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Changes on the oxygen reduction rate induced on aluminium brass by cell-free bacterial cultures of an isolate belonging to thegenus Pseudomonas were studied in relation to the bacteria phase of growth and to the surface oxide layer composition aftervarious electrochemical pre-treatments of the metal samples. Cultures isolated from the stationary phase of growth stronglyinfluenced the oxygen reduction kinetics. Cathodic currents increased throughout the potential range tested when Cu2O and CuOwere present simultaneously in the surface film (so-called aged surfaces). In this case, the maximum increment (35%) was observedwithin the oxygen reduction limiting current region. On pre-oxidised surfaces, when the oxide film was composed mainly by CuO,the effect induced by stationary phase cultures was even higher, with the limiting current density increasing by almost 60%. Onpre-reduced surfaces on the other hand, when only a submonolayer of Cu2O was covering the surface, there was no effect ascurrent density values were similar to those obtained in control experiments. Exponential phase cell-free cultures did not modifythe limiting current values in any of the surfaces investigated. Results were in agreement with the participation of catalase as abacterial catalyst for the oxygen reduction process. The normalised catalase activity from different stationary phase cell-freecultures ranged from 0.88 to 4.02 mg ml−1 U−1, while there was no observable activity in exponential phase cultures. Theincidence of the catalase mechanism in microbiologically influenced corrosion processes induced by aerobic biofilms is highlightedon the basis of the results obtained using metabolites from planktonic cells and their agreement with most of the experimentalevidences so far reported by other authors. © 2002 Elsevier Science Ltd. All rights reserved.

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