Alkenone and alkenoic acid compositions of the membrane fractions of Emiliania huxleyi

Abstract

The lipid classes and unsaturation ratios of long-chain alkenones ( nC 37–C 39), related alkyl alkenoate compounds ( nC 37–C 38) and alkenoic acids ( nC 14–C 22) were determined in isolated membrane and organelle fractions of Emiliania huxleyi. The percentage distribution of these compounds was predominantly high in the endoplasmic reticulum (ER) and coccolith-producing compartment (CPC)-rich membrane fraction, although alkenones and alkenoates could be detected in all membrane fractions. In particular, the alkenones were mainly located in CPC, since their distribution was closely correlated with that of uronic acids which are markers of CPC. In contrast, the alkenoic acids seemed to be mainly located in chloroplast (thylakoid)-rich fractions. The alkenone unsaturation ratio and the ratio of alkenoates to alkenones were similar in all fractions, while the unsaturation ratio of alkenoic acids in the thylakoid-rich and plasma membrane (PM)/Golgi body-rich fractions was overwhelmingly higher than that in the ER/CPC-rich fractions. Thus, alkenoic acids seemed to be typical membrane-bound lipids, and could be closely related to photosynthesis and involved in regulating membrane fluidity and rigidity in E. huxleyi. It is presumed from these results that the alkenones and alkenoates were membrane-unbound lipids that might be associated with the function of CPC.