Growth phase dependent hydrogen isotopic fractionation in alkenone-producing haptophytes

摘要

Recent works have investigated use of the hydrogen isotopic composition ofC₃₇ alkenones (δD_K37s, lipid biomarkers of certainhaptophyte microalgae, as an independent paleosalinity proxy. We discussherein the factors impeding the success of such an application and identifythe potential alternative use of δD_K37s measurements as a proxyfor non-thermal, physiological stress impacts on the U₃₇K'paleotemperature index. Batch-culture experiments with the haptophyteEmiliania huxleyi (CCMP 1742) were conducted to determine the magnitude and variability ofthe isotopic contrasts between individual C₃₇ alkenones. Furtherexperiments were conducted with Emiliania huxleyi (CCMP 1742) andGephyrocapsa oceanica (PZ3-1) to determinewhether, and to what extent, δD_K37s varies between thephysiological extremes of nutrient-replete exponential growth andnutrient-depleted senescence. Emiliania huxleyi was observed to exhibit an isotopic contrastbetween di- and tri-unsaturated C₃₇ alkenones (α_K37:3-K37:2≈0.97) that is nearly identical to that reportedrecently by others for environmental samples. Furthermore, this contrastappears to be constant with growth stage. The consistency of the offsetacross different growth stages suggests that a single, well-defined valuefor α_K37:3-K37:2 may exist and that its use in an isotopemass-balance will allow accurate determination of δD values forindividual alkenones without having to rely on time- and labor-intensivechemical separations. The isotopic fractionation between growth medium andC₃₇ alkenones was observed to increase dramatically upon the onset ofnutrient-depletion-induced senescence, suggesting that δD_K37smay serve as an objective tool for recognizing and potentially correcting,at least semi-quantitatively, for the effects of nutrient stress onU₃₇K' temperature records.