2005 Volume 20 Issue 1 Pages 25-33
A laboratory-scale solid-phase denitrification process for nitrogen removal was constructed by acclimating sewage activated sludge with poly(ε-caprolactone) (PCL) as the sole substrate under denitrifying conditions. The sludge thus acclimated exhibited a nitrogen removal rate of 6-7 mg NO3-N g-1 h-1 with PCL as the sole source of reducing power. The microbial community and the distribution of denitrifying bacteria in this process were studied by rRNA-targeted fluorescence in situ hybridization, quinone profiling and standard cultivation methods. The culture-independent molecular and biomarker approaches demonstrated that members of the class Alphaproteobacteria predominated and those of Betaproteobacteria were the second most abundant group of bacteria in the process. The plate counts of denitrifying bacteria with a non-selective agar medium accounted for 6% of the total count and 10% of the direct viable count on average. The most probable number (MPN) obtained with PCL-containing medium under denitrifying conditions was one order of magnitude lower than the plate count. Most of the denitrifying isolates from the MPN enrichment tubes and 10% of the predominant denitrifying bacteria isolated by the plate-counting method were capable of degrading PCL. 16S rRNA gene sequence comparisons showed that the greater majority of the predominant denitrifiers were members of the genera Comamonas, Diaphorobacter and Paracoccus. All of the PCL-degrading denitrifying strains isolated were assigned to a previously unknown species of the genus Comamonas. The results of this study suggest that, apart from their PCL-degrading capacity, members of Alphaproteobacteria and Betaproteobacteria are mainly responsible for nitrogen removal in the PCL-acclimated denitrification process.
