Alkenone

Alkenones are long-chain unsaturated methyl and ethyl ''n''-ketones produced by a few phytoplankton species of the class Prymnesiophyceae.Marlowe, I.T., Green, J.C., Neal, A.C., Brassell, S.C., Eglinton, G. and Course, P.A. (1984) "Long-chain (''n''-C₃₇-C₃₉) alkenones in the Prymnesiophyceae. Distribution of alkenones and other lipids and their taxonomic significance." ''British Phycological Journal'' 19, 203-216; doi:10.1080/00071618400650221. Alkenones typically contain between 35 and 41 carbon atoms and with between two and four double bonds.Rontani J.F., Prahl F.G. and Volkman J.K. (2006) "Re-examination of the double bond positions in alkenones and derivatives: biosynthetic implications." ''Journal of Phycology'' 42, 800–813; doi:10.1111/j.1529-8817.2006.00251.x. Uniquely for biolipids, alkenones have a spacing of five methylene groups between double bonds, which are of the less common E configuration. The biological function of alkenones remains under debate although it is likely that they are storage lipids.Epstein, B. L., D'Hondt, S. and Hargraves, P.E. (2001) "The possible metabolic role of C₃₇ alkenones in ''Emiliania huxleyi''." ''Organic Geochemistry'' 32 (6), 867–875; doi:10.1016/S0146-6380(01)00026-2.Eltgroth, M.L., Watwood, R.L., and Wolfe G.V. (2005) "Production and cellular localization of neutral long-chain lipids in the haptophyte algae ''Isochrysis galbana'' and ''Emiliania huxleyi''" ''Journal of Phycology'' 41 (5), 1000–1009; doi:10.1111/j.1529-8817.2005.00128.x. Alkenones were first described in ocean sediments recovered from Walvis Ridgede Leeuw, J.W., van der Meer, F.W., Rijpstra, W.I.C. and Schenk, P.A. (1980) "On the occurrence and structural identification of long chain unsaturated ketones and hydrocarbons in sediments." In ''Advances in Organic Geochemistry 1979'' (eds. A.G. Douglas and J.R. Maxwell). Pergamon Press, Oxford, pp. 211–217; doi:10.1016/0079-1946(79)90105-8. and then shortly afterwards in cultures of the marine coccolithophore ''Emiliania huxleyi''.Volkman, J.K., Eglinton, G., Corner, E.D.S. and Sargent J.R. (1980) "Novel unsaturated straight-chain C₃₇–C₃₉ methyl and ethyl ketones in marine sediments and a coccolithophore ''Emiliania huxleyi''." In ''Advances in Organic Geochemistry 1979'' (eds. A.G. Douglas and J.R. Maxwell). Pergamon Press, Oxford, pp. 219–227; doi:10.1016/0079-1946(79)90106-X. The earliest known occurrence of alkenones is during the Aptian 120 million years ago.Brassell S.C., M. Dumitrescu, and ODP Leg 198 Shipboard Science Party (2004) "Recognition of alkenones in a lower Aptian porcellanite from the west-central Pacific." ''Organic Geochemistry'' 35, 181–188; doi:10.1016/j.orggeochem.2003.09.003. They are used in organic geochemistry as a proxy for past sea surface temperature.

Alkenone-producing species respond to changes in their environment — including to changes in water temperature — by altering the relative proportions of the different alkenones they produce. At higher temperatures more saturated alkenones are produced proportionally. This means that the relative degree of unsaturation of alkenones can be used to estimate the temperature of the water in which the alkenone-producing organisms grew.Brassell, S.C., Eglinton, G., Marlowe, I.T., Pflaumann,U. and Sarnthein, M. (1986) "Molecular stratigraphy: a new tool for climatic assessment" ''Nature'' 320, 129–133; doi:10.1038/320129a0. The relative degree of unsaturation as first described (''U''^''K''₃₇) included the tetra unsaturated C₃₇ alkenone:

''U''^''K''₃₇ = (C_37:2 - C_37:4)/(C_37:2 + C_37:3 + C_37:4)

However, a simplified Unsaturation Index (''U''^''K''′₃₇), generally more useful in marine settings, is based on di- versus tri- unsaturated C₃₇ alkenones and defined as:

:''U''^''K''′₃₇ = C_37:2/(C_37:2 + C_37:3) Prahl, F.G. and Wakeham, S.G. (1987) "Calibration of unsaturation patterns in long-chain ketone compositions for palaeotemperature assessment." ''Nature'' 330, 367–369 doi:10.1038/330367a0.

The ''U''^''K''′₃₇ can then be used to estimate sea surface temperature according to an empirical relationship determined from core-top calibrations. The most commonly used calibration is that of Müller et al., 1998:

:''U''^''K''′₃₇ = 0.033''T'' °C+ 0.044 Müller, P.J., Kirst, G., Ruhland, G., von Storch, I., Rosell-Melé, A., 1998. "Calibration of the alkenone paleotemperature index ''U''^''K''′₃₇ based on core-tops from the eastern South Atlantic and the global ocean (60°N–60°S)." ''Geochimica et Cosmochimica Acta'' 62, 1757–1772. doi:10.1016/S0016-7037(98)00097-0.

The Müller et al. (1998) calibration is not suitable for all environments and, in particular, different calibrations are required for high latitudes and lacustrine settings.

References

* Bradley, S R. (1999) ''Paleoclimatology: Reconstructing Climates of the Quaternary.'' Second edition. Academic Press

External links

*{{Commons category-inline, Alkenone biolipids

Paleoclimatology