Alkenone

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Alkenones are long-chain unsaturated methyl and ethyl n-ketones produced by a few phytoplankton species of the class Prymnesiophyceae. [1] Alkenones typically contain between 35 and 41 carbon atoms and with between two and four double bonds. [2] 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. [3] [4] Alkenones were first described in ocean sediments recovered from Walvis Ridge [5] and then shortly afterwards in cultures of the marine coccolithophore Gephyrocapsa huxleyi . [6] The earliest known occurrence of alkenones is during the Aptian 120 million years ago. [7] They are used in organic geochemistry as a proxy for past sea surface temperature.

Contents

The chemical structure of a 37:3 alkenone, (8E,15E,22E)-heptatriaconta-8,15,22-trien-2-one, C37H68O Alkenone-37-3-2D-skeletal.png
The chemical structure of a 37:3 alkenone, (8E,15E,22E)-heptatriaconta-8,15,22-trien-2-one, C37H68O

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. [8] The relative degree of unsaturation as first described (UK37) included the tetra unsaturated C37 alkenone:

UK37 = (C37:2 - C37:4)/(C37:2 + C37:3 + C37:4) [8]

However, a simplified Unsaturation Index (UK37), generally more useful in marine settings, is based on di- versus tri- unsaturated C37 alkenones and defined as:

UK37 = C37:2/(C37:2 + C37:3) [9]

The UK37 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:

UK37 = 0.033T [°C] + 0.044 [10]

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.

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Coccolithophores, or coccolithophorids, are single-celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the plankton community. They form a group of about 200 species, and belong either to the kingdom Protista, according to Robert Whittaker's five-kingdom system, or clade Hacrobia, according to a newer biological classification system. Within the Hacrobia, the coccolithophores are in the phylum or division Haptophyta, class Prymnesiophyceae. Coccolithophores are almost exclusively marine, are photosynthetic, and exist in large numbers throughout the sunlight zone of the ocean.

<span class="mw-page-title-main">Haptophyte</span> Type of algae

The haptophytes, classified either as the Haptophyta, Haptophytina or Prymnesiophyta, are a clade of algae.

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<i>Gephyrocapsa huxleyi</i> Unicellular algae responsible for the formation of chalk

Gephyrocapsa huxleyi, formerly called Emiliania huxleyi, is a species of coccolithophore found in almost all ocean ecosystems from the equator to sub-polar regions, and from nutrient rich upwelling zones to nutrient poor oligotrophic waters. It is one of thousands of different photosynthetic plankton that freely drift in the photic zone of the ocean, forming the basis of virtually all marine food webs. It is studied for the extensive blooms it forms in nutrient-depleted waters after the reformation of the summer thermocline. Like other coccolithophores, E. huxleyi is a single-celled phytoplankton covered with uniquely ornamented calcite disks called coccoliths. Individual coccoliths are abundant in marine sediments although complete coccospheres are more unusual. In the case of E. huxleyi, not only the shell, but also the soft part of the organism may be recorded in sediments. It produces a group of chemical compounds that are very resistant to decomposition. These chemical compounds, known as alkenones, can be found in marine sediments long after other soft parts of the organisms have decomposed. Alkenones are most commonly used by earth scientists as a means to estimate past sea surface temperatures.

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References

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Further reading