Charge transport in films of Geobacter sulfurreducens on graphite electrodes as a function of film thickness

Abstract

Harnessing, and understanding the mechanisms of growth and activity of, biofilms of electroactive bacteria (EAB) on solid electrodes is of increasing interest, for application to microbial fuel and electrolysis cells. Microbial electrochemical cell technology can be used to generate electricity, or higher value chemicals, from organic waste. The capability of biofilms of electroactive bacteria to transfer electrons to solid anodes is a key feature of this emerging technology, yet the electron transfer mechanism is not fully characterized as yet. Acetate oxidation current generated from biofilms of an EAB, Geobacter sulfurreducens, on graphite electrodes as a function of time does not correlate with film thickness. Values of film thickness, and the number and local concentration of electrically connected redox sites within Geobacter sulfurreducens biofilms as well as a charge transport diffusion co-efficient for the biofilm can be estimated from non-turnover voltammetry. The thicker biofilms, of 50 ± 9 μm, display higher charge transport diffusion co-efficient than that in thinner films, as increased film porosity of these films improves ion transport, required to maintain electro-neutrality upon electrolysis.

The Planktonic Relationship Between Fluid‐Like Electrodes and Bacteria: Wiring in Motion

“…The capacity of some microorganisms to exchange electrons with electrical conductive materials as part of their metabolism is one of the most fascinating mechanisms in the field of microbiology . This phenomenon typically occurs in an electroactive biofilm in which all bacterial layers contribute to converting oxidative metabolism into electrical current …”

“…In principle, this seems reasonable owing to the requirement of the existence of redox chemistry between the cells and an insoluble material, which would be satisfied by a biofilm architecture. Unfortunately, the main drawback of this scenario is the limited reactivity of the cells, which depends on the active area of the electrode in which the bioelectrochemical reaction is occurring (commonly reduced to the first 50 μm layer of the biofilm) . Interestingly, in electroactive biofilms, the cells located in the outer layers are at lower redox potentials and electron transfer is proposed to occur between bacteria under a different protein‐based scenario .…”

“…Interestingly, in electroactive biofilms, the cells located in the outer layers are at lower redox potentials and electron transfer is proposed to occur between bacteria under a different protein‐based scenario . In addition to the electroactivity of the cell, electrode surface limitation and diffusion through the biofilm together with its internal resistance represent real limitations with respect to the performance of biofilm‐based electroactive systems . In contrast, some of those limitations could be reduced using a configuration in which every single cell in the culturing medium could be wired to the electrode.…”

“…[1] This phenomenon typically occurs in an electroactive biofilm in which all bacterial layers contribute to converting oxidative metabolism into electrical current. [2][3][4][5] An umber of studies have extensively reported the versatile catalytic capacity of electroactiveb acteria, coupling electrochemistry with energetic metabolism. [6,7] From the very beginning, bacteria from the genus Geobacter have been widely used as model bacteria to investigate the mechanisms of this extracellular redox process.…”

Comparative Proteomics Implicates a Role for Multiple Secretion Systems in Electrode-Respiring Geobacter sulfurreducens Biofilms

“…Our previous work examining the charge transfer properties of G. sulfurreducens biofilms, grown under controlled applied potentials, has shown that biocatalytic current, generated from oxidation of acetate, scales with applied potential 3 . The acetate oxidation currents do not scale directly to biofilm thickness 4 , however, suggesting that either mass or charge transport limits the overall current or that the biofilm formed over time displays differences in bacterial cell densities, viability and/or redox response. Besides this uncertainty, the mechanism by which electrons are transferred over long distances (>50 µm) through G. sulfurreducens biofilm to the solid electrode still remains unclear.…”