To develop novel magnetic composites with acid functionalities, iron oxide produced by iron-oxidizing bacterium, Leptothrix ochracea, was used as a precursor. The composites were prepared by thermal treatment and subsequent reduction of the bacterial iron oxide. alpha-Fe2O3 nanoparticles obtained in air were thermally reduced to Fe3O4 and gradually oxidized to gamma-Fe2O3. The obtained composites with Fe3O4 and gamma-Fe2O3 have high magnetization values that are approximately half those of pure Fe3O4 and gamma-Fe2O3. All samples have an interesting nano-micro-architectural structure: similar to 40-nm-diameter crystalline particles covered with similar to 8-nm-thick amorphous silicate are intricately interconnected into similar to 1-mu m-diameter microtubules. Pyridine adsorption spectra indicate that the surface amorphous silicate has both Bronsted and Lewis acid properties. (c) 2012 Elsevier Ltd. All rights reserved.