Assessing the role of microbes and the chemical conditions necessary for the formation of microbialites

Microbialites are organosedimentary deposits formed by benthic microbial communities that mediate mineral precipitation. They have received particular attention since they are found throughout the geological record up to 3.43 billion years ago and are considered as among the oldest traces of life on Earth. It has been known for several decades that the abundance of microbialites has varied over geological time with an overall decline during the Phanerozoic. Yet, the question of what between biological and/or physico-chemical variations may have changed the abundance of microbialites over geological times remains debated. This calls for a better appraisal of the conditions necessary for the formation of modern microbialites. Here, I will first review (omics) studies about the importance of specific microorganisms in microbialite formation (e.g., Iniesto et al., 2020 & 2021). Then, I will present a recent database listing the physico-chemical parameters of more than a hundred aqueous environments where modern microbialites form. The analysis of this dataset highlights that specific chemical conditions in seem to be always met, thus appearing as necessary for microbialite formation. Moreover, the comparison of this database with existing databases listing close to 8000 continental freshwater systems allows further insight on the potential of modern environment for microbialite formation. Overall, I will discuss how this suggests that the change of the chemical composition of the ocean might be a primary driver of the decline of microbialite abundance over geological times. I will conclude by tentative speculations about the fate of microbialites on an evolving planet.