Enhancement of microbial current production by riboflavin requires the reduced heme centers in outer membrane cytochromes in Shewanella oneidensis MR-1

•Riboflavin (RF) enhances current production via outer-membrane cytochromes (OMCs).•Whole-cell circular dichroism spectroscopy reveals heme oxidation state in OMCs.•Fumarate or dimethyl sulfoxide oxidizes OMCs even in the presence of lactate.•Bound RF as semiquinone state in reduced OMCs highly increases current production.•Upon the oxidation of OMCs, Ic sharply decreases, and RF dissociates from OMCs. The rate enhancement of extracellular electron transfer is critical for microbial fuel cells (MFCs). Flavins have been a promising additive to enhance current production even with a few µM through the formation of bound semiquinone cofactor in the outer membrane cytochromes (OMCs). However, the effect of flavins vastly decreases upon the dissociation from OMCs to be soluble electron shuttle. Therefore, identifying a critical factor to stabilize the bound flavin cofactor is essential. Herein, we demonstrated that the reduced heme centers in OMCs promote riboflavin (RF) binding to OMCs by modulating the intracellular electron pathways in Shewanella oneidensis MR-1. UV–Vis and circular dichroism spectroscopy in situ showed that fumarate or dimethyl sulfoxide (DMSO) oxidizes heme centers in OMCs even at low concentrations with lactate as an electron donor, resulted in the defect of current enhancement by RF. Differential pulse voltammetry detected more soluble flavins and less bound semiquinone in the presence of fumarate or DMSO in the wild type. However, mutant strains lacking a reductase for fumarate or DMSO recovered the effect of RF. These results strongly suggest that reduced heme centers promote RF to bind OMCs, and alternative electron acceptors suppress power generation in MFCs even more than the stoichiometric ratio.