Abstract
Graphene oxide (GO) is reduced by certain exoelectrogenic bacteria, but its effects on bacterial growth and metabolism are a controversial issue. This study aimed to determine whether GO functions as the terminal electron acceptor to allow specific growth of and electricity production by exoelectrogenic bacteria. Cultivation of environmental samples with GO and acetate as the sole substrate could specifically enrich exoelectrogenic bacteria with Geobacter species predominating (51–68% of the total populations). Interestingly, bacteria in these cultures self-aggregated into a conductive hydrogel complex together with biologically reduced GO (rGO). A novel GO-respiring bacterium designated Geobacter sp. strain R4 was isolated from this hydrogel complex. This organism exhibited stable electricity production at >1000 μA/cm3 (at 200 mV vs Ag/AgCl) for more than 60 d via rGO while temporary electricity production using graphite felt. The better electricity production depends upon the characteristics of rGO such as a large surface area for biofilm growth, greater capacitance, and smaller internal resistance. This is the first report to demonstrate GO-dependent growth of exoelectrogenic bacteria while forming a conductive hydrogel complex with rGO. The simple put-and-wait process leading to the formation of hydrogel complexes of rGO and exoelectrogens will enable wider applications of GO to bioelectrochemical systems.
Highlights
Graphene oxide (GO) is reduced by certain exoelectrogenic bacteria, but its effects on bacterial growth and metabolism are a controversial issue
The prepared graphene oxide (GO) almost consisted of single-layered sheets with 1.5 nm thickness as seen in AFM images, and had abundant C = O peaks at 287 eV and a C-C/C-H peak at 285 eV in X-ray photoelectron spectroscopy (XPS) (Supplementary Fig. S1)
GO is a nanomaterial having great promise in application to Bioelectrochemical systems (BESs), it has been in dispute in connection with its effects on bacterial growth and metabolism
Summary
Graphene oxide (GO) is reduced by certain exoelectrogenic bacteria, but its effects on bacterial growth and metabolism are a controversial issue. Cultivation of environmental samples with GO and acetate as the sole substrate could enrich exoelectrogenic bacteria with Geobacter species predominating (51–68% of the total populations) Bacteria in these cultures self-aggregated into a conductive hydrogel complex together with biologically reduced GO (rGO). The better electricity production depends upon the characteristics of rGO such as a large surface area for biofilm growth, greater capacitance, and smaller internal resistance This is the first report to demonstrate GO-dependent growth of exoelectrogenic bacteria while forming a conductive hydrogel complex with rGO. GO was reduced in the cell-free extract from a Shewanella culture[17], possibly by small biomolecules like vitamin C21 These findings raise a question of whether GO serves as an electron acceptor for EET coupling with oxidation of the substrate in exoelectrogenic bacteria, thereby allowing their growth. This makes us expect that GO is usable for the formation of a high-dense assembling complex of EET-driving exoelectrogens and conductive rGO
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