Abstract
Twisted stalks are organo-mineral structures produced by some microaerophilic Fe(II)-oxidizing bacteria at O2 concentrations as low as 3 μM. The presence of these structures in rocks having experienced a diagenetic history could indicate microbial Fe(II)-oxidizing activity as well as localized abundance of oxygen at the time of sediment deposition. Here we use spectroscopy and analytical microscopy to evaluate if--and what kind of--transformations occur in twisted stalks through experimental diagenesis. Unique mineral textures appear on stalks as temperature and pressure conditions increase. Haematite and magnetite form from ferrihydrite at 170 °C-120 MPa. Yet the twisted morphology of the stalks, and the organic matrix, mainly composed of long-chain saturated aliphatic compounds, are preserved at 250 °C-140 MPa. Our results suggest that iron minerals might play a role in maintaining the structural and chemical integrity of stalks under diagenetic conditions and provide spectroscopic signatures for the search of ancient life in the rock record.
Highlights
Twisted stalks are organo-mineral structures produced by some microaerophilic Fe(II)-oxidizing bacteria at O2 concentrations as low as 3 mM
In scanning electron microscopy (SEM) images, stalks had a smooth surface with round aggregates (Fig. 1c), to samples recovered in another underground environment[24]
The structural integrity of the twisted stalks appears maintained by Fe phases (Fig. 3), similar to the shape of microbial cells preserved by Fe3 þ during fossilization experiments at ambient conditions
Summary
Twisted stalks are organo-mineral structures produced by some microaerophilic Fe(II)-oxidizing bacteria at O2 concentrations as low as 3 mM The presence of these structures in rocks having experienced a diagenetic history could indicate microbial Fe(II)oxidizing activity as well as localized abundance of oxygen at the time of sediment deposition. Twisted stalks are extracellular ribbon-like organo-mineral structures produced by neutrophilic microaerophilic, freshwater and marine Fe(II)-oxidizing bacteria (FeOB) in the b-Proteobacteria and z-Proteobacteria phyla, respectively, for example, see ref. Mineralized twisted filaments are commonly described as ancient microaerophilic FeOB in the rock record or in hydrothermal deposits, for example, see refs 18–20, it is unknown how the twisted stalk organic matrix and mineralogy evolve as a function of T and P conditions typical of diagenesis. As stalk formation is induced by O2 in cultures of Gallionella ferruginea[22], and occurs at O2 concentrations as low as 3 mM in cultures of Mariprofundus ferrooxydans[18], stalks could serve as evidence for the presence of localized oxygen in ancient environments
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