Interactions between Nitrogen and Phosphorus metabolism

Nitrogen and phosphorus are essential for all organisms, and are often the elements that limit primary productivity. Both, nitrogen and phosphorus have many roles in photosynthetic organisms. In some roles (e.g. in nucleic acids and proteins) neither nitrogen nor phosphorus can be substituted by other elements. By contrast, polar lipids containing either nitrogen or phosphorus can be replaced by polar lipids lacking nitrogen or phosphorus. The nitrogen:phosphorus ratio in photosynthetic organisms is quite variable, and interpretation in terms of metabolically active nitrogen and phosphorus compounds is made difficult by variations in the storage pools of nitrogen and phosphorus. Geographical variations in organism nitrogen:phosphorus ratios have been rationalised in terms of external nitrogen and phosphorus availability or internal responses to (for example) the length of the growing season. Many interactions of nitrogen and phosphorus occur in their acquisition from the medium; examples include the production of ectoenzymes that require phosphorus (for RNA in protein synthesis) and nitrogen (in the RNA and ectoenzymes). These are involved in converting otherwise inaccessible organic nitrogen and phosphorus into forms that can be taken up.