Reaction kinetics of anodic biofilms under changing substrate concentrations : Uncovering shifts in Nernst‐Monod curves via substrate pulses

In the present study, it is shown that the concentration dependency of undefined mixed culture anodic biofilms does not follow a single kinetic curve, such as the Nernst-Monod curve. The biofilms adapt to concentration changes, which inevitably have to be applied to record kinetic curves, resulting in strong shifts of the kinetic parameters. The substrate concentration in a continuously operated bioelectrochemical system was changed rapidly via acetate pulses to record Nernst-Monod curves which are not influenced by biofilm adaptation processes. The values of the maximum current density j_max and apparent half-saturation rate constant K_s increased from 0.5 to 1 mA cm⁻² and from 0.5 to 1.6 mmol L⁻¹, respectively, within approximately 5 h. Double pulse experiments with a starvation phase between the two acetate pulses showed that j_max and K_s decrease reversibly through an adaptation process when no acetate is available. Pseudo-capacitive charge values estimated from non-turnover cyclic voltammograms (CV) led to the hypothesis that biofilm adaptation and the observed shift of the Nernst-Monod curves occurred due to changes in the concentration of active redox proteins in the biofilm. It is argued that concentration-related parameters of kinetic models for electroactive biofilms are only valid for the operating points where they have been determined and should always be reported with those conditions.