Unification of cell-scale metabolic activity with biofilm behavior by integration of advanced flow and reactive-transport modeling and microfluidic experiments
The bacteria Geobacter sulfurreducens (GS) is a promising candidate for broad applications involving bioelectrochemical systems (BES), such as environmental bioremediation and energy production. To date, most GS studies have reported biofilm-scale metrics, which fail to capture the interactions between cells and their local environments via the complex metabolism at the cellular level. Moreover, the dominance of studies considering diffusion-only molecular mass transport models within the biofilm has ignored the role of internal advection though the biofilm in flow BES. Among other things, this incomplete picture of anode-adhered GS biofilms has led to missed opportunities in optimizing the operational parameters for BES. To address these gaps, we have modernized a GS genome-scale metabolic model (GEM) and complemented it with local flow and reactive-transport models (FRTM). We tuned certain interactions within the model that were critical to reproducing the experimental results from a pure-culture GS biofilm in a microfluidic bioelectrochemical cell under precisely controlled conditions. The model provided insights into the role of mass transport in determining the spatial availability of nutrient molecules within the biofilm. Thus, we verified that fluid advection within biofilms was significantly more important and complex than previously thought. Coupling these new transport mechanisms to GEM revealed adjustments in intracellular metabolisms based on cellular position within the biofilm. Three findings require immediate dissemination to the BES community: (i) Michaelis-Menten kinetics overestimate acetate conversion in biofilm positions where acetate concentration is high, whereas Coulombic efficiencies should be nearly 10% lower than is assumed by most authors; (ii) unification of the empirically observed flow sensitivity of biofilm-scale kinetic parameters and cell-scale values are finally achieved; and (iii) accounting for advection leads to estimations of diffusion coefficients which are much lower than proposed elsewhere in the literature. In conclusion, in-depth spatiotemporal understanding of mechanisms within GS biofilm across relevant size scales opens the door to new avenues for BES optimization, from fine-scale processes to large-scale applications, including improved techno-economic analyses.