Geomicrobiological study of modern microbialites from Mexico: towards a better understanding of the ancient fossil record

Abstract

Microbialites are sedimentary formations that are found throughout the geological record and are usually considered as one of the oldest traces of life on Earth. Although they have been known for more than a century and hold as an emblematic object in Earth Sciences, we yet do not understand in details how they form and how microbial processes impact their chemistry, structure and macroscopic morphology. Here, we show recent advances achieved owing to funding provided by the EPOV program on the formation of modern microbialites in the crater Lake Alchichica (Mexico). We first show that very diverse microbial communities populate these microbialites, including diverse microbial groups able to induce carbonate precipitation. We demonstrate that this microbial diversity can be preserved for several years in laboratory aquaria offering a nice opportunity for future studies to assess the relationships between biodiversity and microbialite formation. We then detail the textural modifications affecting cyanobacterial cells during the first steps of fossilization as captured in Alchichica microbialites. Finally, we report the discovery of a new deep- branching cyanobacterium species, Candidatus Gloeomargarita lithophora, able to form intracellular Ca-, Mg-, Sr- and Ba-rich carbonates and discuss the implications for the interpretation of the fossil record.