Coexistence of crystals with multiple facets on an individual particle or different crystals is prevalent in real environments. However, the reduction efficiency of individual crystal facets in facet-combined system is still enigmatic. This study aimed to determine the reduction efficiencies of different facets of hematite by Shewanella oneidensis MR-1. An iron isotope tracer was used to distinguish Fe(II) produced from reduction of the combined and separate hematite {001} and {100} facets, i.e., Hem {001} nanoplates and Hem {100} nanorods. Hem {001} nanoplates was reduced 1.8 times more significantly than Hem {100} nanorods when the two facets combined, whereas Hem {001} nanoplates was reduced only 1.2 times more significantly than Hem {100} nanorods in separate reactors. Initial - 72% and final - 63% of reduced atoms from Hem {001} nanoplate demonstrated the facet-preferential reduction of hematite crystal when the facets combined. The pseudo-first-order rate constants (k) of reduced Fe(II) for Hem {001} nanoplates and {100} nanorods were 3.2 and 2.0 x 10-2 d-1, respectively. We attributed the more efficient reduction of Hem {001} nanoplates than Hem {100} nanorods to their differences in surface hydroxyl groups, surface charges, ligand-bound conformation and steric effects. These findings provide new insights into microbe-mineral interaction based on the crystal facet and the overall role of Fe oxides nanocrystals in the environments.

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