Dissolved oxygen in microbial slimes
Some years ago, our research group used tiny microelectrodes to measure the dissolved oxygen concentrations near and inside a microbial slime bathed in flowing nutrient. Our first data showed a curve similar to the blue lines in the next sketch:
We thought that the microelectrode was no good because textbooks showed figures like the red lines. Ours were the first real data. We quickly realized that a microbial slime in contact with very dilute nutrients has little demand for oxygen, and ordinary diffusion provide fairly high concentrations throughout the slime. The situation is quite different when there is a high concentration of organic nutrients. Now there is a high demand within the slime, but diffusion cannot keep up. The concentration within the slime and remote from the interface can be very low.
Why doesn't the D.O. approach zero ?