MICROBIAL PRODUCTION OF COPROSTANOL: IMPLICATIONS FOR EARLY ANIMALS AND REAPPRAISAL OF THE BIOMARKER

Studies on the chemical composition of early animal fossils during the Ediacaran (635-541 Ma) have recovered many informative lipid biomolecules. One of these is coprostane, which in other contexts is assumed to be the product of ‘higher vertebrate’ gut microbiomes. Human microbiome studies have identified Clostridium symbionts being responsible for producing coprostanol (by reducing cholesterol in the gut), specifically via the ismA gene. Interestingly, coprostane was recently found in the Ediacaran fossil Dickinsonia, one of the oldest putative animals, and whose digestive anatomy remains highly contentious. There are several competing hypotheses for where the Ediacaran coprostane came from, specifically: a gut microbiome, decay of the animal carcass, and from the environment. To better understand the presence of coprostane in Ediacaran fossils and whether it could suggest a fossilized microbiome signature, we analyzed metagenomic data to explore the distribution of ismA-bearing bacteria in modern invertebrate guts and in their environments. An additional study was conducted to identify any other microbial taxa beyond Clostridium that have the gene. To further test the functionality of the invertebrate results, protein modeling via ColabFold and pymol are used to compare the known Clostridium ismA proteins to the putative sequences. The results suggest that ismA-bearing microbes, and therefore the capacity to produce this biomarker, are present in many groups of invertebrates. The presence of coprostanol therefore doesn’t necessarily imply only decomposition of the Dickinsonia carcass - as is the prevailing hypothesis - but could instead represent a source endogenous to the living animal. The environmental levels of coprostane suggest that the Ediacaran featured an unusual sterol cycle with few modern analogues. By combining lipidomics and metagenomics we can ask new questions in the fossil record as more data on the chemical composition of fossils becomes available.