Hydrogen isotope fractionation in freshwater algae: I. Variations among lipids and species

摘要

Five species of freshwater green algae, including three strains of Botryococcus braunii (two A Race, one B Race), Eudorina unicocca and Volvox aureus, were cultured under controlled conditions in media containing different concentrations of deuterium. The hydrogen isotopic ratios of lipids in the algae, including alkadienes, botryococcenes, heptadecenes, fatty acids, and phytadiene, were measured by gas chromatography–isotope ratio-mass spectrometry (GC–IRMS) and found to closely track water δD values. While correlation coefficients (R2) in excess of 0.99 for all lipids in all species suggest that lipid δD values can be used to determine water δD values, hydrogen isotope fractionation was found to vary systematically between lipids and lipid homologues within a single alga, as well as for the same lipid between species of algae. Under similar growth conditions, two species of Chlorophyceae (Eudorina unicocca and Volvox aureus) and three species of Trebouxiophyceae (Botryococcus braunii) produced palmitic acid (C16 fatty acid) that differed by 90–100‰ relative to water. Ubiquitous lipids such as palmitic acid, with a multitude of aquatic and terrestrial sources, are therefore not good targets for D/H-based paleohydrologic reconstructions. In addition to the use of source-specific biomarkers that derive unambiguously from a single family or species, paleohydrologic applications of lipid D/H ratios will need to consider the as yet unstudied potential influence that environmental parameters such as nutrients, light and temperature, etc., may have on D/H fractionation during lipid synthesis.