Quantification of nitrifying bacteria of the genus Nitrobacter in an aquatic system (Seine estuary, France)

In the Seine estuary, nitrification is a major process involved in the oxygen budget, together with photosynthesis, heterotrophic bacterial degradation and reaeration. Within the framework of the Seine-Aval programme, we attempted to further understand the process using a molecular approach that was expected to detect nitrifying bacteria, and also to quantify them. On the basis of the PCR approach using specific primers of the genus Nitrobacter, we coupled a competitive PCR method in order to quantify these Nitrobacter, a genus widely distributed in freshwater aquatic ecosystems. This molecular approach was conducted in parallel with a measurement of potential nitrifying with a (14)C radiotracing method. We showed that Nitrobacter represent less than 0.1% of the total bacterial community, as determined by epifluorescence microscopy, and that potential nitrifying activity increased with Nitrobacter numbers. Following that methodological step demonstrating a real feasibility, a possible relationship between these two measurements is expected on the basis of five longitudinal field studies already realised. (C) Academie des Sciences / Elsevier, Paris.