Lipid biomarkers preserved in ancient sedimentary rocks provide a key record of early life on Earth. They carry structural and isotopic information directly reflecting the biochemical pathways of their associated organisms. This information can shed light on early evolution and environments and provides one of the few available windows to the composition of ancient ecosystems. Among the most ancestral lipid biomarkers are the hopanes, diagenetic products of bacterial hopanoids. The ability to synthesize hopanoids or similar molecules probably was present in the last common ancestor of Bacteria, and this pathway subsequently was lost from most taxonomic groups. Hopanoids are one of the few biological products that directly record biological activity on the early Earth. This project will take a physiological approach to understanding the evolution and function of hopanoids with the goal to increase our understanding of conditions on early Earth. The PIs will undertake a three-part study: 1) Biosynthesis and cellular setting of hopanoids, 2) Phenotype and physiology, 3) Adaptation to hopanoid modification and implications for evolution of early life.
The broader impacts resulting from this project will be realized through mentoring, community outreach, and undergraduate education. PIs Marx and Pearson will mentor postdoctoral fellow Alexander Bradley throughout the project. In turn, Dr. Bradley will mentor an undergraduate student through Harvard's Microbial Sciences Initiative, which makes a strong effort to recruit students from economically and ethnically diverse backgrounds. Through the Microbial Sciences Initiative we will also participate in a series of day-long professional development workshops for K-12 science teachers. The PIs and Dr. Bradley will participate in developing the curriculum and leading laboratory activities to communicate cutting-edge results to teachers from diverse backgrounds. Finally, this project will form the basis for a new module of a project-based research course designed to teach research-skills to undergraduates within the broader theme of microbial evolution.
Our record of the past environment on Earth comes from our ability to interpret signals that have survived in the rock record. Amongst the most remarkable of these are the remnants of lipid molecules – once part of the membranes of cells – that have survived for over two billion years in some sedimentary rocks. There are many such molecules that can be detected, but perhaps the most remarkable of these are remnants of sterols and hopanoids, both classes being complex, multi-ring structures with interesting side groups. These are undeniably from organisms such as bacteria, but what do they mean? What should we infer if we find them? Can we say anything about what type of organism contained them or what they used them for? This project had as its goal using a modern, living organism – the pink, leaf-colonizing Methylobacterium – as a model system to understand just what role hopanoids can play in bacteria. One major goal was to begin to infer the role of hopanoids by finding out what happens to cells if they are removed genetically by deleting the genes that are needed to make the enzymes that catalyze their formation. The answer: lots of things are different. Cells become clumpy and grow poorly on some compounds. Notably, they become sensitive to a wide variety of toxic chemicals, suggesting that hopanoids help confer a barrier function that is missing in the mutant strains. A second major goal was to determine how cells deal with losing the ability to make hopanoids, as this has occurred many times during Earth’s history. We evolved populations of hopanoid-lacking strains in the environment they fared the most poorly in relative to the normal cells, and kept increasing the concentration of the two toxins we used to keep selection upon their weakness. Not a very nice thing to do, but they improved tremendously. From having sequenced the genomes of several evolved isolates we have determined the genetic targets of the beneficial mutations that allowed them to adapt. Work is ongoing to figure out just why these changes were beneficial, and we have discovered several interesting connections between hopanoids and other cell functions. All of these data suggest that hopanoids play a very general permeability role in Methylobacterium, and challenge the idea that they can be interpreted in the rock record as having contributed to a single, particular lifestyle. Throughout this project, we have interacted with the general public through open presentations to the community and used this project to train high school students, as well as Harvard undergraduates, in how to conduct science. Perhaps the greatest message is that evolution in but a few short months can be used to better understand the evolution that has occurred from an early, purely microbial planet through to the modern day.
