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Metals Binding by the Early Earth Analogue Microorganism, Archaea Methanocaldococcus jannaschu and its Effects on Silicification
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International audience. In order to improve our knowledge of the experimental silicification of microorganisms, we previously undertook the experimental silicification of several microorganisms, representative of the kinds of anaerobic, thermophilic microorganisms that could have existed in the environmental conditions of early Earth and early Mars (Orange et al. 2009). Among them, the experimental silicification the Archaea Methanocaldococcus jannaschii was unsuccessful, due to a natural rapid lysis of the cells, and a silicification process which preserve efficiently only a few cells remains. Several studies had previously mentioned the important role of metal binding in stabilising bacterial cell walls after death to prevent immediate lysis, for instance Fe(III) ions, prior to silicification (Ferris et al. 1988; Fein et al. 2002). Thus, the aim of this study was to determine if preliminary Fe(III) ion binding also had a positive effect on the silicification of M. jannaschii.The results showed that M. jannaschii can rapidly bind several metal cations (Fe3+, Ca2+, Pb2+, Zn2+, Cu2+). However, preliminary quantitative data suggest that the metal uptake by M. jannaschii cells is significantly lower than for Gram-positive or Gram-negative Bacteria. Preliminary exposure to dissolved Fe(III) ions spectacularly improved the results of the experimental silicification, with the preservation of many M. jannaschii cells, and the coating of the cellular structures with a Fe-Si precipitate.This study brings new elements to the understanding of the fossilisation processes, showing that the positive effect of Fe(III) on silicification, already evidenced for Bacteria, can also apply to Archaea, and that this preliminary binding can be decisive for the success of the silicification. Knowledge of these mechanisms can be helpful for the search and the identification of microfossils in both terrestrial and extraterrestrials rocks, and in particular Mars.
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