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Activity and Community Composition of Denitrifying Bacteria in Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)-Using Solid-phase Denitrification Processes

Two laboratory-scale solid-phase denitrification (SPD) reactors, designated reactors A and B, for nitrogen removal were constructed by acclimating activated sludge with pellets and flakes of poly(3-hydoxybutyrate-co-3hydroxyvalerate) (PHBV) as the sole added substrate under denitrifying conditions, respectively. The average denitrification rate in both reactors was 60 mg NO3−-N g (dry wt) h under steady-state conditions, whereas washed sludge taken from the reactors showed an average denitrification rate of 20 mg NO3-N g (dry wt) h with fresh PHBV as the sole substrate. The difference in the denitrification rate between the two might be due to the bioavailability of intermediate metabolites as the substrate for denitrification, because acetate and 3-hydroxybutyrate were detected in the reactors. Most of the predominant denitrifiers isolated quantitatively by the platecounting method using non-selective agar medium were unable to degrade PHBV and were identified as members of genera of the class Betaproteobacteria by studying 16S rRNA gene sequence information. nirS and nosZ gene clone library-based analyses of the microbial community from SPD reactor A showed that most of the nirS and nosZ clones proved to be derived from members of the family Comamonadaceae and other phylogenetic groups of the Betaproteobacteria. These results suggest that the efficiency of denitrification in the PHBV-SPD process is affected by the availability of intermediate metabolites as possible reducing-power sources as well as of the solid substrate, and that particular species of the Betaproteobacteria play the primary role in denitrification in this process.