Seminar on Sponge Sterol Lipid Fossils on May 5, 2022

When: May 5, 2022 01:00 PM
In this public seminar, Dr. Paula Welander will talk about a useful yet controversial lipid biomarker (24-isopropylcholestane) for studying past eukaryotic life, its synthesis mechanism and variety, and implications for our interpretation of the geologic record. The seminar will take place in Steele 006; for those who wish to attend virtually, please email kmw.ears [at] dartmouth [dot] edu using a dartmouth email.

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. This event is supported by the Society of Fellows and the Department of Earth Sciences.