Graphene oxide-dependent growth and self-aggregation into a hydrogel complex of exoelectrogenic bacteria

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.

Microbially reduced graphene oxide shows efficient electricity ecovery from artificial dialysis wastewater

“…We observed an 89% removal of the 1170 mg/L of COD Cr , determined by using chromium as the oxidant, in ADWW in the MFC (Goto and Yoshida, 2016). However, the amount of electricity recovered was remarkably lower than that expected from the superior electricity-production system with rGO with acetate (Yoshida et al, 2016b). A crucial concern is the dependency of GO reduction and electricity recovery using rGO on the composition of synthetic medium or wastewater.…”

“…For SEM imaging, the complexes were cut and fixed with 2% glutaraldehyde and 1% osmium tetroxide, as described previously (Yoshida et al, 2016b). The fixed samples were sputter-coated with gold and observed under a JSM-7001F field-emission, scanning electron microscope (JEOL Ltd., Tokyo, Japan) operating at 1.0 kV.…”

“…Microbial communities in ADWW and rGO/GF complex before/after 20 days of polarization were phylogenetically identified by the sequencing of 16S rRNA genes, as described previously (Yoshida et al, 2016b). For Illumina-MiSeq sequencing, 16S rRNA genes were first amplified using the bacterial and archaeal consensus primers, 515F (5¢-GTGCCAGCMGCCGCGGTAA-3¢) and 806R (5¢-GGACTACHVGGGTWTCTAAT-3¢).…”

“…Among them, GO offers additional advantages for microbial electricity production from organic substances. GO itself is not conductive, but can serve as an excellent anode following microbial reduction of GO to the reduced form (rGO) (Yoshida et al, 2016b). During GO reduction, exoelectrogens are selectively enriched and embedded in rGO (Yoshida et al, 2016a, b).…”

“…floccular aggregates in E. coli (Akhavan and Ghaderi, 2012) and a semi-solid complex in Geobacter sp. R4 (Yoshida et al, 2016b) produced via the p-p stacked multilayers of rGO (Xu et al, 2010). The rGO in the MFC showed lower electricity production with ADWW in the form of floccular aggregates (Goto and Yoshida, 2016).…”