Diffusion in biofilms respiring on electrodes

Abstract

The goal of this study was to measure spatially and temporally resolved effective diffusion coefficients (D_e) in biofilms respiring on electrodes. Two model electrochemically active biofilms, Geobacter sulfurreducens PCA and Shewanella oneidensis MR-1, were investigated. A novel nuclear magnetic resonance microimaging perfusion probe capable of simultaneous electrochemical and pulsed-field gradient nuclear magnetic resonance (PFG-NMR) techniques was used. PFG-NMR allowed for noninvasive, nondestructive, high spatial resolution in situ D_e measurements in living biofilms respiring on electrodes. The electrodes were polarized so that they would act as the sole terminal electron acceptor for microbial metabolism. We present our results as both two-dimensional D_e heat maps and surface-averaged relative effective diffusion coefficient (D_rs) depth profiles. We found that (1) D_rs decreases with depth in G. sulfurreducens biofilms, following a sigmoid shape; (2) D_rs at a given location decreases with G. sulfurreducens biofilm age; (3) average D_e and D_rs profiles in G. sulfurreducens biofilms are lower than those in S. oneidensis biofilms—the G. sulfurreducens biofilms studied here were on average 10 times denser than the S. oneidensis biofilms; and (4) halting the respiration of a G. sulfurreducens biofilm decreases the D_e values. Density, reflected by D_e, plays a major role in the extracellular electron transfer strategies of electrochemically active biofilms.