Ecological study of a bioaugmentation failure

A nitrifying sequencing batch reactor was inoculated twice with the aerobic denitrifying bacterium Microvirgula aerodenitrificans and fed with acetate . No improvement was obtained on nitrogen removal. The second more massive inoculation was even followed by a nitrification breakdown, while at the sa me time, nitrification remained stable in a second reactor operated under t he same conditions without bioaugmentation, Fluorescent in situ hybridizati on with rRNA-targeted probes revealed that the added bacteria almost disapp eared from the reactor within 2 days, and that digestive vacuoles of protoz oa gave strong hybridization signals with the M. aerodenitrificans-specific probe. An overgrowth of protozoa, coincident with the disappearance of fre e-living bacteria, was monitored by radioactive dot-blot hybridization only in the bioaugmented reactor. Population dynamics were analysed with a newl y developed in situ quantification procedure of the probe-targeted bacteria . The nitrifying groups of bacteria decreased in a similar way in the bioau gmented and non-bioaugmented reactors. Other bacterial groups evolved diffe rently. The involvement of different ecological parameters are discussed se parately for each reactor. These results underline the importance of predat or-prey interaction and illustrate the undesirable effects of massive bioau gmentation.