Biogenic iron minerals as a potential survival mechanism for nitrate-reducing iron-oxidizing bacteria

Microaerophilic and phototrophic iron-oxidizing bacteria (FeOB) deal with the threat of cell encrustation by developing effective survival strategies, such as the production of extracellular organic structures for preferential nucleation. However, nitrate-reducing iron-oxidizing (NRFO) bacteria are not reported to have similar survival strategies. In this context, goethite and magnetite were chosen as biogenic iron minerals to evaluate their role in the survival mechanism of Acidovorax sp. BoFeN1. The NRFO kinetics results revealed that the added goethite and magnetite increased Fe(II) oxidation extent from 2.85 mM to 4.22 mM and 4.21 mM, while the extent of nitrate reduction was increased from 0.66 mM to 1.07 mM and 1.03 mM. The addition of goethite and magnetite did not promote microbial heterotrophic nitrate reduction without Fe(II), but accelerated the reaction between nitrite and Fe(II) in the chemodenitrification experiments. This suggested that goethite and magnetite facilitated the biological and chemical Fe(II) oxidation during the microbial NRFO process. Mineralogical analyses revealed that goethite and magnetite serve as nucleation sites for Fe precipitation. Electrochemical measurements revealed that goethite and magnetite elevated Fe(II) reactivity by decreasing its redox potential and corrosion potential. Therefore, goethite and magnetite promoted microbial NRFO process by mitigating cell encrustation and elevating Fe(II) reactivity. Since goethite and magnetite are typical secondary minerals of Acidovorax sp. BoFeN1, the production of biogenic iron mineral may be a survival strategy to prevent cell encrustation and maintain metabolic activity. The current findings provide novel insights into the survival mechanism of NRFO bacteria and related Fe-N cycling.