Diffusion in biofilms respiring on electrodes

Abstract

The goal of this study was to measure spatially and temporally resolved effective diffusion coefficients (De) 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 noninvasive, nondestructive, high spatial resolution in situ De 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 De heat maps and surface-averaged relative effective diffusion coefficient (Drs) depth profiles. We found that 1) Drs decreases with depth in G. sulfurreducens biofilms, following a sigmoid shape; 2) Drs at a given location decreases with G. sulfurreducens biofilm age; 3) average De and Drs 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 De values. Density, reflected by De, plays a major role in the extracellular electron transfer strategies of electrochemically active biofilms.

Local Current Variation by Depth inGeobacter SulfurreducensBiofilms

“…Generally, pH shifts, biofilm density, acetate limitations, and more generally mass transfer have been cited as the cause. 5,9,11,20,21 We note that these mass transfer considerations are distinct from electron transfer limitations caused by significant resistances in the extracellular electron transfer pathways. Although both processes play a central role, mass transfer limitations would primarily affect only the long-term steady current whereas electron transfer limitations would affect both the maximum current and long-term Figure 7.…”

“…These are the general concerns regarding thick G. sulfurreducens biofilms, in which biofilm density increases significantly with age. 5 It is reasonable to expect, based on the two behaviors cited above, that as the microelectrode-biofilm is exposed to the electrode-biofilm, the current response will decrease. The effect is completely reversible by turning off the current at the electrode-biofilm and should increase in magnitude when the microelectrode tip is moved deeper into the biofilm.…”

“…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 7 studies that measured the conductivity of EABs grown across micron-scale distances between electrodes.…”

“…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.…”

“…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][7][8][9][10][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.…”