Influence of the distribution of the manganese-oxidizing bacterium, Leptothrix discophora, on ennoblement of type 316L stainless steel

Abstract

Type 316L (UNS 31603) stainless steel (SS) was ennobled to an open-circuit potential (OCP) of 323 mV vs saturated calomel electrode (SCE) within a 5-day period due to the deposition of manganese oxides on the metal surface by a monospecies biofilm of Leptothrix discophora. However, the same metal experienced only partial ennoblement, achieving a maximum potential of 143 mVSCE during the same period when colonized by a three-species biofilm containing L. discophora, and even this level of ennoblement by the mixed-species biofilm was only transient since the potential decreased to 122 mVSCE shortly after achieving the maximum potential. The mixed-species biofilm was significantly thicker (>200 µm) than the L. discophora monospecies biofilm (120 µm). Using 16s rRNA probes specific for L. discophora, fluorescent in-situ hybridization revealed cells to be heterogeneously distributed throughout the monospecies biofilm. In the three-species biofilm, 16s rRNA probes revealed a homogeneous layer of L. discophora that resided proximal to the biofilm-bulk solution phase. At the most distal position from the biofilm-bulk solution phase near the glass-biofilm interface there existed very few to no L. discophora. Microelectrode studies revealed the presence of oxygen (3.85 mg/L to 4.35 mg/L) at the monospecies-glass substratum interface in 50% of the areas of the substratum assayed. No oxygen was detected at the three-species biofilm-glass interface where the thickness of the overlying biofilm exceeded 200 µm. The results suggest that ennoblement of Type 316L SS by L. discophora depends on oxygen accessibility to cells proximal to the substratum.