Effects of Organic Substrate Concentrations on Biological Mn Oxidation and Reduction under Aerobic Conditions

Biological Mn oxidation mediated by manganese-oxidizing bacteria (MnOB) plays a central role in Mn cycling in natural and engineered systems. Although most MnOB are heterotrophic and frequently associated with oligotrophic environments, the quantitative relationship between organic substrate concentrations and Mn oxidation activity remains unclear. We herein systematically evaluated the effects of organic substrate concentrations on MnOB activity using a down-flow hanging sponge (DHS) reactor (0-1,000 mg COD L^-1), batch experiments with an enriched biomass, and the pure MnOB strain Pseudomonas resinovorans MO-1 (0-2,000 mg COD L^-1). Mn(II) oxidation was favored at organic substrate concentrations below approximately 50 mg COD L^-1, whereas concentrations around 100 mg COD L^-1 were more favorable for MnOB growth. In contrast, elevated organic substrate concentrations induced substrate inhibition and suppressed Mn(II) oxidation. At high organic substrate concentrations (≥1,000 mg COD L^-1), net Mn(IV) reduction by P. resinovorans MO-1 occurred even under aerobic conditions. An exploratory transcriptomic comparison showed patterns that were broadly consistent with the observed shifts in Mn redox behavior. Collectively, these results define an operational organic substrate concentration window for stable MnOB enrichment and efficient Mn(II) oxidation, and indicate the potential of excessive organic substrate concentrations to shift MnOB activity toward net Mn(IV) reduction under aerobic conditions.