Nitrogen is an extremely important element for life on Earth. Along with carbon, oxygen and hydrogen it is necessary for all living organisms. While the nitrogen gas that makes up nearly 80% of the Earth's atmosphere is largely inert, the element is combined into essential molecules of proteins (and their component amino acids) and nucleic acids.

In both natural and artificial systems, nitrogen passes through many stages and forms both food and waste for many organisms. The various stages that nitrogen passes through are known collectively as the nitrogen cycle. In simplistic terms the nitrogen cycle can be summarised as:

However, the nitrogen cycle is quite complex and it would not be easy to show all the combinations of paths and intermediate steps in the cycle. A more comprehensive, but still simple view of the nitrogen cycle is shown in Figure 1.


Figure 1: A simplistic view of the nitrogen cycle in the aquarium. The fish represents any or all animals in the aquarium. The arrows represent different paths nitrogen can flow and in some cases includes transformations.

Central to the nitrogen cycle are organisms that utilize and produce nitrogenous compounds. Algae and plants as autotrophs can utilize inorganic nitrogen compounds and animals as heterotrophs utilize organic nitrogen compounds. Bacteria, as a group, are able to utilize both organic and inorganic nitrogen compounds.

Feeding, excretion and death

As aquarists, we add organic matter as food for the animals in our care. Organic matter is matter that is formed by living organisms, contains carbon and may be animal, algal or plant in origin. The animals digest the food and utilise the carbohydrates, fats and proteins that make up the organic matter. The proteins are broken down into amino acids which can be used as is to form new proteins or metabolised. The products of metabolism of amino acids are carbon dioxide, water and ammonia (and some other compounds depending on the amino acid groups). Most marine animals excrete ammonia directly into the water. Not all of the ingested organic matter is utilised by the animals and it is excreted in faeces. While most of this food we add is eaten by the animals, either the ones we target feed, or other animals in the aquarium, some food is left uneaten by any animals.

As all organisms are largely composed of organic matter, any organisms that die in the aquarium, be that animal, plant, algal, bacterial, etc., will contribute to the organic matter in the aquarium.

Mineralisation

Mineralisation is the process by which organic matter is broken down into inorganic matter. Organic matter that is not eaten (and even matter that is eaten) is quickly broken down by bacteria which utilize some of the organic matter for their own use an release inorganic compounds. Mineralisation of organic nitrogenous compounds normally results in ammonia (NH3) and is also called ammonification.

The process of mineralisation of proteins can be summarised as follows:

Protein is first broken up into amino acid compounds known as peptides using specific enzymes (proteinases). The peptides are then broken into their component amino acids using other enzymes (peptidases). Finally the amino acids are deaminated to produce ammonium (NH4+) and organic acid. Ammonia (NH3) and ammonium are in equilibrium with the ratio of each dependent mainly on the pH and to a lesser extent on the temperature of the water.

Bacteria responsible for ammonification probably belong to the genera Pseudomonas, Vibrio, Proteus, Serratia, Bacillus and Clostridium (Herbert, 1999) and the process of ammonification provides them with energy. Additionally, actinomycetes and fungi may also be responsible for ammonification.

Ammonia is toxic and it is important it is utilised by organisms in the tank before it can build up cause problems for the other organisms in the tank. It can be nitrified by bacteria or assimilated by algae, plants and bacteria. In an established aquarium, ammonia be utilised as soon as it is produced and should never be detectable.

Nitrification

Nitrification is a two stage process in the oxidation of ammonia to nitrate. In the first stage, chemoautotrophic bacteria from the genera Nitrosomonas, Nitrosococcus, Nitrosospira, Nitrosolobus and Nitrosovibrio(Herbert, 1999) utilize ammonia as an energy source and produce nitrite (NO2-) as a by-product. The process of ammonia oxidation can be shown by the following formula:

In the second stage, chemoautotrophic bacteria of the genera Nitrobacter, Nitrosococcus, Nitrospina and Nitrospira(Herbert, 1999) oxidise nitrite to nitrate (NO3-). This process also provides energy to the bacteria and is shown by the following formula: Both of these stages require oxygen and so must be performed in the presence of free (dissolved) oxygen.

While nitrite is toxic to most organisms, it is generally less toxic than ammonia and far less toxic in a marine situation than in freshwater. However, like ammonia, it should be removed (through nitrification, denitrification or assimilation) before it can damage organisms that are sensitive to it. Established aquariums should not have detectable levels of nitrite.

Denitrification

Denitrification involves the reduction of nitrate and nitrite to the gases nitrous oxide (N2O) and nitrogen (N2). Anaerobic bacteria of a number of genera, including Pseudomonas, Bacillus and Alcaligines(Hargreaves, 1998) use the oxygen from nitrate and nitrite for their respiration as there is very little free oxygen available. These bacteria are generally facultative anaerobes (can live with or without free oxygen) (Hargreaves, 1998) and oxidise organic carbon (e.g. glucose) as they would in the presence of free oxygen. The energetic yield from under anaerobic conditions using nitrate is almost the same as that under aerobic conditions (Hargreaves, 1998). The process can be summarised as follows:

It can be seen that oxygen is made available at each step.

Assimilation

Autotrophs are able to utilize inorganic nitrogen compounds such as ammonia, nitrite and nitrate. Autotrophs include bacteria, algae and plants. Photoautotrophs use light as an energy source to assimilate ammonia, nitrite and nitrate into organic nitrogen compounds such as amino acids and proteins. While ammonia is the preferred, nitrite and nitrate will also be assimilated. In order to assimilate nitrate and nitrite, autotrophs must first reduce the nitrate to nitrite and then to ammonia and this requires more energy that assimilating ammonia directly. As there is much competition for ammonia, most autotrophs will have the "satisfied" with the oxidised versions.

Nitrogen fixation (not shown in diagram)

The addition of organic matter through feeding or live organisms is not the only way for nitrogen to enter the aquarium. Nitrogen gas (N2) makes up nearly 80% of the Earth's atmosphere. Nitrogen gas is only half as soluble in water as oxygen, but as there is four times as much nitrogen as oxygen in the atmosphere, there is usually twice as much nitrogen dissolved in aquarium water. Despite the large amounts of dissolved nitrogen in the water, it is largely unavailable to most organisms in the aquarium. An exception to this are specialised groups of prokaryotes (bacteria).

Some bacteria, including most representatives of cyanobacteria, are able to take up nitrogen (N2) directly from their environment, break apart the N-N bonds and fix the nitrogen into ammonia which can then be used for biological processes. This basically means that many cyanobacteria can survive and thrive in the absence of inorganic nitrogen compounds.

Summary

In summary, the nitrogen cycle is very important to organisms in our aquaria. It ensure that toxic compounds such as ammonia and nitrite are converted to harmless compounds such as nitrate and nitrogen gas. It also ensure that all the organisms in our aquaria are able to acquire the necessary nutrients for survival and growth.

References

Hargreaves J.A. 1998. Nitrogen biogeochemistry of aquaculture ponds. Aquaculture 166(3-4):181-212.

Herbert R.A. 1999. Nitrogen cycling in coastal marine ecosystems. FEMS Microbiology Reviews 23(5):563-590.


Last updated: October 28, 2004