Biodiversity of N-cycle bacteria in nitrogen removing moving bed biofilters for freshwater recirculating aquaculture systems

Maintenance of optimal water quality and removal of nitrogen compounds pose challenges to aquaculture worldwide. Presence and activity of different bacteria involved in nitrogen cycling in the biofilm of a moving bed biofilm reactor (MBBR) connected to a Koi carp tank were investigated experimentally. For this MBBR system, a nitrogen removing rate of 3.5 g nitrogen per day was found in activity assays, whereas the predicted nitrogen removal rate was calculated to be 58 g nitrogen per day. The presence of ammonium-oxidizing and nitrite-oxidizing bacteria in the biofilm was demonstrated by situ hybridization (FISH) analysis. A large number of planctomycetes were shown to be present in the biofilm of the reactor, but they did not hybridize to oligonucleotide probes of known genera of anammox bacteria. Molecular 16S rRNA gene analysis of the MBBR biofilm revealed a novel group of planctomycete cells was present in this biofilter, which may explain the difference in experimentally found and calculated nitrogen removal of the system. The novel 16S rRNA gene sequences showed a low similarity to described anammox bacteria. This and other studies may implicate that the anammox reaction can be incorporated in biofilter systems for aquacultural freshwater systems, which makes it possible to remove nitrogen compounds from these systems in a relatively inexpensive and sustainable manner.