Dartmouth Events

The seminar will take place in Steele 006. For those who wish to attend virtually, please use this Zoom registration link: https://dartmouth.zoom.us/meeting/register/tJYld-2orDItG9Y1JNY14SsuthrKo6ocCHcF

Title: Synthesis of sponge sterol lipid fossils – who makes what and why does it matter?

Abstract: Molecular fossils or biomarkers are recalcitrant organic molecules, typically lipids, preserved in ancient sediments that can be used to trace the occurrence of ancient life in the geologic record. Fossilized sterols, or steranes, are one group of lipid biomarkers that are used broadly as biomarkers for past eukaryotic life and can be preserved in rocks up to 1.6 billion years old. One useful and highly controversial sterane biomarker is 24-isopropylcholestane (24-ipc) – a sterol molecule propylated at the C-24 position that is used  as a proxy for demosponges as they are the only extant organisms known to produce large amounts of 24-isopropyl sterols. However, this interpretation has been challenged as other potential sources of 24-ipc have been recently identified and the biochemical mechanism behind the propylation at C-24 remains unknown. In this study, we experimentally characterized a variety of sterol 24-C-methyltransferase (SMT) homologs, the only enzymes known to alkylate at the C-24 position. We show that sponge SMTs are functional and can promiscuously alkylate the sterol side chain. However, no analyzed sponge SMTs could produce 24-propyl sterols. Further, we demonstrate that SMTs from bacterial sources are functional and can produce 24-propyl sterols - the first instance demonstrating the bacterial proteins are able to alkylate sterols at the C-24 position. Taken together, our study indicates that alternative sources of 24-ipc are possible and that a more nuanced interpretation of 24-ipc biomarkers in the rock record is necessary.