Sterol biomarkers for microalgae from the green algal class Prasinophyceae

Abstract

Microscopic green algae from the class Prasinophyceae have recently been recognized as major constituents of oceanic phytoplankton and hence a source of sterols and other lipids in seawater and sediments. To augment 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. Relatively simple distributions of Δ 5-steroids predominated, which allowed the species to be divided into 4 chemotaxonomic groups. The major sterols were 24-methylcholesta-5,24(28)-dien-3β-ol (which also occurs in diatoms), 24-methylcholest-5-en-3β-ol and 24-ethylcholesta-5,24(28)Z-dien-3β-ol (28-isofucosterol). 24-Methylcholest-5-en-3β-ol may be a useful marker for these microalgae, except in those coastal areas where higher plant inputs of sterols could be expected. Three species contained small amounts of a dihydroxylated C 29 sterol identified at 24-ethylcholesta-5,28(29)-dien-3β,24-diol (saringosterol), with the highest abundance in the tropical Australian isolate Micromonas aff. pusilla (14.2%). This is the first report of this unusual sterol in microalgae and it seems likely that saringosterol is formed from enzymatic oxidation of 28-isofucosterol. Hydrogenation of the neutral lipid fraction yielded C 27C 29 stenanes, plus minor amounts of C 30 n-propylcholestane indicating the presence of uncharacterized C 30 sterols. Tricyclic hydrocarbons were not found supporting other work which suggested that prasinophytes are not the source of these compounds in geological samples.