Environmental Phylogeny, Physiology, and Evolutionary Significance of Polycyclic Triterpenoids

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The record of lipid biomarkers preserved in ancient sedimentary rocks is a fundamental geochemical tool used to decipher early environments on Earth. In this project we seek to achieve a better understanding of the (i) genetic diversity, (ii) evolutionary history and (iii) the physiological benefit that drove the evolution of a particular class of these biomarkers, the polycyclic triterpenoids. These compounds include the steroids (e.g., cholesterol) and the analogous microbial compounds known as hopanoids. This project will have three primary components. We will examine contrasts between aerobic and anaerobic environments and between terrestrial and aquatic system, by sequencing genes for hopanoid biosynthesis. We hope to determine the primary organisms and settings that are sources for these important geological biomarkers. We will study whether these compounds confer a phenotype that is related to the presence of oxygen, by using genetically tractable strains of common laboratory microorganisms. Finally, we will use large-insert clone libraries to look for sequences for other varieties of triterpenoid cyclases. This may help us understand the evolutionary history of this gene family. This project represents the first systematic study to utilize molecular biological approaches to survey a major lipid biosynthetic pathway of interest to geobiology. We hope to provide new and robust information about the evolution of aerobic organisms, oxygenation of Earth surface environments, and the geologic record of lipid biomarkers.