The impact of Thalassospira sp., an iron-reducing bacterium isolated from marine corrosion product layers, on oxygen reduction reaction (ORR) has been investigated in the present study. ORR catalysis has been observed on a glassy carbon (GC) electrode when it is exposed to phosphate-buffered saline containing Thalassospira sp., and the adhesive biofilm consisting of bacterial cells and metabolites is believed to be responsible for the catalysis. A certain time of contact between the electrode and live cells is necessary for ORR catalysis, but the metabolic activity of cells after adhesion is not vital for the catalysis, and adsorbed compounds excreted by bacteria play a more important role. The ORR catalytic activity of Thalassospira sp. does not depend directly on bacterial surface coverage given the results of cyclic voltammetry and scanning electron microscopy. For the ORR kinetics analysis, the 4-electron pathway with H2O as the final product is achieved by the biofilm, which is different from the predominant 2-electron transformation on the bare GC electrode. (C) 2016 Elsevier B.V. All rights reserved.