Changes in Nitrobacter serotypes biodiversity in a river: Impact of a wastewater treatment plant discharge

We have studied the impact of a wastewater treatment plant (WTP) effluent on a freshwater biofilm autochtonous bacterial microflora (Nitrobacter sp.). Physicochemical parameters showed drastic changes after wastewater discharge in the river: decrease of dissolved oxygen concentration, increase of N---NH+4 as well as dissolved organic nitrogen and dissolved organic carbon. Nevertheless the standard conditions of nitrification (pH, temperature, oxygen level) downstream of the discharge point were not limiting for the nitrification process. The benthic Nitrobacter populations, involved in the nitrification process, changed qualitatively and quantitatively after the WTP discharge. Immunofluorescence labelling techniques allowed us to identify 6 different serotypes (AG, DE30, DE11, X14, LL and W) and our surveys showed important variations of their relative abundance. WTP discharges represented great quantities of Nitrobacter (from 106 to 108 cells•ml−1) according to the serotypes which were all present in the effluent. The definition of a qualitative serotype index showed that the Nitrobacter serotype biodiversity upstream of the WTP discharge was constant all year long: the downstream biodiversity was likely linked to the wastewater input and to its consequent environmental disruptions.