Controls on Carbon Isotopic Fractionation in Emiliania Huxleyi

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Although laboratory and field studies suggest that isotopic analyses of alkenones show great potential as a CO2 proxy because cell geometry may be estimated, the relationship between specific growth rate and carbon isotopic fractionation in natural samples has not been well defined. For this reason, the principal investigators will develop and evaluate a novel method based on combining 13C labeling and isotope-ratio-monitoring gas chromatography-mass spectrometry to determine in situ growth rates of Emiliania huxleyi and Gephyrocapsa oceanica in the ocean. The objectives of this research are to define the range of growth conditions where the 13C-alkenone-labeling technique gives reliable estimates of growth rate and to identify under what conditions bias may be anticipated. In additions, the principal investigators will evaluate the effects of nutrient and light limitation on carbon isotopic fractionation and explore how culture techniques affect 13C labeling and the relationship between carbon isotopic fractionation, growth rate and aqueous CO2 concentrations using dilute batch versus continuous culturing experiments. Lastly, in a select suite of experiments, the rate of incorporation of 13C into alkenones will be compared with other lipids for E. huxleyi and G. oceanica to provide insight into the physiological function(s) and biosynthetic pathway(s) of alkenones. A field component will be carried out to compare directly the fractionation relationship in natural alkenone-producing algae with those developed in laboratory experiments. Results from this study will help calibrate and validate the use of carbon isotopes in alkenones as a CO2 proxy, essential knowledge if the paleoceanographic community is to use this proxy to better understand mechanisms of climate change.