Surface activity is important to mass transfer. Oxygen is a small molecule and diffuses much more rapidly than an organic molecule such as glucose. This means that a barrier film of microorganisms at an air/water interface retards nutrient transfer so that the advantage of proximity to the oxygen supply cannot be fully exploited. 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. It would seem wise to use free cells in a bioprocess in order to maximize mass transfer, but other factors may be more important. For example, the activated sludge process for biotreatment of wastes must employ cell recycle, and the organisms that are not recovered by sedimentation are lost. There are also cases where activity of closely packed cells is higher than that of the same type of cells in a relatively thin suspension. For example, on a per cell basis, ethanol production by immobilized cells can be surprisingly high.
Attachment may be initiated when the concentration of some nutrient adsorbed at a surface is greater than its concentration in the medium. After colonies coat the surface, there is no reason why adsorption of nutrients should differ much from that for freely suspended cells. However, colonies or slimes on the benthos of a stream or lake can benefit from nutrient particulate matter that settles.
High density of cells in a bioreactor can give excellent productivity. Furthermore, using the cells over and over means that less nutrient must be expended for cell synthesis and more can go into making the product. There are several types of bioreactor designs that aim for very high cell density. One such method is collection of cells and recycle to the bioreactor. Another is simple retention in the bioreactor when the cells are attached to a surface. Cells that do not adhere well to surfaces may be bound, encapsulated, or entrapped. Mass transfer into aggregated cells and the gradients of oxygen and nutrients that are created are of great concern to biochemical engineers. Cells in the interior of aggregates may starve and die because other cells closer to the outside have intercepted the oxygen and nutrients.