"Carbon isotope fractionation in Archaea using the 3HP/4HB pathway: Prospects for paleo-geochemistry and paleo-barometry"

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A better understanding of atmospheric carbon dioxide (CO2) concentration and its relationship to Earth?s temperature is essential to climate research. Records from ice cores show that the partial pressure of carbon dioxide (pCO2) correlates with global temperatures -- high pCO2 implies a warm Earth. However, ice core records are limited to the age of Earth's oldest ice, which is only ~1 million years (Ma). All estimates of pCO2 from older than 1 Ma rely on other indirect measurements that track CO2. These indirect measurements are called proxies. This project will test a new pCO2 proxy called the Archaea (EpsilonAr) proxy, which is based on lipids (a class of biochemicals) recovered from microbes living in marine systems. The basis for the EpsilonAr proxy originated from some limited field observations and from a theoretical model. The primary objective of the current research is to test the theory behind the EpsilonAr proxy, both using controlled laboratory experiments and by studying marine sediments that record a recent glacial-interglacial cycle. The project crosses disciplinary lines, requiring geobiological methods along with isotopic analyses and applications to the sedimentary record. The project will train a post-doctoral investigator and two summer undergraduate and/or high school interns.A combination of laboratory experiments and natural samples will be used to test the hypothesis that the carbon isotope fractionation recorded in lipids of marine Thaumarchaeota reflects the extracellular CO2 concentrations during growth. Thaumarchaeota use the 3-hydroxypropionate/4-hydroxybutyrate (3HP/4HB) pathway of carbon fixation in which the primary enzyme, acetyl/propionyl-CoA carboxylase, is specific for the substrate HCO3-. The current hypothesis is that Thaumarchaeota are dependent on CO2 diffusion, followed by intracellular catalysis to HCO3 . The initial focus of the project will be to conduct controlled experiments on the model taxon Nitrosopumilis maritimus to test a quantitative model of the carbon isotope effect and its dependence on CO2 (by varying pH, dissolved inorganic carbon (DIC), and growth rate). The results will be compared to control experiments with the thermophile Metallosphaera sedula, a 3HP/4HB taxon predicted to be CO2-insensitive. The second stage of the project will be to examine whether the predicted isotopic sensitivity of EpsilonAr is observed in a glacial-interglacial sedimentary record from the subtropical Atlantic Ocean. Outcomes will be evaluated by comparing the observed EpsilonAr response to CO2 and growth rate with the existing theoretical framework, updating and refining the theory as needed to reflect the experiments and observations.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.