Reduced graphene oxide increases cells with enlarged outer membrane of Citrifermentans bremense and exopolysaccharides secretion

Abstract

Conductive carbons can boost anaerobic microbial metabolism by assisting extracellular electron transfer (EET), and their chemistry affects microbial metabolism. Graphene oxide (GO), a chemically oxidized sheet of graphite, has been used in various bioelectrochemical systems, although its mechanism is rarely understood. This study revealed specific metabolic responses to reduced GO (rGO) in an electrogenic strain R4 of Citrifermentans bremense, recently renamed from “Geobacter bremensis,” in comparison to that with graphite felt (GF). Specifically, the change in growth from planktonic cells to biofilm with an enlarged outer membrane. The mRNA profile supported the fact that rGO upregulated the 14 genes related to the exopolysaccharides (EPS) secretion and biofilm formation, which is more than that in GF (4 genes). While GF upregulated the 35 genes involved in cell motility, which is more than that in rGO (8 genes). The heme protein profile suggested that both carbons induced similar EET pathways involving OmcA/MtrC and OmcS; however, GO specifically induced PilQ. These findings show that the chemistry of conductive carbon differentiates metabolism, especially affecting cellular morphology or living form, rather than electron transfer metabolism.