STRUCTURE AND FUNCTION OF A NITRIFYING BIOFILM AS DETERMINED BY MICROELECTRODES AND FLUORESCENT OLIGONUCLEOTIDE PROBES

Microelectrodes for O-2 and NO2-/NO3- and fluorescently labelled 16S r RNA-targeted oligonucleotide probes were combined to examine the activ ity and stratification of nitrifying bacteria in a trickling filter bi ofilm, Microprofiles showed that O-2 consumption and NO3-/NO2- product ion were restricted to the upper 50-100 mu m of the biofilm, The verti cal distribution of the nitrifying bacteria Nitrosomonas sp. and Nitro bacter sp. was investigated by fluorescent in situ hybridisation (FISH ) with specific oligonucleotides. Nitrifiers formed a dense layer of c ells and cell clusters in the upper part of the biofilm, This correlat es well with the measured activity profiles, Ammonia-and nitrite-oxidi sers occurred in close vicinity to each other supporting a fast sequen tial metabolism from ammonia to nitrate. Both species were not restric ted to the oxic part of the biofilm, but also appeared -in lower numbe rs-in the anoxic layers on the bottom of the biofilm, A short term dec rease in the O-2 concentration of the bulk water resulted in a quick d ecrease in O-2 penetration and metabolic rates inside the biofilm. How ever, neither the stratification nor the cellular ribosome content of nitrifiers changed within a few hours. (C) 1997 IAWQ. Published by Els evier Science Ltd.