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.
Highlights
Microbialites are sedimentary structures, often composed of carbonates that are constructed by microorganisms [e.g., 1]
This has been significantly questioned over the last few years: First, other microbial metabolisms inducing carbonate precipitation operate as well in modern microbialites, in particular sulphate-reduction [e.g. 6] and may be key drivers in the formation of microbialites; Bosak et al [7] proposed that anoxygenic phototrophs may have contributed to the formation of ancient microbialites
This is crucial to understand if the study of past microbialites can tell us something about microbial metabolisms that existed at that time and how their activity is recorded
Summary
Microbialites are sedimentary structures, often composed of carbonates that are constructed by microorganisms [e.g., 1]. The old paradigm is that microbialite formation is based mostly on the activity of cyanobacteria as a trigger to precipitation of carbonates. The process of microbialite formation most likely results from the interplay between microorganisms forming complex communities and their metabolic activities under the influence of environmental conditions (e.g. photoperiod, temperature) and local chemistry (ion availability).
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