MSA AWARD LECTURE: BIOMINERALIZATION AND FOSSILIZATION OF BACTERIA: WHAT DO WE LEARN FROM FIELD AND EXPERIMENTAL STUDIES
Here, I will review several approaches that we have been conducting in the recent past. First, I will show how we can use a combination of focused ion beam milling, TEM and synchrotron-based x-ray microscopy to characterize down to the nm-scale some biomorphs in Paleocene phosphorites. While SEM observations provide ambiguous hints, our analyses strongly suggest that these objects are Gram-negative bacteria, with cell walls preserved by incipient biomineralization of Ca-phosphates. In a second approach, we induced biomineralization of diverse bacterial strains in the laboratory. I will show results obtained on anaerobic Fe-oxidizing bacteria and on Ca-phosphate biomineralizing mutant strains affected in their capability to produce a phosphatase enzyme. This improves our understanding of the first steps of fossilization and the ultrastructural details that can be preserved. It moreover provides a source of mineralized cells that are processed through thermal and pressure treatments to simulate the transformations of bacterial structures upon aging. Finally, I will show a study of modern biomineralizing systems in the field, e.g., calcifying microbial communities from a hyperalkaline lake. This allows investigating the link between microbial diversity and fossilization, a key parameter for the study of Precambrian rocks. One lesson is that the use of actualism might be delusive if our knowledge of modern diversity is too limited.