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Continuous cultures of Pseudomonas aeruginosa (ATCC 9027) maintained at different dissolved oxygen concentrations (DO) were studied for the effects of DO on various culture properties, especially aerobic respiration and denitrification. The DO was varied from 0 mg/liter (completely anoxic conditions) to 1.3 mg/liter and measured with optical sensors that could accurately determine very low DO based on oxygenquenched luminescence. The strain was found to perform aerobic denitrification; while the specific rate decreased with increasing DO, denitrification persisted at approximately 1/8 of the maximum rate (1.7 mmol/g of cells/h) even at relatively high DO (1 to 1.3 mg/liter). In the presence of nitrate, the culture's Monod half-rate saturation constant for O 2 was very small, <0.1 mg/liter. Aerobic denitrification appeared to function as an electron-accepting mechanism supplementary to or competitive with aerobic respiration. The shift of the culture's respiratory mechanism was also clearly detected with a fluorometer targeting intracellular NAD(P)H, i.e., the reduced forms of the NAD(P) coenzymes. Comparatively, the NAD(P)H fluorescence under the anoxic, denitrifying conditions (NFU DN ) was highest, that under fully aerobic conditions (NFU OX ) was lowest, and that under conditions in which both denitrification and aerobic respiration occurred ( Oxygen is only sparingly soluble in water. Consequently, the uneven distributions of water flow, nutrients, and microbial populations create a continuous and dynamic spectrum of aerobic, microaerobic, and anaerobic or anoxic conditions in the often heterogeneous, complex environments. The ecological fate of different organic compounds varies differently with changing dissolved oxygen concentrations (DO) (17). For example, oxygenated compounds are largely biodegradable in all conditions (at different rates) (13), while highly chlorinated hydrocarbons are more susceptible to sequential degradation, a reductive dechlorination under anoxic conditions (e.g., by sulfate-reducing bacteria or methanogens) followed by aerobic mineralization (25). Knowing how microbial metabolisms for different organic materials change with varying DO is essential for the modeling of their ecological fate for risk assessment and management as well as for the development of advanced bioremediation technology.Microaerobic conditions are, however, ill defined. From the simple point of view of microbial respiration, aerobic conditions correspond to those in which the organism(s) uses O 2 as the terminal electron acceptor (aerobic respiration); anaerobic or anoxic conditions correspond to those in which the organism(s) performs fermentation (without external terminal electron acceptors) or uses chemicals other than O 2 as terminal electron acceptors (anaerobic respiration) (17). Accordingly, microaerobic conditions may be defined as the transition conditions in which the organism(s) performs simultaneous aerobic and anaerobic respiration or fermentation.The lack of accurate and stable devices for meas...

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