Kikuchi S, Makita H, Konno U, Shiraishi F, Ijiri A, Takai K, Maeda M, Takahashi Y
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.
Geobiology. 2016 Jul;14(4):374-89. doi: 10.1111/gbi.12181. Epub 2016 Mar 30.
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.
许多物理和化学过程控制着异化铁还原菌对氢氧化铁（III）的还原程度。氢氧化铁（III）表面次生铁矿物的沉淀限制了微生物对铁（III）的还原程度，但这种现象在自然界中尚未被观察到。本文报道了在深度达10厘米的淡水沉积物中，水铁矿表面存在次生铁矿物（针铁矿）结壳的观察结果。沉积物表面的特征是具有生物成因的茎和鞘的水铁矿占主导地位。通过16S rRNA基因分析在沉积物深度1厘米和2厘米处检测到一种铁（II）氧化细菌（嘉利翁氏菌科）。沉积物孔隙水中的铁（2+）浓度在2-4厘米深度处相对较高。在1厘米、2厘米和4厘米深度处检测到与异化铁还原菌相关的16S rRNA基因。铁K边扩展X射线吸收精细结构（EXAFS）分析结果表明，在3厘米以下深度存在针铁矿和菱铁矿。然而，尽管在4厘米以下深度存在大量水铁矿，但铁矿物组成的变化仅限于3至4厘米的沉积物深度。在6厘米以下观察到甲烷浓度增加。孔隙水中甲烷的稳定同位素分析表明，乙酸裂解甲烷发生在7厘米以下深度。透射电子显微镜观察表明，在3厘米以下深度的茎和鞘上存在针铁矿和菱铁矿。转换电子产率EXAFS分析结果表明，在10厘米深度处针铁矿占主导地位，从而表明在该深度水铁矿被针铁矿结壳。此外，4厘米以下深度的水铁矿未完全还原并非由于有机碳缺乏，而是可能由于针铁矿在水铁矿表面结壳所致。
