A window on ancient systems: Understanding the environmental distributions of hopanoids using compound-specific multi-isotope analysis and metagenomics

Harvard University, Cambridge MA

Investigators

Abstract

Technical overview: A better understanding of the modern sources and distributions of hopanoids is essential for interpreting ancient sedimentary records. Here we will address questions of hopanoid provenance in marine settings using interdisciplinary analytical approaches, namely compound-specific multi-isotope (13C, D/H, and 14C) measurements combined with analysis of functional genes involved in hopanoid biosynthesis. The project will make a systematic survey of hopanoid isotope measurements and DNA metagenomes. The samples will cover locations ranging from arctic to tropical, and three sets of samples will span gradients of deltaic to pelagic depositional conditions, forming mini-transects. The samples will permit use of the established latitudinal patterns in 14C residence times and source water D/H to distinguish land-based from marine sources. Known patterns of 13C and D/H isotopic fractionation associated with different types of microbial metabolism will be used to determine the metabolic sources of individual compounds, because autotrophic or heterotrophic metabolism is reflected in isotopic variability. The diversity of hopanoid biosynthesis genes (sqhC, hpnP, hpnR, hpnG, hpnH) will be related to hopanoid compound distributions in an independent approach to test if local production is or is not related to local deposition of lipid assemblages. General importance: Organic molecules preserved in sedimentary rocks provide a record of Earth?s early biosphere. They help us understand ancient Earth environments and the expansion, diversification, and maintenance of microbial communities. Such compounds are called 'molecular fossils', and among the earliest and most abundant examples are compounds called hopanoids. Derived primarily from bacteria, hopanoids are detectable over timescales of billions of years and have been proposed to be among the most abundantly preserved molecules on Earth. However, to date we lack an overall picture of their environmental, physiological, and/or species origins. This project aims to answer basic questions about the hopanoids preserved in marine sedimentary environments. Are they primarily of marine origin, or do they come primarily from land? Are they likely to reflect particular sub-groups of microbial taxa, or are they broadly distributed among diverse species, habitats, and lifestyles? In this project we will use the natural distribution of the isotopes of carbon and hydrogen to help answer these questions.