What's living in your filter?

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Dr Timothy Hovanec is a world expert on the bacteria found in aquarium filtration equipment. Here he explains a little more about what's living in our filters.

How has our knowledge about filter bacteria changed over the years?
The identity of major bacteria associated with aquarium biological filters was once thought already known. Nitrosomonas europaea was the ammonia-oxidising bacterium and Nitrobacter winogradskyii the nitrite-oxidising bacterium.

It did not matter if the aquarium was freshwater or saltwater, warm or cold, low pH (Amazonian) or high (Lake Tanganyika) — the bacteria were the same because bacteria are bacteria are bacteria!

This simplistic view changed in the 1990s with the advent of modern molecular methods of PCR, DNA fingerprinting and molecular probes. This allowed microbial experts to study the ecology of nitrifying bacteria at the species level, much like territorial ecologists could study plants or animals.

As someone using the above methods I am not completely unbiased with regard to the importance of nitrifying bacteria in aquaria, but the fish and corals are nice too!  

So what species of bacteria are responsible for the nitrogen cycle?

The results of many studies have shown that nitrite oxidation in aquaria is dominated by bacteria belonging to the genus Nitrospira.  

Ammonia is oxidised by newly discovered species of Nitrosomonas that are not N. europaea. Further, the environment plays a major role in determining which species of nitrifying bacteria are present in the biological filters.  

Does this mean that bacteria in freshwater systems are different to those in marine ones?

Freshwater systems have a different species mix than saltwater ones.  

Even more interesting is that in cold saltwater systems of less than 10°C/50°F ammonia oxidation is carried out by members of the Archaea domain and consists of organisms very different to bacteria.

Research on nitrifying bacteria in aquaria continues at universities and private companies, with new insights announced regularly.

It seems our 'simple fish tanks' are not quite so simple, instead playing a major role in microbial ecology of nitrification.