Flow‐Based Deacidification of Geobacter sulfurreducens Biofilms Depends on Nutrient Conditions: a Microfluidic Bioelectrochemical Study

Abstract

Biofilms from Geobacter sulfurreducens are promising materials for new bioelectrochemical systems. To improve the performance of such systems, limitations related to biofilm acidification should be addressed. This work examines a long‐held assumption that liquid flow can deacidify biofilm pH by enhancing molecular mass transport in the biofilm subdomain. A microfluidic electrochemical system was used to measure changes to biofilm pH in situ while accurately modulating hydrodynamic conditions under turnover, nutrient‐limited and starvation conditions. We discovered that increased flow rates could indeed mitigate biofilm acidification, but not under turnover concentrations, which are the predominant conditions used in research studies. This effect is demonstrated with the observation that relative increases to bio‐current under increased flow rates were stronger for experiments conducted under nutrient‐limited concentrations compared to turnover concentrations. This can open the way for a solution to poor performance of some bioelectrochemical systems at low concentrations.

A Generalized Kinetic Framework Applied to Whole‐Cell Bioelectrocatalysis in Bioflow Reactors Clarifies Performance Enhancements for Geobacter Sulfurreducens Biofilms

“…Therefore, an increase in either or both of k cat or [E] could explain the peak I values (Figure b), which increased by 24 % when Q was increased from 0.4 to 3 mL h −1 . Despite previous suggestions that accelerated turnover under flow could be the result of EAB deacidification, a likely route to increases in k cat , pH was recently shown not to change for similar conditions at [Ac]=10 mM . Alternatively, flow‐induced increases to current might be at least partially related to changes to [E].…”

“…Despite previous suggestions that accelerated turnover under flow could be the result of EAB deacidification, [25] a likely route to increases in k cat , pH was recently shown not to change for similar conditions at [Ac] = 10 mM. [26] Alternatively, flow-induced increases to current might be at least partially related to changes to [E]. In this vein, consider the measurements of device reaction capacity (C), which increased by 19 % when increasing flow from Q = 0.4 to Q = 3 mL h À 1 (SI, Table S1 and Figure S6).…”

“…The channel-embedded electrodes were fabricated as shown previously. [26,30] Graphite was used for both the working (WE) and counter (CE) electrodes and a stable gold (Au) pseudo reference electrode (RE). The device was operated in an anaerobic enclosure with controlled temperature of 23 � 0.5°C.…”

“…Frozen samples of Geobacter sulfurreducens (strain PCA, ATCC 51573), were cultured under controlled temperature and deoxygenated conditions before being injected into the microfluidic electrochemical device. The channel‐embedded electrodes were fabricated as shown previously ,. Graphite was used for both the working (WE) and counter (CE) electrodes and a stable gold (Au) pseudo reference electrode (RE).…”

Engineered living conductive biofilms as functional materials

“…[97,98] These systems ranged from ∼0.5 mL to 1.5 µL and were either operated in batch or continuous flow, but did not contain a built-in reference electrode for control over the electrode potential, which is required to precisely and quantitatively gate EET. In other studies, G. sulfurreducens was grown in a 0.3-24 µL flow cell chamber in either a two-or three-electrode configuration, [99,100] but only a single organism could be tested at a time in these setups. When microfluidics and nanoliter scale reaction volumes are used for microbial electrochemistry, initial electrochemical characterization on previously characterized strains, such as G. sulfurreducens, should be carried out to determine optimal flow rates and inoculation conditions before testing engineered strains.…”