ABSTRACT During aerobic respiration, microorganisms consume oxygen (O ₂ ) through the use of different types of terminal oxidases which have a wide range of affinities for O ₂ . The K _m values for O ₂ of these enzymes have been determined to be in the range of 3 to 200 nmol liter ⁻¹ . In this study, we examined the time course of development of aerobic respiratory kinetics of four marine bacterial species ( Dinoroseobacter shibae , Roseobacter denitrificans , Idiomarina loihiensis , and Marinobacter daepoensis ) during exposure to decreasing O ₂ concentrations. The genomes of all four species have genes for both high-affinity and low-affinity terminal oxidases. The respiration rate of the bacteria was measured by the use of extremely sensitive optical trace O ₂ sensors (range, 1 to 1,000 nmol liter ⁻¹ ). Three of the four isolates exhibited apparent K _m values of 30 to 60 nmol liter ⁻¹ when exposed to submicromolar O ₂ concentrations, but a decrease to values below 10 nmol liter ⁻¹ was observed when the respiration rate per cell was lowered and the cell size was decreased due to starvation. The fourth isolate did not reach a low respiration rate per cell during starvation and exhibited apparent K _m values of about 20 nmol liter ⁻¹ throughout the experiment. The results clearly demonstrate not only that enzyme kinetics may limit O ₂ uptake but also that even individual cells may be diffusion limited and that this diffusion limitation is the most pronounced at high respiration rates. A decrease in cell size by starvation, due to limiting organic carbon, and thereby more efficient diffusion uptake may also contribute to lower apparent K _m values.