The bacterial communities that live in our grow beds play an essential role in the operations of all aquaponic systems. They remove fish ammonia and supply essential nutrients to the plants. Aquaponics bacteria form a diverse and complex community of microorganisms unique to each individual system. The Nitrogen Cycle is a complex web of relationships formed according to the local environmental conditions and food supply. There are two main groups of bacteria that are of prime importance.
- Autotrophic Ammonia Oxidising Bacteria. (AOB) These are often referred to as nitrifiers as they convert ammonia into nitrite and then nitrate through the process of nitrification. They are called autotrophic as they are capable of metabolising ammonia without a source of carbon for energy. They play a vital role in removing toxic ammonia. Find out how to keep nitrifiers happy.
- Heterotrophic Bacteria. This group are able to utilise an organic source of carbon for their energy needs. They are capable of much faster growth rates and are more aggressive that AOB.
These two groups have very different needs and occupy different zones within a biofilter. Competition between the two is common. Which species dominates is very much related to the presence of organic carbon compounds, more commonly known as fish poo.
AOB, or nitrifying bacteria, have a strong desire to attach themselves to solid surfaces. To help keep themselves attached to these surfaces they will produce a slimy matrix of sugars and proteins otherwise known as a biofilm. The resultant micro-habitats are able to harbour an extremely diverse and complex population of living organisms. Typically nitrifies represent just 5% of this community. (Burell et al 2001) Nevertheless they still play a dominant role in nitrification.
The makeup of the rest of the community is dependent upon the local environmental conditions. If there is plenty of organic carbon available the heterotrophic bacteria will rapidly reproduce and grow over the top of the underlying population. This will reduce the ability of oxygen and ammonia to reach any underlying AOB and, by so doing, reduce the rate of nitrification. Eventually these heterotrophic bacteria may outcompete the nitrifiers. As the level of organic carbon declines then so does the thickness of the heterotrophic layer. The underlying AOB are then better able to reach their own food supply, i.e. ammonia.
This overcoating is not as disastrous as it may sound . The ‘overcoat’ of slime and bacteria provide a level of stability and resistance for the AOB. In particular they are able to survive several days of starvation without loss of viability. In addition, a level of protection is afforded against pollutants, including therapeutic drugs, that may be present in the fish tanks. It is rare to find any antibiotic that is capable of completely killing off a biofilter.
Nitrosomonas sp. is the most common AOB and tends to be found throughout the biofilm. Nitrite reducing Nitrospira sp tends to favour the inner reaches of the biofilm. This may be a refection of the fact that Nitrosomons has a doubling time of around 26 hours whereas Nitrospira exhibits much slower growth rates with a doubling time of around 60 hours. (Belser, (1984); Hagopian and Reily (1998). Nitrite oxidisers also have a much higher affinity for dissolved oxygen than ammonia oxidisers and can therefore live in the innermost oxygen depleted zones of a biofilm. (Aoi et al, 2004).