Date of Award

2018

Document Type

Open Access Dissertation

Department

Marine Science

First Advisor

Susan Lang

Abstract

Signatures of organic molecules in the environment are widely used to identify microbial metabolic processes and to track the cycling of carbon. The lipid biomarkers of methane cycling archaea are of particular interest as they are unique, preserved over geologic time scales, and reflect processes that impact an important greenhouse gas. Their isotopic compositions have been used to distinguish regions where archaea produce and anaerobically consume CH4. Previous work has demonstrated that energy availability impacts the stable carbon isotopes of CH4 during microbial synthesis from H2 and CO2. Here, we investigated whether this relationship could be extended to lipids and amino acids. The isotopic distributions of carbon metabolized and synthesized by the hyperthermophile Methanocaldococcus jannaschii were quantified following growth at 82°C in a chemostat with high (~80 µM) and low (15-27 µM) H2 concentrations. The stable carbon isotope fractionation factors for CH4 were >15‰ larger in low H2 experiments than in high H2 experiments. Lipid biomarkers and amino acids were similarly impacted, with approximately 10‰ larger fractionation factors under low H2 conditions. Simultaneously, substantial changes were observed with the relative amounts of carbon shunted to catabolic (CH4) versus anabolic (amino acids, lipids, biomass) pathways. These data can be used to assess the underlying mechanisms that determine the isotopic composition of long-lived biomarkers and, therefore, provide constraints for interpreting these signatures in the environment.

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