Diauxic Lag of Denitrifying Bacteria in a Continuous Flow Reactor Under Alternating Oxic/Anoxic Conditions

Pseudomonas denitrificans were exposed to alternating oxic/anoxic cycles in a continuous flow bioreactor. In the first trial, the length of diauxic lag measured after a preliminary switch from oxic to anoxic conditions was 9.5 hours. Subsequently, a period of oxic/anoxic cycling was imposed, with 8-hr oxic phases and 4-hr anoxic phases. During the period of cycling, there was slight growth before the end of first anoxic phase, but no growth in subsequent anoxic phases. Substantial growth was observed in all oxic phases. In the second trial, the preliminary switch showed a dauxic lag of 9.7 hours, similar to the first trial. Oxic/anoxic cycling was then imposed, with oxic phases of 16 hours and anoxic phases of 8 hours. Significant growth was observed before the end of the first anoxic phase, somewhat less in the second anoxic phase, and almost none in the third anoxic phase. By the fifth cycle, no growth was observed under anoxic conditions. Robust growth was observed during the oxic phases. A final oxic/anoxic switch, with extended anoxic phase, revealed a diauxic lag of 15 hours. Extended oxic/anoxic cycling is thus seen to increase the length of diauxic lag exhibited by P. denitrificans, a phenomenon not predicted by current models of denitrification.