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The use of immobilized cells is advocated for a number of bioengineering purposes, e.g. volumetric production rate, retention of expensive biomass, product quality, and convenient downstream processing. Up to now, succesfull application of such biofilms is rather empirical, since detailed knowledge of the internal structure and performance usually is not available. Due to inhomogeneous biomass density, variable kinetics, limited substrate penetration, starvation, high internal product concentrations and inhibitory effects, mathematical modelling of concentration gradients in active bioflims is of limited value. Therefore, direct measurements inside biofilms are needed to verify these models and to improve the mechanistic understanding of biofilm processes. The spatial resolution of classical analytical techniques, such as concentration measurements in extracted pore water from sliced biofilms, amounts to about 1 mm and is insufficient for biofilms with high activity and a concomitant substrate penetration depth of about 100 jim. Microelectrodes, i.e. needle-shaped devices with a sensitive tip, have proven to be most suitable and indispensable tools for these kind of measurements. Interesting studies with oxygen microelectrodes have been performed in e.g. penicillium pellets (Wittier et al., 1986) and trickling filter biofilms (Kuenen et al., 1986), revealing internal convective flows and local oxygen production by algae, respectively. |
