COPROSTANOL PRODUCTION IN INVERTEBRATE MICROBIOMES: IMPLICATIONS FOR BIOMARKERS IN EDIACARAN FOSSILS

Several recent studies suggest that lipid biomarkers can be preserved in Ediacaran animal fossils. One of the more interesting molecules recovered from these fossils is coprostane (also known as 5β-cholestane) which is a known product of vertebrate gut symbionts. In humans, Clostridium cluster IV gut symbionts are responsible for producing coprostanol (by reducing cholesterol in the gut), specifically via the ismA gene. Whether invertebrate animals might also harbor microbes with ismA genes is currently unknown. To better understand the presence of coprostane in Ediacaran fossils and whether it suggests a fossil microbiome or environmental microbes, this study uses published metagenomic data to explore the distribution of ismA-bearing bacteria in modern invertebrate guts and in their environments.

Preliminary results suggest that ismA-bearing microbes, and therefore the capacity to produce coprostanol, are widely distributed throughout the animal kingdom, including: sponges, coral, tunicates, lancelets, chordates, echinoderms, arthropods, nematodes, rotifers, and molluscs. The ismA gene was also found in marine environmental samples and in sequences from a whale fall carcass, emphasizing that the bacteria do not need to be associated with a gut microbiome to be associated with animal remains. To further test the functionality of the invertebrate results, protein modeling using I-TASSER and pymol are used to compare the Clostridium ismA to proteins from several metagenomic sequences.

If correct, our results suggest that gut microbiomes could have been responsible for some of the coprostanol recovered in Ediacaran animal fossils. This metabolic capacity could prove highly informative to our interpretation of some of these organisms - particularly Dickinsonia - where the method of feeding remains highly contentious. Without more information we cannot adjudicate between cholesterol metabolism in either a fossil microbiome vs free living microbes - however the results of this study produce the first data that supports either explanation for this unusual compound in the Ediacaran.