Prediction of the minimum aggregate size required for oxygen depletion within an aggregate. (A, B) A previously described calculation that predicts the minimum size of a spherical aggregate necessary to deplete a solute at its center was used (). (A) The diffusion coefficient (De) of oxygen through a population of densely packed bacteria is 1.12 × 10−5 cm2 s−1 (, ), and the concentration of oxygen (So) in the aqueous environment at 25°C is 8.24 mg liter−1, the maximum amount that can be dissolved in water under ambient conditions. (B) The volumetric reaction rate of oxygen within the aggregate (ko = 46 mg s−1 liter−1) was calculated by using a P. aeruginosa specific growth rate of 0.56 h−1 (75 min) and the density of cells within the aggregate, 250 mg cm−3 (1012 cells ml−1) (for a more thorough explanation see ; see in the supplemental material) (, ). On the basis of these values, an Rmin of 35 µm was calculated. (C, D) Surface-attached 15- and 60-pl populations (radii of 17 and 27 µm, respectively) generated from representative aggregate measurements were used to predict the steady-state oxygen concentration profile within the aggregate microenvironment, given by ∇2c = −ko, where c is the oxygen concentration and ko is the oxygen uptake rate per unit volume of cells. The oxygen concentration in the external medium is assumed to be at saturation. The equation was solved via finite-element simulations in three dimensions, assuming that there was no penetration at the glass coverslip boundary. The highly porous microtrap wall was assumed not to pose a significant diffusive barrier to oxygen (, ). The simulations for each representative aggregate size and shape were implemented in COMSOL.