A growing interest in using lipid D/H ratios in biogeochemistry and paleoclimatology has resulted in the application of this new tool advancing more rapidly than our understanding of the processes that control D/H fractionation in algae. A quantitative framework in which to evaluate measured D/H values is still lacking.
In this project, researchers at the University of Washington will conduct continuous culture experiments with the marine diatom Thalassiosira psuedonana and the coccolithophorid Emiliania huxleyi at a range of nitrate- and light-limited growth rates, temperatures and salinities to determine how these environmental conditions affect D/H fractionation in lipids. Unlike batch cultures, chemostats permit the evaluation of cells at steady state, under controlled laboratory conditions. These results will be validated with natural populations of phytoplankton in three field experiments. In situ incubation experiments will be conducted in the euphotic zone of subpolar and subtropical North Pacific Ocean sites to evaluate the influence of light- and nutrient-limited growth rate on D/H fractionation in lipids from E. huxleyi. The influence of temperature on D/H fractionation in a natural phytoplankton population will be evaluated by monthly sampling of Lake Washington over the course of a year. D/H fractionation will be determined by comparing the isotopic difference between individual lipids and growth water. Both acetogenic (linear) and isoprenoid (branched) lipids will be analyzed in every experiment. Differences in D/H fractionation in lipids from different biosynthetic pathways will provide the means to formulate and test hypotheses of D/H fractionation mechanisms.
Broader Impacts: The environmental education of high school students, undergraduates, women and the public will be advanced by this research. Of 10 lab group members working with the principal investigator during the last year, 4 were women and 3 were undergraduates. Two undergraduates joined the principal investigator's expedition to the Marshall Islands and Kosrae in Summer 2009. The lab website has averaged more than 2000 hits per day in recent months.
The use of hydrogen isotope ratios in lipids to decipher biogeochemical and climatic processes is expanding rapidly. The relative success of these efforts depends on an understanding of the environmental conditions that influence hydrogen isotope fractionation. During this project we conducted 38 controlled laboratory culture experiments, or chemostats, with two species of marine phytoplankton, the coccolithophorid Emiliania huxleyi and the diatom Thalassiosira pseudonana. In each chemostat culture one of four environmental parameters--light, nutrients, salinity and temperature--was changed while all others were held constant. At the termination of each culture experiment the cells were harvested on filters, the lipids were extracted and purified, and their hydrogen isotope composition was measured on a gas chromatograph-isotope ratio-monitoring mass spectrometer (GC-irMS). In addition we conducted two oceanographic research cruises to determine how light and nutrient limitation in natural populations of phytoplankton would effect hydrogen isotope fractionation in lipids. These experiments demonstrated that environmental conditions exert a large and systematic effect on hydrogen isotope ratios in phytoplankton lipids. Specifically, we found that the heavy isotope of hydrogen, deuterium, is discriminated against to a greater extent as temperature increases and as growth rate increases. On the other hand, discrimination against deuterium declines as light levels increase (in most lipids) and as the salt concentration of growth water increases. The results of these experiments are facilitating the use of hydrogen isotope ratios in phytoplankton lipids to reconstruct rainfall, climate and circulation changes in the past, particularly during the time before weather instruments and satellites were widely used. For example, we are now able to use the hydrogen isotope ratio of algal lipids extracted from sediment cores in tropical oceans and lakes to determine the salinity and water isotopic composition, which together indicate how wet or dry it was at the time the sediment was deposited. In this way the natural range of climate variability in the pre-industrial era can be established, an essential baseline for assessing when the modern climate is outside the range of natural variability. Beyond the scientific and technical implications of this research project it provided the opportunity for eight University of Washington undergraduate students to participate on an oceanographic research cruise to the northeast Pacific Ocean, it funded six undergraduate researchers who obtained skills in the culturing of marine phytoplankton and organic geochemistry, and funded in part the PhD research of three female graduate students.
