Phosphorus is another essential element for living organisms. Phosphate ions are needed to make phospholipids, ATP, and nucleotides (the monomers of DNA and RNA). Phosphates are transferred through an ecosystem in the phosphorus cycle. In this article we will look at the cycle itself, and the role of mycorrhizae and saprobionts.
The phosphorus cycle
Many phosphate ions are locked up as part of rocks. Rocks are broken down or dissolved by weathering, and the phosphate ions enter the soil and oceans (also lakes and rivers). From the soil, phosphate ions can be absorbed by plants. This process is helped by fungi called mycorrhizae (see section below). The plant uses the phosphate ions to synthesise phospholipids and other organic compounds. When animals feed on plants, the phosphate is transferred to organic compounds in the animals. Dead plant and animal material, and waste from animals (e.g. faeces), is decomposed by our old friends the saprobionts by extracellular digestion. Read the nitrogen cycle article for more information about these.
One way that phosphate ions can return to land from the oceans is via guano (bird poo!). Phosphate ions in the oceans are used to make organic compounds in primary producers such as algae, and sea birds feed on the algae. They then fly over land and excrete phosphate ions in guano.
Mycorrhizae
Mycorrhizae are fungi which have a symbiotic mutualistic relationship with plants. They associate with the plant roots and spread out long thin strands called hyphae into the soil. This increases the surface area that the plant has available to absorb water and mineral ions (such as phosphate ions) from the soil. In return, the fungi can get carbohydrates, amino acids and vitamins from the plant.
Summary
- Phosphate ions are needed to synthesise phospholipids, ATP and nucleic acids.
- Phosphate ions can enter the soil through weathering, deposition in guano, and decomposition by saprobionts.
- Mycorrhizae are fungi which increase the surface area of plant root systems.
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