Development of stable denitrifying cultures during repeated aerobic-anaerobic transient periods

The influence of repeated alternating aerobic-anaerobic growth conditions on the development of stable denitrification activity was studied with continuous cultures of Paracoccus denitrificans and with activated sludge. Denitrification activity was determined by measuring the formation of intermediates and by analysis of mRNAs for the individual dentrification enzymes. Primarily, the length of the anaerobic period influenced the synthesis of the denitrification enzymes of P. denitrificans. When the anaerobic period was 24 h, the cultures reached stable denitrification activity within the first anaerobic phase. Then, dinitrogen was the only denitrification product released. In contrast, when the anaerobic periods were short (1.5 h), P. denitrificans was not able to completely synthesize a stable denitrifying enzyme system within the course of the experiment, and a semi-steady stale was reached. Nitrite accumulated in the medium as the predominant denitrification product. The transient characteristics of the activated sludge during the repeated aerobic-anaerobic conditions were similar to those observed with P. denitrificans. This finding indicates that information on the induction process of denitrification in P. denitrificans can be used reasonably well to assess induction of denitrifying activity in more complex systems. Finally, the usefulness of the application of mRNA probes are discussed. (C) 1997 Elsevier Science Ltd.