New alkenones and derivatives in haptophytes
Abstract
The alkenones comprise an unusual class of long-chain unsaturated methyl and ethyl ketones that are synthesized by a limited number of haptophyte microalgae. The relative abundance of the two main C37:2 and C37:3 alkenones expressed as the index [C37:2]/([C37:2] + [C37:3]) is now widely used for reconstructing sea surface paleotemperatures. A new technique involving NaBH4 reduction of alkenones to the corresponding alkenols and subsequent silylation was recently proposed for the characterisation and quantification of alkenones in natural samples (1). This reduction-silylation technique has proven to be very useful for the characterisation of previously unreported alkenones in microalgae, sediments and seawater. A re-examination of extracts of several haptophytes after this treatment allowed us to detect small amounts of C35-C38 monounsaturated alkenones. We could show that the double bonds of these compounds are present at w18 position instead of the expected w15 position. We have found large amounts of alkenols in extracts of some non-reduced sediments from Camargue (France); these compounds may result from a natural contribution of haptophytes or a non-selective bacterial reduction of alkenones under anoxic conditions. The possibility of a natural contribution of alkenols from haptophytes was tested and we were able to detect traces of alkenols in non-reduced extracts of Emiliania huxleyi, Gephyrocapsa oceanica and Isochrysis galbana. We examined also a benthic haptophyte Chrysotila lamellosa. The amounts of alkenols appeared to increase significantly during the senescence of C. lamellosa to reach 7% of the corresponding alkenones. In this species, we have also found non-negligible amounts of alkendiones and epoxyalkenones. LiAlD4 reduction of these compounds allowed us to characterise the positions of their additional keto and epoxy groups. Bacterial incubations under denitrifying conditions were also carried out in order to test the possibility of a bacterial reduction of alkenones to the corresponding alkenols. On the basis of these different results, the origin of alkenols in sediments is discussed. 1- Rontani, J.-F., Marchand, D., Volkman, J.K., 2001. NaBH4 reduction of alkenones to the corresponding alkenols: a useful tool for their characterisation in natural samples. Organic Geochemistry 32, 1329-1341.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.......79R