The computer exercise demonstrates an air-water interface with microorganisms attracted to the surface, but it does not show the movement of cells that makes this program interesting. Notice that the concentration of cells is high near the surface and decreases with depth, and that the organisms near the surface are relatively immobile compared with those in the bulk of the medium. Surface activity is important to mass transfer. Oxygen is a small molecule and diffuses much more rapidly than an organic molecule such as glucose. It also illustrates the fallacy of focussing exclusively on oxygen transfer rates, because the diffusion of a substrate or product may be rate controlling.
Real systems have many different surface active agents present.
Little is known about their interactions at an interface and even less is known about how microbial cells form at an interface when lipids, protein, and the like are packed in the same region. Consumption and excretion by the cells can alter the amounts of surface active substances. The effects of mixtures of cells and surface active agents on mass transfer should be a fertile research area. Some microorganisms are freely suspended in their medium while others aggregate or attach to a surface. Attachment imparts great directionality to mass transfer.