Shewanella algae BrY uses insoluble mineral oxides as terminal electron acceptors, but the mechanism of electron transfer from cell surface to mineral surface is not well understood. We tested the hypothesis that cell-associated melanin produced by S. algae BrY serves as an electron conduit for bacterial-mineral reduction. Results from Fourier transform infrared spectroscopy and cell surface hydrophobicity assays indicated that extracellular melanin was associated with the cell surface. With H(2) as electron donor, washed cell suspensions of melanin-coated S. algae BrY reduced hydrous ferric oxide (HFO) 10 times faster than cells without melanin. The addition of melanin (20 microg ml(-1)) to these melanin-free cells increased their HFO reduction rate two-fold. These results suggest that cell-associated melanin acts as an electron conduit for iron mineral reduction by S. algae BrY.