Ennoblement of stainless steel by the manganese-depositing bacterium Leptothrix discophora

The noble shift in open-circuit potential exhibited by microbially colonized stainless steel (ennoblement) was investigated by examining the relationship among surface colonization, manganese deposition, and open-circuit potential for stainless steel coupons exposed to batch cultures of the manganese-depositing bacterium Leptothrix discophora. Open-circuit potential shifted from -100 to +330 mV(SCE) as a biofilm containing 75 nmol of MnOx cm(-2) formed on the coupon surface but changed little further with continued MnOx deposition up to 270 nmol cm(-2). Increased open-circuit potential corresponded to decreasing Mn(II) concentration in solution and to increased MnOx accumulation and attached cell density on the coupon surfaces. MnOx deposition was attributable to biological activity, and Mn(II) was observed to enhance cell attachment. The experimental results support a mechanism of ennoblement in which open-circuit potential is fixed near +350 mV(SCE) by the cathodic activity of biomineralized MnOx.