New evidences on the catalase mechanism of microbial corrosion

Changes on the oxygen reduction rate induced on aluminium brass by cell-free bacterial cultures of an isolate belonging to the genus Pseudomonas were studied in relation to the bacteria phase of growth and to the surface oxide layer composition after various electrochemical pre-treatments of the metal samples. Cultures isolated from the stationary phase of growth strongly influenced the oxygen reduction kinetics. Cathodic currents increased throughout the potential range tested when Cu₂O and CuO were present simultaneously in the surface film (so-called aged surfaces). In this case, the maximum increment (35%) was observed within 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%. On pre-reduced surfaces on the other hand, when only a submonolayer of Cu₂O was covering the surface, there was no effect as current density values were similar to those obtained in control experiments. Exponential phase cell-free cultures did not modify the limiting current values in any of the surfaces investigated. Results were in agreement with the participation of catalase as a bacterial catalyst for the oxygen reduction process. The normalised catalase activity from different stationary phase cell-free cultures ranged from 0.88 to 4.02 mg ml⁻¹ U⁻¹, while there was no observable activity in exponential phase cultures. The incidence of the catalase mechanism in microbiologically influenced corrosion processes induced by aerobic biofilms is highlighted on the basis of the results obtained using metabolites from planktonic cells and their agreement with most of the experimental evidences so far reported by other authors.