A Rechargeable Biomineral Induced by the Sulfate-reducing Bacterium <i>Nitratidesulfovibrio</i> sp. HK-II

A sulfate-reducing bacterium was isolated from the anode surface of a microbial fuel cell (MFC) producing a high current density. 16S rRNA gene ana-lyses showed that the isolate was affiliated with the genus Nitratidesulfovibrio, and the strain was named HK-II. When Nitratidesulfovibrio sp. strain HK-II was incubated anaerobically under sulfate-reducing conditions with Fe(III) citrate, a black precipitate formed. The resulting black precipitate was investigated using multidisciplinary methods. An X-ray diffraction (XRD) ana-lysis revealed that the black precipitate was mainly composed of mackinawite. A cyclic voltammetry ana-lysis showed clear redox peaks, and biogenic mackinawite possessed rechargeable properties. The XRD ana-lysis also showed that the form of the rechargeable biogenic mineral induced by strain HK-II (RBM-II) was changed by discharge and recharge treatments. Field-emission transmission electron microscopy revealed that lepidocrocite and amorphous iron oxide formed from mackinawite under discharged conditions, and the three mineral types were intermingled via charge and discharge cycles. Physicochemical parameters regularly changed under the treatments, suggesting that discharge occurred via iron oxidation followed by sulfur reduction and vice versa. These results indicate that sulfur dynamics are important key processes in charge and discharge mechanisms. MFCs equipped with lactate, strain HK-II, and an anode containing RBM-II consumed lactate under open-circuit conditions, after which MFCs generated a higher current density under reclosed-circuit conditions. These results demonstrate that RBM-II is a rechargeable material that enables the capture of electrons produced by bacterial cells and is useful for enhancing the performance of MFCs.