One of the most important cycles in your reef aquarium is the nitrogen cycle. Nitrogen (N ₂ ) is a fundamental component of many biomolecules and occurs, for example, in proteins and amino acids.

The algae in your aquarium use inorganic nitrogen compounds to build up biomass and grow. If fish or other reef dwellers eat these algae, the N compounds they contain are broken down and the excess nitrogen is released into the environment in the form of ammonia (NH ₃ ), ammonium (NH ₄ ⁺ ), urea (CH ₄ N ₂ O) or uric acid (C ₅ H ₄ N ₄ O ₃ ). These are therefore the temporary end products or waste materials that cannot be further utilized by the organisms.

Bacteria now begin to convert the excreted nitrogen compounds into inorganic compounds. They do this in order to access the carbon contained in the organic compounds. The resulting inorganic nutrients are then available to algae and corals, which means that the nitrogen cycle is largely self-contained.

Nitrification vs. denitrification

Within the nitrogen cycle, two metabolic pathways carried out by bacteria are of particular importance for your reef aquarium. These are nitrification and denitrification.

During nitrification, ammonium (NH ₄ ⁺ ) is converted via nitrite (NO ₂ ^- ) to nitrate (NO ₃ ^- ). This reaction is mainly carried out by the two bacteria Nitrosomonas and Nitrobacter in the presence of oxygen. It is therefore referred to as oxidation.

The nitrate formed during nitrification is converted to molecular nitrogen (N ₂ ) during denitrification. This reaction is a so-called reduction. It can only take place in the absence of oxygen, i.e. under anoxic conditions.

Ammonium vs. Ammonia

The conversion of ammonium to nitrate is of great importance because, depending on the pH value, ammonium can be converted into toxic ammonia, while nitrate is harmless.

Ammonia is toxic to almost all organisms in your reef aquarium. The actual concentration of ammonia can be calculated indirectly using the pH value and the ammonium concentration. The concentration of ammonia must not exceed 0.01 mg/l. The ammonium content should also not be higher than 0.1 mg/l.

nitrite

The oxidation of ammonium produces nitrite. Although this is less dangerous than ammonia in seawater, it is still considered a toxic compound whose concentration should not exceed 1 mg/l.

This is especially true for fish, since nitrite oxidizes the iron contained in the red blood pigment hemoglobin and thus negatively affects the transport of vital oxygen (O ₂ ).

However, high nitrite concentrations are less harmful in saltwater aquariums than in freshwater. This is because the chloride ions (Cl ^- ) contained in seawater compete with the nitrite molecules on the fish's gills for absorption into the blood. Since the concentration of chloride ions is much higher, less nitrite is absorbed into the fish's blood.

nitrate

Nitrate is produced through the oxidation of nitrite. Although this is harmless to the organisms in your reef aquarium, it can be absorbed particularly easily by algae and can therefore potentially lead to unwanted and excessive algae growth.

This also applies to the zooxanthellae of your corals. Due to the increased nitrate concentrations, they begin to multiply rapidly and compete directly with the corals for important substances and compounds. This ultimately leads to your corals growing more slowly and their tissue turning increasingly brown.

To counteract this, it is recommended to remove or reduce the nitrate in the aquarium so that its value does not exceed a concentration of 5 to 10 mg/l.

As already described, nitrate can be converted into molecular atmospheric nitrogen under anoxic conditions using what is known as denitrification. This process takes place inside your live rock, for example, while nitrification processes take place on the surface.

Factors influencing the nitrogen cycle

First of all, the type and number of bacteria are crucial for a functioning cycle. Bacterial preparations such as Tropic Marin Nitribiotic can help to accelerate or support the nitrogen cycle. A fine-pored filter material offers a particularly large settlement area and promotes the formation of a large bacterial population. The oxygen supply also plays a major role. The aeration or flushing of filter material or the substrate should be checked regularly and adjusted if necessary. You can also increase bacterial activity by feeding the bacteria. Examples of bacterial food are various carbon sources such as ethanol (keyword "vodka method") or commercial products such as Tropic Marin Elimi-Phos .