Continuous shear stress alters metabolism, mass-transport, and growth in electroactive biofilms independent of surface substrate transport

Cullen R Buie,A-Andrew D Jones

doi:10.1038/s41598-019-39267-2

Cullen R Buie, A-Andrew D Jones

Open Access

https://doi.org/10.1038/s41598-019-39267-2

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Abstract

Electroactive bacteria such as Geobacter sulfurreducens and Shewanella onedensis produce electrical current during their respiration; this has been exploited in bioelectrochemical systems. These bacteria form thicker biofilms and stay more active than soluble-respiring bacteria biofilms because their electron acceptor is always accessible. In bioelectrochemical systems such as microbial fuel cells, corrosion-resistant metals uptake current from the bacteria, producing power. While beneficial for engineering applications, collecting current using corrosion resistant metals induces pH stress in the biofilm, unlike the naturally occurring process where a reduced metal combines with protons released during respiration. To reduce pH stress, some bioelectrochemical systems use forced convection to enhance mass transport of both nutrients and byproducts; however, biofilms’ small pore size limits convective transport, thus, reducing pH stress in these systems remains a challenge. Understanding how convection is necessary but not sufficient for maintaining biofilm health requires decoupling mass transport from momentum transport (i.e. fluidic shear stress). In this study we use a rotating disc electrode to emulate a practical bioelectrochemical system, while decoupling mass transport from shear stress. This is the first study to isolate the metabolic and structural changes in electroactive biofilms due to shear stress. We find that increased shear stress reduces biofilm development time while increasing its metabolic rate. Furthermore, we find biofilm health is negatively affected by higher metabolic rates over long-term growth due to the biofilm’s memory of the fluid flow conditions during the initial biofilm development phases. These results not only provide guidelines for improving performance of bioelectrochemical systems, but also reveal features of biofilm behavior. Results of this study suggest that optimized reactors may initiate operation at high shear to decrease development time before decreasing shear for steady-state operation. Furthermore, this biofilm memory discovered will help explain the presence of channels within biofilms observed in other studies.

Highlights

Bacteria exist in biofilms more than in planktonic states to (1) protect bacteria from predation and chemical attack and (2) to allow bacteria to manipulate their environment[1]
We find high metabolic rate increases with increased shear that may improve the performance of bioelectrochemical systems during startup
We find that maximum current and rates of current increase follow increasing shear stress for continuous growth of G. sulfurreducens PCA with fixed mass flux to the surface of a rotating disk, Fig. 1a

Summary

Introduction

Bacteria exist in biofilms more than in planktonic states to (1) protect bacteria from predation and chemical attack and (2) to allow bacteria to manipulate their environment[1]. Though bioelectrochemical systems employ flow to improve nutrient delivery and metabolic waste removal[6,7,8,9], which is known to influence biofilm behavior, the independent influence of shear on electrochemically active biofilms remains largely unexplored[6,7,8,9,10,11]. Short-term exposures of electroactive biofilms to shear showed expected improvements in salt transport[12] and unexpected resilience to high shear (635 s−1)[13] despite forming thicker biofilms than aerobic bacteria[5]. Using the analytic equations for fluid flow and mass flux that exist for a rotating disk electrode, the absolute concentration of nutrients is reduced in a way that decouples the influence of mass flux and shear stress. We used a pure culture of Geobacter sulfurreducens, as it has been shown to produce the highest current[5] in a bioelectrochemical system and is the most abundant species in mixed-culture bioelectrochemical systems when selected through time, shear, or voltage methods

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