Sponges, Symbionts or Side-Chain Scrambling: What are the Major Sources of Neoproterozoic C30 Steranes?

Abstract

Summary Three C30 sterane compounds are typically found together in the Cryogenian and Ediacaran rock record. This represents only a small subset of all structural possibilities from altering the side-chain chemistry. Both 24-isopropylcholestane (24-ipc) and 26-methylstigmastane (26-mes) are produced in abundance by some groups of demosponges and have been used as early animal biomarkers. Recently, it has been suggested that these C30 steranes are principally derived from diagenetic modification of C29 sterol precursors made by ancient green algae. If correct, this undermines their robustness and the interpretative value of these steranes as early animal markers. To assess which mechanism (biogenic vs. side-chain alteration) can account for the patterns of C30 steranes found in the Neoproterozoic rock record, we need to know the range of steroid structures that can be made in abundance by modern sponges. The sterols in over fifty sponge species were analyzed to identify those groups that make C30 and higher sterols. Catalytic hydyropyrolysis (HyPy) of sponges was conducted to convert sterols into steranes to elucidate the sterane elution patterns and six resolvable C30 steranes have already been detected. We will discuss the implications of these new sponge steroid findings for understanding the origins of Neoproterozoic C30 steranes.