title: Limited reduction of ferrihydrite encrusted by goethite in freshwater sediment.
abstract: Many physical and chemical processes control the extent of Fe(III) oxyhydroxide reduction by dissimilatory Fe(III)-reducing bacteria. The surface precipitation of secondary Fe minerals on Fe(III) oxyhydroxides limits the extent of microbial Fe(III) reduction, but this phenomenon has not yet been observed in nature. This paper reports the observation of secondary Fe-mineral (goethite) encrustation on ferrihydrite surface within freshwater sediment up to 10 cm deep. The sediment surface was characterized by the predominance of ferrihydrites with biogenic stalks and sheaths. An Fe(II)-oxidizing bacterium (Gallionellaceae) was detected by 16S rRNA gene analysis at sediment depths of 1 and 2 cm. Fe(2+) concentration in the sediment pore water was relatively higher at 2-4 cm depths. The 16S rRNA genes affiliated with dissimilatory Fe(III)-reducing bacteria were detected at 1, 2, and 4 cm depths. The results of the Fe K-edge extended X-ray absorption fine structure (EXAFS) analysis suggested the presence of goethite and siderite at depths below 3 cm. However, the change in the Fe-mineral composition was restricted to sediment depths between 3 and 4 cm, despite the presence of abundant ferrihydrite at depths below 4 cm. An increase in CH4 concentration was observed at deeper than 6 cm. Stable isotopic analysis of CH4 in the pore water indicated that acetoclastic CH4 occurred at depths below 7 cm. Transmission electron microscope observations suggested the presence of goethite and siderite on stalks and sheaths at depths below 3 cm. Results from conversion electron yield EXAFS analysis suggested that goethite dominated at 10 cm depth, thereby indicating that ferrihydrite was encrusted by goethite at this depth. Moreover, the incomplete reduction of ferrihydrite below depths of 4 cm was not due to the lack of organic carbon, but was possibly due to the surface encrustation of goethite on ferrihydrite.
authors.affiliation: Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Hiroshima, Japan.|Project Team for Development of New-Generation Research Protocol for Submarine Resources, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan.
authors.forename: S
authors.initials: S
authors.lastname: Kikuchi
authors.affiliation: Department of Subsurface Geobiological Analysis and Research (d-SUGAR), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan.
authors.forename: H
authors.initials: H
authors.lastname: Makita
authors.affiliation: Department of Subsurface Geobiological Analysis and Research (d-SUGAR), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan.
authors.forename: U
authors.initials: U
authors.lastname: Konno
authors.affiliation: Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Hiroshima, Japan.
authors.forename: F
authors.initials: F
authors.lastname: Shiraishi
authors.affiliation: Kochi Institute for Core Sample Research, Agency for Marine-Earth Science and Technology (JAMSTEC), Nankoku, Kochi, Japan.
authors.forename: A
authors.initials: A
authors.lastname: Ijiri
authors.affiliation: Department of Subsurface Geobiological Analysis and Research (d-SUGAR), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Takai
authors.affiliation: Department of Technical Center, Hiroshima University, Higashi-Hiroshima, Japan.
authors.forename: M
authors.initials: M
authors.lastname: Maeda
authors.affiliation: Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Hiroshima, Japan.|Department of Earth and Planetary Science, The University of Tokyo, Bunkyo-ku, Tokyo, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Takahashi
mesh_terms: D001105:Archaea / Q000145:classification / Q000302:isolation & purification* / Q000378:metabolism; D001419:Bacteria / Q000145:classification / Q000302:isolation & purification* / Q000378:metabolism; D016000:Cluster Analysis; D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics; D004275:DNA, Ribosomal / Q000737:chemistry / Q000235:genetics; D005290:Ferric Compounds / Q000378:metabolism*; D005618:Fresh Water; D019015:Geologic Sediments / Q000737:chemistry* / Q000382:microbiology*; D058085:Iron Compounds / Q000737:chemistry*; D008903:Minerals / Q000737:chemistry*; D010084:Oxidation-Reduction; D010802:Phylogeny; D012336:RNA, Ribosomal, 16S / Q000235:genetics; D017422:Sequence Analysis, DNA; D056928:X-Ray Absorption Spectroscopy
chemical_list: D004269:DNA, Bacterial; D004275:DNA, Ribosomal; D005290:Ferric Compounds; D058085:Iron Compounds; D008903:Minerals; D012336:RNA, Ribosomal, 16S; C094886:goethite; C092844:ferric oxyhydroxide