Oxidation of Fe(II)–Organic Matter Complexes in the Presence of the Mixotrophic Nitrate-Reducing Fe(II)-Oxidizing Bacterium Acidovorax sp. BoFeN1

Fe­(II)–organic matter (Fe­(II)–OM) complexes are abundant in the environment and may play a key role for the behavior of Fe and pollutants. Mixotrophic nitrate-reducing Fe­(II)-oxidizing bacteria (NRFeOx) reduce nitrate coupled to the oxidation of organic compounds and Fe­(II). Fe­(II) oxidation may occur enzymatically or abiotically by reaction with nitrite that forms during heterotrophic denitrification. However, it is unknown whether Fe­(II)–OM complexes can be oxidized by NRFeOx. We used cell-suspension experiments with the mixotrophic nitrate-reducing Fe­(II)-oxidizing bacterium Acidovorax sp. strain BoFeN1 to reveal the role of nonorganically bound Fe­(II) (aqueous Fe­(II)) and nitrite for the rates and extent of oxidation of Fe­(II)–OM complexes (Fe­(II)–citrate, Fe­(II)–EDTA, Fe­(II)–humic acid, and Fe­(II)–fulvic acid). We found that Fe­(II)–OM complexation inhibited microbial nitrate-reducing Fe­(II) oxidation; large colloidal and negatively charged complexes showed lower oxidation rates than aqueous Fe­(II). Accumulation of nitrite and fast abiotic oxidation of Fe­(II)–OM complexes only happened in the presence of aqueous Fe­(II) that probably interacted with (nitrite-reducing) enzymes in the periplasm causing nitrite accumulation in the periplasm and outside of the cells, whereas Fe­(II)–OM complexes probably could not enter the periplasm and cause nitrite accumulation. These results suggest that Fe­(II) oxidation by mixotrophic nitrate reducers in the environment depends on Fe­(II) speciation, and that aqueous Fe­(II) potentially plays a critical role in regulating microbial denitrification processes