Earth's history is marked by dramatic climate changes that have had a significant impact on the evolution of life. Of particular interest to earth scientists is how ancient microbes have responded to such dramatic events. Because microbial organisms do not leave readily recognizable fossils, alternate strategies are needed to investigate ancient microbial communities. One approach is to study certain biological molecules (lipids) that can be preserved in ancient rocks. These lipids function as molecular fossils, or biomarkers, that can inform us about ancient microbial communities and environmental conditions under which they used to live. One such group of biomarkers is the 3-methylhopanes, cyclic molecules that are often assumed to be indicators for methane-oxidizing bacteria. However, these assumptions may not be robust and further studies are needed. This proposal will utilize molecular and biochemical tools to study how 3-methylhopane production is genetically and environmentally controlled in modern bacteria. This research will provide valuable information to better interpret 3-methylhopane patterns in the rock record and will ultimately lead to a better understanding of how ancient microbial communities responded to atmospheric change in the past and how similar microbial communities may respond to future climate events. The investigator will involve summer students recruited through the Summer Undergraduate Research in Geosciences and Engineering (SURGE) program at Stanford University. This proposal will provide funding for low-income summer high school interns to perform research in the researcher's laboratory through the General Earth Sciences Internship program at Stanford. These activities will provide underrepresented students with the opportunity to experience geobiology research first hand and encourage them to pursue a career in the Earth Sciences.
Methylhopanoids are pentacyclic bacterial lipids whose diagenetic products, methylhopanes, are readily detected in sedimentary rocks. Based on the production of methylhopanoids in modern bacteria, methylhopane signatures in the rock record have the potential to function as biomarkers that can link bacterial taxa and their metabolisms to a specific time or event in Earth?s history. In particular, it has been proposed that 3-methylhopanes are reliable proxies for aerobic methanotrophs and indicators for significant methane biogeochemical cycling. However, recent studies have demonstrated that the diversity of extant 3-methylhopanoid producers extends beyond aerobic methanotrophs. These findings have also revealed that an understanding of the function and regulation of methylhopanoids in modern bacteria is needed in order to properly interpret the occurrence of methylhopane biosignatures in the rock record. To this end, the investigator and collaborators have begun investigating 3-methylhopanoid physiology in the aerobic methanotroph Methylococcus capsulatus. In this organism, a gene (hpnR) required for C-3 methylation has been identified and subsequently deleted. Physiological studies of the methylase mutant, which only produces unmethylated hopanoids, have revealed a potential role for 3-methylhopanoids in stationary phase survival. In this project, the investigator aims to directly pinpoint the role of 3-methylhopanoids in stationary phase survival in hopes of providing the foundation needed to properly interpret 3-methylhopane signatures in the rock record. The following three lines of investigation are proposed: 1. Physiological studies will be pursued with the M. capsulatus 3-methylhopanoid mutant to determine whether 3-methylhopanoids are involved in stationary phase survival under oxygen limited conditions, 2. ÄhpnR suppressor mutants that exhibit stationary phase survival levels similar to the wild type strain will be isolated. Utilizing high-throughput whole genome sequencing the investigator will identify intergenic mutations that restore viability of the methylase mutant, and 3. To determine if 3-methylhopanoid function is conserved in other bacterial taxa, physiological and genetic analyses of 3-methylhopanoid physiology will be undertaken in the alkaliphilic, halotolerant aerobic methanotroph Methylobacterium alcaliphilum and the actinomycete Streptomyces ghanaensis.
