Formation of Biogenic Fe-Oxyhydroxides in an Extreme Thermal Environment

Abstract:

Biogenic Fe-oxyhydroxides have been widely found in freshwater and marine environments. Many studies have suggested a microbial role in iron precipitation in these settings, through either direct metabolic activities of bacteria or passive sorption and nucleation reaction. Due to the complex origin of biogenic Fe-oxyhydroxides, however, it is still a great challenge to ascertain the exact role of microorganisms in the formation of biogenic Fe-oxyhydroxides in nature environments, especially in Fe-rich neutral pH environments. Here, we report the geomicrobiological characterization of Fe-rich reddish precipitates from a high Fe, near-neutral pH hot spring with a temperature of 42 to 73°C located in the Rehai Geothermal Field, Tengchong, China. Abundant sheath-like Fe-oxyhydroxides, which are composed largely of Fe, Si, O and other trace elements, are scattered in the reddish precipitates and exhibit a diversity of morphologies and sizes. The sheath-like Fe-oxyhydroxides consist of ferrihydrites rather than more crystalline Fe oxides. Molecular evidences show that no chemolithotrophic Fe oxidizers were identified. Various thermophiles, mainly including cyanobacteria, Planctomycetes, β-proteobacteria, Deinococci-Thermus and Chlorobi, may be involved in the formation of the sheath-like Fe-oxyhydroxides, through simply acting as binding and nucleation surface for Fe-oxyhydroxides. The oxygen produced by cyanobacteria that dominate the microbial community may greatly accelerate the oxidization of Fe(II) in the spring. Biogenic sheath-like Fe-oxyhydroxides in such a hot, near-neutral pH, Fe-rich 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.