{{Short description|Class of chemical compounds}} {{about|alkenone biomolecules having applications in paleothermometry|the class of organic compounds in general|Enone}} '''Alkenones''' are long-chain unsaturated methyl and ethyl ''n''-ketones produced by a few phytoplankton species of the class Prymnesiophyceae.<ref name="Malowe et al. (1984)">{{cite journal | doi = 10.1080/00071618400650221 | title = Long chain (''n''-C<sub>37</sub>–C<sub>39</sub>) alkenones in the Prymnesiophyceae. Distribution of alkenones and other lipids and their taxonomic significance | date = 1984 | last1 = Marlowe | first1 = I.T. | last2 = Green | first2 = J.C. | last3 = Neal | first3 = A.C. | last4 = Brassell | first4 = S.C. | last5 = Eglinton | first5 = G. | last6 = Course | first6 = P.A. | journal = British Phycological Journal | volume = 19 | issue = 3 | pages = 203–216 | doi-access = free }}</ref> Alkenones typically contain between 35 and 41 carbon atoms and with between two and four double bonds.<ref name="Rontani et al. (2006)">{{cite journal | doi = 10.1111/j.1529-8817.2006.00251.x | title = RE‐EXAMINATION OF THE DOUBLE BOND POSITIONS IN ALKENONES AND DERIVATIVES: BIOSYNTHETIC IMPLICATIONS | date = 2006 | last1 = Rontani | first1 = Jean‐François | last2 = Prahl | first2 = Fredrick G. | last3 = Volkman | first3 = John K. | journal = Journal of Phycology | volume = 42 | issue = 4 | pages = 800–813 | s2cid = 84316762 | doi-access = free}}</ref> 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.<ref name="Epstein et al. (2001)">{{cite journal | doi = 10.1016/S0146-6380(01)00026-2 | title = The possible metabolic role of C37 alkenones in Emiliania huxleyi | date = 2001 | last1 = Epstein | first1 = B.L. | last2 = d'Hondt | first2 = S. | last3 = Hargraves | first3 = P.E. | journal = Organic Geochemistry | volume = 32 | issue = 6 | pages = 867–875 | bibcode = 2001OrGeo..32..867E }}</ref><ref name="Eltgroth et al. (2005)">{{cite journal | doi = 10.1111/j.1529-8817.2005.00128.x | title = PRODUCTION AND CELLULAR LOCALIZATION OF NEUTRAL LONG‐CHAIN LIPIDS IN THE HAPTOPHYTE Algae ''ISOCHRYSIS GALBANA'' AND ''EMILIANIA HUXLEYI''| date = 2005 | last1 = Eltgroth | first1 = Matthew L. | last2 = Watwood | first2 = Robin L. | last3 = Wolfe | first3 = Gordon V. | journal = Journal of Phycology | volume = 41 | issue = 5 | pages = 1000–1009 | s2cid = 22092773}}</ref> Alkenones were first described in ocean sediments recovered from Walvis Ridge<ref name="De Leeuw et al. (1980)">{{cite journal | doi = 10.1016/0079-1946(79)90105-8 | title = On the occurrence and structural identification of long chain unsaturated ketones and hydrocarbons in sediments | date = 1980 | last1 = De Leeuw | first1 = J.W. | last2 = v.d. Meer | first2 = F.W. | last3 = Rijpstra | first3 = W.I.C. | last4 = Schenck | first4 = P.A. | journal = Physics and Chemistry of the Earth | volume = 12 | pages = 211–217 | bibcode = 1980PCE....12..211D }}</ref> and then shortly afterwards in cultures of the marine coccolithophore ''Gephyrocapsa huxleyi''.<ref name="Volkman et al. (1980)">{{cite journal | doi = 10.1016/0079-1946(79)90106-X | title = Novel unsaturated straight-chain C37C39 methyl and ethyl ketones in marine sediments and a coccolithophore Emiliania huxleyi | date = 1980 | last1 = Volkman | first1 = J.K. | last2 = Eglinton | first2 = G. | last3 = Corner | first3 = E.D.S. | last4 = Sargent | first4 = J.R. | journal = Physics and Chemistry of the Earth | volume = 12 | pages = 219–227 | bibcode = 1980PCE....12..219V }}</ref> The earliest known occurrence of alkenones is during the Aptian 120 million years ago.<ref>{{cite journal | doi = 10.1016/j.orggeochem.2003.09.003 | title = Recognition of alkenones in a lower Aptian porcellanite from the west-central Pacific | date = 2004 | last1 = Brassell | first1 = Simon C. | last2 = Dumitrescu | first2 = Mirela | journal = Organic Geochemistry | volume = 35 | issue = 2 | pages = 181–188 }}</ref> They are used in organic geochemistry as a proxy for past sea surface temperature.

:thumb|left|600px|The chemical structure of a 37:3 alkenone, (8E,15E,22E)-heptatriaconta-8,15,22-trien-2-one, C<sub>37</sub>H<sub>68</sub>O{{clear-left}}

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. <!--Replaced sentence: At higher temperatures a greater relative proportion of less unsaturated alkenones is produced. -->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.<ref name="Brassel et al. (1986)">{{cite journal | doi = 10.1038/320129a0 | title = Molecular stratigraphy: A new tool for climatic assessment | date = 1986 | last1 = Brassell | first1 = S. C. | last2 = Eglinton | first2 = G. | last3 = Marlowe | first3 = I. T. | last4 = Pflaumann | first4 = U. | last5 = Sarnthein | first5 = M. | journal = Nature | volume = 320 | issue = 6058 | pages = 129–133 | bibcode = 1986Natur.320..129B | s2cid = 4366905 }}</ref> The relative degree of unsaturation as first described (''U''<sup>''K''</sup><sub>37</sub>) included the tetra unsaturated C<sub>37</sub> alkenone:

''U''<sup>''K''</sup><sub>37</sub>&nbsp;= (C<sub>37:2</sub> - C<sub>37:4</sub>)/(C<sub>37:2</sub>&nbsp;+ C<sub>37:3</sub> + C<sub>37:4</sub>) <ref name="Brassel et al. (1986)" />

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

:''U''<sup>''K''′</sup><sub>37</sub>&nbsp;= C<sub>37:2</sub>/(C<sub>37:2</sub>&nbsp;+ C<sub>37:3</sub>) <ref name="Prahl and Wakenham (1987)">{{cite journal | doi = 10.1038/330367a0 | title = Calibration of unsaturation patterns in long-chain ketone compositions for palaeotemperature assessment | date = 1987 | last1 = Prahl | first1 = F. G. | last2 = Wakeham | first2 = S. G. | journal = Nature | volume = 330 | issue = 6146 | pages = 367–369 | bibcode = 1987Natur.330..367P }}</ref>

The ''U''<sup>''K''′</sup><sub>37</sub> 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''<sup>''K''′</sup><sub>37</sub> = 0.033''T'' [°C] + 0.044 <ref name="Müller et al. (1988)">{{cite journal | doi = 10.1016/S0016-7037(98)00097-0 | title = Calibration of the alkenone paleotemperature index U37K′ based on core-tops from the eastern South Atlantic and the global ocean (60°N-60°S) | date = 1998 | last1 = Müller | first1 = Peter J. | last2 = Kirst | first2 = Georg | last3 = Ruhland | first3 = Götz | last4 = von Storch | first4 = Isabel | last5 = Rosell-Melé | first5 = Antoni | journal = Geochimica et Cosmochimica Acta | volume = 62 | issue = 10 | pages = 1757–1772 }}</ref>

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 == {{Reflist}}

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

==External links== *{{Commons category-inline}}

Category:Enones Category:Paleoclimatology