Local Current Variation by Depth inGeobacter SulfurreducensBiofilms

Abstract

New methods of detecting local extracellular electron transfer rates in electrochemically active biofilms (EABs) are needed to quantify the role of microscale gradients in extracellular electron transfer. We utilized electrode-respiring Geobacter sulfurreducens biofilms as a model EAB to demonstrate local current measurement by depth using microelectrodes with a graphite tip covered by biofilm. We grew G. sulfurreducens biofilms on an electrode (electrode-biofilm) and on the tip of a microelectrode (microelectrodebiofilm) and monitored current generation. The microelectrode-biofilm current was negatively affected by the current produced in the electrode-biofilm; this resulted in a change in current, . We found a 30% decrease in current from the top of the electrode-biofilm to the bottom of the electrode-biofilm. We attribute the current loss at the microelectrode-biofilm to exposure to local microscale conditions that reduced the ability of the microelectrode-biofilm to participate in extracellular electron transfer. It is possible that such microscale gradients as pH, redox and acetate caused a 30% current drop between top and bottom. Electrochemically active biofilms (EABs) are a specific subset of biofilms that participate in extracellular electron transfer to and from a solid, conductive surface.1 Starting from initial cell attachment, the colonizing cells multiply to take advantage of the solid electron acceptor and eventually form thicker, mature biofilms with cells spatially distributed in the xy-plane (parallel to the surface) and by depth. Typically, extracellular electron transfer is described as occurring through three generic mechanisms: electron shuttling, 1 electron hopping, 2 and metallic-like conductivity.3 Extracellular electron transfer to and from electrodes is generally studied using strains of Shewanella oneidensis and Geobacter sulfurreducens. Although the extracellular electron transfer mechanism may vary among EABs, diffusion and mass transfer processes must also be accounted for. 4 In thicker, mature EABs, diffusion coefficients inside the biofilm have been shown to deviate noticeably from the bulk diffusion coefficients. 5,6 Many techniques exist that can directly measure the microscale physicochemical gradients within live EABs and in biofilms in general.6-11 However, there are comparatively few techniques that directly measure electrochemical gradients within live EABs; these are generally optical-based methods tracking the redox state of c-type cytochromes.12,13 Recently, we used an amperometric microelectrode to enrich microbes locally within a complex microbial mat system that could harness a polarized microelectrode using extracellular electron transfer.14 Because of this, we explored the possibility that a similar method could be used to measure local current by depth in EABs where a variation in local current would indicate some form of electrochemical interaction and possibly limitation. The method is similar to that in previous split-anode 3 and interdigitated electrode array ...

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