Experimental Silicification of Thermophilic Microorganisms. Relevance for Early Life on Earth and Mars
Résumé
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 Orig Life Evol Biosph (2009) 39:179-392 361 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 silicification was monitored by electron microscopy for a morphological study, and by chemical analysis (GC, GC-MS, HPLC) for a preliminary study of the preservation or degradation of the 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 particular Mars.