Alkenones are long-chain unsaturated methyl and ethyl n-ketones produced by a few phytoplankton species of the class Prymnesiophyceae. Alkenones have been observed containing between 35 and 41 carbon atoms and with between one and four double bonds. 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. Alkenones were first described in ocean sediments recovered from Walvis Ridge and then shortly afterwards in cultures of the marine coccolithophore Emiliania huxleyi. The earliest known occurrence of alkenones is during the Aptian 120 million years ago. 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. The relative degree of unsaturation is typically described as an Unsaturation Index of di- versus tri- unsaturated C37 alkenones according to:
- UK′37 = C37:2/(C37:2 + C37:3) 
The UK′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:
- UK′37 = 0.033T [°C] + 0.044 
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.
- 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.
- 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.
- 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.
- de 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.
- 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.
- 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.
- 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.
- 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.
- Müller, P.J., Kirst, G., Ruhland, G., von Storch, I., Rosell-Melé, A., 1998. "Calibration of the alkenone paleotemperature index UK′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.
- Bradley, S R. (1999) Paleoclimatology: Reconstructing Climates of the Quaternary. Second edition. Academic Press
- Media related to Alkenones at Wikimedia Commons