Investigation of acetate uptake kinetics of mature Geobacter sulfurreducens biofilms in continuous bioelectrochemical systems reveals unexpected challenges

Mature Geobacter sulfurreducens biofilms cultivated at two distinct anode potentials, representing unlimited and limited electron acceptor availability, were subjected to different acetate concentrations in a continuous reactor system. The Nernst-Michaelis-Menten equation, being modified to consider the conditions of a continuous stirred-tank reactor, was applied to analyze the acetate uptake kinetics. For the first time, the kinetics of pure G. sulfurreducens biofilms based on quantification of the acetate consumption were determined. Acetate uptake parameters ($K_{M,\mathrm{Ac}}$ = 1.41 ± 0.42 mM, $v_{\text{max},\mathrm{Ac}}$ = 1.10 ± 0.12 mmolAc⁻ h⁻¹ g_DW⁻¹, n = 4) obtained from cultivation at 0.4 V (vs. SHE), showed significant differences compared to an approach that derived acetate uptake parameters from the current production. This deviation is likely related to the coulombic efficiency, which exhibited a dependence on the acetate concentration. The coulombic efficiency was 30–50 % for acetate concentrations <1 mM, and saturated at 78.6 ± 4.0 % for more than 2 mM acetate. Despite the Nernst-Michaelis-Menten equation being developed to cover limiting terminal electron acceptor conditions, its application to biofilms cultivated at −0.1 V yielded contradictory results indicating application limits. Considering the complexity and plasticity of acetate uptake kinetics together with the complex extracellular electron transfer machinery, Geobacter sulfurreducens biofilms are a model system that is shown to provide unexpected challenges and demanding highly controlled experimental conditions.