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Formation of biogenic sheath‐like Fe oxyhydroxides in a near‐neutral pH hot spring: Implications for the origin of microfossils in high‐temperature, Fe‐rich environments
Author(s) -
Peng Xiaotong,
Chen Shun,
Xu Hengchao
Publication year - 2013
Publication title -
journal of geophysical research: biogeosciences
Language(s) - English
Resource type - Journals
eISSN - 2169-8961
pISSN - 2169-8953
DOI - 10.1002/jgrg.20119
Subject(s) - lepidocrocite , ferrihydrite , goethite , hot spring , transmission electron microscopy , chemistry , analytical chemistry (journal) , mineralogy , materials science , geology , environmental chemistry , adsorption , nanotechnology , paleontology , organic chemistry
A small hot spring that is informally called “Fe‐waterfall spring” and is located in the Rehai geothermal area discharges hot (42 to 73°C), near‐neutral (pH = 7.65) Fe‐rich water. Submerged reddish precipitates are composed largely of ferrihydrite, goethite, lepidocrocite, opal‐A, quartz, and anorthite, as revealed by X‐ray diffraction (XRD) and Mössbauer spectroscopy. Molecular phylogenetic analysis demonstrates that the bacterial community in these precipitates is mainly composed of Cyanobacteria, Planctomycetes, β‐proteobacteria, Deinococci‐Thermus, and Chlorobi. Scanning electron microscopy and high‐resolution transmission electron microscopy examinations show that abundant sheath‐like Fe oxyhydroxides, which exhibit different morphologies and sizes, are present in Fe‐rich precipitates. These sheath‐like structures are composed of ferrihydrite rather than more crystalline lepidocrocite or goethite. Energy‐dispersive X‐ray spectrometer, scanning transmission electron microscopy, and nano secondary ion mass spectrometry reveal that they are mainly composed of Fe, Si, and O, together with some trace elements. Most of the sheath‐like structures are not morphologically comparable to biogenic Fe oxyhydroxides produced by known chemolithotrophic Fe oxidizers, which is consistent with the fact that no chemolithotrophic Fe oxidizers were identified by molecular analysis in the precipitates. We suggest that the sheath‐like Fe oxyhydroxides are formed through passive Fe sorption and nucleation onto the cell walls of various thermophiles rather than by the direct metabolic activities of chemolithotrophic Fe oxidizers. Biogenic sheath‐like Fe oxyhydroxides in Fe‐waterfall spring have important implications for geochemical cycles driven by microorganisms, the origin of microfossils, and the formation of banded iron formations (BIFs) in the Archean ocean.