The Nitrogen Cycle plays an essential role in all aquaponic systems, ponds and aquaria. Nearly all recirculating aquaculture systems (RAS) also rely on it to maintain healthy fish. It’s importance helps explain why biofilters are such an essential component of these systems.
What is Nitrogen
Gaseous nitrogen is all around us, making up 78% of the Earth’s atmosphere. It is the most common element on our planet. It is inert, remarkably stable but unavailable to plants. Nevertheless, it is absolutely essential for all life. It is a key ingredient of proteins, amino and nucleic acids. Nitrogen really is in our DNA. Fortunately, nitrogen can exist in a number of different interchangeable forms that together make up what is called the Nitrogen Cycle. Progress through the circle can be brought about by physical and biological systems.
The Nitrogen Cycle consists of a number of interlinked, but distinct, stages. A simplified version involving five key processes is shown above.
In reality the Nitrogen Cycle is an extremely complex set of processes involving a huge range of different organisms, The more modern understanding is that it is less of circle and more of a web. Progression through the web is largely possible by the actions of different groups of bacteria. Scientists love to give each group their own descriptive but mind-numbing name. The important thing to remember is that each one of these bacteria have their own particular set of requirements in which they thrive. In an effort to simplify the understanding of these processes the Nitrogen Cycle can be sub-divided into the following main processes.
It has long been recognised that the process of nitrification plays a fundamental role in the success of a biofilter. It is essentially a two-step process that is performed by different groups of bacteria. The first stage involves the conversion of fish ammonia to nitrite. the second , the nitrite to nitrate. It is a vitally important process for fish because nitrite is less toxic than ammonia and nitrate considerably less toxic than nitrite.
There are numerous groups of bacteria that carry out the first stage of ammonia breakdown but Nitrosomonas is probably the most important. Certainly it is the one most commonly cited and used to represent all ammonia oxidising bacteria (AOB). Other species known to also carry out this reaction include, Nitrosospira and Nitrosococcus sp. The chemical reaction that occurs can be simplified as below;
During this reaction there is a release of 84 kcal/mol of energy. (Hagopian and Riley, 1998) The second stage involves another group of bacteria of which Nictrobacter is the species most commonly cited as the predominant species. More recent research has established that Nitrospira spp, is in fact more prevalent. Here the chemical reaction involved is;
During this reaction there is a release of 17.8 kcal/mol, roughly 25% of reaction (1)
Two important things to note;
- Both reactions require the presence of oxygen. If this is absent nitrification cannot proceed.
- H+ are produced and hence the pH tends to fall.
It is therefore important that fish keepers consider the conditions under which both Nitrosomona and Nitrospira spp. will thrive.
This is the process of converting organic nitrogen into ammonia. This can happen if a fish, or a plant, dies in a tank and is not removed. Fish faeces also contain a significant amount of organic nitrogen. This is largely in the form of undigested protein, and is a much more common ingredient of ammonification. Bacteria will seize on this food supply and ultimately produce ammonia through the process of ammonification. This is a key reason why faeces should be removed on a regular basis if the filter is unable to cope with this additional load.
Many fish keepers have a good appreciation of the advantages that nitrification can bring by reducing levels of ammonia. Few, on the other hand, have heard of ammonification. It is the science behind the recommendation that dead fish, uneaten food and fish faeces should be removed as soon as is practically possible. This is not only important from the ammonia viewpoint. It makes even more sense when dissolved oxygen levels are taken into consideration.
Another potential promoter of ammonification is uneaten food. The quantity of nitrogen contained within food is higher than fish faeces so it has the potential for even higher production of ammonia.
This stage completes the cycle by converting the nitrate produced by nitrification, as shown in equation (2), into gaseous nitrogen that returns to the atmosphere. In common with fixation this process only occurs in the absence of oxygen.
The above description is very much an oversimplification of what is actually happening. It is intended to highlight the more important stages for fish keepers. Denitrification is far more complex than nitrification as it involves many more species of bacteria. There are a number of key points to note;
- Denitrification occurs in the absence of oxygen
- A separate source of carbon is required for denitrification to proceed.
- Ammonia and nitrite may also be produced under certain conditions.
In the early days methanol was a popular source of carbon for denitrification filters. Since then a whole variety of other sources have been tested including sugar, starch and even vodka! Critical control methods were needed as the cost was often prohibitive. Attention has more recently centred fish waste as the carbon source. This usually involves a digester with the liquid produced being drip fed into the denitrifying filter. Work in Israel looked at using cotton wool mixed with plastic beads. There is still much to discover with regards to practical denitrification filters.
This stage involves converting gaseous nitrogen into organic nitrogen. Nitrogen is extremely stable and much energy is needed to convert it. Combustion, volcanoes and lightening are some of the physical processes that have the capacity of breaking down gaseous nitrogen. Luckily for life on Earth there is a small group of bacteria that are able to carry out this process, with the aid of the enzyme nitrogenase. A key requirement for nitrogenase to work is the absence of oxygen, Some plants, such as legumes, are able to provide the conditions within special nodules. Bacteria living within these nodules can ‘fix’ nitrogen directly from the atmosphere. Most fixation however is carried out by soil-borne bacteria.
Nitrogen assimilation is the formation of organic nitrogen compounds, such as amino acids, from inorganic nitrogen compounds like nitrate and ammonia. These are the building blocks of proteins and essential for growth.
Plants take up these inorganic compounds via their roots before incorporating them into proteins. If a plant is unable to fix nitrogen directly it is dependent upon the process of assimilation.
If oxygen is present then nitrate is the end product of nitrification and is consequently the most abundant nitrogen compound absorbed by plants. Ammonium ions, on the other hand, are also capable of being absorbed under certain conditions..