Free radical oxidation (autoxidation) of alkenones and other lipids in cells of Emiliania huxleyi

Cells of the coccolithophorid Emiliania huxleyi strain CS-57 grown under an atmosphere of air + 0.5% CO2 showed oxidative damage after 10 days growth with concomitant and major changes to the lipid composition. The fatty acid profile was strongly altered and lacked appreciable amounts of the polyunsaturated fatty acids (PUFA: C18:5, C18:3 and C22:6) typical of healthy cells. Oxidation products of these PUFA could not be detected, but monounsaturated fatty acids proved to be good indicators of oxidative processes. The presence (after NaBH4-reduction) of a high proportion of 11-hydroxyoctadec-cis-9-enoic and 8-hydroxyoctadec-cis-9-enoic acids showed that the degradation of oleic acid involved mainly free radical oxidation processes (70-75% autoxidation and 20-25% photooxidation). We also detected large amounts of degradation products of the oxidation product 9,10-epoxyoctadecanoic acid including diols, methoxyhydrins and chlorohydrins. These oxidative effects were found in all the lipid classes examined. Products included significant amounts of chlorophyll side-chain autooxidation products Z- and E-3,7,11,15-tetramethylhexadec-3-ene-1,2-diols and Z-and E-3,7,11,15-tetramethylhexadec-2-ene-1,4-diols, while phytyldiol was present in relatively low proportions. Δ5-3β,7-epimeric unsaturated steroidal diols arising from the autooxidation of the Δ5 double bond of epi-brassicasterol and minor amounts of Δ4-3β,6-diols were also detected. Long-chain unsaturated ketone (alkenone) content per cell was much higher in the presence of 0.5% CO2 likely due to carbon storage under these conditions. The proportions of di- and tri-unsaturated alkenones was relatively stable throughout the growth cycle in the absence of additional CO2, but not when grown with 0.5% CO2. The detection of characteristic alkenone autoxidation products in cells grown under these latter conditions allowed us to attribute the significant increase in index observed to the involvement of free radical oxidation processes.