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Over the past two years we made significant progress understanding the influences on hydrogen isotope fractionation in algal lipids. Our study sites in Palau, Christmas Island and the Chesapeake Bay have proven to be well-suited to addressing the central question of how water isotope composition and salinity are preserved in lipid D/H ratios, offering large environmental gradients with few extraneous influences.

We published papers in Spring 2008 in Geochimica et Cosmochimica Acta with post-doctoral investigator Dirk Sachse on Christmas Island (Sachse & Sachs, 2008), Autumn 2007 with post-doctoral investigator Rienk Smittenberg on the purification of dinosterol by HPLC-MS in Journal of Chromatography A (Smittenberg & Sachs, 2007), and in Spring 2009 with post-doctoral investigator Valerie Schwab on alkenone purification from Chesapeake Bay in Organic Geochemistry (Schwab & Sachs, 2009). We have a paper in preparation reporting on the influence of water D/H and salinity on D/H ratios in dinosterol and alkenones (Schwab & Sachs, in prep). It is the results in this last paper that we discuss below since the results of the other three papers were addressed in large part in our previous Progress Report.

Our study shows that the ?D value of individual alkenones from the Chesapeake Bay are primarily controlled by the hydrogen isotope composition of the source water showing no significant influence of salinity. This feature suggests that the ?D values of individual alkenones from estuaries may be a robust tool for the reconstruction of D/H ratios of paleo-water. In contrast, ?D values of dinosterol clearly indicated an influence of salinity suggesting a possible use of this compound as a paleo-salinity proxy.

The use of hydrogen isotope ratios in algal lipids as paleo-proxies relies on the fact that the isotopic fractionation is largely determined by a small number of environmental and physiological conditions. It has recently been established that the hydrogen isotope composition of algal lipids is not controlled entirely by the ?D values of source water, but may be additionally influenced by growth rate (Schouten et al., 2006; Zhang et al., 2009), temperature (Zhang & Sachs, 2007; Zhang et al., 2009), salinity (Sachse & Sachs, 2008; Schouten et al., 2006), and species (Zhang & Sachs, 2007). Sachse and Sachs (2008) suggested that the D-enrichment of cyanobacterial lipids with increasing salinity resulted from a decrease of membrane permeability toward water. If so, hydrogen isotopic composition of marine organisms will additionally depend on many factors related to the process of osmoregulation. Exposed to a wide variation of salinities, natural genetic selection may have favored haptophyte algae in the Chesapeake Bay that had higher osmoregulatory capacity thereby decreasing the influence of salt on the hydrogen isotope composition of alkenones. In a diluted medium, organisms must reduce the influx of water and the loss of ions by decreasing the permeability of the apical membrane (i.e., the side facing water). This may be achieved by increasing the proportion of long chain saturated lipids.

This project has provided substantial training and opportunities for postdoctoral scholars Dr. Valerie Schwab and Dr. Dirk Sachse to become independent and well-regarded researchers in their own right. It has also provided support for graduate student Dan Nelson to develop a very promising thesis topic for his dissertation. Furthermore, the results of the research made possible by this ACS-PRF grant have made a substantial impact on the fields of geochemistry, plant and algal physiology, and paleoclimatology by elucidating the influence of salinity on lipid D/H ratios in phytoplankton.

REFERENCES

Sachse, D., Sachs, J.P., (2008) Inverse relationship between D/H fractionation in cyanobacterial lipids and salinity in Christmas Island saline ponds. Geochimica et Cosmochimica Acta, 72(3), 793-806.

Schouten, S., Ossebaar, J., Schreiber, K., Kienhuis, M.V.M., Langer, G., Benthien, A., Bijma, J., (2006) The effect of temperature, salinity and growth rate on the stable hydrogen isotopic composition of long chain alkenones produced by Emiliania huxleyi and Gephyrocapsa oceanica. Biogeosciences, 3(1), 113-119.

Schwab, V.F., Sachs, J.P., (2009) The measurement of D/H ratio in alkenones and their isotopic heterogeneity. Organic Geochemistry, 40(1), 111-118.

Schwab, V.F., Sachs, J.P., (in prep) The influence of salinity on D/H fractionation in algal lipids in the Chesapeake Bay estuary. Geochimica et Cosmochimica Acta.

Smittenberg, R.H., Sachs, J.P., (2007) Purification of dinosterol for hydrogen isotopic analysis using high-performance liquid chromatography-mass spectrometry. Journal of Chromatography A, 1169(1-2), 70-76.

Zhang, Z., Sachs, J.P., (2007) Hydrogen isotope fractionation in freshwater algae: I. Variations among lipids and species. Organic Geochemistry, 38(4), 582-608.

Zhang, Z., Sachs, J.P., Marchetti, A., (2009) Hydrogen isotope fractionation in freshwater and marine algae: II. Temperature and nitrogen limited growth rate effects. Organic Geochemistry, 40(3), 428-439.