Respiratory interactions of soil bacteria with (semi)conductive iron-oxide minerals

Authors

  • Souichiro Kato,

    1. Hashimoto Light Energy Conversion Project, ERATO, JST, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
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  • Ryuhei Nakamura,

    1. Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
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  • Fumiyoshi Kai,

    1. Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
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  • Kazuya Watanabe,

    Corresponding author
    1. Hashimoto Light Energy Conversion Project, ERATO, JST, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
    2. Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan.
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  • Kazuhito Hashimoto

    Corresponding author
    1. Hashimoto Light Energy Conversion Project, ERATO, JST, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
    2. Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
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E-mail watanabe@light.t.u-tokyo.ac.jp; Tel. (+81) 3 5452 5749; Fax (+81) 3 5452 5749; E-mail hashimoto@light.t.u-tokyo.ac.jp; Tel. (+81) 3 5452 5749; Fax (+81) 3 5452 5749.

Summary

Pure-culture studies have shown that dissimilatory metal-reducing bacteria are able to utilize iron-oxide nanoparticles as electron conduits for reducing distant terminal acceptors; however, the ecological relevance of such energy metabolism is poorly understood. Here, soil microbial communities were grown in electrochemical cells with acetate as the electron donor and electrodes (poised at 0.2 V versus Ag/AgCl) as the electron acceptors in the presence and absence of iron-oxide nanoparticles, and respiratory current generation and community structures were analysed. Irrespective of the iron-oxide species (hematite, magnetite or ferrihydrite), the supplementation with iron-oxide minerals resulted in large increases (over 30-fold) in current, while only a moderate increase (∼10-fold) was observed in the presence of soluble ferric/ferrous irons. During the current generation, insulative ferrihydrite was transformed into semiconductive goethite. Clone-library analyses of 16S rRNA gene fragments PCR-amplified from the soil microbial communities revealed that iron-oxide supplementation facilitated the occurrence of Geobacter species affiliated with subsurface clades 1 and 2. We suggest that subsurface-clade Geobacter species preferentially thrive in soil by utilizing (semi)conductive iron oxides for their respiration.

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