosphorus has no significant gaseous form, so the phosphorus cycle has no atmospheric stage and is very slow.Most phosphorus is held in rocks as phosphate. Weathering and erosion release phosphate ions (PO₄³⁻) into soils and water.Plants absorb phosphate ions from the soil through their roots, and aquatic producers take them up from the water. The phosphorus is incorporated into organic molecules and passed along food chains when animals eat plants or other animals.Phosphate is returned to the soil and water in waste (for example urine, faeces and the guano of sea birds) and when organisms die. Saprobionts (bacteria and fungi) decompose the dead organic matter and waste by extracellular digestion, releasing phosphate ions again. This step is essential: without saprobionts, phosphorus would stay locked in dead remains.Phosphate ions in rivers and seas are carried into sediments. Over millions of years these sediments are compressed into new sedimentary rock, and geological uplift eventually exposes the rock to weathering, completing the cycle.