Experimental silicification of microorganisms. Applications in the search for evidence of life in early Earth and extraterrestrial rocks. ; Fossilisation expérimentale de bactéries : appui à l?identification de signatures microbiologiques terrestres et extraterrestres

Since the earliest life forms known to date (> 3 Gyr) were preserved due to the precipitation of dissolved silica on cellular structures (silicification), we undertook an experiment to silicify several microbial species (the Archaea Methanocaldococcus jannaschii and Pyrococcus abyssi, and the Bacteria Chloroflexus aurantiacus and Geobacillus sp.), representative of anaerobic, thermophilic microorganisms that could have existed in the environmental conditions of early Earth and early Mars. This is the first time that Archaea have been used in a simulated fossilisation experiment and one of the very first fossilisations of thermophilic microorganisms. The experimental fossilisation was monitored by electron microscopy (SEM, TEM, Cryo-SEM) for the morphological study, and by chemical analysis (GC, GC-MS, HPLC) for the study of the preservation or degradation of organic matter during silicification. This experiment demonstrated that not all microorganisms silicify under the same conditions. M. jannaschii cells lysed rapidly, although the EPS (extracellular polymeric substances) were preserved, as opposed to P. abyssi, Geobacillus sp. and C. aurantiacus where the cells were preserved and fossilized with differing degrees of silicification between species. The microorganisms apparently used active mechanisms to protect themselves temporarily from silicification, such as EPS production or silica repulsion. These results suggest that differences between species have a strong influence on the potential for different microorganisms to be preserved by fossilisation. This study provides valuable insight into the silicification and preservation processes of the kind of microorganisms that could have existed on the early Earth. Knowledge of these mechanisms can be helpful for the search and the identification of microfossils in both terrestrial and extraterrestrials rocks, and in the particular case of Mars. ; Puisque les premières formes de vie connues à ce jour (> 3 Ga) ont été préservées grâce à la précipitation de ...