Iron-oxidizing bacteria are important actors of the geochemical cycle of iron in modern environments and may
have played a key role all over Earth’s history. However, in order to better assess that role on the modern and the
past Earth, there is a need for better understanding the mechanisms of bacterial iron oxidation and for defining
potential biosignatures to be looked for in the geologic record. In this study, we investigated experimentally and
at the nanometre scale the mineralization of iron-oxidizing bacteria with a combination of synchrotron-based
scanning transmission X-ray microscopy (STXM), scanning transmission electron microscopy (STEM) and cryotransmission
electron microscopy (cryo-TEM). We show that the use of cryo-TEM instead of conventional
microscopy provides detailed information of the successive iron biomineralization stages in anaerobic nitratereducing
iron-oxidizing bacteria. These results suggest the existence of preferential Fe-binding and Fe-oxidizing
sites on the outer face of the plasma membrane leading to the nucleation and growth of Fe minerals within the
periplasm of these cells that eventually become completely encrusted. In contrast, the septa of dividing cells
remain nonmineralized. In addition, the use of cryo-TEM offers a detailed view of the exceptional preservation of
protein globules and the peptidoglycan within the Fe-mineralized cell walls of these bacteria. These organic
molecules and ultrastructural details might be protected from further degradation by entrapment in the mineral
matrix down to the nanometre scale. This is discussed in the light of previous studies on the properties of
Fe–organic interactions and more generally on the fossilization of mineral–organic assemblies.
Iron-oxidizing bacteria are important actors of the geochemical cycle of iron in modern environments and mayhave played a key role all over Earth’s history. However, in order to better assess that role on the modern and thepast Earth, there is a need for better understanding the mechanisms of bacterial iron oxidation and for definingpotential biosignatures to be looked for in the geologic record. In this study, we investigated experimentally andat the nanometre scale the mineralization of iron-oxidizing bacteria with a combination of synchrotron-basedscanning transmission X-ray microscopy (STXM), scanning transmission electron microscopy (STEM) and cryotransmissionelectron microscopy (cryo-TEM). We show that the use of cryo-TEM instead of conventionalmicroscopy provides detailed information of the successive iron biomineralization stages in anaerobic nitratereducingiron-oxidizing bacteria. These results suggest the existence of preferential Fe-binding and Fe-oxidizingsites on the outer face of the plasma membrane leading to the nucleation and growth of Fe minerals within theperiplasm of these cells that eventually become completely encrusted. In contrast, the septa of dividing cellsremain nonmineralized. In addition, the use of cryo-TEM offers a detailed view of the exceptional preservation ofprotein globules and the peptidoglycan within the Fe-mineralized cell walls of these bacteria. These organicmolecules and ultrastructural details might be protected from further degradation by entrapment in the mineral
matrix down to the nanometre scale. This is discussed in the light of previous studies on the properties of
Fe–organic interactions and more generally on the fossilization of mineral–organic assemblies.
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