STRUCTURE AND FUNCTION OF A NITRIFYING BIOFILM AS DETERMINED BY IN-SITU HYBRIDIZATION AND THE USE OF MICROELECTRODES

Microprofiles of O-2 and NO3- were measured in nitrifying biofilms fro m the trickling filter of an aquaculture water recirculation system. B y use of a newly developed biosensor for NO3-, it was possible to avoi d conventional interference from other ions. Nitrification was restric ted to a narrow zone of 50 mu m on the very top of the film. In the sa me biofilms, the vertical distributions of members of the lithoautotro phic ammonia-oxidizing genus Nitrosomonas and of the nitrite-oxidizing genus Nitrobacter were investigated by applying fluorescence in situ hybridization of whole fixed cells with 16S rRNA-targeted oligonucleot ide probes in combination with confocal laser-scanning microscopy. Amm onia oxidizers formed a dense layer of cell clusters in the upper part of the biofilm, whereas the nitrite oxidizers showed less-dense aggre gates in close vicinity to the Nitrosomonas clusters, Both species wer e not restricted to the oxic zone of the biofilm but were also detecte d in substantially lower numbers in the anoxic layers and even occasio nally at the bottom of the biofilm.