Direct extracellular electron transfer to an indium tin doped oxide electrode via heme redox reactions in Desulfovibrio ferrophilus IS5

Soluble redox mediators have been hypothesized to play a major role in cellular electron transportation to extracellular insoluble electron acceptors in sulfate-reducing bacteria (SRB) through a diffusion-based electron shuttle mechanism. Herein, in vivo electrochemistry shows that outer-membrane cytochromes (OMCs) directly mediated extracellular electron transfer (EET) in Desulfovibrio ferrophilus IS5. Anodic current production by IS5, coupled with lactate oxidation, was detected on indium-tin-doped oxide electrodes poised at +400 mV (vs. standard hydrogen electrode), which was the sole electron acceptor. Subsequent electrolyte replacement ex-periments with a fresh one showed that cells attached to the electrode contributed to 73% of current production by direct EET, while the electron shuttle mechanism may have contributed to the rest. Differential pulse vol-tammetry detected two redox species at E =-180 and 20 mV, with the former being most likely assignable to cell-surface cytochromes because nitric oxide decreased the peak current at-180 mV, as previously demon-strated with OMCs in iron-reducing bacteria. The EET onset potential at-200 mV further supports the notion that OMCs mediated EET in IS5, and that EET is thermodynamically favorable for reducing Fe(III) oxide. These results suggest that the direct EET mechanism regulates the reduction rate of insoluble substrates in SRB.