Nitrobacter and Nitrospira genera as representatives of nitrite-oxidizing bacteria: detection, quantification and growth along the lower Seine River (France)

Pollution from agriculture and urban effluents influences the ecology and biochemical functioning of the Seine River. Nitrification dominates nitrogen transformations downstream of the effluents of the Paris wastewater treatment plant (WWTP) at Achères, treating, by activated sludge the wastewater of 6.5 million inhabitant equivalents from Paris and its suburbs, without nitrification and denitrification treatment. It discharges effluents containing large amounts of nitrogen, ammonium mostly (30 mg L⁻¹ N-NH₄⁺ L⁻¹), on average 45 mg L⁻¹ of suspended particulate matter, high quantities of total organic carbon (30 mg C L⁻¹) largely biodegradable (40%), and high concentration in total phosphorus (3 mg Tot P L⁻¹), as well as microorganisms. Ammonium, brought into the river system, is slowly nitrified in the lower Seine River and especially in the freshwater estuary. The nitrifying activities can be observed by measuring inorganic nitrogen compound concentrations and potential activities. To understand the contributions of the WWTP effluents, the upstream agricultural runoff water and the Seine tributaries, it is useful to investigate the bacterial community. Whereas ammonia oxidation has been widely studied, the second step, i.e. nitrite oxidation, is less well understood. We have previously analysed the ammonium-oxidizing bacterial (AOB) community in the Seine (Cébron, A., Berthe, T., Garnier, J., 2003. Nitrification and nitrifying bacteria in the lower Seine River and estuary (France). Appl. Environ. Microbiol. 69, 7091–7100; Cébron, A., Coci, M., Garnier, J., Laanbroek, H.J., 2004. DGGE analysis of the ammonia oxidizing bacterial community structure in the lower Seine River: impact of the Paris wastewater effluents. Appl. Environ. Microbiol. 70, 6726–6737), and focus here on the composition of the nitrite-oxidizing bacterial (NOB) community. As no general molecular probe targeting all known NOBs is currently available, we chose to target and quantify (by competitive PCR) the two genera Nitrobacter and Nitrospira assumed to be the major players in nitrite oxidation in freshwater environments. Nitrobacter species were dominant in the upstream Seine River basin but Nitrospira was the dominant NOB downstream of the WWTP. These two genera were equally represented in WWTP effluents. In the Seine River estuary, especially in the salinity gradient, the Nitrobacter proportion increases and that of Nitrospira disappears, possibly due dilution by seawater.