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

Abstract

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. Pseudocapacitive 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.

Comparing theoretical and practical biomass yields calls for revisiting thermodynamic growth models for electroactive microorganisms

“…The obtained results on biomass yields can be used to energetically optimize biotechnological applications based on anodic electroactive biofilms. Together with substrate uptake rates (Korth et al, 2020a;Kubannek et al, 2022) and an optimized anode potential (i.e., minimizing energy input for applying a potential but providing sufficient driving force for growth of EAM), a tailor-made feeding strategy for EAM could be designed optimizing biomass production. After EAM established sufficient biomass for a particular MET process, the anode potential (or the external resistance of a MFC) can be modulated for steering the energy fluxes towards improved energy efficiency of MET by decreasing the energy flux to the biomass.…”