title: Finding a family for an orphan biomarker - searching for modern, biological sources of cheilanthane molecular fossils
Creators: name: Liyanage, Tharika
Publisher: name: The Australian National University
Year: 2024

description: Molecular fossils are the hydrocarbon skeletons of functionalised biological molecules. Diagenetic processes in sediments alter these precursor biomolecules to form stable hydrocarbon molecules that can be preserved in the geological record for billions of years. Some molecular fossils can be linked to functionalised biological molecules and specific clades of microorganisms based on similarities in the hydrocarbon skeleton. However, there are still many orphan biomarkers, such as cheilanthanes (tricyclic terpanes), with no known microbial sources or precursor biomolecules. Until cheilanthanes are linked to microbial sources, we are left with inaccurate reconstructions of the ecosystems and environments in Earth's history and possible misinterpretations about the co-evolution of life and Earth. The objectives of this doctoral research were to determine whether there were any living microbial sources of cheilanthanes and to characterise the original biomolecule to contextualise the entire record of cheilanthane biomarkers in the history of life on Earth. First, I assembled the first record of C19-C46 cheilanthanes through the Proterozoic and Phanerozoic Eons. I describe the oldest, demonstrably syngenetic cheilanthanes in the 1.641 Ga Teena Dolostone. Based on the relative abundance of cheilanthanes to steranes and hopanes through the geological record, we propose that bacterial or archaeal sources of cheilanthanes are most likely rather than eukaryotic sources such as Tasmanites green alga, which has been previously proposed. I examined the relative abundance of C19-C46 cheilanthane pseudohomologues to discern patterns that may be indicative of cheilanthane precursor biomolecules' chemical nature. The absence of notable relative abundance maxima indicative of parent molecules' carbon number suggests that cleavage of the regular isoprenoid side chain of cheilanthane precursor biomolecules is efficient during diagenesis or that precursors could be part of a large macromolecular structure. These findings provide testable hypotheses in the search for living microbial sources and precursor biomolecules of cheilanthanes. Second, I conducted closed-system pyrolysis experiments to test whether cleaved hopanoid molecules could produce tetracyclic and tricyclic terpanes. Pyrolysis experiments on C30 diplopterol, a widespread bacterial membrane molecule, generated hopanoid breakdown products, which included tricyclic compounds that coelute with C19-C22 cheilanthanes and a tetracyclic compound that coelutes with or is identical to C24 tetracyclic terpane. Examination of geological samples revealed that these hopanoid breakdown products are common in oils and bitumens. Statistical analyses suggest that these hopanoid breakdown products can strongly influence biomarker proxies utilising cheilanthanes and tetracyclic terpane. This underscores the value of understanding why these cheilanthane abundance ratios are indicative of certain depositional environments. Third, to determine whether there were any living microbial sources of cheilanthanes, I developed a rapid screening technique that makes cheilanthanes detectable in modern environmental samples. This new pyrolysis technique simplifies functionalised biological molecules to their GC-amenable hydrocarbon skeletons. I discovered cheilanthane precursor biomolecules and living microbial sources in several aquatic settings, including Lake Cadagno in Switzerland and Ace Lake in Antarctica. My investigations into the chemical structure of the precursor biomolecules have suggested that there are multiple precursor molecules that bear an acid moiety. This discovery indicates that the cheilanthane precursor biomolecules are a new class of biomolecule. This provides a novel opportunity to bridge this gap between the geological record and past microbial ecology and can begin to contextualise the entire record of cheilanthane biomarkers in the molecular fossil record of life on Earth.