Nitrogen

Nitrogen (abbreviation N) is widespread and relevant in aquaristics in various forms. It occurs as atmospheric nitrogen, ammonia, nitrite, nitrate and as organic nitrogen.

Nitrogen is present in the atmosphere as molecular nitrogen N₂ and accordingly also dissolved in water. It can be used as a nitrogen source by a few aquatic organisms, e.g. cyanobacteria with heterocysts. This process is called nitrogen fixation. Molecular nitrogen is also the end product in the denitrification of nitrite and nitrate to nitrogen.

Heterotrophic organisms (animals, many bacteria) release the excess nitrogen taken up as ammonia/ammonium NH₃ / NH₄ (ammoniotelic). Preferably plants, algae, bacteria and corals take up the NH₄ form of nitrogen = ammonia assimilation. Ammonia and ammonium are in equilibrium, which is strongly dependent on the pH value, as well as on temperature and salinity. Ammonia is quickly toxic at higher pH values, whereas ammonium is not.

The first stage of nitrification converts ammonia/ammonium to nitrite/salpetrous acid.

Nitrite NO₂ is in equilibrium with nitrous acid HNO₂ (pH, temperature, salinity). At low pH values - many freshwater biotopes - the toxic nitrous acid is present - caution should be exercised with nitrite measurements above 0.1 mg/l in freshwater. In seawater with high pH, concentrations up to 10 mg/l nitrite do not matter. Nitrite can also be absorbed and utilised by algae and zooxanthellae (nitrite assimilation). Nevertheless, high nitrite concentrations in seawater should be regarded as a sign of incomplete filter performance and corrected.

The second stage of nitrification converts nitrite/salpetrous acid to nitrate/nitric acid.

Nitrate NO₃ is also in equilibrium with an acid, namely nitric acid HNO₃. However, the pH value must be so low for the toxic nitric acid to be present that it does not play a role in practice. Nitrate is tolerated in high concentrations, but indicates the condition of an over-fertilised (eutrophic) aquarium system.

In addition to the inorganic nitrogen compounds listed above, there are also organic nitrogen compounds. The number of compounds is so enormous that only the most important groups are mentioned here: Amino acids, urea, proteins, humic substances (yellow substances), amino sugars and many biopolymers.
Important for reef aquaristics is the role of dissolved organic nitrogen DON and particulate organic nitrogen PON. They serve as food for corals and filter feeders. Of course, any form of plankton and aquarium food is also considered an available organic nitrogen source.

The sum of organic and inorganic nitrogen is called total nitrogen (TN).