STEROL BIOMARKERS FOR MICROALGAE FROM THE GREEN ALGAL CLASS PRASINOPHYCEAE

Microscopic green algae from the class Prasinophyceae have recently be en recognized as major constituents of oceanic phytoplankton and hence a source of sterols and other lipids in seawater and sediments. To au gment the limited biochemical data available for this algal class, the sterols in five species (viz. Tetraselmis chui, Pyramimonas cordata, Micromonas pusilla, Micromonas aff. pusilla and Pycnococcus provasolii ) were analysed by capillary gas chromatography-mass spectrometry. Rel atively simple distributions of Delta(5)-steroids predominated, which allowed the species to be divided into 4 chemotaxonomic groups. The ma jor sterols were 24-methylcholesta-5,24(28)-dien-3 beta-ol (which also occurs in diatoms), 24-methylcholest-5-en-3 beta-ol and 24-ethylchole sta-5,24(28)Z-dien-3 beta-ol (28-isofucosterol). 24-Methylcholest-5-en -3 beta-ol may be a useful marker for these microalgae, except in thos e coastal areas where higher plant inputs of sterols could be expected . Three species contained small amounts of a dihydroxylated C-29 stero l identified at 24-ethylcholesta-5,28(29)-dien-3 beta,24-diol (saringo sterol), with the highest abundance in the tropical Australian isolate Micromonas aff. pusilla (14.2%). This is the first report of this unu sual sterol in microalgae and it seems likely that saringosterol is fo rmed from enzymatic oxidation of 28-isofucosterol. Hydrogenation of th e neutral lipid fraction yielded C-C-27-(29) steranes, plus minor amou nts of C-30 n-propylcholestane indicating the presence of uncharacteri zed C-30 sterols. Tricyclic hydrocarbons were not found supporting oth er work which suggested that prasinophytes are not the source of these compounds in geological samples.