Colloquium: Alexander S. Bradley
Abstract: Molecular biomarkers in geochemistry are organic compounds that record fossil evidence of life. Biomarkers can derive from many biochemical precursors, but lipids have the highest preservation potential the structures and isotopic compositions of lipid carbon skeletons can be preserved in rocks for billions of years.
The first part of this talk will focus on the how the analysis of lipids from geological samples can reveal information about microbial populations and activity in the environment. I will discuss an example from the Lost City Hydrothermal Field (Mid-Atlantic Ridge), where serpentinization reactions produce highly reducing vent fluids. These fluids provide a rich source of energy for chemotrophic microbes, and also provide reducing power for the near-quantitative conversion of inorganic carbon to methane. Molecular biomarkers provide the key piece of information needed to uncover the role of microbes in the Lost City carbon cycle. The result has implications for understanding i) the record of 13C abundance in sedimentary organic material, ii) the Archean carbon cycle, and iii) the origin of methane on other terrestrial planets such as Mars.
The second part of the talk will look in more detail at the origins of molecular biomarkers and at how geochemists arrive at an understanding of the information they carry. Traditionally, biomarkers have been interpreted as unambiguous records of particular organisms or groups of organisms. However, this approach relies on demonstrating the presence or absence of precursor compounds in laboratory cultures, and is therefore hampered by small sample sizes and uncultivable organisms. I propose a new definition of biomarkers, where the link is not to the organism but to a suite of genes. This approach harnesses cutting-edge genetics, genomics, and bioinformatics to robustly constrain biomarker sources, and interprets sedimentary biomarkers as a fossil record of gene expression. By understanding the genetic basis and physiological role of biomarker lipids, we can better understand their link to organisms, environments, and evolution. To demonstrate the feasibility of this approach, I provide an example from recent results examining the genetics and physiology of hopanoids in the model organism Methylobacterium. I will propose several new lines of inquiry where a similar approach might be useful, with applications ranging from animal evolution to paleoclimate.
Alexander S. Bradley, Agouron Institute Fellow
Agouron Institute, Harvard University