Organic Nitrogen

Abstract

Dissolved organic nitrogen (DON) compounds in lakes and rivers originate from photosynthetic organisms (algae and plants) and excretion of nitrogenous waste by animals, but leachate from soil, sewage discharge, and atmospheric deposition can also contribute organic nitrogen to the water. Since most of the DON substances originate from biological matter, largely all nitrogenous compounds in living organisms can be found in the DON pool. Thus, dominant DON compounds in freshwater are proteins, free amino acids, amino sugars (from cell walls) and nucleic acids (from RNA and DNA), but excretion products such as urea and methylamines are also commonly found. In addition, some DON compounds are found in dissolved humic matter, but the chemical composition in uncertain. DON serves as valuable sources of nitrogen (and also carbon) to bacteria and some microalgae. The microorganisms readily take up and assimilate small DON molecules such as free amino acids and urea, but larger molecules like proteins and DNA must first be degraded to smaller molecules by extracellular enzymes before uptake. The microbial utilization of DON ensures that dissolved nitrogen compounds are cycled efficiently in the ecosystem and that the dissolved organic nitrogen becomes available to particle-feeding organisms at higher levels in the food web.