EXPERIMENTAL FOSSILIZATION OF BACTERIA AND BIOMINERALS: APPLICATIONS IN THE SEARCH FOR MICROFOSSILS IN ANCIENT ROCKS

The fossil record of microorganisms and biominerals contains key information about the evolution of life on Earth. However, it is challenging to recognize and decode this record since the structural and chemical signatures of microorganisms and biominerals are inevitably altered by fossilization processes. A large number of purported microfossils have been suggested in ancient rocks. The biogenicity of some of them has been debated. To improve the reliability of these interpretations, we need to investigate how microorganisms and biominerals eventually look like once they have been processed in the laboratory in a way that simulates a plausible geological history.

Here, we investigated the structural and chemical evolution upon heating of four samples consisting in three different bacterial strains associated with different biominerals: an E. coli strain with no biomineral as a control; E. coli cells that formed Ca-phosphates in their cell walls; Fe-oxidizing bacteria cells that formed Fe-phosphates in their cell walls; and the magnetotactic bacterial strain AMB-1 that formed intracellular magnetite chains. Fossilization experiments were carried out on few day old bacteria cultures, embedded or not within silica gel. The influence on the geochemical evolution of biomolecules and biominerals of different parameters, including temperature, pressure, the nature of the organic precursor and the associated mineral phase, the oxygen fugacity and the experiment duration were tested.

We characterized the cell morphologies and ultrastructures, and the chemical composition of the organic and inorganic phases using a combination of microscopic and spectroscopic techniques including FIB-SEM, TEM, EPR, Infrared and Raman spectroscopies and synchrotron-based STXM coupled with XANES spectroscopy. We evidenced the varying potentialities of various bacteria to fossilize and the importance of biominerals and inorganic matrix in the preservation of biosignatures during fossilization processes. Altogether, our results open new perspectives for the search of biosignatures in ancient rocks.