Our understanding of the evolution of life on Earth is based primarily on the fossil record of multi-cellular organisms. Yet the majority of life’s history has been dominated by microbes whose metabolic inventions have significantly altered the Earth’s environment and in turn impacted the evolution of life on Earth. Because microbial organisms do not usually leave diagnostic morphological fossils, alternative strategies are often employed for studying microbial communities in the context of the Earth’s distant past. One such strategy is to correlate organic compounds deposited by ancient microbes and preserved in sedimentary rocks with lipid molecules produced by modern bacteria. Because rocks provide the primary record of ancient events, these biomarkers are powerful microbial fossils that have the potential to physically link microbes and their metabolisms to the past and provide insight into the evolution of life billions of years ago.
Biomarker signatures in ancient rocks have been used to reconstruct important events in the Earth’s past, such as the first appearance of major groups of organisms, the catastrophic loss of biodiversity and the evolution of cyanobacteria and oxygenic photosynthesis. Despite the significant implications biomarker studies have on our interpretation of microbial evolution and of the Earth’s ancient environment, our understanding of the phylogenetic distribution and physiological function of most of these molecules in modern bacteria is quite limited. Our research utilizes a combination of bioinformatics, microbial genetics, organic geochemistry, and biochemistry to address three general questions that can be applied to any biomarker: 1) What is its phylogenetic distribution in modern bacteria? 2) What are its physiological roles in modern bacteria? 3) What is the evolutionary history of its biosynthetic pathway?
We specifically focus on understanding the biosynthesis and function of hopanoids and sterols in aerobic methanotrophs. One of the most abundant and ubiquitous types of extractable organic compounds in ancient rocks are the polycyclic triterpenoid molecules known generally as steranes and hopanes. These molecules are clearly recognized as the diagenetic products of sterol and hopanoid lipids found in modern organisms today and are robust geochemical proxies for eukaryotes and bacteria, respectively. Further, because of their structural similarities, sterols and hopanoids are also considered to have a common evolutionary and biochemical heritage. However, the production of sterols in a few bacteria has raised questions about the evolutionary significance of this eukaryotic pathway in the bacterial domain. It is clear that our interpretation of the evolutionary history of this important biosynthetic pathway would greatly benefit from understanding the function of both sterols and hopanoids in bacterial taxa. While several studies have emerged in the last few years focused on the biosynthesis and function of hopanoids, very little work has been done to understand the function of sterols in bacterial cells.