C-30-C-32 ALKYL DIOLS AND UNSATURATED ALCOHOLS IN MICROALGAE OF THE CLASS EUSTIGMATOPHYCEAE

A study of the neutral lipids of the marine microalgae Nannochloropsis oculata Droop (2 strains), Nannochloropsis salina Hibberd and an unnamed species, all from the little-studied yellow-green algal class Eustigmatophyceae (Chromophyta), has shown that these microalgae contain unusual C30-C32 1,15-alkyl diols and a monounsaturated C32 1,15-diol. Alkane diols are abundant in many marine sediments, but this is the first report of their occurrence in cultured microalgae. This result argues against the suggestion, based on the identification of the same diols in a natural bloom of the cyanobacterium Aphanizomenon flos-aquae from the Baltic Sea, that these compounds are specific cyanobacterial markers. C32 homologues were more abundant than C30 homologues in the eustigmatophytes, as in the field sample of A. flos -aquae, whereas in most sediments the reverse is found. This suggests that the biological origin for these compounds in sediments may well be microalgae other than those studied here. The diols do not occur as free lipids in the eustigmatophytes, at least under the culture conditions used, but they could be liberated from more polar lipids by acid hydrolysis. In contrast, the major form of the alkyl diols in sediments appears to be as free lipids and not esters. Acid hydrolysis also liberated C30-C32 mono and diunsaturated straight-chain alcohols. The carbon number distributions of the alcohols and diols are identical, and the relative proportions of homologues are similar, suggesting that both compound classes are formed by the same biosynthetic pathway. The sterol compositions are also unusual with cholesterol comprising about 75% of the total sterols. C28 sterols were not detected, but small amounts of the C29 sterols 24-ethylcholesta-5,24(28)E-dien-3-beta-ol and 24-ethylcholesta-5,24(28)Z-dien-3-beta-ol were found in all algae. N. salina also contained 24-ethylcholesterol.