Determination of unsaturation-, growth phase-, and growth rate-dependent hydrogen isotopic fractionation in C₃₇ alkenones produced by Emiliania huxleyi

Graduation date: 2009Recent works have investigated use of the hydrogen isotopic composition of C₃₇ alkenones (δD[subscript K37s]), lipid biomarkers of certain haptophyte microalgae, as an independent paleosalinity proxy. I discuss herein the factors impeding such an application and identify the potential alternative use of δD[subscript K37s] measurements as a proxy for non-thermal, physiological stress impacts on the U [superscript K'][subscript 37] paleotemperature index. Batch-culture experiments with the haptophyte Emiliania huxleyi (CCMP 1742) were conducted to determine the magnitude and variability of the isotopic contrasts between individual C₃₇ alkenones. Further experiments were conducted with E. huxleyi (CCMP 1742) and Gephyrocapsa oceanica (PZ3-1) to determine whether, and to what extent, δD[subscript K37s] varies between the physiological extremes of nutrient-replete exponential growth and nutrient-depleted senescence, the basis for our proposed use of the measurement as an indicator of stress. A third set of E. huxleyi cultures was grown with variable, light-controlled growth rates to observe whether or not this physiological factor affects fractionation when varied independently of temperature and salinity.\ud \ud Emiliania huxleyi was observed to exhibit an isotopic contrast between di- and tri-unsaturated C₃₇ alkenones (α[subscript K37:3-K37:2] ≈ 0.97) that is nearly identical to that reported by others for environmental samples. Furthermore, this contrast appears to be constant with growth stage. The consistency of the offset across different growth stages suggests that a single, well-defined value for α[subscript K37:3-K37:2] exists and that its use in an isotope mass-balance will allow accurate determination δD values for individual alkenones without having to rely on time- and labor-intensive chemical separations. The isotopic fractionation between growth medium and C₃₇ alkenones was observed to increase dramatically upon the onset of nutrient-depletion-induced senescence, but not to vary with different rates of nutrient-replete exponential growth. These results suggest that δD[subscript K37s] may serve as an objective tool for recognizing and potentially correcting the effects of nutrient stress on U[superscript K'][subscript 37] temperature records