Long-chain alkenes of the haptophytes Isochrysis galbana and Emiliania huxleyi

The major alkenes of the haptophytes Isochrysis galbana (strain CCAP 927/14) and Emiliania huxleyi (strains CCAP 920/2 and VAN 556) have been identified by nuclear magnetic resonance spectroscopy and by mass spectrometric analysis of their dimethyl disulfide adducts. The dominant alkene in I. galbana is (22Z)-1,22-hentriacontadiene, with 1,24-hentriacontadiene and 1,24-tritriacontadiene present in much lower abundance; (22Z)-1,22-hentriacontadiene also occurs in E. huxleyi (strain CCAP 920/2), together with (2Z,22Z)-2,22-hentriacontadiene (the major hydrocarbon) and (3Z,22Z)-3,22-hentriacontadiene. Minor abundances of 2,24-hentriacontadiene and 2,24-tritriacontadiene are also present in this strain. In contrast, the dominant alkene in E. huxleyi (strain VAN 556) is (15E,22E)-1,16,23-heptatriacontatriene with the related alkatriene 1,15,22-octatriacontatriene also present and (22Z)-1,22-hentriacontadiene occurring as a minor component. From structural relationships (15E,22E)-1,15,22-heptatriacontatriene is proposed to derive from the same biosynthetic pathway as that of the characteristic C37 alkenones which occur in both E. huxleyi and I. galbana. The C31 and C33 dienes likely derive from chain extension and decarboxylation of (Z)-9-octadecenoic acid or (Z)-7-hexadecenoic acid, using a pathway analogous to that elucidated previously in the chlorophyte Botryococcus braunii. Therefore, long-chain dienes and trienes, which can co-occur in haptophytes, may have distinct biosynthetic pathways.