Geobacter Sulfurreducens Biofilm Growth Kinetics on High Surface Area, Flow-through Anodes Under Ion-Transport Limitations

Anode-respiring bacteria like Geobacter sulfurreducens have been used to generate electrical current in many different bioelectrochemical systems, including microbial fuel cells and microbial electrolysis cells. Various cell designs have led to increases in the current densities of these systems when normalized to the projected surface area of the anodes by using 3D architecture and various convection mechanisms, with the result being an increase in current density by around five-fold. However, this increase does not nearly match the increase in available surface area. We hypothesized that the surface area is in excess in these systems, and that the current density is probably limited by ion-transport. To investigate this hypothesis, we designed a flow-through, 3D anode bioelectrochemical reactor to investigate the effect of the ion-transport limitation on the growth kinetics of G. sulfurreducens. 

Using our three-electrode bioelectrochemical reactor system, we ran experiments to test how the current production, acetate utilization, hydrogen production, and biomass production were affected by changing Nernst-Planck variables, including ion transport and electrode spacing. In our system, we found that ion transport can become limiting after a 3D electrode is sufficiently colonized by Geobacter sulfurreducens, and that beyond this point the growth kinetics are no longer defined by the Nernst-Monod equation. Under these conditions, we found that the maximum current is controlled by mass transport of acetate when advection is absent, and by the mass transport of ions when advection is present. We also found that on a graphite felt electrode, in a system where ion transport is limiting (i.e., when addition of salt or convection increases the current), the acetate consumption and growth kinetics of Geobacter sulfurreducens are arithmetic, with the respective rates controllable using the variables in the Nernst-Planck equation. In this arithmetic growth regime, the acetate consumption rate, hydrogen productivity and the growth rate were decoupled from instantaneous biomass, and instead coupled solely to the steady-state current. Though arithmetic growth patterns in bacteria and algae have been described historically when the cells are under certain conditions, this the first time that an arithmetic growth regime of a microorganism has been demonstrated to be caused by ion transport in an electrochemical system, and should add to our understanding of the growth kinetics of anode-respiring biofilms. The presentation will discuss the implications of our results for future reactor designs.