Electron donor concentration- and pH-dependent, biogenic Fe(II)-facilitated biotransformation of ferrihydrite to various iron oxide nanomaterials by Shewanella sp. strain HN-41

Abstract

Dissimilatory iron-reducing bacteria catalyze the biotransformation of ferrihydrite into iron oxide nanomaterials by producing aqueous Fe(II), which subsequently transfers electrons to ferrihydrite. However, the specific environmental factors that determine the biotransformation of ferrihydrite to distinct iron oxide nanomaterials remain poorly understood. In this study, we investigated the biotransformation of ferrihydrite to various iron oxide nanomaterials using the iron-reducing bacterium Shewanella sp. strain HN-41, depending on lactate concentrations from 0.1 to 10 mM at pH 6.8 and 7.5. SEM and XRD analyses revealed that lepidocrocite nanosheets were generated at 0.1-1 mM lactate and pH 6.8. In comparison, at pH 7.5, secondary clusters of hematite nanoparticles and nanorod clusters of goethite were generated at 0.1 mM and 1 mM lactate, respectively. However, at a higher concentration of 10 mM lactate, magnetite nanoparticles, and nanorods were generated, regardless of pH. Strain HN-41 produced higher aqueous Fe(II) concentrations and lowered Eh values as lactate concentration increased. In addition, overall aqueous Fe(II) concentrations and Eh values were higher at pH 6.8 than at pH 7.5. The differences in aqueous Fe(II) concentration and the system's redox potential (Eh value) altered the fate of aqueous Fe(II), resulting in the biotransformation products of various iron oxide nanomaterials. This study provides insight into how nutrient availability and pH affect the geochemical composition of iron minerals by regulating the activity of iron-reducing bacteria.IMPORTANCEFerrihydrite is ubiquitous in the environment and a favorable electron acceptor for iron-reducing bacteria due to its low crystallinity and high surface area. After reduction of ferrihydrite by the iron-reducing bacteria, it was transformed into thermodynamically more stable crystalline iron oxide nanomaterials by electron transfer from biogenic aqueous Fe(II) to ferrihydrite. This study highlighted that electron donor concentration and pH conditions strongly influenced the biotransformation products of distinct iron oxide nanomaterials by affecting the activity of dissimilatory iron-reducing bacteria. Ferrihydrite is ubiquitous in the environment and a favorable electron acceptor for iron-reducing bacteria due to its low crystallinity and high surface area. After reduction of ferrihydrite by the iron-reducing bacteria, it was transformed into thermodynamically more stable crystalline iron oxide nanomaterials by electron transfer from biogenic aqueous Fe(II) to ferrihydrite. This study highlighted that electron donor concentration and pH conditions strongly influenced the biotransformation products of distinct iron oxide nanomaterials by affecting the activity of dissimilatory iron-reducing bacteria.