Free radical oxidation (autoxidation) of alkenones and other microalgal lipids in seawater

The free radical oxidation (autoxidation) of long chain alkenones derived from haptophyte microalgae was studied in the laboratory in the presence of a radical initiator (di-tert-butyl nitroxide) and a radical enhancer (tert-butyl hydroperoxide). Alkenones were autoxidized faster than analogues of other common marine lipids such as phytyl acetate, methyl oleate and cholesteryl acetate. Alkenone autoxidation rates increased in an approximately linear fashion with the number of double bonds present in the molecule and so autoxidation has the potential to affect values of . The process seems to involve allylic hydrogen abstraction and subsequent oxidation of the allylic radical to form products that can be analysed as alkene polyols after NaBH4 reduction. Evidence for the autoxidation of lipids in seawater was obtained from analysis of organic matter in sediment traps deployed at the DYFAMED site in the northwestern Mediterranean Sea. High contents of specific autoxidation products of monounsaturated fatty acids and the chlorophyll phytyl side chain were recognized in samples collected during post-bloom conditions, but not during a phytoplankton bloom event. Biomarkers for autoxidation include the presence of 8- and 11-hydroxyoctadec-cis-9-enoic acids and various phytol diols and triols. These same samples also showed anomalously high values (equivalent to an inferred temperature change of at least 2 °C), consistent with the results of the autoxidation experiments, but opposite to that expected for any effect due to nutrient limitation on haptophyte cells. Our work demonstrates that autoxidation of lipids may occur in seawater and that this has the potential to affect alkenone distributions, especially where residence times of particles are long. In such circumstances this might lead to a warm bias in estimates of palaeotemperature derived from alkenone ratios in sediments.